1. Optical Trapping and Quantum Sensing with
Nanodiamonds
Kazuma Wittick
z3460734
Supervisor: Dr. Peter J. Reece
Honours Thesis
The University of New South Wales
Nov 2016
2. Statement of Originality
‘I hereby declare that this submission is my own work and to the best of my knowl-
edge it contains no materials previously published or written by another person, or
substantial proportions of material which have been accepted for the award of any other
degree or diploma at UNSW or any other educational institution, except where due
acknowledgement is made in the thesis. Any contribution made to the research by
others, which whom I have worked at UNSW or elsewhere, is explictly acknowledged in
the thesis. I also declare that the intellectual content of this thesis is the product of my
own work, except to the extent that assistance from others in the project’s design and
conception in style, presentation and linguistic expression is acknowledged.’
Signed Date
3. Statement of Contribution
The following list details my contributions to this project.
ˆ All measurements (T1, ODMR, Spectroscopy, Single Photon Counting) that have
produced graphs in the results section of this thesis.
ˆ Schematics representing the experimental set-up that have not been referenced to
any person.
ˆ LabView program that communicates to the AOTF, with assistance from Dr. Peter
Reece.
ˆ Installation of the new MCS64A single photon counter and subsequent re-wiring
of cables to accommodate the change, with assistance from Drs Peter Reece and
David Simpson.
ˆ Mathematical modeling of population evolutions using Matlab, with kind help of
Yaroslav Kharkov (PhD, Theoretical Physics), who assisted with some mathemat-
ics and programming.
The following list details who has assisted in this project, or has provided relevant
material in full that I have not been a part of creating.
ˆ LabView programs for spin-lattice relaxation measurements and optically detected
magnetic resonance measurements were absolutely critical to the progress f this
project and have been generously provided by Dr. David Simpson from the physics
department of the University of Melbourne.
ˆ LabView programs that control the AOD, SLM and spectrometer were the back-
bone in operating the optical trap, and were created by Dr. Ana Andres Arroyo,
PhD (completed 2016).
ˆ Apart from very minor additions I made to the optical table (adjustable slits
used to stop the laser beam when required - this addition does not change the
configuration of the experimental set-up, and was only added for efficiency of
taking measurements), all credit for the set up goes to Drs Ana Andres-Arroyo,
Fan Wang, Wen Jun Toe, and previous honours student Simon Ralph.
ˆ An informal thanks to Simon Ralph, whose immense efforts last year has provided
me with a strong platform. A great deal of my knowledge can be attributed to his
work.
4. ‘Equipped with his five senses, man explores the universe around him and calls
the adventure Science.’
– Edwin Powell Hubble
Acknowledgements
First and foremost, I would like to extend my biggest thank you to Dr. Peter Reece,
who supervised my thesis project this year. I could not have picked a better supervisor
to share my honours year with - thank you for your patience, care and constant guidance
throughout the toughest year of university.
To David Simpson, the master of LabView programming and NV centers, I cannot
fathom how much time it would have taken me to develop a program to carry out these
measurements this year (probably forever). Thank you for that amazing program, and
for your consistent helping hand with providing insight and direction.
To the friends that helped me get through 2016 alive. Those countless moments of
laughter shared with during intense ping pong or card game sessions (Jarrod, Tristan,
Harry, Lachlan...the list goes on!). To those who could answer any and every question
from coursework that I could manage to throw at them - Yaroslav, you are an absolute
genius! One day, I will create a machine to sap all that quantum mechanics out of your
brain and into mine. Jarrod, you are the only friend who has taught me quantum me-
chanics AND game mechanics. I hope our gaming sessions never cease! Simon, I think
I would still be twiddling my thumbs trying to learn NV centers if it wasn’t for your
brilliantly written thesis and constant answers to my questions. Thank you to all!
And most importantly, my family and Wennie, who have undoubtedly been the best
support that any student could dream of. Thank you for putting up with me during
such a long education career (you’re lucky I’m not considering a PhD!) and for your
provision in all aspects not only during university time, but of course the 18 years prior.
5. Abstract
The Optical Trapping of nano-particles provides a strong platform for enhancing preci-
sion measurements on the nano-scale. In recent years, the Nitrogen-Vacancy (NV) defect
center in diamond has attracted significant attention for its great potential in quan-
tum sensing applications in the field of biomedicine and quantum information. This
robust nano-probe demonstrates remarkable sensitivity and high spatial resolution in
thermometry and magnetometry, whilst producing photo-stable fluorescence in ambient
and extreme conditions unlike most alternatives. This project investigated firstly the
Spin-Lattice Relaxation (T1) Times of nano-diamond (ND) under three-dimensional con-
trol of the optical trap. Upon observing a non-trivial interaction of the Infra-Red (IR)
trapping laser with the optical dynamics of the NV center, baseline T1 measurements
were performed without the trap. Mathematical modelling of rate equations governing
an isolated NV− energy level system suggest the IR laser directly affects the population
evolution of the ground state triplet electrons. These results also affect the Optically De-
tected Magnetic Resonance (ODMR) measurements performed in this project. ODMR
of NDs were characterised without the trap for external influences such as magnetic
fields, microwave fields, IR laser and lattice strain. This groundwork, supported by any
further study on the role of the trapping laser regarding optical dynamics will contribute
to elucidating this field of research that is currently still developing.
6. Frequent Abbreviations
ND — NanoDiamond
NV — Nitrogen-Vacancy
PL — PhotoLuminescence
RF — Radio Frequency
IR — Infra-Red
ODMR — Optically Detected Magnetic Resonance
ZPL — Zero Phonon Line
NP — NanoParticle
ZFS — Zero Field Splitting
OT — Optical Trap/Trapping
SLM — Spatial Light Modulator
AOD — Acousto-Optic Deflector
AOTF — Acousto-Optic Tunable Filter
7. List of Figures
1 The Nitrogen Vacancy Center in a diamond lattice. . . . . . . . . . . . . . 12
2 Energy level diagram of NV Center. . . . . . . . . . . . . . . . . . . . . . 14
3 Photon anti-bunching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4 Comparison of Temperature and Nano-sensor size. . . . . . . . . . . . . . 17
5 Relationship between ND PL and trapping power. . . . . . . . . . . . . . 18
6 Band Diagram describing photo-ionisation of NV− into the NV0 center. . 19
7 NV Center Photoluminescence Spectrum. . . . . . . . . . . . . . . . . . . 20
8 Energy level diagram with corresponding transition rates . . . . . . . . . 21
9 Optical Trap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10 Holographic Optical Trap. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11 ODMR Spectrum with and without magnetic field. . . . . . . . . . . . . . 25
12 Relationship between angle of applied magnetic and the resonance frequency. 26
13 Affect of temperature on ODMR signal. . . . . . . . . . . . . . . . . . . . 27
14 ZFS parameter vs temperature for 5K - 296K. . . . . . . . . . . . . . . . . 28
15 Pulse Sequence and T1 decay time. . . . . . . . . . . . . . . . . . . . . . . 29
16 Complex pulse sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
17 Change in temperature with change in laser power, as detected by ND. . . 32
18 Scanning probe microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . 35
19 T1 decay affected by the presence of Gadolinium ions. . . . . . . . . . . . 36
20 Affect of magnetic fields on ODMR spectra. Vectorial Magnetometry. . . 37
21 Cell Tracking in HeLa cells. . . . . . . . . . . . . . . . . . . . . . . . . . . 39
22 UNSW Optical Tweezers Experimental Set-up. . . . . . . . . . . . . . . . 44
23 CCD Image of trapped ND. . . . . . . . . . . . . . . . . . . . . . . . . . . 50
24 Photon counting system set-up. . . . . . . . . . . . . . . . . . . . . . . . . 52
25 Timing for excitation laser pulse sequence and photon counting periods. . 56
26 Circuitry of equipment for ODMR experiment. . . . . . . . . . . . . . . . 59
27 NV− PL Spectrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
28 Comparison of PL between ND and dye molecules. . . . . . . . . . . . . . 63
29 PL Counts vs Time as NDs drift into the trap. . . . . . . . . . . . . . . . 64
30 The effect of increasing/decreasing the trapping laser power on the PL. . 65
31 Normalised pulse profiles during T1 experiment. . . . . . . . . . . . . . . . 68
32 Average number of PL counts per bin per second vs trapping power. . . . 69
33 T1 decays for decreasing trapping powers. . . . . . . . . . . . . . . . . . . 70
34 T1 Decays for different NDs without trap. . . . . . . . . . . . . . . . . . . 71
35 Modified Energy Levels of the NV center with notations included. . . . . 73
36 Population evolution of all energy levels. . . . . . . . . . . . . . . . . . . . 74
37 Pulse profile comparison for high and low IR power. . . . . . . . . . . . . 76
38 Mathematical model for population evolution. . . . . . . . . . . . . . . . . 77
39 Population evolution for various initial conditions and fixed b. . . . . . . . 78
40 Systematic error of drift in ODMR signal. . . . . . . . . . . . . . . . . . . 80
41 Broad ODMR signal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
42 ODMR spectrum for decreasing RF power. . . . . . . . . . . . . . . . . . 83
43 ODMR spectrum with two dips due to strain. . . . . . . . . . . . . . . . . 84
44 Effect of Magnetic Field on ODMR splitting. . . . . . . . . . . . . . . . . 85
45 ODMR Signal with and without IR laser . . . . . . . . . . . . . . . . . . . 87
10. 1 Motivation
In order to advance our understanding of complex biological and condensed matter
systems at the nano-scale, direct measurements with extreme precision are imperative.
Nano-diamond (ND) has recently demonstrated its promising abilities as a nano-probe in
thermometry and magnetometry, due to its capability in optically reading out quantum
states in the Nitrogen-Vacancy (NV) center. The NV center is a defect in diamond whose
electron energy levels have temperature and magnetic field dependence which directly
affects its fluorescence properties. Foundational control and measurement techniques
utilise the NV center’s extraordinarily robust properties to use it as a quantum sensor.
The NV center’s inertness, photo-stability and bio-compatibility opens doors to broader
applications in various fields, some of which are quantum information and computing,
quantum optics, biomedicine and the semi-conductor industry.
In the realm of nano-scale sensing where precision control of nanoparticles (NP)
are becoming pre-requisite, the Optical Trapping (OT) method is a novel approach to
the problem. Offering three-dimensional positional and orientation control, this tech-
nique facilitates further development of ND as a flexible nano-probe. The sensitivity
demonstrated by NV centers are remarkably comparable to other nano-probes, despite
its ability to operate at room temperature without the need of vacuum or cryogenic
temperatures, which the others require. So far, OT has allowed Geiselmann et al to
perform vectorial magnetometry alongside monitoring of NV axis orientation over ex-
tended periods of trap time [12]. It has also facilitated thermometry in living cells [23].
There exists an agreement with studies [19, 32, 45] that a magnetometer sensitivity of
5-50 µT/
√
Hz can be achieved. To put this into perspective, at a distance of 10 nm
an electron spin produces a magnetic field of about 1µT. For thermometry, a 5-10 mK
sensitivity has been recorded [23, 39]. However, an outstanding challenge has been the
stability of the trap due to brownian motion and external pertubations. Attempts to
overcome this can be made by performing novel spin relaxation measurements with the
optical tweezers, due to the prospect of increasing the nano-scale sensitivity of ND’s.
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2 The Nitrogen-Vacancy Center in Nano-Diamond
The Nitrogen-Vacancy (NV) colour center is one of several hundred natural defects in
diamond [36, 44]. A substitutional nitrogen adjacent to a carbon vacancy (see Figure 1)
in the diamond lattice [36] gives rise to excellent fluorescent properties. These charac-
teristics give significance to the NV center in ND, through demonstrations of quantum
behaviour at room temperature.
NV centers are produced naturally in diamond, however synthetic materials have
become increasingly available since the 1950’s through the introduction of methods such
as chemical vapour deposition (CVD), detonation synthesis and laser ablation [44, 35].
NV centers utilised for experimental purposes are now commonly created using type Ib
diamond crystals [24] with 100 ppm of nitrogen. When they are irradiated with a high
energy electron beam greater than 200 keV, vacancies form in the lattice [30]. Subse-
quent annealing at 800° encourages the vacancy to be trapped adjacent to the nitrogen
impurity, thus forming the NV center [53]. This way, the size of diamonds and concen-
tration of NV centers can be accurately controlled. It is important to note here that the
irradiation method inherently carries a change to induce strain in the ND lattice due to
the high energies involved [44] - some photo-physical parameters can be altered and thus
this effect is investigated in this thesis. The 1990’s was a decade of crucial development
in ND control - techniques were developed to colloidally suspend individual 4-5nm NDs
and implement them as non-toxic alternatives to biomedical imaging purposes [35].
Figure 1: The Nitrogen-Vacancy center. The diamond lattice of carbon atoms features
a substitutional nitrogen atom represented by the orange region, and adjacent to it is
the vacancy, represented by the transparent section. Figure from [2].
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13. Kazuma Wittick z3460734 Honours Thesis
3 Promising Properties
3.1 Opto-electronic properties
3.1.1 Energy Levels
The NV Center exists in one of two charge states - NV− and NV0, where the neutral
state is optically inactive. Thus, the negatively charged state is desired for fluorescence
experiments (The NV− will be written as NV from here onwards for convenience unless
specification is necessary). The electron ground and excited state energy levels are shown
in Figure 2. As shown, there are two paths for electron excitation and subsequent relax-
ation, depending from which spin angular momenta ms = 0, ±1 the electron is excited.
The zero-field splitting (ZFS, the energy difference between the ground state sub-levels
in the absence of an external magnetic field) is a well known value of 2.87 GHz. Due to
the quantum mechanical selection rule | l| = 0, an excitation from ground state ms = 0
would result in radiative decay from excited state ms = 0, producing the NV center’s
characteristic bright fluorescence in the 550-800nm region. [11].
Of course, its usefulness is limited without its corresponding non-radiative decay
route from the ms = ±1 excitation. Note that both pathways have probabilities to
undergo radiative or non-radiative decay - however, as shown in figure 2, one dominates
over the other.
The configuration of these NV energy levels allow us to observe the spin states of
the electrons through the amount of fluorescence. External parameters such as tem-
perature and magnetic field directly influence these energy levels and their subsequent
fluorescence. Thus, this circular relationship forms a strong platform for quantum sens-
ing. Furthermore, lasers can be used to optically ’pump’ the electrons into either state,
allowing manipulation of population levels. This adds a significant amount of control
over the nano-probe and offers improvements to precision in its already successful career
in nano-magnetometry and nano-thermometry.
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14. Kazuma Wittick z3460734 Honours Thesis
Figure 2: Schematic from [44] depicting the energy levels of NV- center. The ground and
excited state is separated by an energy of 1.945 eV, while the valence-to-conduction band
energy gap is 5.5 eV. The energy difference between the ms = ±1 states experiences
Zeeman splitting under the influence of an external magnetic field - given by 2γB.
Straight lines represent non-radiative decays, while squiggly lines represent radiative
decays.
3.1.2 Single Photon Emitter
An ideal Single Photon Emitter (SPE) would produce light such that if it passed a beam
splitter, two equidistant detectors behind it would never register simultaneous events
[24]. Although single photon detection has been available for the past 60 years or so,
the on-demand production of individual photons have only appeared recently [29], some
of which are discussed in section 3.3.
The finite time required for excitation and radiative decay naturally allows the two-
level NV energy system to be a great candidate as a single photon source. Hanbury
Brown and Twiss (HBT) experimental set ups are commonly used to analyse and con-
firm the SPE characteristics of the NV center. [46, 12, 24] This is done with two detectors
on the other side of a coherent light source, separated by a half-silvered mirror. The
two detectors measure the correlation and anti-correlation in the amount of photons
received, and produce graphs shown in figure 3.
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15. Kazuma Wittick z3460734 Honours Thesis
Figure 3: The measured correlation function is plotted against time τ, for different
excitation powers (a) 0.16 Psat, (b) 1.6 Psat, and (c) 30 Psat. The correlation function
is essentially the measure of probability that another photon will arrive at a detector,
given that a photon is received at τ = 0. The characteristic dip of SPE’s are shown in
all three graphs. Figure from [24].
SPE’s have applications in quantum cryptography, quantum optics, and optical quan-
tum computation [51, 29]. One particularly interesting application proposed by Rarity
et al was the quantum mechanical random number generator. A single photon passing a
50:50 beam splitter has equal probability to be reflected or transmitted. The quantum
mechanical nature of this scenario makes it truly random - it is worthy to note how chal-
lenging it is to completely remove external biases from any random generator. Thus, it
is speculated that efficient single photon emitters will improve the random generation
rate [42].
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16. Kazuma Wittick z3460734 Honours Thesis
3.2 Physical/Electrochemical flexibility
While the opto-electronic properties aforementioned allow for immaculate control over
the NV center, the robust nature of its physical and chemical properties are extremely
useful in applications such as biomedicine. The well known surface chemistry of com-
mercially available ND contains different functional groups such as hydroxyl, carboxyl,
lactone and ketone. The resulting affinity of ND for protein absorption (and cellular
uptake in general) [18] makes it relatively easy to conjugate with biomolecules, vaccines,
and polymers [3]. The bio-compatibility that ND offers allows biomedical processes such
as wide-field imaging of neuron activity to be non-invasive [17].
Furthermore, the size of ND’s are easily modifiable during synthesis or post process
by etching or oxidation [3]. Coupled with its inertness, ND proves its significance in
biological environments where it can be used unrestrictedly as a magnetic and fluorescent
nano-probe [33].
3.3 Comparisons to alternative nano-probes
As mentioned previously, various nano-probes such as semiconductor quantum dots
(QDs) and dye molecules have been popular candidates in recent research for their
fluorescence properties. Excitons (electron-hole pairs) in QDs give photo-luminescence
(PL) that can be used to probe local electro-magnetic fields, [47], and dye molecules
work through the deviation in radiative (and non-radiative) decay rates depending on
the adjacent NP [10]. However, they both suffer photo-bleaching [47] (the permanent
loss of the ability to fluoresce due to chemical damage) and are limited by their toxicity
to biological environments. Furthermore, dye molecules undergo rapid degradation at
room temperature [24]. However, Yu et al successfully demonstrated that single NV
centers did not show any sign of photo-bleaching even under excitation intensities of 5
MegaWatts (MW)/cm2 [53].
This great advantage that NV centers hold over other nano-probes is strengthened
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17. Kazuma Wittick z3460734 Honours Thesis
by NV’s ultra-long spin coherence time (at room temperature) which originates from its
nuclear spin-zero lattice made of 98.9 % Carbon-12 [43]. Furthermore, ND’s extreme
photo-stability towers over its competitors, making it a robust, on-demand SPE, capable
of quantum sensing with high precision.
Figure 4 is a plot describing the temperature accuracy and the relative size of several
viable quantum sensors. It is clear that even in the developing stages, NV centers are
superior in the accuracy-to-size ratio, and that they have huge potential in future works,
carrying a projected thermometry sensitivity on the order of micro-kelvins.
Figure 4: A temperature accuracy (K) vs sensor size (µm) plot, comparing numerous
quantum sensors. Note that sensing methods such as quantum dots or Scanning Thermal
Microscopy (SThM) are smaller in size, but have less accuracy than ND’s. Conversely,
although bulk diamond offers better accuracy, it does not have the nano-scale spatial
resolution and mobility that ND provides. Furthermore, ND’s are projected to reach an
advanced level of accuracy and size, shown by the open red circle. Figure from [23].
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18. Kazuma Wittick z3460734 Honours Thesis
4 Optical Dynamics
4.1 Interaction of IR laser in Optical Processes
The field of optical dynamics regarding the NV center is still developing, especially in
characterising the interplay between the IR trapping laser (1064 nm), the excitation laser
and the ND PL. This includes the population evolution of electrons in the energy levels
as well as the dynamics that govern which charge state (NV− or NV0) these electrons
are occupying. While there are results in literature that greatly support well-accepted
facts about the NV dynamics, there still exists debate - some are presented in this section.
Firstly, figure 5 displays the well-known phenomena of increasing PL with decreasing
IR trapping power, commonly seen in literature [37, 25, 13, 7]. The exponential rate at
which this PL changes with laser power is shown by figure 32 in the results section of
this thesis.
Figure 5: The relationship between ND PL and the optical trapping power. The char-
acteristic NV− spectrum clearly increases in magnitude (whilst keeping its shape) with
decrease in IR laser power. Figure from [41].
This phenomenon follows from two competing optical processes postulated in liter-
ature involving the interaction of the 1064nm trapping laser and the energy levels of
both NV charge states. Firstly, there has been evidence for quenching to be attributed
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19. Kazuma Wittick z3460734 Honours Thesis
to heating of the ND lattice via IR absorption, leading to temperature increases signifi-
cant enough to change its photo-physical parameters [25]. Alongside this, quenching is
suspected to occur as a result of the ionisation of NV− to NV0. This NV0 charge state
is often referred to as the ‘dark state’ because of its lack of spin-dependent fluorescence
- i.e. unlike NV−, you cannot spin-polarise NV0 in order to modulate its fluorescence.
The current understanding is that the photo-ionisation is a two-step (or two-photon)
procedure, whereby the first photon from the green excitation laser will provide energy
> 1.95 eV for the electron to reach the excited state (as shown by the green arrows in
figure 6), and a second photon with sufficient energy to excite into the conduction band.
Figure 6: Band diagram configuration of the NV− photo-ionisation process (right hand
side) from [21]. The left hand side describes the NV0 recombination back to NV−.
Green arrows represent excitations, whereas orange/red arrows indicate relaxations. The
relative energies between energy levels and bands are provided, except for some values
on the NV0 diagram where they currently remain unknown.
For there to be an optical cycle between both charge states, there must be a mech-
anism for the recombination of the neutral state back to the negative state. Ji et al
explains this recombination to be similar to the ionisation effect - whereby a photon will
excite NV0 to its excited state, followed by a promotion of an electron from the valence
band by a subsequent photon.
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20. Kazuma Wittick z3460734 Honours Thesis
Figure 7: Diagram from [21] displaying the photoluminescence behaviour of NV- and
NV0 centers. The PL from each charge state is highlighted in its respective region. The
green line indicates PL with no IR laser, and red vice versa. NV − ZPL = 638nm.
As seen in figure 5, it had been made quite clear the fact that IR illumination causes
a decrease in PL from NV− center. However, Ji et al have acquired results that show
the exact opposite to current literature (as well as the characteristics observed at UNSW
for this project). Figure 7 clearly shows an increase in counts for the negatively charged
state with the presence of the IR laser (red line) [21]. These new found observations
dated at April 7th 2016 present a mystery to this field of research, and the explanation
regarding the explicit differences between this and previous studies are still developing.
4.2 Population Dynamics
A great way to model the fluorescence dynamics of the NV center is through its energy
level population. Since the characteristic red PL of NV centers originates from the
relaxation transition from the 3E excited state triplet down to the 3A ground state
triplet, the fluorescence response to an excitation laser can be modelled by investigating
the populations of these excited states. Tetienne et al have taken steps along this path by
experimentally measuring radiative rates of transitions for each possible path in the NV−
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system [49] from the fit of time-resolved PL traces. These values, and their corresponding
transition are marked in figure 8. Notice here that several assumptions have been made
with the rates k0
ij. Firstly, the rate of excitations from ground states and relaxations
from excited states (whether it be from ms = 0 or ±1) are all equal, and denoted k0
r .
Secondly, ignore any differences in transition rates between the ±1 sublevels and the
metastable singlet state - that is, k0
57 = k0
67, k0
72 = k0
73. Thirdly, account for the optical
pumping provided by an excitation laser source by the parameter β, multiplied onto the
corresponding transition rate k0
r . These experimental results by Tetienne et al will be
referred to in the results section of this thesis.
Figure 8: Energy level diagram from [49] labelled with corresponding transition rates,
alongside experimental values of these rates. (b) describes a 5-level system in absense of
a magnetic field, whereas in (c) a non-zero magnetic field splits the ±1 levels to make
it a 7-level system. Four different NV defects were investigated, oriented at an angle θ
along the magnetic field.
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5 Existing Foundational Measurement Techniques
The experimental techniques discussed in this section are foundational and imperative to
NV-based magnetometry and thermometry. They utilise the properties aforementioned
to initialise, three-dimensionally manipulate, and optically read-out spin states of the
nano-diamond.
5.1 Optical Trapping
The fundamentals of optical trapping rely on a high numerical aperture objective lens
to tightly focus a laser beam [38]. A particle in vicinity of the beam focus will absorb
and spontaneously re-radiate photons, resulting in a net driving force aligned to the
direction of incident light. This force is broken down into two main forces - radiation
pressure, and gradient force. The radiation pressure is proportional to the light intensity,
while the gradient force is related to the spatial gradient of light intensity over the trap
area [5, 9]. In fact, these forces were experimentally demonstrated in 1901 by Lebedev,
Nichols and Hull - a torsion balance was moved from its equilibrium position as a result
of focused thermal light sources [31]. Unfortunately, this effect was not utilised until
Arthur Ashkin in 1970 optically manipulated micro-scale particles.
A steady state position for the trap exists only if the gradient force dominates over
the radiation pressure - this is shown in figure 9. To do this, a high numerical aperture
(NA) and magnification objective is required. Furthermore, a particle with favourable
polarisability, dielectric constant and refractive index should be used [41]. If the trapping
method involves suspension of NDs in a fluid on a microscope slide, viscosity should be
considered as a damping mechanism to reduce the uncertainty in measurements caused
by brownian motion.
The aforementioned spatial gradient can be described by the gradient of electric field
density and spatial polarisation gradient. All forces combined give a time averaged force
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23. Kazuma Wittick z3460734 Honours Thesis
given by:
< F >=
1
2
Re(α) |E|2
+
σ
2c
Re(E × H∗
) +
σc 0
4ωi
× E × E∗
(1)
Where α is the polarisability of the particle, ω is the angular frequency of the optical
field, E is the electric field, and H is the magnetic field [31]. This force is parallel to the
direction of propagation, acting in such a way that the NP is attracted to the region of
highest light intensity.
The OT serves as a great platform in improving the precision of measurements that
had previously been done without them. It is complemented by the NV center’s remark-
able stability in the OT, which was demonstrated by Geiselmann et al. They found that
the orientation of the trapped ND remained almost fixed over a 30 minute time interval
- within ± 5°. The stability allowed for precise manipulation of the NV center axis by
changing the polarisation of the trapping laser [12].
Figure 9: An optical trap, with the laser beam drawn in a hyperboloid structure. As
mentioned previously, the colloidal particle will experience a force towards the focus of
the laser beam if the gradient force dominates. If the radiation pressure dominates the
particle will be pushed in the direction of propagation of light. Figure from [14].
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5.1.1 Dual Trapping Techniques
Research in 2012 by Curtis et al has significantly boosted the capabilities of optical traps
by allowing more than one trapping site [9]. These ‘holographic’ optical tweezers feature
a diffractive optical element (designed on the computer) which splits a single trapping
laser beam into separate beams in such a way that it creates multiple trapping locations.
The ability to control more than one particle at a time has huge implications on future
work in quantum sensing due to the utmost control of the local environment of an NV
probe. Figure 10 demonstrates the capability of the holographic optical trap.
Figure 10: 26 colloidal silica spheres of 0.99 µm diameter are individually control to
transform from a star shaped pattern to a circular pattern. (a) and (c) represent the
start and end totaling 38 steps, (b) describes the shape after 16 steps. Figure from [9].
5.2 Optically Detected Magnetic Resonance
Optically Detected Magnetic Resonance (ODMR) is an optical technique capable of
ultra-sensitive measurements, by utilising the temperature dependence of the relative
energy difference between the ms = 0 and ms = ±1 states [1]. It utilises Electron
Spin Resonance (ESR) in the NV center by examining the change in fluorescence when
under a microwave field. ESR is analogous to nuclear magnetic resonance (NMR) -
unpaired electron spins exhibit paramagnetism, and undergo Zeeman splitting under
a magnetic field [44, 32]. For a certain range of frequencies, a transition with energy
hν = geµbB0 can occur if an unpaired free electron jumps between ms = 0 and ms = ±1.
Thus, the common approach of ODMR is to slowly sweep an auxiliary microwave
field (usually with a copper wire) over a frequency area around 2.87GHz [40, 12, 44],
which is well known to resonate with this transition frequency. As discussed in section
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25. Kazuma Wittick z3460734 Honours Thesis
3.1.1, we know that an excitation from the ms = ±1 level is followed by a non-radiative
decay route [34], giving a Lorentzian dip in the fluorescence rate. This is clearly seen in
figure 11, which describes the fluorescence signal against the frequency.
Figure 11: Figure from [26] displaying the standard ODMR spectrum of the NV center in
a ND. Photoluminescence intensity is plotted against microwave frequency (GHz). The
characteristic dip in intensity in these graphs are due to the preferential non-radiative
decay when electrons are transferred to the ± 1 ground states by the RF. Note that
(a) is in zero-field conditions - no external magnetic fields. However, (b) introduces a
non-zero magnetic field that splits the dip into two, with an energy gap of 2γB, where
B is the magnetic field strength. Note that the dip on the left represents drop in PL
intensity as the RF sweeps through the resonant frequency corresponding to the ms = 0
to ms = −1 transition, and ms = 0 to ms = 1 transition on the right.
The first demonstration of ODMR was by Gruber et al - the signal from an ensemble
of 10 NV centers had indeed experienced a 10 % decrease in fluorescence as a result of the
magnetic resonance [15]. This effect has since been further explored and improved. Two
key factors influence the ODMR phenomena - magnetic field and temperature. These
have been explored in recent research.
5.2.1 Effect of Magnetic Field on ODMR Spectra
As seen in Figure 11, N.D Lai et al have demonstrated the effect of applying a magnetic
field to the local environment of the NV center. It has been observed that the original
dip at 2.87 GHz splits into two separate dips at two different frequencies (above and
below 2.87 GHz). This is due to the Zeeman Splitting of the ms = ±1 into two distinct
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26. Kazuma Wittick z3460734 Honours Thesis
energy levels. ms = −1 comes closer to the ms = 0 energy level, and thus a slightly
smaller microwave frequency is required to be resonant with this energy. In this way,
two dips are formed.
Lai et al have further demonstrated the utility of the ODMR technique by deter-
mining the orientation of the NV spin axis relative to the applied magnetic field. This
was done by measuring the shift in the resonance frequency as the magnetic field was
increased [26]. The way in which this frequency responds to the applied field is governed
by the spin Hamiltonian of the NV system and from this, the angle of the NV spin
axis can be inferred. In this experiment summarised in figure 12, the angle that the
magnetic field is applied can be varied (while its strength is kept constant) to determine
the maximal splitting angle.
Figure 12: The graphs investigating the relations between the NV spin axis, the res-
onance frequency and the angle of the applied magnetic field [26]. (c) relates the ex-
perimental (red squares) and theoretical (blue line) response of the resonance frequency
(GHz) when the applied magnetic field strength is increased from 0 mT to 25 mT. From
this, the NV spin axis of 57 degrees is inferred. In (d), the angle of the magnetic field (in
the plane described in the figure) at which the resonance frequency splitting is maximum
is determined to be 105 degrees.
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27. Kazuma Wittick z3460734 Honours Thesis
The unique electron energy configuration of the NV center provides a method of
detecting external magnetic fields, due to its affect on the fluorescence rate. Furthermore,
their temperature dependence allows the use of NDs as nano-thermometers (discussed
in section 6).
5.2.2 Effects of Temperature on ODMR Spectra
Nano-diamond’s thermometry capabilities arise from a shift in resonance frequency as
its local temperature is varied. Acosta et al have attributed this phenomenon to the
local lattice expansion which alters the spin-spin coupling between the two unpaired
electrons in the NV center. The change in D, a parameter associated with the ZFS of
the NV, with respect to temperature, was calculated to be dD/dT = −74.2(7) kHz/K
[1]. This relatively small number in fact has noticeable effects when measured above
room temperature, as displayed in Figure 13. Not only is there a change in resonance
frequency as expected, but less contrast as well (shallower dip) - this is most likely due
to relatively small PL quenching effects that arise with increased temperature [25] as
discussed in section 4.1.
Figure 13: A figure from [1] displaying the effects of temperature on the ODMR spec-
trum. In particular, the change in ZFS parameter D with temperature.
Although the change in ZFS parameter D was considered to be linear with a neg-
ative slope, further investigation by Chen et al over the larger temperature range of
5.6 K and 295 K have indicated a negative parabolic shape instead. Figure 14 displays
D ≈ 7 MHz over the entire range [8], whilst also demonstrating that assuming a linear
relationship over the 280-330 K range that Acosta et al measured was logical.
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28. Kazuma Wittick z3460734 Honours Thesis
Figure 14: The ZFS parameter D (GHz) as a function of temperature (K) over the range
5.6K - 295K [8]. The second y-axis gives the energy (eV) values of the ZFS.
5.3 Spin-Lattice Relaxation Times
Spin-Lattice Relaxation times, or T1 times, represent the time it takes for NV center
spins to ‘relax’ into an equilibrium state after being aligned in a certain direction due to
a external influence such as a polarising laser pulse or magnetic field. Since these times
are directly affected by any magnetic noise that happen to be around the spin transition
frequency [44], the NV center can thus be utilised as an ultra-sensitive magnetometer.
To reiterate section 3.1.1, the ground and excited state of the NV center both exist
as triplets. Now, whether or not the NV spin is initially in the ms = 0 or ms = ±1 state
determines the subsequent relaxation route. So, a clever way to bias the population of
electrons into a certain state can be done by optically ‘pumping’ them with a sequence
of laser pulses and/or microwave sources. This is seen in (b) of Figure 15 [36], where
the population of the ground state ms = 0, P0(t) is maximised by a 1 ms green light
pulse, and a subsequent microwave π-pulse (180°) transfers them into the ms = ±1 state.
With this ability, the pulse sequence is utilised in the following way: the initial
(highly fluorescent) spin state is set by a polarisation pulse, then after time tau, the spin
relaxes to natural state and measuring the fluorescence intensity tells us the relative
population of spin states. This process is done with and without the microwave pulse in
the middle, giving a reference point. This measurement as a function of delay time will
map out the desired T1 time.
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29. Kazuma Wittick z3460734 Honours Thesis
Figure 15: (a) describes the 532nm laser “on” (green) and “off” (nothing) state, the
former for the polarisation of spin states, the latter for read-out of spin states. The
first and last laser pulse is separated by evolution time τ. Underneath is the 180° (π)
microwave pulse, which pumps the NV centers into the ms = ±1 state. This phenomenon
is clearly shown in (b) which describes the population of the ms = 0 ground state. Note,
that as the spin ’relaxes’, the population of this state increases. After a time interval τ
the subsequent spin precession is stopped with a second pulse, after which the spin state
is optically read-out. Finally, (c) gives the time dependence of luminescence intensity,
with and without the microwave pulse. Note - the graphs on the right are control data
without the microwave pulse. The characteristic exponential decay (T1 time) is obtained
after subtracting the two data sets - with and without the MW, at different τ’s. Figure
from [36].
This technique does not necessarily have to be conducted with a polarising microwave
field - in fact, it is not included in the UNSW laboratory set up. Section 10.7 discusses
how the spin-lattice relaxation time will be measured with a sequence of excitation laser
pulses with increasing evolution time τ. The polarisation and subsequent read-out will
occur in the same pulse, by taking the ratio of the fluorescence at its head and tail.
The unique advantage of this measurement technique is the flexibility of pulse se-
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30. Kazuma Wittick z3460734 Honours Thesis
quences, allowing customised investigation of the NV center optical processes. For ex-
ample, figure 16 by SushKov et al presents a complex ‘spin-echo’ sequence involving
multiple microwave pulses of varying length. These increasingly detailed sequences al-
low experiments to account for external unwanted magnetic noise by canceling them
out via well-timed microwave pulses. This is the basis of T2 (spin decoherence) time
measurements, which allow more accurate investigations of the NV center.
Figure 16: As shown in the inset, Sushkov et al’s [48] pulse sequence exceed standard
spin-echo sequences by having combinations of π
2 and π pulses.
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31. Kazuma Wittick z3460734 Honours Thesis
6 Applications in Nano-thermometry
Nano-resolution thermometry has been an under-explored area of research, but is ex-
tremely beneficial in biomedicine. Techniques such as scanning thermal microscopy are
invasive in nature and are not ideal in bio-thermometry [39]. Another approach to this
outstanding challenge was via quantum dots (QD). Li et al utilised single QD’s as lo-
cal temperature markers by optically reading out their temperature-dependent emission
spectrum [27]. Albeit being a viable method of local temperature probing, quantum dots
are ultimately limited by low sensitivity and errors from fluctuations in fluorescence rate
[3]. For biological applications, intracellular temperatures are required to be known with
nano-scale precision in order to understand the dynamical state of the surrounding sys-
tem. However, a complication is the requirement that nano-probes have to be coupled
to cells without modifying their functionality [20].
Nanodiamonds with NV centers have recently been utilised in novel methods as a
temperature nano-probe. The benefit of NV-based thermometry is that ND’s excellent
thermal conductivity allows for the sensitivity of measurements to not be constrained
by proximity to the source, unlike NV-based magnetometry.
6.1 Thermometry in Living Cells
G. Kucsko et al demonstrated the techniques that utilise the novel manipulation of elec-
trons spins in NV centers. They displayed ND’s ability to detect temperature variations
down to 1.8 mK (sensitivity of 9 mK /
√
Hz) [23]. This was done by exploiting the tem-
perature dependence of the zero-field splitting of NV centers in ND. More specifically,
the relative splitting position between ms = 0 and ±1 shifts energy proportionally to
the local temperature. Kucsko et al used gold NP’s as a heat source, due to their effi-
cient absorption properties in the visible spectrum (and thus, from a green laser). They
then co-localised the ND adjacent to the gold NP with a confocal microscope with two
independent scanning beams, and locally measured the temperature variations around
the ND via ESR spectroscopy.
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32. Kazuma Wittick z3460734 Honours Thesis
This approach to temperature gradient mapping was then applied to biological en-
vironment. Introducing NDs and gold NPs into human embryonic fibroplast (via silicon
nano-wire assisted delivery) allowed temperature mapping at the sub-cellular level on
length scales down to 100nm. This sub-degree temperature resolution is a powerful new
tool in biological research that can be utilised for temperature induced control of gene
expression and cell-selective treatment of disease. Other practical uses include monitor-
ing tumour metabolisms and heat dissipation in integrated circuits [23].
Figure 17 re-iterates the basis of particle heating under laser illumination by showing
the relation between the laser power and NP temperature.
Figure 17: Change in temperature (K) plotted against laser power (mW) [23]. It is
clear that with higher laser power, more heating of the gold NP occurs. The red line
represents the change in temperature with power when the laser is directed onto the
gold NP. The blue line correctly indicates no change in temperature since the laser was
not directed at the gold NP and thus there was no heating.
For N colour centers, the temperature sensitivity of a sensor is inversely proportional
to the temperature dependence of the zero-field splitting:
η =
C
d /dT
1
√
TcohNt
(2)
Where Tcoh is the NV spin coherence time, is the zero-field splitting, and C is a
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33. Kazuma Wittick z3460734 Honours Thesis
factor that accounts for imperfect read-out and initialisation [23].
Characterising the dynamical system in question is imperative also to ensure that any
temperature increases due to absorption of radiation isn’t harmful to the surrounding
biological environment [6]. Thus, Bendix et al performed direct temperature measure-
ments of the local environment of single gold NP, optically trapped in a lipid bilayer
(a two-layered polar membrane that marks the boundaries of a cell). They utilised the
temperature dependent transitions of the bilayers to map the heating characteristics
experienced in the trap [6]. Although they used fluorophores, the usefulness of optical
trapping is demonstrated in this paper. Gold NP’s absorption of visible wavelengths is
very efficient and thus have great potential in use for photothermal applications, such as
photothermal cancer therapy and gene delivery. Even though their experimental data
was for gold NPs, Bendix et al claim that the method is easily applicable to any type of
NP.
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34. Kazuma Wittick z3460734 Honours Thesis
7 Applications in Nano-magnetometry
7.1 Magnetic Sensing with T1
The unique interaction between a NV center and fluctuating magnetic field is encoded
in its spin-relaxation (T1) time. That is, a local environment ‘rich in spin’ will facilitate
a quicker relaxation of the NV center spin. This is the basis of ultra-sensitive magnetic
sensing with T1 times, and will be discussed in the following sections.
7.1.1 Spin-Lattice Relaxation (T1) Time
A simple method demonstrated by Hall et al has characterised the relationship between
ground state population and local fluctuating magnetic fields. The NV center was utilised
in scanning probe microscopy by embedding it at the apex of a very sharp tip of less
than 10 nm in radius [44] and scanning this tip over an area of interest. The subtle yet
different response of the NV’s spin-relaxation time as it interacts with magnetic fields
would determine its frequencies and amplitudes. This is displayed in figure 18 (d) and (e)
below. Note here, that the ability to estimate electron populations in the NV center via
fluorescence allows this method to determine magnetic field characteristics that would
otherwise be inaccessible with the use of DC and AC magnetometry techniques [16].
7.1.2 Single Spin Detection
Gadolinium ions have recently been used as a tool to test single spin sensitivities of
nanoprobes. Gd3+ ions produce characteristic magnetic noise due to the high frequency
fluctuations of the 7
2 spin [22] that directly influence the T1 times of a NV center in
proximity. Kaufmann et al used gadolinium-labelled lipids to form a supported lipid
bilayer (SLB) around the NDs. An average of 74 ± 6 % reduction in the NV relaxation
time was observed for five Gd labelled lipids, revealing very clear correlation between
the magnetic noise and the relaxation times. This trend is shown clearly in figure 19.
The effective number of spins detected was subsequently estimated to be near-individual
spin sensitivity - a remarkable 4 ± 2 spins. This result served to give motivation to
future endeavours in single spin detections.
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35. Kazuma Wittick z3460734 Honours Thesis
Figure 18: Figure from [16] detailing typical results for a scanning probe microscope of
this configuration. The familiar NV lattice and energy levels are shown in (a) and (b),
along with the 2.88GHz microwave (c) commonly used in experiments. The scanning
probe tip is seen in the center of the image. The green section translates the magnetic
field signals from (d) into physical representations of amplitude and frequency. Region
I - Strong, rapid fluctuations. Region II - Strong, slow fluctuations. Region III - weak,
rapid fluctuations. Region IV - weak, slow fluctuations. The subsequent influence that
these different magnetic fields have on the ground state population is described in (e).
Note, each graph shows either exponential decay or gaussian decay. Rapid fluctuations
cause exponential decay, and slow fluctuations cause gaussian decay. The rate at which
these graphs drop to 0 depends on the magnitude of the field.
Surely enough, in recent works, Sushkov et al have made progress in this field with
the detection of single proton spins using MRI techniques involving qubits as quantum
’reporters’ on the surface of high purity diamond [48]. Simply put, the measurements
of larmor precession frequency induced on the reporters were consistent with the proton
gyromagnetic ratio of 2π × 4.26 kHz/G, thus confirming the detection of the magnetic
field created by protons.
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36. Kazuma Wittick z3460734 Honours Thesis
Figure 19: Spin-relaxation time measurement with a pulse sequence as shown (note, no
microwave pulses were used). A definitive reduction is seen in a 25 nm nanodiamond in
the presence of the paramagnetic gadolinium. Figure from [44]
7.2 Vectorial Magnetometry
A brilliant demonstration of vectorial magnetometry using optically trapped NV centers
was shown by Geiselman et al. Initial findings were the remarkable long term stability of
the NV axis orientation in the optical trap over extended periods of time (30 minutes) - it
remained the same inside an error range of ±5°. This was attributed to the asymmetrical
shape of the nanodiamonds which created a preferential equilibrium axis in the optical
trap. Next, they incrementally aligned an external magnetic field with respect to the
orientation of the NV axis to investigate the effect on the ODMR spectrum [12]. From
(a) - (c) in figure 20 it is clear that total alignment of the magnetic field with the NV
axis induces maximum splitting between the two dips.
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37. Kazuma Wittick z3460734 Honours Thesis
Figure 20: (a) No magnetic field, thus the degeneracy of the energy levels corresponds
to ’overlapping’ dips. (b) A perpendicular magnetic field is applied to the NV axis, and
as mentioned previously, the energy difference between ms = 1 and ms = −1 increases,
thus splitting the dip into two minima. Maximum splitting is shown in (c) when the
magnetic field is applied parallel to the NV axis, allowing vector magnetometry to be
possible. (d)-(f) demonstrates the effect of polarising electric fields on the orientation of
the NV axis. It is clear that one can control the NV axis by changing the polarisation
axis of the trapping laser. Diagrams on the right describe the relative axes - NV, Bx,y,
and Etrap. Figure from [12].
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38. Kazuma Wittick z3460734 Honours Thesis
8 Cell Tracking and Labelling using NV fluorescence
To date, numerous optical imaging methods have been employed in attempt to track
cells in biological environment using probes such as quantum dots or dye molecules.
However, as mentioned previously they suffer from photobleaching and are hazardous
to the body due to their toxicity [52]. Thus, nanodiamond’s excellent bio-compatibility
with other nano-scale carbon materials makes it a promising candidate for many biolog-
ical applications - in particular, cell tracking and labelling [28]. The inertness and ease
of conjugation with biological molecules mentioned earlier allow NDs to have full access
and control of the environment. In aiding these tracking and labelling processes, they
indirectly advance the progress made in areas such as cell therapy by repairing damaged
or lost tissue.
The bio-compatibility and mechanism of cellular uptake of ND’s HeLa cancer cells
has been evaluated by Vaijayanthimala et al using flow cytometry, a technique where
fluorescent cells pass through a laser in order to determine its physical and chemical
properties. They attained results that certain cells (namely 3T3-L1 pre-adipocytes and
489-2 osteoprogenitors) were not affected with the attached ND’s. Likewise, Wu et al
found that spontaneously labelling lung stem cells with NDs did not affect or elimi-
nate cellular functions such as division, proliferation and differentiation [52]. Thus they
demonstrated NV centers to be ideal candidates for cell labelling [50].
After confirming ND’s biocompatability, Wu et al performed cell tracking on ND-
labelled lung stem cells in live mice. Furthermore, McGuiness et al [34] performed
orientation tracking of NV centers in ND-attached HeLa cells as shown in figure 21.
They achieved tracking precision that was within 1° /
√
Hz.
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39. Kazuma Wittick z3460734 Honours Thesis
Figure 21: The NV center in HeLa cells being tracked over a three hour period in
four dimensions - three dimensional position as well as orientation [34]. The colour
gradient labels the corresponding angle of the NV axis relative to the magnetic field.
The positional axes are in nano-meters.
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40. Kazuma Wittick z3460734 Honours Thesis
9 Progression and Discussion of Literature
Research in nano-scale sensing was born primarily in the 1900’s - however, only recently
has there been manipulation techniques such as optical trapping that have escalated
progress in the field. ESR spectra were obtained by Horowitz et al on an ensemble of
approx. 3500 optically trapped NV centers [19]. More recent applications of optical
trapping were demonstrated by Geiselmann et al in 2013 with a single trapped 60-70 nm
diamond crystal with a single NV center [12]. This progress from ensemble to individual
NV center measurements has marked a huge leap in research and is a testament to the
capabilities of optical trapping techniques.
Although NV centers have achieved such capabilities, numerous ways to improve
its sensitivity are still being suggested by current literature. Firstly, it is important to
note that the absolute sensitivity of the ND probe (whether it be in thermometry or
magnetometry) is dependent on the signal to noise ratio in the optical read-out of the
NV spin state. An easy yet effective approach is given by Kucsko et al - they took
multiple fluorescence measurements in the vicinity of the zero-field splitting in ODMR
measurements. They did this to cancel any unwanted noise caused by fluctuations in
the total fluorescence rate [23].
This is limited by the photon collection efficiency (PCE, which is 0.1 %). However,
this PCE can be improved by lengthening the NV spin coherence time and increasing the
number of NV centers. Kucsko et al claim that by doing this, sensitivities of 80µK/
√
Hz
can be achieved [23]. Neumann et al suggests an equipment-oriented alternative to im-
prove the PCE, with the use of solid immersion lenses and diamond pillar structures.
Also, due to the fluctuating magnetic fields produced by nuclear spin baths of Carbon-
13, faster NV center spin decoherence is observed. To combat this problem, they suggest
a reduction in carbon-13 concentration and an increase of external magnetic field [39].
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41. Kazuma Wittick z3460734 Honours Thesis
T1 and ODMR Measurements with the optical trap are novel and in some ways an
unknown territory of optical dynamics that is still developing. In particular, the in-
teraction of the IR trapping laser with the population dynamics of the NV− and NV0
have been characterised most notably by [49, 25, 21]. However, a sizeable portion of the
literature are observations followed by inconclusive (but still very enlightening) attempts
to have a fully working and widely accepted model. It is worth noting that while most
research give evidence for the decrease in PL as the IR laser is illuminating the ND
[25, 7, 13], exact opposites of this effect have been observed [21].
Optical trapping technique offers a unique method of micro-manipulation of the ND
during measurement. Further investigation into the interplay of the IR laser and the
optical dynamics of the NV center may hold great implications for the future of research,
considering it has already produced excellent results such as vectorial magnetometry and
cell thermometry/tracking for biological systems.
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43. Kazuma Wittick z3460734 Honours Thesis
10 Experimental Methods
The optical table used for experiments in this project had many components working
together. Though figure 22 looks rather convoluted, its functionality can be separated
into 6 crucial parts:
ˆ Optical trapping (IR) laser
ˆ Lamp and CCD for imaging
ˆ Spectrometer
ˆ Stage with sample
ˆ Avalanche photodiode (APD) for single photon counting
ˆ Excitation (Green) laser
A condensed description of this project is as follows: a ND trapped in 3D space
by an IR laser is illuminated with an excitation laser, to investigate firstly its PL spec-
trum with the spectrometer and more importantly its opto-electronic characterstics with
the single photon counting system. Each of these components contribute their part in
shaping and directing the laser beams in order to maximise their use in single photon
counting measurements (such as T1 times or ODMR), and are detailed in this section.
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44. Kazuma Wittick z3460734 Honours Thesis
10.1 Optical Bench Description
Figure 22: A schematic depicting the experimental set-up [4]. Important sections of
equipment are circled and labeled.
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45. Kazuma Wittick z3460734 Honours Thesis
The optical trapping laser is a 1064 nm Nd:YAG (Laser Quantum, 1064 Ventus,
TEM00) laser pumped by a 808nm laser diode. A long pass filter transmits >1000nm
wavelengths in order to block out any 808nm coming through. After passing a half
wave-plate (λ/2) which shifts the polarisation direction of the linearly polarised incident
beam, a 2-Dimensional Acousto-Optic Deflector (Gooch & Housego, 45035 AOBD) is
employed to have controlled bi-directional diffraction. The first order diffracted beam
then passes a digitally configurable Spatial Light Modulator (SLM) (Hamamatsu LCOS-
SLM), again used as a tool for diffraction. The beam then passes multiple lenses and
beam steering mirrors, past a 100x E-Plan Nikon Objective (NA 1.25).
Two other light sources also go through the sample. A lamp (THORLABS OSL1
Fiber Illuminator) is used for Dark Field Illumination and a 561 nm diode laser acts as
the excitation laser that passes a similar ensemble of optical equipment on its way to
the sample. For illumination uniformity, a pinhole is used for spatial light filtering and
the SLM is used to correct for aberrations caused by the mirrors/lenses.
10.2 Sample Preparation
10.2.1 Desirable Concentrations
The NDs used for this project are type Ib (meaning atomically dispersed NV centers),
categorised as ‘brFND-100’ and manufactured at the Institude of Atomic and Molecular
Sciences, Academia Sinica, Taiwan. Compared to the electron irradiation process de-
scribed in section 2, these NDs have more NV centers produced more efficiently, as they
are irradiated with a He+ ion beam that have 40 keV energy as compared to the 2 MeV
electrons.
It is vital to pick the right concentration when trapping, to suit the needs of the
experiment. For some cases where the number of particles trapped can be arbitrary,
a more concentrated solution is desired for the ease of finding and trapping a NP. For
example, a 1:200 solution is a high enough concentration that multiple NPs will drift in
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46. Kazuma Wittick z3460734 Honours Thesis
after only a few seconds of enabling the optical trap. However, experiments like the T1
decay measurements require single NPs, in which case the concentration should be on
the order of one-to-several thousand. It is reasonably easy to find an efficient concen-
tration such that one NP is not too hard to find, and once found, will not be disturbed
by other NPs also drifting into the trap.
For example, to dilute a sample by a factor of 10, one will take 1 ‘part’ sample, 9
‘parts’ de-ionised water. Whatever volume of water one takes as 1 ‘part’ is arbitrary -
perhaps 100 µL and 900 µL, for example.
10.2.2 Microscope Slide
One of the fundamentals in optical trapping is facilitating the space in which the nanopar-
ticle was confined, such that it would be relatively easy to search and trap said particle.
The sample is prepared in the following way:
1. A ring shaped adhesive spacer ‘Avery Transparent Invis’ is placed on the center of
the microscope slide.
2. The desired amount of diluted NP solution is transferred in the central part of the
ring using a pipette. Usually between 6µL and 10µL.
3. A type 0 square glass cover slip (0.18 mm thick) is placed on top. Although the
liquid will remain in the central area, some of it will disperse between the adhesive
spacer and the cover slip.
4. A small amount of acrylic varnish is smeared along the edges of the cover slip to
seal the edges and prevent liquid from drying up and/or escaping.
5. A drop of immersion oil (ProSciTech, IC116-LDF) is placed on the center.
10.2.3 Preparation for experiments without Optical Trap
Initial analysis of spin-relaxation time measurements revealed that the trapping laser
itself had been affecting the results. To combat this problem, a sample had to be
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47. Kazuma Wittick z3460734 Honours Thesis
prepared in such a way that the ND will be locatable in a predetermined position and
remain stationary throughout experiments. This was achieved by doing measurements
on NDs that were essentially precipitates stuck on the surface of the cover slip:
1. Pipette approximately 7µL of ND solution onto a type 0 square cover slip.
2. Wait 1 minute to allow the NDs to settle to the bottom of the droplet.
3. Carefully position the pipette perpendicular to the cover slip and slowly remove
approximately 3
4 of the solution back up into the pipette.
4. Let the remaining solution evaporate to leave the NDs stuck to the surface of the
cover slip.
5. Place the ring shaped adhesive spacer on the microscope slide.
6. Pipette between 6µL and 10µL of de-ionised water.
7. Place the cover slip prepared earlier upside down, such that the NDs come into
contact with the fluid.
8. Seal the edges and place the immersion oil as described in the previous section.
10.3 Optical Tweezers
10.3.1 Trapping Laser
An optically pumped 1064 nm wavelength Nd:YAG is employed as the trapping laser.
This Diode-Pumped Solid State Laser (DPSSL) works analogously to other lasers. Where
a standard Helium Neon (HeNe) laser would be pumped by an electrical discharge be-
tween cathode and anode, the DPSSL is pumped by another laser diode - 808 nm in this
case. It has been chosen due to its sufficiently low photon energy that prevents ND− to
be transferred into the optically inactive NV0 state. Also, its low absorption coefficient
in water allows for more of the trapping energy to be transferred into the NPs instead
of being lost in the suspension medium.
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48. Kazuma Wittick z3460734 Honours Thesis
The process of trapping a nanoparticle goes as follows:
1. Place the prepared sample on the stage, making sure that the output from the
condenser below penetrates the center of the ringed adhesive tape, where the fluid
is located.
2. Carefully wind the z axis knob of the stage, such that the drop of immersion oil
makes contact with the condenser. The aim is to wind the z position of the stage
until the focus of the trap coincides with the region where the NPs are suspended
in the fluid.
3. There will be noticeable flashes as the z axis sweeps through the sample, as there
will be changes in refractive indices causing reflection. Notably, from air to im-
mersion oil, from oil to glass, and from glass to liquid.
4. The most stable trapping position occurs immediately after entering the liquid
section of the sample. Adjustments in the x-y direction will be required until a
particle is found and trapped.
It is worth noting that the trap is not always reliable and stable, and there are
several factors that contribute to any instabilities of the trapped particle. Firstly, the
AOD ‘factor’ that determines the strength of the IR laser at the trapping site by the
amount of diffraction it undergoes. Also, the countless factors that define the pixel
elements of the SLM which make corrections to imperfections in the beam such as
spherical aberrations and asymmetry. In some cases, using a concentrated solution will
see numerous particles ‘fighting’ for the trap, producing large fluctuations in any PL they
produce. Before all measurements are conducted it is advised that one follows a routine
check and make any adjustments to any of the aforementioned factors if necessary.
10.3.2 Adjacent Trapping
In our current Optical Tweezers, the Acousto-Optic Deflector (AOD) has time-sharing
capabilities that allow for the trapping beam to be switched between two adjacent po-
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sitions fast enough such that the reactional movement of the particle (already reduced
by viscous drag) is negligible. In this way, two adjacent nanoparticles can be optically
trapped simultaneously. Although this technique was not utilised in this project due to
time constraints, it has great potential in local temperature probing, whereby a ND is
trapped adjacent to a metallic NP, and the temperature differences induced by external
sources (IR laser, for example) could be detected using ODMR.
This technique is not limited to dual trapping. In fact, time sharing capability can
just as easily be utilised on a single NP. This way, it is possible to modulate the amount
of time that the IR laser is hitting the particle (as half the time is spent in its adjacent
location), and has potential in experiments investigating the optical dynamics of the NV
center.
10.4 Imaging and Photon Collection
10.4.1 Dark-field Microscopy
It is essential for the optical trap set up that there is real-time imaging of inside the
trap. Finding nanoparticles and moving the stage such that they fall into the trap would
otherwise be a near-impossible task. Thus, we view our image via dark-field illumination
- an imaging method that displays scattered light from objects in a dark background,
since it is ideal to have a bright spot in a dark background (see figure 23) , rather than
a small grey unresolved figure in a bright background.
This is done through creating a ring of illumination by shining the light source on an
axicon (conical surface lens). After the beam enters the sample, scattered light (trans-
mitted light is blocked) goes on to produce the image. This technique is advantageous
for particles such as diamond that are quite transparent to visible light, making it hard
to view under bright illumination due to low contrast between background and object.
However, the final quality of the image captured by the Allied Technologies Stingray
CCD camera is inherently limited by the low light environment, along with low resolu-
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tion due to the nature of imaging scattered light. Fortunately, image resolution is not a
key aspect in the optical tweezers - the navigation of the NP towards the trap focus and
the subsequent alignment of the excitation laser is all that is required.
Figure 23: Image from the CCD camera of the optically trapped ND - the bright spot
in the middle of the picture.
10.4.2 Avalanche Photodiode
In all measurements apart from spectroscopy, the Avalanche Photodiode (PicoQuant
τ-SPAD) is utilised (alongside a single photon counting device) to facilitate the in-
vestigation of the optical dynamics of the NV center. It captures scattered light or
photoluminescence from the sample that is coupled into a multi-mode optical fibre and
directed towards the sensor. The need for an optical fibre arises from the inherent need
for the APD to be positioned away from light sources such as the lamp or laser. Due
to the high sensitivity of the APD, several precautionary steps are taken. Firstly, a
bandpass filter (Semrock 675/67) is placed before the optical fibre that allows a 67nm
window of wavelengths through (641nm - 708nm). Fortunately, this wavelength region
is perfect as it blocks not only the green excitation laser but the IR trapping laser as
well. The sensor itself is covered by several layers of absorbent material to minimise any
background light.
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Aligning the optical path to the APD was essential in obtaining the necessary in-
formation for measurements, but was quite a challenging task as it experiences regular
misalignment (which is the nature of optics). Firstly, the excitation laser will be aligned
to overlap with the trapped ND - if the spectrum observed is characteristic of the ND,
we can then proceed to aligning the path through two objectives. There are three-
dimensional controls for adjusting the input to the objective as well as the optical fibre,
both of which require micro-precision. The same process then applies in aligning the
path to the sensor upon exiting the fiber.
On an experimental standpoint, it is essential to check before each measurement
that the counts received on the APD are indeed from the PL of the ND itself. This can
easily be done by blocking the excitation laser and observing the drop in counts as a
result. Generally, there will be some IR laser and background light leaking in, but are
usually very weak and negligible compared to the PL. They can easily be accounted for
by observing the increase in counts as the trapping laser and background light (if any)
is turned on.
10.4.3 Single Photon Counter
A photon detection device requires its counterpart - a photon counter. A Time-to-Live
(TTL) signal sent from the APD acts as a triggering mechanism for the counter it is
connected to - in this project, the Stanford Research Systems SRS SR400, the Fast
ComTec’s MultiChannel Scalar (MCS64A) and the Data Acquisition (DAQ) card were
used. In particular, the MCS for the T1 decay measurements and the DAQ for ODMR. A
Labview program (generously made available by David Simpson) was capable of handling
both T1 and ODMR measurements. For the purposes of efficiency, the SRS SR400 which
was utilised by Simon Ralph’s ODMR program was replaced by the MCS. Note that each
single photon counter connects to the PulseBlasterESR-PRO (SpinCore Technologies,
Inc.), a high speed programmable pulse generating device that will be discussed in the
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52. Kazuma Wittick z3460734 Honours Thesis
following section. Figure 24 describes this set-up.
Figure 24: Schematic describing the path that the photon counting data follows. The
dashed lines indicate possible routes for the TTL signal from the APD. The SR400
communicates with S. Ralph’s photon counting program, whereas the MCS64A and
DAQ communicate with Dr. Simpson’s program for the T1 and ODMR measurements,
respectively.
This single photon counting system was highly crucial in preparation for T1 and
ODMR experiments. This was because David Simpson’s program utilised this system
to included a real-time update of PL counts. This graph updates the current PL count
at 200 Hz (5 ms sampling time, 20 samples per shot), giving an accurate and quick
response to any changes in PL. Since a large proportion of the time spent in this project
went towards aligning optical equipment for the best photon-collection efficiency, this
program was highly beneficial.
10.5 Excitation of Nano-diamond
10.5.1 Excitation Laser
As discussed in the literature review, the excitation laser has to be carefully picked cor-
responding to the energies required for excitation of the ND (see figure 2 for the energy
levels). Thus, a wavelength shorter than the ND ZPL (638nm) was required, however
not too short, so as to ionise the NV− into the optically in-active NV0 center. This is
precisely the reason why the 12W Argon Laser (Coherent Innova 70) was not used and
instead, a 561nm (green) 10 mW Melles Griot diode laser.
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53. Kazuma Wittick z3460734 Honours Thesis
This continuous wave (CW) laser had to pulse in a predetermined sequence for the
T1 time measurements - a 3 ms polarisation pulse followed by multiple 3 µs pulses that
become increasingly spaced in between each iteration (see section 10.7). A sequence such
as that cannot be easily done by physical means without any computer control. A great
way to micro-control this laser is through AOD-switching. This utilises the AOD that
diffracts the beam and a circular aperture that only lets through the 1st order diffraction
beam. This means that if the AOD is switched off, the non-diffracted beam will be
blocked by the circular aperture. The PulseBlasterESR-PRO (PB) takes care of this
process with its programmable pulse generating capabilities. The PB has a processor
architecture embedded into the PC albeit working independently to the computer’s
processes. The 500 MHz internal clock frequency (2 billion p/second) evidently has the
precision necessary for these nano-scale measurements. Communication to the AOD
from port 2 allow the pulse sequence to be generated and nano-controlled.
10.5.2 Acousto-Optic Tunable Filter
The Acousto-Optic Tunable Filter (AOTF) is a device that at its core acts for laser
wavelength selection, by driving its own AOD at frequencies that support the desired
wavelength output. While it has great utility on an optical bench running many laser
sources, the NV center required only the 561 nm wavelength and thus was not used for
its potential in this project. The AOTF was computer controlled by a terminal based
input of command characters, which would then communicate via GPIB to set up the
required wavelength output. However, since only wavelength was ever required in this
project, the process of entering the sequence of commands (‘x - 5, i - 0, o - 1’) became
an tedious and redundant process. To clarify, the ‘x’ command is typed for the terminal
to prompt the user for the wavelength, to which the user answers ‘5’ (a number for a
predetermined wavelength). This process will be repeated a few more times to interface
with the AOTF to set the right properties.
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54. Kazuma Wittick z3460734 Honours Thesis
This process was quite esoteric without any instructions for what each command and
response represents. In an endeavour to learn introductory LabView programming, an
initial task was to interface between the AOTF and computer through Labview. The
end result was a program that gave simple visual instructions to set the right parameters
in one click. This was done by programming LabView to send the right set of characters
as a automated signal upon the click of a button. Of course, the option for other laser
wavelengths can be easily added in future following the exact same code but with dif-
ferent characters and numbers in the signal, rendering it a quicker and more convenient
process for current and future use.
10.6 Spectroscopy
In all measurements being conducted, it was imperative that nothing else apart from a
ND was in the optical trap. Occasionally, a speckle of dust, or any other microscopic
particle could become trapped. Measurements will then be inaccurate or totally wrong if
taken without confirming the true nature of the NP - thus, the Acton SP2300 and PIXIS
256 (Princeton Instruments) was utilised as a tool to do exactly that. Nano-diamond
has a characteristic fluorescence curve under illumination by the green laser, with well
known peaks given by the Zero Phonon Line (ZPL) of the NV− center and NV0 center.
10.7 Spin-Relaxation Measurements
As of several months into the project, UNSW’s opto-electronics lab in LG48 installed
Fast ComTec’s MultiChannel Scalar (MCS), a new photon-counting device. The pre-
vious SRS SR400 photon counter used in accordance with a real time photon counting
program made in Labview by Simon Ralph was utilised for preliminary NV characterisa-
tion measurements. However, receiving a LabView program by David Simpson dedicated
for T1 with the MCS triggered a change in circuitry of equipment. As shown in Figure
24 the APD was connected via BNC cable to the PulseBlaster - however a T-connector
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55. Kazuma Wittick z3460734 Honours Thesis
also allowed the connection of the APD to the MCS STOP, which registers an event
input (i.e. the count from the APD). The MCS START connected to port 1 of the PB
is the triggering mechanism to sync the measurements between the PB controlling the
AOD and the MCS photon counting device.
The optical dynamics of the NV center dictate that there will be more PL (than the
steady-state) when there is a larger population of electrons in the ground state ms = 0
state. So, we expect a ND that has been spin-polarised into the ms = 0 state to fluoresce
more than one that has even mixing (i.e. steady-state). This is exactly the reason why
there is ‘dead’ time between each excitation laser pulse - to give the ND a certain amount
of time to relax into a steady-state population of even mixing. Figure 25 b) displays
the total sequence (for arbitrary length of 20 pulses) making it clear that the dead time
increases with each pulse - this is so that we can probe the influence of the length of
dead’ time to the population of electrons in each ground state sub-level. This population
can be directly investigated by observing the immediate PL response of the NV center
under illumination by the excitation laser. The subtle shape of the sub-microsecond
response as seen by a) of figure 25 allows this.
The 3 µs pulse length is strategically chosen to be able to view this subtle PL vari-
ation - too long, and the small changes will be lost; too short, and the full picture will
not be considered. In theory, by the end of the pulse, the steady-state value should be
constant across all pulses. Even though this is the case for most measurements, there
are factors such as the constant IR laser illumination that have to be taken into account
and may cause discrepancies as shown in a), which is a real measurement. The grey
areas (the head and tail of the pulse) contain important information about the relative
populations of the energy levels. Thus, the ratios of PL between the two areas for each
pulse produce the characteristic T1 exponential decay.
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56. Kazuma Wittick z3460734 Honours Thesis
Figure 25: a) Comparison of the first (red) and last (blue) pulse of a T1 measurement.
Actual data is displayed here for the purposes of explaining the measurement. Shaded
areas indicate the regions taken as a ratio to produce the T1 exponential decay. b) The
total pulse sequence - each spike represents a pulse exactly matching what is displayed
in a). Between each pulse is the ‘dead’ time for the NV center to relax into a steady-
state population mix. c) The ON/OFF states for the excitation laser and photon counter.
Each 2 µs ON state of the excitation laser is followed by evolution time τ which increases
in length. The photon counter is ON for the head and tail 300 ns for each pulse.
10.7.1 With the Optical Trap
With measurements of an optically trapped ND, it is imperative that they are done on
single particles throughout the duration of the acquisition time. This is clear because
the aim of this experiment is to investigate the spin relaxation time of the NV centers
being illuminated by the excitation laser. If a new particle was to enter the trap during
the measurement, they will fluoresce differently as they do not have the same population
of electrons in their energy levels to those that have prior been pumped and influenced
by the excitation laser. The entire premise of probing the population of states becomes
skewed.
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57. Kazuma Wittick z3460734 Honours Thesis
Technically, the requirement is such that the number of NV centers in the trap stay
constant during the experiment and not that only one ND is allowed. However, by allow-
ing many NDs to be part of the measurement, several sources of error and inaccuracy
arise. Firstly, the fact that the spot size of the focus of the excitation laser and IR
trapping laser is only several times the size of the ND implies that a ND on the surface
of an aggregation of trapped particles will experience different powers of laser light, thus
reacting differently in PL. An unstably trapped ND being illuminated with the dimmest
section of the excitation laser will skew results obtained by stable NDs fluorescing prop-
erly. Another issue is the increasing possibility than an unstably trapped ND will leave
the trap and be replaced by a new ND, which is not ideal, as explained in the previous
paragraph.
10.7.2 Without the Optical Trap
The key difference in experiments with and without the optical trap is the method of
preparing the sample, as described in section 10.2. The conceptual basis on which these
experiments were undertaken was the suspicion that the presence of the trapping laser
could affect the optical dynamics of the NV system.
As aforementioned, having the NDs stuck on the cover slip allowed microscopy and
optical probing of a spatially fixed NV center without using an optical trap.
A source of error in this experimental set up came from a mild drift in the relative
position of the excitation laser and the spatially fixed ND. This became apparent as the
ND PL linearly decreased over time, and a noticeable difference in the visual position
of the ND with respect to the excitation laser after said time. Although this drift was
consistent, predictable and only noticeable after t > 5 mins, it was still a source of error
that made the experiment subtly difficult to make accurate. It remains unknown if the
ND optical dynamics is affected negatively due to the excitation laser slowly moving over
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58. Kazuma Wittick z3460734 Honours Thesis
different part of the ND. However, under intuitive and educated guesses it is suitable to
say that the excitation laser spot size is bigger than the ND and had no substantial affect
on measurements. More importantly, a gradual decrease in the ND PL due to this drift
does not matter due to the fact that the ultimate requirement for this T1 measurement
was the acquisition of PL signal over time - and the rate at which this is achieved does
not matter.
10.8 Optically Detected Magnetic Resonance
Again, acknowledgements to David Simpson for being kind enough to spend time and
effort in providing a Labview program (as well as modifying and de-bugging) to run
measurements with the experimental set up at UNSW. Along with this was the Ohm
shaped resonator chamber that was used to apply RF to the NPs inside it.
As explained in section 5.2, the ODMR experiment revolves around detecting the
decrease in PL of ND in response to an external RF field at the resonant frequency of
2.87 GHz. Thus the experimental set-up (figure 26) requires a computer-controlled RF
generator to sweep over a predetermined frequency range and to detect the PL response
via the single photon counting system aforementioned (section 10.4.3). The DAQ card
facilitated the photon counting process in this measurement with the use of its internal
clock by having ON periods of data acquisition during which it will collect counts from
the APD via a TTL high signal. During the OFF periods it offloads the stored data to
the computer and reset its clock and memory to repeat the process again.
As explained in diagram 26, the RF signal produced upon a TTL trigger is amplified
before entering the sample. A RF circulator and attenuator ensure no back-signal. This
assembly is the same as what S. Ralph used in the previous year apart from switching
to a stronger 20W amp in hopes of removing any suspicions of weak RF signal being
the reason that an ODMR signal fails to show. The differences arise in the LabView
programming as Dr. Simpson’s program was utilised for this measurement. To create a
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59. Kazuma Wittick z3460734 Honours Thesis
Figure 26: The Circuitry for the ODMR experiment [41]. The computer controlled RF
generator (labelled SYNTH) produces a signal that is amplified before entering the Ohm
resonator chamber. An RF circulator and two attenuators ensure safety and termination
of the signal.
normalised PL count, a baseline measurement with RF OFF must be taken so that it
can be compared with RF ON. In order to do this, the program modifies the frequency
of the RF to a value far away from the ZFS of 2.87 GHz, such that it has no effect on the
NV center. This was done instead of turning the generator on and off due to the quicker
response time. The default RF sweep is set to occur along a frequency window of 2.82
GHz - 2.92 GHz that is then divided into 101 points (this is another default value that
can be changed). The program spends 30 ms at each frequency points along this inter-
val, during which the counts are recorded for RF ON and RF OFF. Each scan therefore
takes roughly 3 seconds (30 ms multiplied by 101) - this is repeated for as many scans
necessary to obtain a ODMR signal. Roughly 20-40 scans are ideal for the best accuracy.
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11 Results and Discussion
11.1 Spectroscopy
As discussed in the methodology, before each session of measurements, alignment proce-
dures and ND identification through spectroscopy is necessary. Courtesy of Ana Andres
Arroyo’s Labview program, these are done with a press of a few buttons.
The spectrum produced by NV− centers have unique characteristic as seen in figure
27 that allow the identification of NDs through its PL. The ZPL located at 638 nm is the
indicator of the presence of the NV− center. This spike is not only used to distinguish a
ND from random NPs producing back reflections into the spectrometer, but as a visual
aid in micro-alignment procedures detailed in the previous section.
Figure 27: The PL spectrum of ND. The characteristic ZPL of the NVV− center located
at 638nm is clearly visible. The region of PL on the right of the ZPL (longer wavelengths)
are phonon-assisted sidebands that make up a significant portion of the total PL. Note
here that the NV0 ZPL at 575nm is omitted along with the rest of the PL in that region
due to a spectral filter employed so that we view NV− specific PL.
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62. Kazuma Wittick z3460734 Honours Thesis
11.2 NV Characteristics probed with single photon counting
Many of the advantageous characteristics of the NV center discussed in literature, such
as its photostability and robustness with the optical tweezers, were confirmed in simple
single photon counting measurements involving the new MCS.
11.2.1 Photo-stability
First of the measurements were the comparison of ND PL to that of a fluorescent
dye molecule (200nm polymer microspheres, Thermo Scientific). These molecules are
polystyrene nanoparticles coated with a sheet of red fluorescent dye that produces PL
that decays exponentially over the course of a couple of minutes. Figure 28 displays the
superiority of the ND PL over that of the dye molecule.
The rapid quenching of the dye coated spheres represent a number of significant
challenges in their use with this technique. These include:
ˆ One would have to make micro adjustments to the positioning of the excitation
laser over the NP without the aid of spectroscopy, as it would result in the quench-
ing of the PL. Although, spectroscopy could be utilised at the expense of having
to find (and trap) another particle each time one as to re-align the optical table
(quite unfortunately). This would inevitably compromise the scientific procedure,
as doing repetitions of an experiment on the same NP becomes near impossible in
the time frame given.
ˆ Any significant variability in the physical/opto-electronic properties of the NP will
also affect any type of measurements that sought to determine the effect of, say,
the IR laser or RF power or magnetic field strength, since it may affect each NP
slightly differently.
ˆ Another issue would be the statistical significance of fluctuations that would arise,
if one chose to continue measurements on a ‘dead’ dye molecule which produces
little to no PL, since
√
N/N will become more prominent for low N.
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Figure 28: The normalised PL counts versus time for ND and dye molecules. The NV
center clearly prevails over the dye molecule in its absolute photostability. The dye
molecule displays rapid quenching PL and possesses a significantly larger fluctuation
range than the ND.
11.2.2 Nano-Diamond and the Optical Trap
For measurements like the T1 decay that strongly rely on the consistency of the number
of NDs in the trap, it is imperative to be able to recognise if and when a ND drifts into the
trap by chance. Now, there are preemptive steps taken, such as using a diluted solution
of NDs to minimise the chances of another particle drifting into the trap. However, it
is common sense that one cannot make the solution too dilute, as the time it takes to
find a particle to trap then becomes too long. The single photon counting system allows
the monitoring of PL counts with time and also provides indirect insight to the optical
dynamics of the NV center. Figure 29 displays the jumps in counts as NDs drift into
the trap, prompting questions about its particular features.
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64. Kazuma Wittick z3460734 Honours Thesis
Figure 29: The PL counts along a 150 second acquisition period, where sudden jumps in
the counts correspond to a ND drifting into the trap. For the purposes of showing this
effect, a concentrated solution of ND was used with high trapping power to ensure NDs
will drift into the trap at a moderate rate. However, the solution still had to be dilute
enough such that each step in counts corresponded to a single ND event.
It is very interesting to note here the variability of each ND as they hop into the
trap. It is clear that some NDs create a much smaller jump in PL counts than others.
This may be attributed to several factors in play. First, the variability of the number
of NV centers in each ND, will easily result in a ‘faint’ or ‘bright’ ND. The third pos-
sibility ties in with the second - it is possible that as the nth ND falls into the trap,
the ensemble of NDs already clumped together in the trap will re-assemble to its lowest
entropy configuration, leading to a different configuration. This is considered due to the
excitation laser width being several hundred nanometers, alongside the trapping laser
whose focal width is 700nm. Each ND being roughly 100nm, and having many NDs in
the optical trap, this phenomenon becomes influential in the scenario where bright NDs
are by coincidence located along the surface of the ‘sphere’ that the ensemble of NDs
create, and the dim NDs in the center. This will lead to a dimmer configuration due to
the maximal excitation laser hitting the dim NDs whereas the non-focal point NDs will
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65. Kazuma Wittick z3460734 Honours Thesis
experience less laser power.
It should also be added here that this form of real-time display of events occurring
inside the optical trap was also used in the ODMR experiment. The constant feed
of PL counts updating at 200 Hz was more than enough information to easily tell if
the ODMR experiment would produce a positive result. Since the RF generator and
sweeping mechanism could be left on outside ODMR data acquisition periods, the PL
will still produce a constant series of dips. This simple photon counting mechanism
saved significant amounts of time, as it was a way to confirm a positive ODMR response
from the ND without formally running the Labview program and taking time to acquire
the data.
Figure 30: The effect of increasing and decreasing the trapping laser power on the PL
of ND. Each sudden increase in the PL counts represents each time a button is clicked
to increase the trapping power by an incremental amount.
Figure 30 displays the observation that has been previously reported in literature
[25, 7, 13]. Increasing the trapping power has been shown to decrease the PL counts in
ND. It has been postulated in literature that there may be an ionisation event occurring
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as a result of the increased trapping power, turning NV− centers into NV0 due to the
electron being kicked up to the valence band. This population shift towards NV0 would
obviously lead to a decrease in NV− fluorescence. It is interesting to note that this is
not an irreversible process - the trapping power can be decreased and there will be a
subsequent increase in PL counts. Alongside this is another interesting phenomena - the
seemingly instantaneous PL response by the ND. This electronic response time has been
characterised as 100 ps by Lai et al [25]. This seems contradictory to the PL response
times to the excitation laser (see figures 37 and 38), however it will be shown that it is
related to the strength of the laser.
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