Terahertz spectroscopy

Terahertz Spectroscopy
Amit Soni
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Topic Seminar

Outline:
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•Introduction
•Basics of Terahertz
•Terahertz Time domain spectroscopy
•Applications
•Conclusion

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What is spectroscopy?
• Study of the quantised interaction between matter and electromagnetic radiation.
Image courtesy: hyperphysics.phy-astr.gsu.edu

What is Terahertz(THz)?
• Ranges from 0.1 to 10THz
• 1THz = 1012 Hz
• wavelength: 3mm to 0.03mm
Chem. Soc. Rev., 2012, 41, 2072–2082
Anal. Chem., 2011, 83, 4342–4368  !4
• Wavenumber: 3.3cm-1 to 333.6cm-1
• Energy: 0.41 to 41 meV
• Temperature: 1 to 100K

What is Terahertz Gap?
Appl. Phys. A, 2010, 100, 591–597
• Non availability of THz generators and Detectors
• Non availability of stable femtosecond lasers
This restricts the application of THz in science and industry, offering complementary or
even alternative methods of material characterization.
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Properties of THz radiations:
• Terahertz waves can penetrate through materials opaque to other parts of the EM
spectrum
• Packaging materials as plastics, ceramic, wood etc. are transparent to some degree
• Does not initiate any changes in chemical structure, as opposed to other forms of
spectroscopy
• Can create images and transmit information
• It lies in the region of rotational transitions of many gas molecules and the
vibrational transition of weak bond.
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Image source: Science, 2002, 297 5582, 763

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Generation of THz radiation:Chapter 3. Generation and detection of THz radiation 29
Figure 3.5. Source [2]: THz radiation from accelerating electrons by frequency up conver-
sion.
accelerators produce extremely bright THz radiation. The process of generating the THz
radiation from accelerating electrons results in both broadband and continuous wave THz
radiation as indicated below;
1. Femtosecond laser pulse incident on the electron source, triggers the electron source,
and sets in motion ultrashort pulses of electrons. The generated electrons are accel-
erated to relativistic speeds, smashing them into a metal target or forcing them into
circular motion by a magnetic field. It is the acceleration of these transient electrons
that generates coherent broadband THz radiation [2].
2. CW THz radiation can be generated using Backward Wave Oscillators (BWOs) and
Stellenbosch University http://scholar.sun.ac.za
THz radiation are mainly two types Broadband and Continuous wave (CW)THz radiation
Principles of terahertz, science and technology. Springer, 2009
3.2.2.1 Photocurrent in semiconductors
THz generation from photocurrent in semiconductors utilising a biased photoconductive
antenna excited by laser beams. Two techniques utilising this process are summarised
in Figure 3.4. In the first technique a femtosecond laser pulse produces broadband THz
radiation by using transient photoconductive switching of the PC antenna. The other
technique is referred to as photomixing, where two laser beams with different frequencies
are mixed forming an optical beat which generates CW THz radiation at the beat frequency
via a PC antenna. More information about the structure and operation of a PC antenna
is given in Section 3.3.
Figure 3.4. Source [2]: THz radiation from accelerating electrons by frequency down
conversion.
THz radiation from
accelerating electrons by
frequency down
conversion
THz radiation from
accelerating electrons
by frequency up
conversion

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• PC switching
• Schottky multiplier chains
• Quantum cascade lasers
• Backward wave oscillator
THz Sources
Terahertz Broadband Terahertz CW
• Schottky diode
• Surface surge currents
• Plasmon, phonon and
coupled mode oscillation
• Photoconductive switching
• Quantum cascade lasers
Backward wave OscillatorQuantum cascade laserSchottky diode
Prog. Quant. Electron., 2010, 34, 278–347

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THz detectors
Semiconductor detectorsCoherent detectorsThermal detectors
• Hot electron bolometer.
• Photo-acoustic detector.
• Pyroelectric
• Quantum well infrared detectors
• Homojunction devices.
• Electro-optic detector
• Photoconductive switch
Prog. Quant. Electron., 2010, 34, 278–347
Schematic diagram of thermal bolometer

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Photoconductive antenna (PC):
Mounted on a hemispherical substrate lens
electrical switch in which
the increase in electrical
conductivity of semiconductors
and insulators when they are
exposed to light
• Most commonly used THz emitter and detector
• Examples: LT-GaAs, RD-SOS, Cr-GaAs, InP, amorphous Si
Int. J. Infrared Millimeter Waves, 2006 , 27, 531–546. 
PC emitter antenna PC detector antenna

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terahertzfrequencies,thevaluesofcapacitanceC,andtheinductan
small(ontheorderofpicofaradandpicohenry).ThePCantennaco
electrodesthatarecoatedonasemi-insulatingsemiconductorsubstr
gap)betweenthetwoelectrodes[3].Thesubstratebeingsemi-insul
thegaparea,theelectricenergyisstoredinthisgaparea[3].Th
withadcvoltageandilluminatedwithafemtosecondlaserpulsew
gap,generatingphoto-excitedchargecarriers.Thesephotoexcited
acceleratedunderthebiasﬁeld(Eb=Vb
D
,whereVbisthebiasvolt
ofthePCgap)producingaphotocurrent[30].Thistransientcurre
ultra-shortpulseofelectromagneticradiation(THzradiation).Ther
theTHzelectricﬁeldamplitudeandthephotocurrentisgiveninequa
Figure3.7.Geometricalparametersoftheantenn
W is width of
antenna( range 10-20 µm)
L is length (30 − 50 µm)
D is gap between two electrodes
(5-10 µm)
The gap area is the active area with
laser excitation and that is where
THz wave generation and
detection occurs.
1

√LC
𝝎=
Angular frequency
3.7. In our practical experimentation we use a PC antenna with the following dimensions,
the PC gap D, of 10 µm, antenna width W, of 20 µm, and antenna length L, of 40 µm for
the Photoconductive switch [31].
A typical PC switch Figure 3.8, has a bias voltage and a load resistor connected in series
with a semiconductor. When laser light illuminates the PC antenna, the photons generate
free electron carriers and holes [2]. However, for this laser light to generate photoinduced
free carriers from the substrate, it must have enough photon energy, that means for gen-
eration to occur, the excitation optical pulse should have photon energy higher than the
band gap of the substrate [3]. The increase in the number of free carriers and holes results
Figure 3.8. Photoconductive switch [2].
in the photoconductivity of the antenna [2].
In the generation and detection of THz radiation, the switching action of the PC antenna
is vital, and should be in the subpicosecond time range. Low-temperature grown gallium
The electric energy is stored in this gap area and biased with a dc voltage and illuminated with a
femtosecond laser pulse which excites the PC gap, generating photo-excited charge carriers. These photo
excited charge carriers are accelerated under the bias producing a photocurrent
Principles of terahertz, science and technology. Springer, 2009  
Meas. Sci. Technol, 2002, 13, 1739–1745. 
Generation of THz using PC:

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Detection of THz using PC:
Chapter 3. Generation and detection of THz radiatio
Figure 3.9. Schematic representation of THz puls
silicon, as will be summarised in Subsubsection 3.4.1
PC antenna excited by femtosecond laser.
Schematic diagram of detection of THz radiations
Int. J. Infrared Millimeter Waves, 2006 , 27, 531–546.

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3.4.1 Materials for Photoconductive switches
A wide variety of materials have been used for the construction of photoconductive anten-
nas. Some examples are shown in Table 3.2. Of all these materials, the most commonly
Photoconductive ma-
terials
Carrier
lifetime
(Ps)
Mobility
(cm2
/(V.s))
Resistivity (⌦.cm)
(Breakdown ﬁeld, V/cm)
Band gap
(ev at R.T)
Cr:doped SI-GaAs 50 100.0 ' 1000 107
1.43
LT-GaAS 0.3 150 200 106
(5 ⇥ 105
) 1.43
SI-InP 50 100.0 ' 1000 4 ⇥ 107
1.34
Ion-Implanted InP 2 4.0 200 > 106
1.34
RD-SOS 0.6 30 1.10
Amorphous Si 0.8 20.0 1 107
1.10
MOCVD CdTe 0.5 180 1.49
LT-In0.52Al0.48As 0.4 5 1.45
Ion-Implanted Ge 0.6 100 0.66
Table 3.2. Characteristics of ultra fast photoconductive materials [32].
used materials for THz emitters and detectors are, radiation-damaged-silicon-on sapphire
(RD-SOS) and low-temperature grown gallium arsenide (LT-GaAs) [2, 32]. RD-SOS is pre-
pared by implanting argon, silicon or oxygen ions into SOS samples. Implanting results in
Materials for Photoconductive switches:
Terahertz optoelectronics. Springer, 2005

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THz time-domain spectroscopy
It uses THz pulsed tradition and consists of an Emitter stimulated by femtosecond laser, a
beam forming optics consisting of focuses lenses and mirrors, a sample holder and optical
delay line and a Detector which uses a femtosecond laser.
Nanotechnology, 2015, 26 (31), 5203
Difference absorption
spectra technique,THz
pulse is measured with and
without the sample.
The absorption and
dispersion of the sample
obtained by Fourier
transformation.
THz-TDS determines both
the amplitude and the phase
Pump Pulse
REVIEW ARTICLES
Femtosecond laser Half mirror
Emitter
Excitationpulses
Off-axis
paraboloidal mirror
Detector
Time delay
Detection pulses
THz
1.0
0.8
–0.2
–0.4
0.0
0.2
0.4
0.6
DAST
p-InAs
DAST
p-InAs
100
10–1
10–2
Normalizedamplitude(a.u.)
1,560 nmc
Probe Pulse
Schematic diagram of THz time domain spectrometer

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Material Type Optical property
Liquid Water High absorption (α = 250 cm−1 at 1
THz)
Metal High reflective (> 99.5 % at 1 THz)
Plastic
Low absorption (α < 0.5 cm−1), low
refractive index (n ∼= 1.5)
Semiconductor Low absorption (α < 1 cm−1 at 1
THz), high refractive index
Optical properties of condensed matter in the THz band:
Principles of terahertz, science and technology. Springer, 2009

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Opt. Commun. 2012, 285, 1868
IR-Spectra THz-Spectra
Chemical Analysis and Molecular Spectroscopy:
1. Differentiating the molecules:

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Due to low energy of THz radiation many vibrational and rotational modes fall within this region.
2. Tracing illegal drugs/ narcotics:
REVIEW ARTICLES
to study the behaviour of light at longer wavelengths. Examples
include the study of multiple diffraction and propagation in
random media84,85
and the group-velocity anomaly of propagating
Figure 6 MOSFET damage detection by LTEM. a, Laser-reflection image of a
series of MOSFETs. Three MOSFETs as indicated by yellow circles have interrupted
connection lines cut by a focused ion beam. One MOSFET is shown magnified
in the large yellow circle. b,c, LTEM images of the normal and damaged sample,
respectively. The negative and positive amplitude of the THz waves, when the delay
time is fixed at the time which gives maximal intensity, are denoted by black and
white respectively. The blue and red boxes indicate p- and n-type respectively.
200 µm
a b c a
Visible THz-reflection mode
b
Codeine Cocaine Sucrose
(60 mg each)
Every explosive and narcotic has a
distinct signature in its THz spectra,
making THz spectroscopy valuable
for security applications.
Nat. Photon., 2007, 1 97105
rtz spectroscopy of explosives and drugs
a range of common drugs-of-abuse. The samples are prepared as compressed pellets, diluting the pure drugs with PTFE powder;
ht) is indicated. A refractive index measurement is shown for cocaine free base, as an example. The cocaine free base and cocaine
s shown in Fig. 2.

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3. Tracing explosive:
Absorption
THz absorption spectra of a range of
common explosives. The pure
explosive samples (PETN and RDX)
were diluted with PTFE powder to a
30:70 (explosive: PTFE) ratio by
weight, and compressed into pellets
range of common explosives. The pure explosive samples (PETN and RDX) were diluted with PTFE powder to a 30:70 (explosive:
essed into pellets. The plastic explosives (Metabel, Semtex, and SX2) were cut into thin slices and measured without further
x and SX2, several spectra are shown, each arising from a different sample batch. A refractive index measurement is shown for
Mater. Today 2008, 11(3)

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Biology and Medicine:
Water is strong observer of
THz radiations in 1-3THz.
So THz radiation can
detect the difference in
water content and density
of a tissue.
Image Source: teraview.com

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Diabetic foot measurement studies using THz. (A) Photograph of the measurement set-up. THz images
plotting the calculated water volume for (B) a control and (C) a diabetic patient. (D) Calculated water
volume for control and diabetic patients using data from the centre of the big toe.
Quant Imaging Med. Surg. 2017;7(3):345-355
Biology and Medicine:

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THz imaging in Biology:
Opt. Lett. 1995, 20 (16)

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Chem. Phy. Lett. 2004, 390, 20–24
rahertz absorption spectra for
orms of CBZ, EM, IM, and FC
from the ﬁgures that diﬀerences
or all four compounds give rise
he terahertz absorption spectra
ectra of CBZ forms III and I
t are polymorph-distinct. The
ibits major peaks at 41, 60 and
eak at 47 cmÀ1
while the form I
peaks at 31, 44, 52 and 70
ty peak at 23 cmÀ1
. The mid-
n spectra of forms III and I are
tive absorptions however, can
d 1680 cmÀ1
in the IR spectra
udies have shown that these
ith the CONH2 moiety. Both
mation with hydrogen-bonding
Fig. 3. Absorbance spectra of EM form I (solid line) and form II
(dashed line) 25% in PE.
10 20 30 40 50 60 70
0.0
0.5
1.0
1.5
2.0
2.5
Absorbance(decadic)
Wavenumber / cm
-1
Fig. 4. Absorbance spectra of IM crystalline (solid line) and amor-
phous (dashed line) 75% in PE.Indomethacin (anti-inflammatory drug)
Amorphous(dotted line) and Crystalline(solid)
Pharmaceuticals:

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THz imaging:
Concealed knife under the sole of shoe
Check before you buy next time..
Crack in ceramic
Image sources: Terasense and Teraview

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Visible image of an integrated circuit THz image of an integrated circuit
Electronic devices:
Opt. Lett. 1995, 20 (16)

!27
Current wireless systems utilise carrier waves less than 5 GHz which restrict their
maximum data rate, 100’s Mbps typically. Higher frequency carriers enable high data rates
and therefore 1000’s Gbps dates rates are on offer with terahertz carrier waves.
J. Phys. D: Appl. Phys. 2017, 50, 043001
Wireless technology:
46 Gbps..😳
using a carrier frequency of 400 GHz

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Read through a book without opening it..
flections of the E-field from the layered
me resolution. The sample consists of nine
single character written on each page. The
the velocity extracted from the waveform, with high energy
corresponding to large amplitudes and low velocities (Supple-
mentary Fig. 2a). This corresponds to candidate points being
Sample
n2n1n0n1n0
1,
92,..2,..
1,
Layered sample
Motorized
stage
z
y
x
THz-TDS
d< , g <0.1
g~20 µm d~300 µm
ba
geometry and sample schematics. (a) Confocal THz time-domain (THz-TDS) measurement is used in reflection geometry. x–y–z
n coordinate system that is kept consistent throughout this study. The 9-page sample is held on an x–y-motorized stage that
r scanning in x–y plane. A THz pulse is transmitted. The electric field is a bipolar pulse as shown schematically in blue. The
n red) has a series of dense reflections (usually more than nine) from the layered sample that provides time-of-flight information
n z. (b) The layered sample is composed of nine packed paper layers. Each layer is 300 mm thick and the non-uniform gaps
B20 mm after pressing the pages together.
NATURE COMMUNICATIONS | 7:12665 | DOI: 10.1038/ncomms12665 | www.nature.com/naturecommunications
timization software. Figure 3c shows that
pite the presence of significant shadowing
deeper layers. The CCSC optimization is
ary Note 7.
n. The proposed three steps work together
deep as possible (Supplementary Movie).
uperior performance (about an order of
dB—SNR is 20log(|Es|.|En|À 1)) of PPEX
of standard deconvolution techniques
Es is the signal component of the mea-
plitude and En is the noise component.
th edge detection techniques and CLEAN
n in Supplementary Figs 4a–e and 5a–d
plied to the data before feeding it to the
4a comparison. The induced dispersive
omain is not because of the dispersion in
itself as in the case of optical waveguides.
ve behaviour induced by the frequency-
the detection process. Water vapour-induced ripples in the
electric field can be partially compensated for deconvolution
methods as well, but the strong reference dependency of these
techniques means that any change in the humidity level can
negatively impact the deconvolution.
Figure 4b shows the difference between amplitude mapping in
the time domain and the time-gated Fourier transform. The time-
gated spectral analysis based on kurtosis provides up to 18 times
more contrast for the eighth layer. For the ninth layer, the signal
level is too low to accurately estimate the contrast improvement
(our estimation is B10.5). However, in Fig. 4b, we see that the
character is now completely recognizable to both the human eye
and the CCSC algorithm.
The signal loss with depth is a major burden on reading deeper
layers. This loss is caused by consecutive reflections at the
material interfaces (both the back and the front of each layer) and
also by the exponential Lambertian absorption of the layers
themselves. The reflected signal level is not the bottleneck to
content extraction at deeper layers (we can detect 15 pages with
the first step of time-gated spectral imaging; PPEX), it is rather
Page number
0
1
0.51
0
5
10
15
20
20100
25
30
30
35
40
40
2 3 4 5 6 7 8 9
x(mm)
t (ps)
a b
c
ontent extraction with THz time-gated spectral imaging. (a) Layers are identified in time based on the statistics of the
ignal. Image is binary. (b) The technique uses kurtosis of the time-gated Fourier transform to contrast the content. Grey scale
rmalized amplitude of the averaged frequency components in arbitrary units that is output from the contrast enhancement
malized separately, and horizontal and vertical axes are omitted for simplicity. (c) Convex cardinal shape composition (CCSC)
uded characters through THz noise down to page 9. The detected letters are highlighted with light orange.
a
c
0
17
0
Page 1 Page 2 Page 3
x (m
y(mm)
Figure 2 | Sample description and raw measurements overview. (a) Nine
are then stacked on top of each other, respectively. The orange highlighted a
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms12665
Nine roman letters T, H, Z, L, A, B, C,
C and G are written on nine pages
(a) THz-TDS measurement is used in reflection geometry
(b) layered sample is composed of nine packed paper layers
uniform thickness
(a) Layers are identified in time based on the
statistics of the reflected bipolar THz field
signal.
(b) Then converted the time-gated Fourier
transform to contrast the content.
(c) Convex cardinal shape composition
algorithm extracts the occluded characters
through THz noise down to page 9.
Nat. Comm. 2016, 7, 12665

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THz technology
Plasmonics
Metamaterials
MEMS
Semiconductor technology
Material properties
Nano- and microtechnology
Electromagnetic wave simulation
Spectroscopic and imaging equipment
Optical components (lenses, filters, polarizers,
waveguides, modulators)
Sensing devices
Superconductor technologyLaser technology
Small light sources (laser-excited radiation devices, electronic devices)
Medium and large light sources (gyrotrons, synchrotron, FELs)
Detectors (bolometers, superconductive detectors, single-electron
transistors, arrays)
Information communications
Security
Materials science Analysis science
Biology
Medical care
Agriculture
Industries
Space technologies
Environment
Drug development
Preservation of
cultural assets
Quantum
information
Nuclear fusion plasma diagnosis
Materials development
Fig. 15. (Color online) Peripheral technologies and applications of THz technology.
54, 120101 (2015) COMPREHEN
Jpn. J. Appl. Phys. 2015, 54, 120101
Conclusions

Terahertz spectroscopy

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