1. Photoinduced Force Microscopy (PiFM)
By: Avinash Warankar
(20143307)
Supervisor: Dr. Pankaj Mandal
1

2. Outline
 Introduction
Atomic Force Microscopy (AFM)
Photoinduced Force Microscopy (PiFM)
 Instrumentation
 PiFM Signal
 Applications of PiFM
 Summary
2

3. Introduction to AFM
 Atomic Force Microscope (AFM)
– one of the type of SPM
developed in 1980’s
 an imaging technique,
measures interaction forces
between tip of cantilever and
sample to map 3D-image
 Major components
 Tip along with Cantilever
 Photo detector (position
sensitive) along with laser
source
 The Piezo electric scanner
 Feedback control module
 Operational Aspects
 Alignment and approach
 Scanning modes & Raster
Scanning
 Data rendering
 Scanning Modes
 Contact mode
 Non contact mode
 Tapping Mode
 Tip diameter : 5-25 nm
 Local forces : pN or less
 X-Y & Z motion is controlled by
piezo material
 Modern AFM uses single piezo
with tube geometry 3

4.  Interaction potential between the tip
and sample
 Experimental setup of AFM
4
U=A/z12 –B/z6
Samples : Conductors, non-conductors,
polymers etc
Resolution mainly depends the tip size
Lateral resolution below 10nm
Atomic Force Microscopy
(AFM)

5. Photoinduced Force Microscopy
(PiFM)
 Many existing techniques features either nanoscale spatial
resolution or chemical sensitivity.
 PiFM: A typical Microscopy with spectroscopic sensitivity in
measurement.
 Photoinduced forces: Gradient & Scattering forces, interactions
between the optically driven tip dipole and the optically induced
polarization of sample material. These forces are in the pN range.
 Carry information on the sample’s optical polarizability & possible to
use this quantity as read-out mechanism for probing spectroscopic
transition with sub-10 nm spatial resolution.
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6. Principles of PiFM
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 PiFM measures the photoinduced forces between the sharp tip &
the sample.
 Resulting force depends on the optically induced polarization with
the gradient of electric field in vicinity of sample.
Schematic of the interaction between the photoinduced tip dipole and the
photoinduced dipole of particle in the focal plane of tightly focused field.

7. Principles of PiFM
7
 According to theory of optical forces between polarizable
particles with dipole approximation a time averaged
photoinduced force, ⟨F⟩ experienced by the sample particle is
given by,
⟨F⟩ ∝⟨ ΣiRe{Pi
*(r)∇E i( (r)} ⟩
 The polarization is complex quantity, and the force can be
rewritten as a gradient force (Fg) ,which arises because of field
inhomogeneities, and a scattering force(Fsc), which is related
to the momentum transfer between the light fields and the
polarizable objects,
⟨F⟩ = Fg+ Fsc

8. Principles of PiFM
 Gradient and Scattering forces are expressed as
Fg ∝ 1/z4 αp′αt′| E0z|2
Fsc ∝ αt ′ ′ | E0x|2 , E0j (j=x,y,z)-polarized component of the
incident electric field.
 The polarizability of tip and the sample are written as αt and αs ,
respectively
αm = αm ′ + i αm ′ ′ , m = t, p
 Gradient force is sensitive to spectroscopic properties of the sample
particle because it depends on polarizability, αm .
 Fg depence on z-4 detectable only over tip-sample distances in the
nanometer range.
 Fsc insensitive to spectroscopic properties of the sample.
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9. Instrumentation of PiFM
 Imaging is achieved moving
sample relative to position of the
focal spot, accomplished with a
piezoelectric stage.
 Tomography is obtained
monitoring the amplitude and
phase variation at resonance
frequency f01 .
 Photoinduced force is detected
by demodulating the registered
motions of the cantilever at
frequency that contains
information on the effective
optical modulation frequency.
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Basic layout of the photoinduced force microscope,
shown here for a dual beam excitation configuration
Piezo
stage
Acc.Chem.Res. 2015, 48, 2671−2679

10. PiFM Signal
 PiFM measures Amplitude A2 (z) and phase of cantilever at
detection frequency
|FPiF(z)| ∝ A2(z)
 But the amplitudeA2 (z) is not linear function of intensity at focal
plane,
10
Amplitude of the cantilever resonance with respect to illumination power

11. Applications of
PiFM
 Photoinduced Forces in the Focal Volume
 Chemical Imaging Application
 Linear Spectroscopy at the Nanoscale
 Nonlinear Optical Contrast in PiFM
11

12. Photoinduced Forces in the Focal Volume
 Focusing laser beam to tightly focal spot at glass/air interface
 Mapping the magnitude of PiF as the tip is scanned through the field
 Electric field distribution in near-field can be measured without relying
on scattering to far field photodetector
12Acc.Chem.Res. 2015, 48, 2671−2679
(a) Schematic of the tip in
the vicinity of the focused
optical field. The z-polarized
part of the field interacts
strongly with tip-dipole,
which has its strongest
component along z.
(b) Simulation of Ez, the z
polarized component of the
focused field by a high
numerical aperture objective
using linearly polarized input
radiation. (c) Topography(d)
PiFM signal

13. Chemical Imaging Application
 PiFM makes it possible to visualise nanoscopic material with
spectroscopic contrast.
 Materials with strong optical cross sections give rise to better contrast
in PiFM.
13
Acc.Chem.Res. 2015, 48, 2671−2679
Chemical imaging
of a thin film of a
coblock polymer
(PS-b-P2VP)
based on IR-
absorption
contrast using a
cw IR laser.
(a)PiFM image
taken at 1492
cm−1. (b) Image
taken at 1589
cm−1. Scale bar
is 100 nm.

14. Linear Spectroscopy at the Nanoscale
14
Spectroscopic sensitivity of
PiFM. (a) Imaginary part
(black dots) and real part
(blue dots) of SiNc optical
response. Red dots
indicate the magnitude of
the PiFM response. (b)
Structural formula of SiNc.
(c) Topography (top) and
PiFM amplitude (bottom) of
two SiNc nanoclusters as a
function of excitation
wavelength
Acc.Chem.Res. 2015, 48, 2671−2679

15. Nonlinear Optical Contrast in PiFM
15
(a) Schematic of the
pump−probe excitation of
SiNc. (b) Time-resolved
excited state absorption
measured with PiFM (solid
dots) and with optical
pump−probe microscopy
(solid line). (c)Topography
(top) and PiFM signal
amplitude (bottom) of a
nanocluster measured at
different time delay settings
of the probe pulse.
Acc.Chem.Res. 2015, 48, 2671−2679
PiF actives only when
presence of both
pulses

16. Summary
 PiFM: based on the principle of AFM i.e. information about the
sample is encoded in the deflection.
 Sub-nanoscale spatial resolution.
 Tomography and PiFM signal measured simultaneously.
 Capability of probing Linear and Nonlinear properties of sample.
 PiFM is relatively new scan probe technique, it may find application
in different branches of science as well in near future.
16

17. Acknowledgment
Supervisor:
Dr. Pankaj Mandal
Lab mambers:
Sohini
Gurivi Reddy
Sneha
Friends:
Shammi Rana
Dinesh
Harshad
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Reference:
1. Linear and Nonlinear Optical Spectroscopy at the Nanoscale with
Photoinduced Force Microscopy, Acc.Chem.Res. 2015, 48, 2671−2679
2. Gradient and scattering forces in photoinduced force microscopy,
PHYSICAL REVIEW B 90, 155417 (2014)
3. Ultrafast pump-probe force microscopy with nanoscale resolution, Appl.
Phys. Lett. 106, 083113 (2015)
4. Advances in Atomic Force Microscopy, Rev. Mod. Phys., Vol. 75, No. 3,
July 2003
5. Resonance optical manipulation of nano-objects based on nonlinear
optical response, Phys. Chem. Chem. Phys., 2013, 15, 14595—14610
6. Atomic force microscope, Phys. Rev. Lett. 1986, 56, 930−933
7. NPTEL lectures on Atomic Force Microscopy by Prof. R. Mukherjee
8. http://www.parkafm.com/index.php/park-spm-modes/standard-imaging-
mode/217-true-non-contact-mode

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20. 20
Samples : Conductors, non-conductors,
polymers etc
Resolution mainly depends the tip size,
sharper the tip ,better the resolution
Lateral resolution below 10nm can be obtained

21. 21
⟨ F ⟩ = (α//2)∇ ⟨ |E|2 ⟩ + ωα// ⟨ E × B ⟩

22. 22
FIG. 3.Sketch of the photoinduced force
microscope.
AFM
controller
Quadrant
detector
f01
f02
Piezo
stage
Objective
Cantilever

23. 23

24. 24

25. 25
⟨ F ⟩ = (α//2)∇ ⟨ |E|2 ⟩ + ωα// ⟨ E × B ⟩
Silicon naphthalo