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Photoinduced Force Microscopy (PiFM)

By: Avinash Warankar(20143307)

Supervisor: Dr. Pankaj Mandal

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Outline

Introduction

Atomic Force Microscopy (AFM) Photoinduced Force Microscopy (PiFM)

Instrumentation

PiFM Signal

Applications of PiFM

Summary

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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

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Interaction potential between the tip and sample

Experimental setup of AFM

U=A/z12 –B/z6

Samples : Conductors, non-conductors, polymers etcResolution mainly depends the tip sizeLateral resolution below 10nm

Atomic Force Microscopy (AFM)

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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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Principles of PiFM

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.

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Principles of PiFM

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 = F⟩ g+ Fsc

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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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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.

Basic layout of the photoinduced force microscope, shown here for a dual beam excitation configuration

Piezo stage

Acc.Chem.Res. 2015, 48, 2671−2679

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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,

Amplitude of the cantilever resonance with respect to illumination power

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Applications of PiFM

Photoinduced Forces in the Focal Volume

Chemical Imaging Application

Linear Spectroscopy at the Nanoscale

Nonlinear Optical Contrast in PiFM

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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 Acc.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

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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.

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.

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Linear Spectroscopy at the Nanoscale

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

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Nonlinear Optical Contrast in PiFM

(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

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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.

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Acknowledgment

Supervisor: Dr. Pankaj Mandal

Lab mambers:SohiniGurivi ReddySneha

Friends:Shammi RanaDineshHarshad

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Reference:1. Linear and Nonlinear Optical Spectroscopy at the Nanoscale with

Photoinduced Force Microscopy, Acc.Chem.Res. 2015, 48, 2671−26792. 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 20035. Resonance optical manipulation of nano-objects based on nonlinear

optical response, Phys. Chem. Chem. Phys., 2013, 15, 14595—146106. Atomic force microscope, Phys. Rev. Lett. 1986, 56, 930−9337. NPTEL lectures on Atomic Force Microscopy by Prof. R. Mukherjee8.

http://www.parkafm.com/index.php/park-spm-modes/standard-imaging-mode/217-true-non-contact-mode

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Samples : Conductors, non-conductors, polymers etcResolution mainly depends the tip size, sharper the tip ,better the resolutionLateral resolution below 10nm can be obtained

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

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FIG. 3.Sketch of the photoinduced forcemicroscope.

AFM controller

Quadrant detector

f01

f02

Piezo stage

Objective

Cantilever

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

Silicon naphthalo