Thermo-elastic properties characterization by photothermal microscopy J.Jumel,F.Taillade and F.Lepoutre Eur. Phys. J. AP 23, Journal Club Presentation 5/15/06 Presenter: AshwinKumar

Outline Motivation Thermal Characterization of bulk isotropic media by photothermal microscopy * Temperature distribution of the surface * Characterization of thermal wave propogation * Photoreflectance Technique Experimental Setup * Photoreflectance Configuration * Interferometer Configuration(Normarski) Microscopic Thermoelastic characterization * Analysis of the interferometric signal * Isotropic media characterization * Anisotropic media chracterization Summary

Motivation A better understanding of the microscopic physical mechanisms is pivotal. Sample response - photothermal experiment - dependant on thermoelastic parameters Photoreflectance Technique allows accurate characterization of thin films, interfaces and composites Determination of thermo-elastic parameters such as thermal diffusivity,elastic anistropy and crystalline orientation - surface displacements by interferometry

Thermal characterization of bulk isotropic media by photothermal microscopy 1 Electromagnetic flux 2 Sample 3 Periodic Temperature rise 4 Periodic Surface Displacement 5 Refractive index Variation 6 Infra Red emissions 7 Acoustic emissions

Description of the Thermal Problem Three dimensional Heat Equation Temperature distribution of the sample T(r,t) - Temperature distribution (K) g(r,t) - (W/m 3 ) K - thermal conductivity of the sample (W/m-K)  - thermal diffusivity of the sample (m 2 /sec)

wseTemperature Distribution of the Sample Solution by Green's function method :  - thermal diffusion length The phase lag varies linearly with r 1 Thermal diffusivity can be obtained from the thermal wave number Thermal waves are heavily damped Higher the modulation frequency, faster the amplitude decreases Typically f ~ 100 KHz,  ~ 1 cm 2 /sec, confinement volume is about a few cubic microns - determines the thermal resolution of the method

Photoreflectance Technique Temperature modulation leads to modulation of the reflection coefficient R Where - coefficient of thermal reflectance Total reflected Light Periodic fluctations of I(t) about I 0 R 0

Experimental Setup Control of dichoric mirror controls the pump-probe position r 1 Pump beam is scanned at sample surface Interferometrer Configuration obtained by the addition of parts 18 and 19

Experimental Results: Sample - Nickel KHz Circular aspect of the isotherms confirms the isotropic behavior distance measurement between consecutive isophase lines gives the thermal diffusion length Thermal diffusivity

Experimental Results: Linear Phase Variation Tantalum Sample Thermal diffusivity mm 2 /sec

Experimental Setup Control of dichoric mirror controls the pump-probe position r 1 Pump beam is scanned at sample surface Interferometrer Configuration obtained by the addition of parts 18 and 19

Nomarski Interferometer 1. Beam Splitter 2. Quarter Wave plate 3. Wollaston Prism 4. Microscope objective 5. Sample Wollaston Prism - Splits probe beam - Two orthogonal polarized beams Two spots are focused onto the sample seperated by a few microns The height difference between the two spots introduces a optical path length difference Wollaston prism produces a static phase lag given by  h - surface altitude variation  - splitting angle of wollaston prism - wavelength of the laser d - distance that can be adjusted by piezo- translation stage

Interference Signal The DC Signal measured at the photodetector is R 1, R 2 - reflection coefficient of the two beams F 0 - ratio of the common surface between the two beams to section surface Of a single beam on the photodiode. The periodic elevation U z of the sample modulates the phase lag about  Produces a harmonic term Usin  where Photothermal effects cause modulation of reflection coefficient R1 Non -uniform surface displacement and a possible thermal lens effect causes the beam to defocus and deviate periodically Makes f 0 to oscillate about it's mean value giving rise to a photodeflection signal

Total Signal at the Photodetector F- photodeflection signal T - photothermal signal A and B are experimental parameters related to interference fringe amplitude and contrast At  = 0 or , a pure interferometric term would have the same value, but spurious effects are seen To extract interferometric signal, we take measurements at  = -  /2 and  /2 U is obtained by

Reconstruction of the Signal Sample : AlPdMn quasi crystal modulated at 100 KHz

Isotropic Media Characterization The position where the phase has a minimum is found by multiparameter least square regression fitting. Phase minimum and cut off as function of thermal diffusion length is plotted For a small pump radius rg Minimum phase: Cut - Off Position

Isotropic Media Characterization Thermal Diffusivity obtained from (AlPdMn sample, 100 KHz) is /- 0.1 mm 2 /sec

Anisotropic Media Characterization Simulation of out-of plane response of Ni [1 1 1] at 500KHz and using a Gaussian beam Of radius 1 micron. Anisotropy not quite evident in the attenuation plot The phase plot shows distinct features of anisotropy

Experimental Results Most Significant contrast is observed for [111] with phase variation- quasi sinusoidal with 3 periods Four periods (cubic symmetry) for [100] Two periods (orthotropic symmetry) for [110] Modulation at 500 KHz Offset Pump- Probe : 10 microns

Experimental Results - Phase Plots -100 KHz [1 0 0] [110] [111]

Summary Simultaneous Thermal and thermoelastic characterizations at a micrometer scale can be performed Experimental Setup allows photoreflectance and interferometric configurations Extraction of thermal diffusivity from photodisplacement and photoreflectance measurements were shown. Phase measurements have shown to be very sensitive to anisotropy in the media