Photothermal Deflection
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Photothermal Deflection Set-up at ISP

Photothermal deflection technique is one of the sensitive laser based measurement methods which we have perfected in our laboratory. We have experimental set-up for pump-probe photothermal deflection (PTD) methode and single beam photothermal deflection methode.In pump-probe methode we can either use a modulated CW laser or a pulsed laser as the pump source. 


The pump-probe PTD set-up, the excitation source is a Argon ion laser  and an intensity stabilized He-Ne laser is used as the probe beam. The sample is placed in a quartz sample cell and a coupling fluid is used to enhance the deflection. The position of the pump and probe beam on the sample can be adjusted using the x-y-z translator. The deflection of the probe beam is detected using a position sensitive detector and the output is analysed using a lock-in-amplifier. In pulsed PTD set-up the pump source is either an optical parametric oscillator or Nd:YAG laser. Some of the material systems that we have already studied is given below.  


Thermal Diffusivity Measurement
We have used the cw PTD set up to determine the thermal diffusivity of various samples available only in small quantities. Thermal diffusivity is an important thermophysical parameter  which controls the rate at which heat may flow through the medium. One of the important material system studied is III-IV semiconductors. We have successfully used PTD to evaluate the thermal diffusivity of bulk and layered semiconductors.  

Variation of PTD Signal Phase

Other Studies
PTD is one of the methods which can be used to determine the phase transition temperatures of the samples with great accuracy and convenience. Since this technique is fully non-contact in nature, the contamination of the samples by attachment of probes or electrodes can be avoided. We observed striking variations in the PTD signal at the temperature at which phase transitions occur in the sample material. PTD studies have been made on phase transition in ceramics, liquid crystals and some metal pthalocynines.

Phase Transition of Liquid Crystals

Laser induced damage threshold is an important parameter of a material which is used to make optical components. The damage threshold and ablation thresholds in some polymers and high temperature superconducting samples have been studied using PTD. Here the pump beam used is a 1.06 micron radiation from a Nd-YAG laser operated in single shot mode. The polymer samples studied are increasingly being used for the manufacture of optical components. 

The single beam photothermal deflection methode has been used to carry out imaging in different samples with internal inhomogeniety and physical defect . Polypyrole thin films doped with iodine at spatially selected locations and at different doping concentrations have also been examined using PTD technique to demonstrate its usefulness to study the presence of dopants and their spatial variation in concentration. We also used PTD to study the behaviour of stratified fluid media. This experiment led to the first direct observation of what is known as internal wave phenomenon in stratified fluids . The diffusion process occurring in such media has been studied using beam deflection technique.

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