Thermal Lens Effect
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Thermal Lens


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Dual Beam Thermal Lens Facility at ISP

Thermal lens effect is a very useful photothermal phenomenon which can be profitably exploited for the measurement of material properties. We have successfully set up dual-beam thermal lens experiment which can be used for the measurement of thermal and optical properties of weakly absorbing materials.

Pulsed Thermal Lens Set-up
The dual beam pulsed thermal lens set-up at ISP consist of an Optical parametric oscillator and a Q-switched Nd:YAG laser as pumping source. One of the excitation source can be used at a time. An intensity stabilized He-Ne laser is used as the probe beam. The pump and probe beams are combined by using a dichroic mirror and are made collinear by carefully adjusting the optical components in the x, y, and z planes. The set-up is usually used to study the thermal lens effect in liquid samples and the sample solution is kept in a quartz cuvette. The pump beam is focused to into the cuvette containing the sample solution. The thermal lens effect is detected by monitoring the intensity fluctuations in the beam center of the probe He-Ne laser through a small aperture. the intensity of the central portion of the transmitted probe beam is detected by using an optical fiber. The optical fiber also serves as a limiting aperture. The output of the optical fiber is detected by a photo multiplayer tube that is coupled to a monochromator, which further filtered out the scattered light from the excitation beam. The time dependent thermal lens signal is processed with a digital storage oscilloscope. Some of the material systems that we have already studied is given below. 


Thermal Lens Set-up
Laser Dyes
Fluorescence quantum yield is one of the key photophysical quantities that are amenable to direct experimental determination. The quantum yield of fluorescence is a measure of the rate of nonradiative transitions that compete with the emission of light. The knowledge of fluorescence quantum efficiency of organic dyes and its concentration dependence are essential for selecting efficient laser media. Conventional measurements require the use of accurate luminescence standard samples and comparison of the given sample with a standard for which the fluorescence yield is known. In order to evaluate absolute quantum efficiency, we have to consider both the radiative and nonradiative processes taking place in the medium. As the contribution from nonradiative processes is not directly measurable using the traditional optical detection methods, thermo-optic techniques such as thermal lens technique can be adopted for this purpose. We have successfully used the dual beam thermal lens technique to evaluate the fluorescence quantum yield of laser dyes in different solvents. Our results demonstrate that fluorescence parameters are influenced by environment of the fluorescing molecule, processes like internal nonradiative conversion, excited singlet state absorption and aggregation of the dye molecule. These are strongly dependent on excitation source, solvent characteristics as well as concentration of the dye solution.
Quantum Yield Measurement
We have used thermal lens technique to study the nonlinear optical proparties of fullerence solutions.Our studies revealed that C60 and C70 molecules in solution exhibit very good optical limiting at 532 nm wavelength.Because of large excited singlet as well as triplet absorption cross sections compared to ground state absorption cross section, the major mechanism for this limiting behavior of these molecules is reverse saturable absorption. Thermal lensing studies in these solutions show a quadratic dependence of the thermal lens signal amplitude versus input laser energy. Hence, thermal lensing studies in fullerenes lead to the conclusion that sequential two photon absorption plays a leading role in the optical limiting properties of C60 and C70.
Nonlinear Studies Using TLS
Thermal Diffusivity Measurement
Thermal diffusivity is an important parameter which controls the rate at which heat may flow through the medium. Pulsed thermal lens technique can be successfully adopted for thermal diffusivity measurements. We measured values of thermal diffusivity of several liquids using the pulsed thermal lens technique. The measured values of thermal diffusivity are in good agreement with the literature values. However aqueous solutions gave results which were strongly dependent on concentration and nature of the dissolved material. This has yielded new understanding on the thermal diffusion processes in ionic solutions .

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