Measurement of the absolute fluorescence quantum yield of rhodamine B solution using a dual-beam thermal lens technique

The dual-beam thermal lens technique has been found to be very effective for the measurement of fluorescence quantum yields of dye solutions. The concentration-dependence of the quantum yield of rhodamine B in methanol is studied here using this technique. The observed results are in line with the conclusion that the reduction in the quantum yield in the quenching region is essentially due to the non-radiative relaxation of the absorbed energy. The thermal lens has been found to become abberated above 40 mW of pump laser power. This low value for the upper limit of pump power is due to the fact that the medium is a resonantly absorbing one.

Measurement of thermal diffusivity of some halogeno benzimidazole complexes of cobalt(II), copper (II) and copper(I) using laser induced photoacoustic effect

Cochin University of Science and Technology

Ultrasonic measurement of the elastic constants of selected nonlinear optical /semiorganic crystals with orthorhombic symmetry

Ultrasonic is a good tool to investigate the elastic properties of crystals. It enables one to determine all the elastic constants, Poisson’s ratios, volume compressibility and bulk modulus of crystals from velocity measurements. It also enables one to demonstrate the anisotropy of elastic properties by plotting sections of the surfaces of phase velocity, slowness, group velocity, Young’s modulus and linear compressibility along the a-b, b-c and a-c planes. They also help one to understand more about phonon amplification and help to interpret various phenomena associated with ultrasonic wave propagation, thermal conductivity, phonon transport etc. Study of nonlinear optical crystals is very important from an application point of view. Hundreds of new NLO materials are synthesized to meet the requirements for various applications. Inorganic, organic and organometallic or semiorganic classes of compounds have been studied for several reasons. Semiorganic compounds have some advantages over their inorganic and inorganic counterparts with regard to their mechanical properties. High damage resistance, high melting point, good transparency and non-hygroscopy are some of the basic requirements for a material to be suitable for device fabrication. New NLO materials are being synthesized and investigation of the mechanical and elastic properties of these crystals is very important to test the suitability of these materials for technological applications

Thermal characterization of methylene blue intercalated montmorillonites by photoacoustic technique

Thermal diffusivity (TD) measurement on commercial K-10 and KSF montmorillonites was carried out by photoacoustic technique. The TD of the montmorillonites after methylene blue adsorption changed with the dye concentration. The repeatedly adsorbed samples showed a lesser TD than the single adsorbed samples. After methylene blue adsorption the acid leached K-10 samples showed well defined changes in TD when compared to the ordered KSF samples.

Photothermal deflection measurement on heat transport in GaAs epitaxial layers

In this paper, we report the in-plane and cross-plane measurements of the thermal diffusivity of double epitaxial layers of n-type GaAs doped with various concentrations of Si and a p-type Be-doped GaAs layer grown on a GaAs substrate by the molecular beam epitaxial method, using the laser-induced nondestructive photothermal deflection technique. The thermal diffusivity value is evaluated from the slope of the graph of the phase of the photothermal deflection signal as a function of pump-probe offset. Analysis of the data shows that the cross-plane thermal diffusivity is less than that of the in-plane thermal diffusivity. It is also seen that the doping concentration has a great influence on the thermal diffusivity value. Measurement of p-type Be-doped samples shows that the nature of the dopant also influences the effective thermal diffusivity value. The results are interpreted in terms of a phonon-assisted heat transfer mechanism and the various scattering process involved in the propagation of phonons.

Photothermal deflection measurement on heat transport in GaAs epitaxial layers

In this paper, we report the in-plane and cross-plane measurements of the thermal diffusivity of double epitaxial layers of n-type GaAs doped with various concentrations of Si and a p-type Be-doped GaAs layer grown on a GaAs substrate by the molecular beam epitaxial method, using the laser-induced nondestructive photothermal deflection technique. The thermal diffusivity value is evaluated from the slope of the graph of the phase of the photothermal deflection signal as a function of pump-probe offset. Analysis of the data shows that the cross-plane thermal diffusivity is less than that of the in-plane thermal diffusivity. It is also seen that the doping concentration has a great influence on the thermal diffusivity value. Measurement of p-type Be-doped samples shows that the nature of the dopant also influences the effective thermal diffusivity value. The results are interpreted in terms of a phonon-assisted heat transfer mechanism and the various scattering process involved in the propagation of phonons

Photothermal deflection measurement on heat transport in GaAs epitaxial layers

In this paper, we report the in-plane and cross-plane measurements of the thermal diffusivity of double epitaxial layers of n-type GaAs doped with various concentrations of Si and a p-type Be-doped GaAs layer grown on a GaAs substrate by the molecular beam epitaxial method, using the laser-induced nondestructive photothermal deflection technique. The thermal diffusivity value is evaluated from the slope of the graph of the phase of the photothermal deflection signal as a function of pump-probe offset. Analysis of the data shows that the cross-plane thermal diffusivity is less than that of the in-plane thermal diffusivity. It is also seen that the doping concentration has a great influence on the thermal diffusivity value. Measurement of p-type Be-doped samples shows that the nature of the dopant also influences the effective thermal diffusivity value. The results are interpreted in terms of a phonon-assisted heat transfer mechanism and the various scattering process involved in the propagation of phonons

Studies on Electrical and Thermal Properties of Certain Selected Photonic Materials

The present thesis is centered around the study of electrical and thermal properties of certain selected photonic materials.We have studied the electrical conduction mechanism in various phases of certain selected photonic materials and those associated with different phase transitions occurring in them. A phase transition leaves its own impressions on the key parameters like electrical conductivity and dielectric constant. However, the activation energy calculation reveals the dominant factor responsible for conduction process.PA measurements of thermal diffusivity in certain other important photonic materials are included in the remaining part of the research work presented in this thesis. PA technique is a promising tool for studying thermal diffusivities of solid samples in any form. Because of its crucial role and common occurrence in heat flow problems, the thermal diffusivity determination is often necessary and knowledge of thermal diffusivity can intum be used to calculate the thermal conductivity. Especially,knowledge of the thermal diffusivity of semiconductors is important due to its relation to the power dissipation problem in microelectronic and optoelectronic devices which limits their performances. More than that, the thermal properties, especially those of thin films are of growing interest in microelectronics and microsystems because of the heat removal problem involved in highly integrated devices. The prescribed chapter of the present theis demonstrates how direct measurement of thermal diffusivity can be carried out in thin films of interest in a simple and elegant manner using PA techniques. Although results of only representative measurements viz; thermal diffusivity values in Indium, Aluminium, Silver and CdS thin films are given here, evaluation of this quantity for any photonic and / electronic material can be carried out using this technique in a very simple and straight forward manner.

Effect of Te doping on thermal diffusivity of Bi2Se3 crystals: A study using open cell photoacoustic technique

An open cell configuration has been employed for the photoacoustic measurement of the thermal diffusivity of undoped Bi2Se3 crystals and Bi2Se3 crystals doped with various concentrations of Te. The amplitude of the photoacoustic signal obtained under heat transmission configuration as a function of chopping frequency is used to evaluate the numerical value of thermal diffusivity, α. Doped samples show a substantial reduction in the value of α compared to undoped samples. The variations in the thermal diffusivity of the doped samples are explained in terms of the phonon assisted heat transfer mechanism. It is seen that α is very sensitive to structural variations arising from doping. The experimentally observed results are correlated with X-ray diffraction studies.

Photoacoustic measurement of transport properties in doped GaAs epitaxial layers

The photoacoustic technique under heat transmission configuration is used to determine the effect of doping on both the thermal and transport properties of p- and n-type GaAs epitaxial layers grown on GaAs substrate by the molecular beam epitaxial method. Analysis of the data is made on the basis of the theoretical model of Rosencwaig and Gersho. Thermal and transport properties of the epitaxial layers are found by fitting the phase of the experimentally obtained photoacoustic signal with that of the theoretical model. It is observed that both the thermal and transport properties, i.e. thermal diffusivity, diffusion coefficient, surface recombination velocity and nonradiative recombination time, depend on the type of doping in the epitaxial layer. The results clearly show that the photoacoustic technique using heat transmission configuration is an excellent tool to study the thermal and transport properties of epitaxial layers under different doping conditions.

Thermal characterization of ceramic tapes using photoacoustic effect

Thermal characterization of alumina–zirconia and zirconia ceramic tapes using a photoacoustic technique is presented. A transmission-mode geometry is employed for the measurement of thermal diffusivity while a reflection-mode geometry is used for the measurement of thermal effusivity. In both these geometries, the same open photoacoustic cell is used. From the measured values of thermal diffusivity and thermal effusivity, the thermal conductivity value has also been evaluated.

Laser Induced Thermal Lens Effect in Rhodamine B — Signature of Resonant Two Photon Absorption

Measurement of thermal lensing signal as a function of laser power made in Rhodamine B solutions in methanol give clear evidence of two photon absorption process within certain concentration ranges when 488 nm Ar+ laser beam is used as the pump source. Only one photon process is found to occur when 514 nm and 476 nm beams are used as the pump.

Realization of a Scanning Photoacoustic Technique: Thermal properties of Bulk and Film Samples

Among the large number of photothcrmal techniques available, photoacoustics assumes a very significant place because of its essential simplicity and the variety of applications it finds in science and technology. The photoacoustic (PA) effect is the generation of an acoustic signal when a sample, kept inside an enclosed volume, is irradiated by an intensity modulated beam of radiation. The radiation absorbed by the sample is converted into thermal waves by nonradiative de-excitation processes. The propagating thermal waves cause a corresponding expansion and contraction of the gas medium surrounding the sample, which in tum can be detected as sound waves by a sensitive microphone. These sound waves have the same frequency as the initial modulation frequency of light. Lock-in detection method enables one to have a sufficiently high signal to noise ratio for the detected signal. The PA signal amplitude depends on the optical absorption coefficient of the sample and its thermal properties. The PA signal phase is a function of the thermal diffusivity of the sample.Measurement of the PA amplitude and phase enables one to get valuable information about the thermal and optical properties of the sample. Since the PA signal depends on the optical and thennal properties of the sample, their variation will get reflected in the PA signal. Therefore, if the PA signal is collected from various points on a sample surface it will give a profile of the variations in the optical/thennal properties across the sample surface. Since the optical and thermal properties are affected by the presence of defects, interfaces, change of material etc. these will get reflected in the PA signal. By varying the modulation frequency, we can get information about the subsurface features also. This is the basic principle of PA imaging or PA depth profiling. It is a quickly expanding field with potential applications in thin film technology, chemical engineering, biology, medical diagnosis etc. Since it is a non-destructive method, PA imaging has added advantages over some of the other imaging techniques. A major part of the work presented in this thesis is concemed with the development of a PA imaging setup that can be used to detect the presence of surface and subsmface defects in solid samples.Determination of thermal transport properties such as thermal diffusivity, effusivity, conductivity and heat capacity of materials is another application of photothennal effect. There are various methods, depending on the nature of the sample, to determine these properties. However, there are only a few methods developed to determine all these properties simultaneously. Even though a few techniques to determine the above thermal properties individually for a coating can be found in literature, no technique is available for the simultaneous measurement of these parameters for a coating. We have developed a scanning photoacoustic technique that can be used to determine all the above thermal transport properties simultaneously in the case of opaque coatings such as paints. Another work that we have presented in this thesis is the determination of thermal effusivity of many bulk solids by a scanning photoacoustic technique. This is one of the very few methods developed to determine thermal effiisivity directly.

Thermal characterization of methylene blue intercalated montmorillonites by photoacoustic technique

Thermal diffusivity (TD) measurement on commercial K-10 and KSF montmorillonites was carried out by photoacoustic technique. The TD of the montmorillonites after methylene blue adsorption changed with the dye concentration. The repeatedly adsorbed samples showed a lesser TD than the single adsorbed samples. After methylene blue adsorption the acid leached K-10 samples showed well defined changes in TD when compared to the ordered KSF samples