Nibu A. George

Chemical sensing with microbent optical fiber

We propose and demonstrate the possibility of using a permanently microbent bare optical fiber for detecting chemical species. Two detection schemes, viz., a bright-field detection scheme (for the core modes), and a dark-field detection scheme (for the cladding modes) have been employed to produce a fiber-optic sensor. The sensor described here is sensitive enough to detect concentrations as low as nanomoles per liter of a chemical species, with a dynamic range of more than 6 orders of magnitude.

Studies on Fluorescence Efficiency and Photodegradation of Rhodamine 6G Doped PMMA Using a Dual Beam Thermal Lens Technique

In this paper we report the use of the dual beam thermal lens technique as a quantitative method to determine absolute fluorescence quantum efficiency and concentration quenching of fluorescence emission from rhodamine 6G doped Poly(methyl methacrylate) (PMMA), prepared with different concentrations of the dye. A comparison of the present data with that reported in the literature indicates that the observed variation of fluorescence quantum yield with respect to the dye concentration follows a similar profile as in the earlier reported observations on rhodamine 6G in solution. The photodegradation of the dye molecules under cw laser excitation is also studied using the present method.

Depth-kymography: high-speed calibrated 3D imaging of human vocal fold vibration dynamics

We designed and developed a laser line-triangulation endoscope compatible with any standard high-speed camera for a complete three-dimensional profiling of human vocal fold vibration dynamics. With this novel device we are able to measure absolute values of vertical and horizontal vibration amplitudes, length and width of vocal folds as well as the opening and closing velocities from a single in vivo measurement. We have studied, for the first time, the generation and propagation of mucosal waves by locating the position of its maximum vertical position and the propagation velocity. Precise knowledge about the absolute dimensions of human vocal folds and their vibration parameters has significant importance in clinical diagnosis and treatment as well as in fundamental research in voice. The new device can be used to investigate different kinds of pathological conditions including periodic or aperiodic vibrations. Consequently, the new device has significant importance in investigating vocal fold paralysis and in phonosurgical applications.

Photoacoustic study on photobleaching of Rhodamine 6G doped in poly(methyl methacrylate)

The photobleaching of the lasing dye Rhodamine 6G embedded in the solid matrix poly(methyl methacrylate) was investigated using a photoacoustic technique. Chopped laser radiation from an argon ion laser at four different wavelengths was used for the study. Experimental results indicate that the photobleaching rate is directly proportional to the incident laser power while it decreases with increase in concentration of the dye molecules. In the present case we have not observed any dependence of photobleaching on the chopping frequency. One-photon absorption is found to be responsible for the photobleaching of the dye within the selected range of laser power.

Thermal diffusivity of liquid crystalline polymers measured using open cell photoacoustic technique

Thermal diffusivity of two side-chain liquid crystalline polymers is reported in this paper. An open photoacoustic cell in the heat transmission configuration is employed for the measurements. The investigation shows that the thermal transport properties of these materials are mainly determined by their molecular structure.

Photoacoustic studies on n-type InP

We discuss an open photoacoustic cell study on sulfer-doped n-type InP wafer. The thermal diffusivity of the sample is evaluated from the phase data associated with the photoacoustic signal as a function of the modulation frequency under heat transmission configuration. Analy- sis is made on the basis of the Rosencwaig-Gersho theory and the re- sults are compared with those from earlier reported photoacoustic stud- ies of semiconductors. Our investigation clearly indicates that the instantaneous thermalization process is the major heat diffusion mecha- nism responsible for the photoacoustic signal generation in an InP sample.

Photoacoustic evaluation of the thermal effusivity in the isotropic phase of certain comb-shaped polymers

The thermal effusivity values in the isotropic phase of certain comb-shaped polymers have been evaluated for the first time using an open photoacoustic cell configuration. The compounds investigated have siloxane and acrylate backbone and they carry mesogenic groups in their side chain. The results indicate that the polymer chain length as well as the side chain length have pronounced influence on the thermal effusivity values in liquid crystalline polymers.

Use of photoacoustic effect for the detection of phase transitions in liquid crystal mixtures

We report on a laser induced photoacoustic study of the nematic-to-isotropic transition in certain commercial nematic liquid crystal mixtures, namely BL001, BL002, BL032 and BL035. A simple analysis of the experimental data using the Rosencwaig-Gersho theory shows that the heat capacities of all these compounds exhibit a sharp peak as the temperature of the sample is varied across the transition region. Also, substantial differences in the photoacoustic signal amplitudes in nematic and isotropic phases have been noticed for all the mixtures. The increased light scattering property of the nematic phase may be the reason for the enhanced photoacoustic signal amplitude in this phase.

Photoacoustic investigation of the effect of excess lead oxide on thermal diffusivity of PLZT ceramic

The versatility as well as the potential of the photoacoustic (PA) technique as a material characterization method has been established by several workers [1–6]. Apart from providing direct optical absorption spectra, the PA technique can also be used to perform depth profile analysis, thermal characterization as well as investigation of nonradiative relaxation processes [7–10]. In a typical PA experimental arrangement the sample enclosed in an airtight cavity is exposed to an intensity modulated light beam. The resulting periodic heating of the sample is strongly dependent on the interplay of three factors, namely, the optical absorption coefficient at the incident radiation wavelength, lightinto-heat conversion efficiency and the heat diffusion through the sample. The light-into-heat conversion efficiency of each material depends on the nonradiative de-excitation processes taking place within the sample. The dependence of the PA signal on the rate of heat diffused through the sample allows us to perform thermal characterization, especially the determination of thermal diffusivity. Ceramics are considered as the optimum materials to solve a number of scientific and technological problems due to the availability of raw material as well as their resistance to corrosion and irradiation. The unique electro-optic and photo-electric properties of transparent ferroelectric ceramics (TFC) have helped them to acquire a significant fraction of the solid state optoelectronic device market, particularly in high speed light modulators and shutters, thermal and light filters, electrically controlled color filters, alphanumeric displays, block data composers, video projectors and optoelectronic voltmeters [11–16]. The majority of the TFC compositions include materials prepared on the basis of the well-known PZT system with the ABO3 perovskite structure. One such material is lanthanum doped lead zirconate-titanate (PLZT), in which some Pb2+ ions in the A sites are replaced by the higher valence La3+ ions. As a result of the difference in the valency between Pb2+ and La3+, some of the A sites and B sites will be vacant in order to maintain electrical neutrality in the structure. Earlier studies show that the nonstoichiometry of lead oxide as well as the doping of lanthanum introduces a large number of vacancies in PLZT [17, 18]. This letter deals with a study of the effect of excess lead oxide on the thermal diffusivity of PLZT ceramic carried out using the laser induced photoacoustic technique. Since the material preparation was done at 1200 ◦C, some fraction of the lead may escape during the preparation, so the actual lead content may be slightly less than the initial concentration. Samples were prepared with excess lead oxide in different weight percentages and the thermal diffusivity was determined for each sample with different amounts of excess lead oxide. The experimental set up used for the present investigation is similar to the one used by George et al. [19] as shown in Fig. 1. The 488 nm line of an argon ion laser (Liconix 5000 series) was used as the pump beam. The laser beam at a power level of 70 mW was intensity modulated using an electromechanical chopper (Ithaco HMS 230) before it was made to fall on the sample. The sample compartment of the non-resonant PA cell made of stainless steel has a diameter of 8 mm and a depth of 5 mm. The PA signal produced in the cavity was detected using a miniature electret microphone kept in a side chamber coupled to the sample compartment. The output of the microphone was processed using a lock-in-amplifier (Stanford Research Systems SR 510). We measured the PA signal amplitude as a function of chopping frequency. Charpentier et al. have presented a frequency analysis of the PA signal for the determination of thermal diffusivity based on the RosencwaigGersho theory for the PA effect [20, 21]. According to this model, knowing the actual thickness (ls) of the sample and the characteristic frequency ( fc) at which the sample becomes thermally thick, the thermal

Photoacoustic evaluation of the thermal diffusivity of coconut shell

In this paper we report the thermal diffusivity of coconut shell measured using a laser-induced photoacoustic (PA) technique. An open PA cell in the heat transmission configuration is employed for the investigation. Laser-induced heating of the coconut shell is found to result in a thermoelastic bending of the sample. Taking into account this effect, an appropriate modification of Rosencwaig-Gersho theory for the PA effect is made for the calculation of the thermal diffusivity. Our investigation shows that coconut shell possesses a larger thermal diffusivity than ordinary wood.