Abstract: | A novel fibre optic sensor for the in situ measurement of the rate of deposition of thin films has been developed. Evanescent wave in the uncladded portion of a multimode fibre is utilised for this sensor development. In the present paper we demonstrate how this sensor is useful for the monitoring of the deposition rate of polypyrrole thin films, deposited by an AC plasma polymerisation method. This technique is simple, accurate and highly sensitive compared with existing techniques. |
URI: | http://dyuthi.cusat.ac.in/purl/2392 |
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Dyuthi-P0201.pdf | (79.28Kb) |
Abstract: | Optical limiting and thermo-optic properties of C60 in toluene are studied using 532 nm, 9 ns pulses from a frequency-doubled Nd:YAG laser. Optical limiting studies in these fullerene molecules lead to the conclusion that reverse saturable absorption is the major mechanism for limiting properties in these molecules. Thermal lensing measurements are also performed in fullerene solutions. The quadratic dependence of thermal lens signal on incident energy confirms that enhanced optical absorption by the sample via excited triplet state absorption may play a leading role in the limiting property. |
URI: | http://dyuthi.cusat.ac.in/purl/2491 |
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Dyuthi-P0216.pdf | (474.4Kb) |
Abstract: | The nanosecond optical-limiting characteristics (at 532 nm) of some rare-earth metallo-phthalocyanines (Sm(Pc)2, Eu(Pc)2, and LaPc) doped in a copolymer matrix of poly(methyl methacrylate) and methyl-2-cyanoacrylate have been studied for the first time to our knowledge. The optical-limiting response is attributed to reverse saturable absorption due to excited-state absorption. The performance of LaPc in a copolymer host is studied at different linear transmissions. The laser damage thresholds of all the samples are also reported. |
URI: | http://dyuthi.cusat.ac.in/purl/2627 |
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Dyuthi-P0186.pdf | (452.0Kb) |
Abstract: | Optical absorption and emission spectral studies of various phthalocyanine molecules, viz., LaPc, NdPc, SmPc, EuPc, CuPc and ZnPc in a polymer matrix of cyano acrylate are reported for the first time. All the absorption spectra show an intense B band (Soret) in the UV region followed by a weaker Q band in the visible region. The positions of the Q and B bands are found to have dependence on the metallic substitution. Values of the important spectral parameters, viz., molar extinction coefficient (ϵ), oscillator strength (f), radiative transition rate and decay time of the excited singlet state are also presented and compared with other solid matrices. The recorded fluorescence spectrum shows two broad emission bands in the case of NdPc, whereas for ZnPc only a very weak band is observed. The absence of emission bands for the other metallated phthalocyanines is attributed to increased spin orbit interaction and intersystem crossing. |
URI: | http://dyuthi.cusat.ac.in/purl/2490 |
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Dyuthi-P0229.pdf | (423.0Kb) |
Abstract: | Optical properties of free and substituted porphyrins (PP) doped borate glass matrix are reported for the first time. Absorption spectral measurements of H2TPP, CdTPP, MgTPP and ZnTPP doped borate glass matrix have been made in the 200–1100 nm region and the spectra obtained are analyzed to obtain the optical bandgap (Eg) and other important spectral parameters viz. oscillator strength (f), molar extinction coefficient (ε), electric dipole strength (q2), absorption cross-section (σa) and molecular concentration (N). Intense fluorescence was observed in the region 668–685 nm for CdTPP, ZnTPP and MgTPP doped matrices, whereas no such fluorescence was observed in H2TPP doped matrix. Fluorescence intensity was observed to be almost similar in all the metallated porphyrine matrices. Fluorescence bandwidth (Δλ), decay time (τ), stimulated emission cross-section (σ) and optical gain (G) of the principal fluorescence transitions corresponding to the Q-band excitation were also evaluated and discussed. |
URI: | http://dyuthi.cusat.ac.in/purl/2614 |
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Dyuthi-P0175.pdf | (443.1Kb) |
Abstract: | Optical absorption and emission spectral studies of various phthalocyanine (Pc) molecules in PVA matrix have been reported for the first time. The recorded spectra are analyzed to get the important spectral parameters, such as optical absorption cross-section (σa), emission cross-section (σe), oscillator strength (f), fluorescence bandwidth (Δλ), emission wavelength (λ), radiative decay time (τ) and optical gain (G). Analysis shows that the emission cross-section and optical gain are maximum in the NdHPc2-doped PVA matrix. However, a comparison of the calculated emission parameters with that of borate glass matrix show that they are many times smaller in the present matrix. |
URI: | http://dyuthi.cusat.ac.in/purl/2400 |
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Dyuthi-P0220.pdf | (95.80Kb) |
Abstract: | The vibrational overtone spectra 0f the liquid phase 1,2-dichloroethane and 1,2-dibromoethane in the spectral regions of CH stretching local mode overtones corresponding to delta v CH= = 2 to delta v CH=5 are reported. The observed spectral features are assigned using the local mode model. LocaI mode frequencies WCH and diagonal local mode anharmonicities XCH are obtained from an analysis of the spectra. The local-local combinations observed are interpreted on the basis of a coupled CH oscillator model hamiltonian. Local-normal combinations show complex structures and their possible assignments are given. |
URI: | http://dyuthi.cusat.ac.in/purl/2606 |
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Dyuthi-P0166.pdf | (472.3Kb) |
Abstract: | Vibrational overtone spectra of styrene (liquid) and polystyrene (solid), studied by the laser-induced thermal lens (for ΔV=6) and the conventional near infrared absorption (for ΔV=3–5) techniques, are reported. For polystyrene, the overtone energy-bond length correlation predicts that the aryl CH bonds are ∼0.0005 Å longer than that in benzene, while no such conclusions could be drawn on styrene. Thesp 3 CH overtones in polystyrene are observed on the low energy side of the aryl CH overtones. |
URI: | http://dyuthi.cusat.ac.in/purl/2631 |
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Dyuthi-P0190.pdf | (256.9Kb) |
Abstract: | The chaotic dynamics of directly modulated semiconductor lasers with delayed optoelectronic feedback is studied numerically. The effects of positive and negative delayed optoelectronic feedback in producing chaotic outputs from such lasers with nonlinear gain reduction in its optimum value range is investigated using bifurcation diagrams. The results are confirmed by calculating the Lyapunov exponents. A negative delayed optoelectronic feedback configuration is found to be more effective in inducing chaotic dynamics to such systems with nonlinear gain reduction factor in the practical value range. |
URI: | http://dyuthi.cusat.ac.in/purl/2529 |
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Dyuthi-P0104.pdf | (1.292Mb) |
Abstract: | Machine tool chatter is an unfavorable phenomenon during metal cutting, which results in heavy vibration of cutting tool. With increase in depth of cut, the cutting regime changes from chatter-free cutting to one with chatter. In this paper, we propose the use of permutation entropy (PE), a conceptually simple and computationally fast measurement to detect the onset of chatter from the time series using sound signal recorded with a unidirectional microphone. PE can efficiently distinguish the regular and complex nature of any signal and extract information about the dynamics of the process by indicating sudden change in its value. Under situations where the data sets are huge and there is no time for preprocessing and fine-tuning, PE can effectively detect dynamical changes of the system. This makes PE an ideal choice for online detection of chatter, which is not possible with other conventional nonlinear methods. In the present study, the variation of PE under two cutting conditions is analyzed. Abrupt variation in the value of PE with increase in depth of cut indicates the onset of chatter vibrations. The results are verified using frequency spectra of the signals and the nonlinear measure, normalized coarse-grained information rate (NCIR). |
URI: | http://dyuthi.cusat.ac.in/purl/2598 |
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Dyuthi-P0158.pdf | (263.3Kb) |
Abstract: | Absorption spectra of formaldehyde molecule in the gas phase have been recorded using photoacoustic (PA) technique with pulsed dye laser at various power levels. The spectral profiles at higher power levels are found to be different from that obtained at lower laser powers. Two photon absorption (TPA) is found to be responsible for the photoacoustic signal at higher laser power while the absorption at lower laser power level is attributed to one photon absorption (OPA) process. Probable assignments for the different transitions are given in this paper. |
URI: | http://dyuthi.cusat.ac.in/purl/2487 |
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Dyuthi-P0283.pdf | (545.1Kb) |
Abstract: | An open cell photoacoustic (PA) configuration has been employed to evaluate the thermal diffusivity of intrinsic InP as well as InP doped with tin and iron. Thermal diffusivity data have been evaluated from variation of phase of PA signal as a function of modulation frequency. In doped samples, we observe a reduced value for thermal diffusivity in comparison with intrinsic InP. We also observed that, while the phase of the PA signal varies linearly with the square root of chopping frequency for doped samples, the intrinsic material does not exhibit such behaviour in the experimental frequency range. These results have been interpreted in terms of the heat generation and phonon assisted heat diffusion mechanisms in semiconductors. |
URI: | http://dyuthi.cusat.ac.in/purl/2406 |
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Dyuthi-P0240.pdf | (147.6Kb) |
Abstract: | An open-cell configuration of the photoacoustic (PA) technique is employed to determine the thermal and transport properties of intrinsic Si and Si doped with B (p-type) and P (n-type). The experimentally obtained phase of the PA signal under heat transmission configuration is fitted to that of theoretical model by taking thermal and transport properties, namely, thermal diffusivity, diffusion coefficient, and surface recombination velocity, as adjustable parameters. It is seen from the analysis that doping and also the nature of dopant have a strong influence on the thermal and transport properties of semiconductors. The results are interpreted in terms of the carrier-assisted and phonon-assisted heat transfer mechanisms in semiconductors as well as the various scattering processes occurring in the propagation of heat carriers. |
URI: | http://dyuthi.cusat.ac.in/purl/2486 |
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Dyuthi-P0255.pdf | (729.3Kb) |
Abstract: | In this paper, we present a laser-induced photoacoustic study on the photostability of laser dye Coumarin 540 doped in PMMA matrix and modified by the incorporation of low-molecular weight additives. The dependence of photostability of the dye on various experimental conditions, such as nature of solvents, incident optical power and dye concentration, is investigated in detail. The activation rates for the bleaching process are calculated for different concentrations and they suggest the possibility of two distinct mechanisms responsible for photodegradation. Further, analysis of the data confirms the linear dependence of photodegradation on the intensity of incident radiation. The role of different externally influencing parameters, such as wavelength and modulation frequency of incident radiation, is also discussed. |
URI: | http://dyuthi.cusat.ac.in/purl/2509 |
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Dyuthi-P00132.pdf | (136.8Kb) |
Abstract: | 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. |
URI: | http://dyuthi.cusat.ac.in/purl/2595 |
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Dyuthi-P0155.pdf | (134.5Kb) |
Abstract: | S1 to S3 excited singlet state absorption and two-photon absorption in Rhodamine 6G at the pump wavelengths of 532 and 1064 nm respectively are investigated. The advantages of employing the pulsed photoacoustic technique for conveniently observing excited singlet state absorption are discussed. It is shown that, since photoacoustics and fluorescence are complementary phenomena, analysis using both techniques will yield a better understanding of optical processes in molecules like Rhodamine 6G. |
URI: | http://dyuthi.cusat.ac.in/purl/2445 |
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Dyuthi-P0306.pdf | (315.7Kb) |
Abstract: | Pulsed photoacoustic studies in solutions of C70 in toluene are made using the 532-nm radiation from a frequency-doubled Nd:YAG laser. It is found that contrary to expectation, there is no photoacoustic (PA) signal enhancement in the power-limiting range of laser fluences. Instead, the PA signal tends to saturate during optical power-limiting phenomenon. This could be due to the enhanced optical absorption from the photoexcited state and hence the depletion of the ground-state population. PA measurements also ruled out the possibility of multiphoton absorption in the C70 solution. We demonstrate that the nonlinear absorption leading to optical limiting is mainly due to reverse saturable absorption. |
URI: | http://dyuthi.cusat.ac.in/purl/2461 |
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Dyuthi-P0323.pdf | (181.4Kb) |
Abstract: | Photoacoustic spectrum of samarium phthalocyanine powder is recorded and compared with previously reported UV–vis absorption spectra of the same dissolved in different liquid and solid host media. The Davidov splitting of Q band is observed in the PA spectrum but the two bands are overlapped considerably and the shorter wavelength band is more intense and dominating one in the powder spectrum. |
URI: | http://dyuthi.cusat.ac.in/purl/2508 |
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Dyuthi-P0131.pdf | (134.0Kb) |
Abstract: | Non-destructive testing d multilayer dielectric coatings (SiO2/TiO2 structure) has been carried out using the photoacoustic technique. This technique makes use d a 10 mW He-Ne laser, a photoacoustic cell and a lock-in amplifier. The chopped He-Ne laser beam is allowed to fall on the sample placed in a photoacoustic cell. The acoustic signals thus generated are detected using a microphone and the resulting output is processed by a lock-in amplifier. The amplitude and phase of the signals were measured as a function of the chopping frequency. Striking step-like variations are observed in me phase against frequency plot which dearly reveals the different layers present in the multilayer structure. |
URI: | http://dyuthi.cusat.ac.in/purl/2441 |
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Dyuthi-P0301.pdf | (194.1Kb) |
Abstract: | 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. Analysis is made on the basis of the Rosencwaig-Gersho theory and the results are compared with those from earlier reported photoacoustic studies of semiconductors. Our investigation clearly indicates that the instantaneous thermalization process is the major heat diffusion mechanism responsible for the photoacoustic signal generation in an InP sample. |
URI: | http://dyuthi.cusat.ac.in/purl/2411 |
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Dyuthi-P0233.pdf | (135.9Kb) |
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