Mohanan, P; Girijavallabhan, C P; Aanandan,C K; K G Nair(Indian Journal of Radio & Space Physics, February , 1984)
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Abstract:
A forward - biased point contact germanium signal diode placed inside a waveguide section along the E -vector is found to
introduce significant phase shift of microwave signals . The usefulness of the arrangement as a phase modulator for microwave
carriers is demonstrated. While there is a less significant amplitude modulation accompanying phase modulation , the insertion
losses are found to be negligible. The observations can be explained on the basis of the capacitance variation of the barrier layer
with forward current in the diode
Chitra, R N; Kuriakose, V C(American Institute of Physics, 2008)
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Abstract:
We consider an array of N Josephson junctions connected in parallel and explore the condition for chaotic synchronization. It is found that the outer junctions can be synchronized while they remain uncorrelated to the inner ones when an external biasing is applied. The stability of the solution is found out for the outer junctions in the synchronization manifold. Symmetry considerations lead to a situation wherein the inner junctions can synchronize for certain values of the parameter. In the presence of a phase difference between the applied fields, all the junctions exhibit phase synchronization. It is also found that chaotic motion changes to periodic in the presence of phase differences.
Louis, Godfrey; Philip, J(The American Physical Society, 1996)
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Abstract:
A detailed ultrasonic study of the elastic properties of lithium ammonium sulfate ~LiNH4SO4! or LAS has
been carried out below room temperature. The elastic constants of LAS at room temperature are reported. The
discrepancy present in earlier elastic constant data associated with the different choice of axes for this orthorhombic
system are clarified. The results of the temperature variation study down to 220 K confirm the
ferroelastic phase transition at 285 K and establish a thermal hysteresis of about 2.5 K between the cooling and
heating cycles. Results of the investigation on the suspected weak phase transition at 256 K suggest that this
transition occurs at 242 K on cooling and at 256 K on heating, thus having a thermal hysteresis of about 14 K.
However, since the observed elastic anomaly for this transition is very small, the nature of this transition still
remains unclear
Ravi Kumar, A V; Padmaja, G; Nampoori, V P N; Vallabhan, C P G(Springer, November , 1989)
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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.
Radhakrishnan, P; Nampoori, V P N; Girijavallabhan, C P; Nibu, A George; George, A K(Journal of Physica: Condensed Matter, Institute of Physics Publishing, November 28, 2000)
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Abstract:
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
Sajan, D George; Vallabhan, C P G; Heck, M; Radhakrishnan, P; Nampoori, V P N(Taylor & Francis, 2002)
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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.
Sajan, D George; Aneeshkumar, B; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Society of Photo-Optical Instrumentation Engineers, 2004)
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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.
Nampoori, V P N; Radhakrishnan, P; Girijavallabhan, C P; George, N A; Paul, T; Sebastian,M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Radhakrishnan, P; Nampoori, V P N; Girijavallabhan, C P; George, N A; Paul, T; Sebastian,M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; George, N A; Paul, T; Sebastian,M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Ritty J, Nedumpara; Binoy, Paul; Santhi, A; Radhakrishnan, P; Nampoori, V P N(Elsevier, January , 2004)
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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.
Sajan, D George; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, Dilna S P(WILEY-VCH Verlag, 2003)
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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.
Sathy, P; Reji, Philip; Nampoori, V P N; Vallabhan, C P G(IOP, 1994)
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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.
Riju, C Issac; Harilal, S S; Geetha K, Varier; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Society of Photo-Optical Instrumentation Engineers, February , 1997)
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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.
Nibu, A George; Jyotsna, R; Thomas Lee, S; Aneeshkumar, B; Jayan,Thomas; Nampoori, V P N; Radhakrishnan, P(Elsevier, 2005)
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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.
Annieta, Philip; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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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.
Nibu, A George; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(Society of Photo-Optical Instrumentation Engineers, January , 2002)
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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.
Radhakrishnan, P; Nampoori, V P N; Girijavallabhan, C P; Sajan, D George; George, A K(Smart Materials and Structures, July 5, 2007)
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Abstract:
Complete thermal characterization of liquid crystal mixtures in the smectic
phase consisting of various relative volume fractions of cholesterol and
1-hexadecanol have been carried out using the photoacoustic technique.
Thermal diffusivity values of these liquid crystal mixtures are evaluated using
the open cell photoacoustic technique whereas the thermal effusivity value is
measured using the conventional photoacoustic technique. From the
measured values of these transient thermophysical parameters, the thermal
conductivity and heat capacity of the sample under investigation are
calculated. Analyses of the results show that all the thermophysical
parameters depend strongly on the volume fraction of the constituents.
Results are interpreted in terms of enhanced hydrogen bonding and the
consequent enhancement in cohesive thermal energy transport with
increasing volume fraction of 1-hexadecanol
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; Sajan, D George; George, A K(Smart Materials and Structures, July 5, 2007)
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Abstract:
Complete thermal characterization of liquid crystal mixtures in the smectic
phase consisting of various relative volume fractions of cholesterol and
1-hexadecanol have been carried out using the photoacoustic technique.
Thermal diffusivity values of these liquid crystal mixtures are evaluated using
the open cell photoacoustic technique whereas the thermal effusivity value is
measured using the conventional photoacoustic technique. From the
measured values of these transient thermophysical parameters, the thermal
conductivity and heat capacity of the sample under investigation are
calculated. Analyses of the results show that all the thermophysical
parameters depend strongly on the volume fraction of the constituents.
Results are interpreted in terms of enhanced hydrogen bonding and the
consequent enhancement in cohesive thermal energy transport with
increasing volume fraction of 1-hexadecanol
Nampoori, V P N; Radhakrishnan, P; Girijavallabhan, C P; Sajan, D George; George, A K(Smart Materials and Structures, July 5, 2007)
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Abstract:
Complete thermal characterization of liquid crystal mixtures in the smectic
phase consisting of various relative volume fractions of cholesterol and
1-hexadecanol have been carried out using the photoacoustic technique.
Thermal diffusivity values of these liquid crystal mixtures are evaluated using
the open cell photoacoustic technique whereas the thermal effusivity value is
measured using the conventional photoacoustic technique. From the
measured values of these transient thermophysical parameters, the thermal
conductivity and heat capacity of the sample under investigation are
calculated. Analyses of the results show that all the thermophysical
parameters depend strongly on the volume fraction of the constituents.
Results are interpreted in terms of enhanced hydrogen bonding and the
consequent enhancement in cohesive thermal energy transport with
increasing volume fraction of 1-hexadecanol