Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; 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.
Radhakrishnan, P; V P N Nampoori; 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
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.
George, S D; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Springer-Verlag, 2003)
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Abstract:
Laser induced transverse photothermal deflection
technique has been employed to determine the thermal parameters
of InP doped with Sn, S and Fe as well as intrinsic
InP. The thermal diffusivity values of these various samples
are evaluated from the slope of the curve plotted between the
phase of photothermal deflection signal and pump-probe offset.
Analysis of the data shows that heat transport and hence
the thermal diffusivity value, is greatly affected by the introduction
of dopant. It is also seen that the direction of heat flow with
respect to the plane of cleavage of semiconductor wafers influences
the thermal diffusivity value. The results are explained in
terms of dominating phonon assisted heat transfer mechanism in semiconductors.
V P N Nampoori; Litty, Irimpan; Radhakrishnan, P(Journal of Applied Physics, American Institute of Physics,, 2008)
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Abstract:
We describe the structure of luminescence spectrum in the visible region in nano-ZnO in colloidal
and thin film forms under weak confinement regime by modeling the transition from excited state
energy levels of excitons to their ground state. Measurements on nanocrystallites indicate the
presence of luminescence due to excitonic emissions when excited with 255 nm. The relevant
energy levels showing the transitions corresponding to the observed peaks in the emission spectrum
of ZnO of particle size 18 nm are identified.
Aneeshkumar, B; Pramod, Gopinath; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(Optical Society of America, July , 2003)
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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.
Achamma, Kurian; Unnikrishnan, K P; Sajan George, D; Pramod, Gopinath; Nampoori, V P N; Vallabhan, C P G(Elsevier, 2003)
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Abstract:
The wavelength dependence of thermal lens signal from organic dyes are studied using dual beam thermal lens
technique. It is found that the profile of thermal lens spectrum widely differ from the conventional absorption spectrum
in the case of rhodamine B unlike in the case of crystal violet. This is explained on the basis of varying contribution of
nonradiative relaxations from the excited vibronic levels.
Lyjo K, Joseph; Dayas, K R; Soniya, Damodar; Bindu, Krishnan; Krishnankutty, K; Nampoori, V P N; Radhakrishnan, P(Elsevier, December 15, 2008)
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Abstract:
The laser-induced luminescence studies of the rare earth titanates (R2Ti2O7) (R = La, Nd and Gd) using 355 nm radiation from an Nd:YAG laser are presented. These samples with submicron or nanometer size are prepared by the self-propagating high temperature synthesis (SHS) method and there is no known fluorescence shown by these rare earths in the visible region. Hence, the luminescence transitions shown by the La2Ti2O7 near 610 nm and Gd2Ti2O7 near 767 nm are quite interesting. Though La3+ ions with no 4f electrons have no electronic energy levels that can induce excitation and luminescence processes in the visible region, the presence of the Ti3+ ions leads to luminescence in this region.
Santhi, A; Vinu V, Namboodiri; Radhakrishnan, P; Nampoori, V P N(American Institute of Physics, 2006)
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Abstract:
Wavelength dependence of saturable absorption (SA) and reverse saturable absorption (RSA) of zinc phthalocyanine was studied using 10 Hz, 8 ns pulses from a tunable laser, in the wavelength range of 520–686 nm, which includes the rising edge of the Q band in the electronic absorption spectrum. The nonlinear response is wavelength dependent and switching from RSA to SA has been observed as the excitation wavelength changes from the low absorption window region to higher absorption regime near the Q band. The SA again changes back to RSA when we further move over to the infrared region. Values of the imaginary part of third order susceptibility are calculated for various wavelengths in this range. This study is important in identifying the spectral range over which the nonlinear material acts as RSA based optical limiter.
Litty, Irimpan; Bindu, Krishnan; Nampoori, V P N; Radhakrishnan, P(Elsevier, 2008)
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Abstract:
In this article we present the nonlinear optical properties of ZnO–TiO2–SiO2 nanocomposites prepared by colloidal chemical synthesis. Nonlinear optical response of these samples is studied using nanosecond laser pulses at an off-resonance wavelength. The nonlinearity of the silica colloid is low and its nonlinear response can be improved by making composites with ZnO and TiO2. These nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behaviour. The nonlinear refractive index and the nonlinear absorption increases with increasing ZnO volume fraction. The observed nonlinear absorption is explained by two photon absorption followed by weak free carrier absorption and nonlinear scattering. ZnO–TiO2–SiO2 is a potential nanocomposite material for the development of nonlinear optical devices with a relatively small limiting threshold.