Radhakrishnan, P; Girijavallabhan, C P; Nampoori, V P N; Thomas, K J; Sheeba, M(Journal of Optics A: Pure and Applied Optics, March 27, 2008)
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Abstract:
Polymethyl methacrylate (PMMA) optical fibres are fabricated by a preform drawing process.
The Raman spectra of PMMA fibres are recorded using a diode pumped solid state laser
emitting at 532 nm and a CCD-spectrograph in the 400–3800 cm−1 range. The variation of the
Raman intensity with the length of the optical fibre is studied. Investigations are carried out on
the variation of FWHM of the Raman peak at 2957 cm−1 with the length of the optical fibre and
pump power. The differential scattering cross section and gain coefficient of the Raman peak at
2957 cm−1 in PMMA are calculated in relation to that of toluene.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Thomas Lee, S; Jose, Gin; Unnikrishnan, N V(Journal of Optics A: Pure and Applied Optics, July 12, 2001)
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Abstract:
The fabrication and characterization of a fibre optic pH sensor based on
evanescent wave absorption is presented. The unclad portion of a
multi-mode optical fibre is coated with a pH sensitive dye, which is
immobilized by the sol–gel route. The sensitivity of the device has been
found to increase when multiple sol–gel coatings are used as the sensing
region. The dynamic range and the temporal response of the sensor are
investigated for two different dyes, namely bromocresol purple and
bromocresol green. The performance of the device is evaluated in terms of
the results obtained during actual measurements
Radhakrishnan, P; V P N Nampoori; Litty, Irimpan(Science of Advanced Materials , American Scientific Publishers, 2010)
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Abstract:
The spectral and nonlinear optical properties of ZnO based nanocomposites prepared by colloidal
chemical synthesis are investigated. Very strong UV emissions are observed from ZnO–Ag, ZnO–
Cu and ZnO–SiO2 nanocomposites. The strongest visible emission of a typical ZnO–Cu nanocomposite
is over ten times stronger than that of pure Cu due to transition from deep donor level to the
copper induced level. The optical band gap of ZnO–CdS and ZnO–TiO2 nanocomposites is tunable
and emission peaks changes almost in proportion to changes in band gap. Nonlinear optical
response of these nanocomposites is studied using nanosecond laser pulses from a tunable laser in
the wavelength range of 450–650 nm at resonance and off-resonance wavelengths. The nonlinear
response is wavelength dependent and switching from RSA to SA has been observed at resonant
wavelengths. Such a change-over is related to the interplay of plasmon/exciton band bleach and
optical limiting mechanisms. The observed nonlinear absorption is explained through two photon
absorption followed by weak free carrier absoption, interband absorption and nonlinear scattering
mechanisms. The nonlinearity of the silica colloid is low and its nonlinear response can be improved
by making composites with ZnO and ZnO–TiO2. The increase of the third-order nonlinearity in the
composites can be attributed to the enhancement of exciton oscillator strength. This study is important
in identifying the spectral range and the composition over which the nonlinear material acts
as an RSA based optical limiter. These nanocomposites can be used as optical limiters and are
potential materials for the light emission and for the development of nonlinear optical devices with
a relatively small limiting threshold.
Radhakrishnan, P; Nampoori, V P N; Girijavallabhan, C P; Nandakumaran, V M; Dann, V J; Mathew, M V(Plasma Science and Technology, August , 2007)
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Abstract:
Optical emission from TiO2 plasma, generated by a nanosecond laser is spectroscopically
analysed. The main chemical species are identified and the spatio-temporal distribution of
the plasma parameters such as electron temperature and density are characterized based on the
study of spectral distribution of the line intensities and their broadening characteristics. The parameters
of laser induced plasma vary quickly owing to its expansion at low background pressure
and the possible deviations from local thermodynamic equilibrium conditions are tested to show
its validity
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Sajan, D George(Journal of Physics D : Applied Physics, Institute of Physics Publishing, April 2, 2003)
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Abstract:
An open photoacoustic cell operating in the low range of chopping
frequency has been employed to evaluate the thermal diffusivity values of
intrinsic InP and InP doped with S, Sn and Fe. The experimental set-up is
calibrated by the evaluation of thermal diffusivity value of pure Si and
GaAs. The present investigation shows that doped samples show a reduced
value for thermal diffusivity compared to intrinsic sample. From the
analysis of data it is also seen that nature of dopant clearly influences the
thermal diffusivity value of semiconductors. The results are explained in
terms of phonon assisted heat transfer mechanism in semiconductors
Radhakrishnan, P; Nampoori, V P N; Girijavallabhan, C P; Nibu, A George; George, A K(Applied Physics B Lasers and Optics, July 18, 2001)
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Abstract:
In this paper, we describe the use of an open cell
photoacoustic configuration for the evaluation of the thermal
effusivity of liquid crystals. The feasibility, precision and reliability
of the method are initially established by measuring
the thermal effusivities of water and glycerol, for which the
effusivity values are known accurately. In order to demonstrate
the use of the present method in the thermal characterization
of liquid crystals, we have measured the thermal
effusivity values in various mesophases of 4-cyano-4 -
octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl
(7OCB) liquid crystals using a variable temperature open
photoacoustic cell. A comparison of the measured values for
the two liquid crystals shows that the thermal effusivities of
7OCB in the nematic and isotropic phases are slightly less
than those of 8OCB in the corresponding phases
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Nibu, A George; George, A K(Journal of Physics D : Applied Physics, October 13, 2000)
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Abstract:
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