Radhakrishnan, P; V P N Nampoori; Litty, Irimpan; Bindu, Krishnan; Deepthy, A(Journal of Physics D : Applied Physics, August 30, 2007)
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Abstract:
In this paper, the fluorescence behaviour of nano colloids of ZnO has been
studied as a function of the excitation wavelength. We have found that
excitation at the tail of the absorption band gives rise to an emission that
shifts with the change of the excitation wavelength. The excitation
wavelength dependent shift of the fluorescence maximum is measured to be
between 60 and 100 nm. This kind of excitation wavelength dependent
fluorescence behaviour, which may appear to be in violation of Kasha’s rule
of excitation wavelength independence of the emission spectrum, has been
observed for nano ZnO colloids prepared by two different chemical routes
and different capping agents. It is shown that the existence of a distribution
of energetically different molecules in the ground state coupled with a low
rate of the excited state relaxation processes, namely, solvation and energy
transfer, are responsible for the excitation wavelength dependent
fluorescence behaviour of the systems.
Radhakrishnan, P; Jayasree, V K; Sandhya, T V(Measurement Science Review, 2008)
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Abstract:
A non-invasive technique is implemented to measure a parameter which is closely related to the distensibility of large arteries,
using the second derivative of the infrared photoplethysmographic waveform. Thirty subjects within the age group of 20-61 years
were involved in this pilot study. Two new parameters, namely the area of the photoplethysmographic waveform under the systolic
peak, and the ratio of the time delay between the systolic and the diastolic peaks and the time period of the waveform (T/T) were
studied as a function of age. It was found that while the parameter which is supposed to be a marker of distensibility of large arteries
and T /T values correlate negatively with age, the area under the systolic peak correlates positively with age. The results suggest that
the derived parameters could provide a simple, non-invasive means for studying the changes in the elastic properties of the vascular
system as a function of age.
Radhakrishnan, P; Girijavallabhan, C P; V P N Nampoori; 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.
Jinesh, Mathew; Mahesh, V V; Radhakrishnan, P(International Frequency Sensor Association, August , 2008)
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Abstract:
The design and fabrication of fiber based ammonia sensors employing Bromothymol blue
and Chitosan as sensing elements are presented in this paper. In the presence of ammonia gas the
absorption of Bromothymol blue changes while in the case of Chitosan the refractive index changes
which in turn modulates the intensity of light propagating through a fiber.
Radhakrishnan, P; V P N Nampoori; Kailasnath, M; Nishant Kumar; Bikas, Ranjan(PHOTONICS-2008: International Conference on Fiber Optics and Photonics, December , 2008)
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Abstract:
The direction dependant wavelength selective transmission mechanism in poly (methyl
methacrylate)(PMMA) rods doped with C 540 dye and C 540:Rh.B dye mixture as a combination has been
investigated. When a polished slice of pure C 540 doped polymer rod was used side by side with a C540:Rh B
doped rod with acceptor concentration [A] = 7x10-4 m/l , we could notice more than 100% change in the
transmitted intensity along opposite directions at the C 540, Rh B emission and the excitation wavelengths . A
blue high bright LED emitting at a peak wavelength 465nm was used as the excitation source.
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.