Riju, C Issac; Bindhu, C V; Harilal, S S; Geetha K, Varier; Nampoori, V P N; Vallabhan, C P G(World Scientific Publishing Company, 1996)
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Abstract:
Pulsed photoacoustic studies in solution of C60 in toluene have been made using the 532 nm radiation from a frequency doubled Nd:YAG laser. Though C60 is found to exhibit the phenomenon of optical limiting, the results on photoacoustic measurements do not give any indication of multiphoton transitions as suggested in some of the earlier works. Results of photoacoustic measurements show that excited state absorption is the dominant process responsible for optical limiting while phenomena like nonlinear scattering may contribute to a lesser extent.
Ritty J, Nedumpara; Thomas, K J; Jayasree, V K; Girijavallabhan, C P; Nampoori, V P N; Radhakrishnan, P(Optical Society of America, July 20, 2007)
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Abstract:
The results of a brief investigation of the amplified spontaneous emission and lasing characteristics of Coumarin 540 dye in as many as ten different solvents are reported. It has been found that C 540 dye solutions contained within a rectangular quartz cuvette give laser emission with well resolved equally spaced modes when pumped with a 476 nm beam. The modes were found to originate from the subcavities formed by the plane-parallel walls of the cuvette containing the high-gain medium. While the quantum yield remains a decisive factor, a clear correlation between the total width of the emission spectra and the refractive indices of the solvents of the respective samples has been demonstrated. The well-resolved mode structure exhibited by the emission spectra gives clear evidence of the lasing action taking place in the gain medium, and the number of modes enables us to compare the gain of the media in different samples. A detailed discussion of the solvent effect in the lasing characteristics of C540 in different solutions is given.
Jayaraj, M K; Vallabhan, C P G(IOP Publishing, 1989)
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Abstract:
The method of preparation of ZnS phosphors doped with praseodymium and copper is given. The electroluminescence (EL) spectrum of ZnS:Pr,Cl has two broad bands at 470 and 570 nm. ZnS:Cu,Pr,Cl gives white emission with spectral peaks at 470, 520, 570 and 640 nm. The EL spectra of both types of phosphor exhibit a conspicuous colour shift as the frequency of the excitation voltage is varied. Detailed investigations show that the relative intensities of spectral peaks are strongly dependent on the frequency of the excitation voltage. The colour shift is explained on the basis of the Schon-Klasens model.
Achamma, Kurian; Sajan, D George; Nampoori, V P N; Vallabhan, C P G(Elsevier, 2005)
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Abstract:
A sensitive method based on the principle of photothermal phenomena to study the energy transfer processes in organic dye mixtures is presented. A dual beam thermal lens method can be very effectively used as an alternate technique to determine the molecular distance between donor and acceptor in fluorescein–rhodamine B mixture using optical parametric oscillator.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(Elsevier, December 26, 1994)
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Abstract:
We report the experimental observation of subcritical Hopf bifurcation and the existence of non-oscillating “windows” in the dynamics of a Ne-Nd hollow cathode discharge current as the control parameter.
Thomas, Kuruvilla; Nandakumaran, V M(Elsevier, April , 1999)
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Abstract:
The effect of coupling on two high frequency modulated semiconductor lasers is numerically studied. The phase diagrams and bifurcatio.n diagrams are drawn. As the coupling constant is increased the system goes from chaotic to periodic behavior through a reverse period doubling sequence. The Lyapunov exponent is calculated to characterize chaotic and periodic regions.
Parvathi, M R; Bindu M, Krishna; Rajesh, S; Manu P, John; Nandakumaran, V M(Elsevier, December , 2008)
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Abstract:
In this Letter we numerically investigate the dynamics of a system of two coupled chaotic multimode Nd:YAG lasers with two mode and three mode outputs. Unidirectional and bidirectional coupling schemes are adopted; intensity time series plots, phase space plots and synchronization plots are used for studying the dynamics. Quality of synchronization is measured using correlation index plots. It is found that for laser with two mode output bidirectional direct coupling scheme is found to be effective in achieving complete synchronization, control of chaos and amplification in output intensity. For laser with three mode output, bidirectional difference coupling scheme gives much better chaotic synchronization as compared to unidirectional difference coupling but at the cost of higher coupling strength. We also conclude that the coupling scheme and system properties play an important role in determining the type of synchronization exhibited by the system.
V P N Nampoori; Mohammed Yusuff, K K; Arun,V; Mathew, S; Robinson, P P; Jose,M(Elsevier, Journal of Dyes and Pigments, March 20, 2010)
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Abstract:
The Schiff base, 3-hydroxyquinoxaline-2-carboxalidine-4-aminoantipyrine, was synthesized by the
condensation of 3-hydroxyquinoxaline-2-carboxaldehyde with 4-aminoantipyrine. HPLC, FT-IR and NMR
spectral data revealed that the compound exists predominantly in the amide tautomeric form and
exhibits both absorption and fluorescence solvatochromism, large stokes shift, two electron quasireversible
redox behaviour and good thermal stability, with a glass transition temperature of 104oC. The
third-order non-linear optical character was studied using open aperture Z-scan methodology employing
7 ns pulses at 532 nm. The third-order non-linear absorption coefficient, b, was 1.48 x 10-6 cm W-1 and
the imaginary part of the third-order non-linear optical susceptibility, Im c(3), was 3.36 x10-10 esu. The
optical limiting threshold for the compound was found to be 340 MW cm-2.
Harilal, S S; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Applied Spectroscopy, 1998)
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Abstract:
Spectroscopic studies of laser -induced plasma from a high-temperature
superconducting material, viz., YBa2Cu3O7 (YBCO), have
been carried out. Electron temperature and electron density measurements
were made from spectral data. The Stark broad ening of
emission lines was used to determine the electron density, and the
ratio of line in tensities was exploited for the determination of electron
temperature. An initial electron temperature of 2.35 eV and
electron density of 2.5 3 1017 cm2 3 were observed. The dependence
on electron temperature and density on different experimental parameters
such as distance from the target, delay time after the in itiation
of the plasm a, and laser irradiance is also discussed in detail.
Index Headings: Laser -plasma spectroscopy; Plasma diagnostics;
Emission spectroscop y; YBa2Cu3O7.
Harilal, S S; Issac, Riju C; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, September 15, 1996)
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Abstract:
Laser ablation of graphite has been carried out using 1.06mm radiation from a Q-switched Nd:YAG
laser and the time of flight distribution of molecular C2 present in the resultant plasma is
investigated in terms of distance from the target as well as laser fluences employing time resolved
spectroscopic technique. At low laser fluences the intensities of the emission lines from C2 exhibit
only single peak structure while beyond a threshold laser fluence, emission from C2 shows a twin
peak distribution in time. The occurrence of the faster velocity component at higher laser fluences
is explained as due to species generated from recombination processes while the delayed peak is
attributed to dissociation of higher carbon clusters resulting in the generation of C2 molecule.
Analysis of measured data provides a fairly complete picture of the evolution and dynamics of
C2 species in the laser induced plasma from graphite.
Harilal, S S; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, June , 1994)
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Abstract:
YBa2Cu307 target was laser ablated, and the time-of-flight (TOF) distributions of Y, Y+., and YO in
the resultant plasma were investigated as functions of distance from the target and laser energy
density using emission spectroscopy. Up to a short distance from the target (-1.5 cm), TOF
distributions show twin peaks for Y and YO, while only single-peak distribution is observed for Y+.
At greater distances (>1.5 cm) all of them exhibit single-peak distribution. The twin peaks are
assigned to species corresponding to those generated directly/m the vicinity of target surface and to
those generated from collisional/recombination process.
Sajan, D George; Achamma, Kurian; Martin, Lase; Nampoori, V P N; Vallabhan, C P G(SPIE, 2001)
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Abstract:
An open cell photoacoustic configuration has been employed to evaluate the thermal diffusivity of pure InP as well as InP doped with sulphur and iron. Chopped optical radiation at 488 nm from an Ar-ion laser has been used to excite photoacoustic signals which been detected by a sensitive electret microphone. Thermal diffusivity value have been calculated from phase versus chopping frequency plots. Doped sample are found to show a reduced value for thermal diffusivity in comparison with intrinsically pure sample. The results have been interpreted in terms of the mechanisms of heat generation and transmission in semiconductors.
Joseph, L K; Sanjay, G; Suja, H; Sugunan, S; Nampoori, V P N; Radhakrishnan, P(Taylor & Francis, April 1, 2009)
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Abstract:
Thermal diffusivity (TD) measurements were performed on some industrially
important dyes – auramine O (AO), malachite green and methylene blue (MB) –
adsorbed K-10 montmorillonites using photoacoustic method. The TD value for
the dye-adsorbed clay mineral was observed to change with a variation in dye
concentration. The contribution of the dye towards TD was also determined. The
repeatedly adsorbed samples with MB and AO exhibited a lower TD than the
single-adsorbed samples. TD values of sintered MB samples were also obtained
experimentally. These sintered samples exhibit a higher TD, although they show
a trend similar to that of non-sintered pellets. A variation in dye concentration
and sintering temperature can be used for tuning the TD value of the clay mineral
to the desired level.
Annieta, Philip K; Lyjo, Joseph K; Litty, Irimpan M; Bindu, Krishnan; Radhakrishnan, P; Nampoori, V P N; Raghu, Natarajan(WILEY-VCH Verlag, 2007)
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Abstract:
Thermal characterization of alumina–zirconia and zirconia ceramic tapes using a photoacoustic technique is presented. A transmission-mode geometry is employed for the measurement of thermal diffusivity while a reflection-mode geometry is used for the measurement of thermal effusivity. In both these geometries, the same open photoacoustic cell is used. From the measured values of thermal diffusivity and thermal effusivity, the thermal conductivity value has also been evaluated.
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Sajan, D George; Saravanan, S; Venkatachalam, S(Physical Review, American Physical Society, June 8, 2004)
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Abstract:
Thermal diffusivity of the composites of camphor sulphonic acid (CSA) doped polyaniline (PANI) and its
composites with cobalt phthalocyanine (CoPc) has been measured using open cell photoacoustic technique.
Analysis of the data shows that the effective thermal diffusivity value can be tuned by varying the relative
volume fraction of the constituents. It is seen that polaron assisted heat transfer mechanism is dominant in CSA
doped PANI and these composites exhibit a thermal diffusivity value which is intermediate to that of CSA
doped PANI and CoPc. The results obtained are correlated with the electrical conductivity and hardness
measurements carried out on the samples.
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; 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; 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
Nampoori, V P N; Radhakrishnan, P; 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
Nampoori, V P N; Lyjo, Joseph K; Suja, H; Sanjay, G; Sugunan, S; Radhakrishnan, P(Elsevier, Journal of Applied Clay Science, June 28, 2008)
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Abstract:
Thermal diffusivity (TD) measurement on commercial K-10 and KSF montmorillonites was carried out by
photoacoustic technique. The TD of the montmorillonites after methylene blue adsorption changed with the
dye concentration. The repeatedly adsorbed samples showed a lesser TD than the single adsorbed samples.
After methylene blue adsorption the acid leached K-10 samples showed well defined changes in TD when
compared to the ordered KSF samples.
Sajan, D George; Rajesh, Komban; Warrier, K G K; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Taylor & Francis, February , 2010)
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Abstract:
A laser-induced photoacoustic technique was employed to investigate
thermal transport through nanocrystalline CePO4 samples prepared via the
sol–gel route. Evaluation of thermal diffusivity was carried out using the
one-dimensional model of Rosencwaig and Gersho for the reflection
configuration of the photoacoustic method. Structural analyses of samples
revealed that they are nanoporous in nature, possessing micron-sized
grains. Analysis of results shows that thermal diffusivity value varies with
sintering temperature. Results are explained in terms of the variation in
porosity with sintering temperature and the effects of various scattering
mechanisms on the propagation of phonons through the nanoporous
ceramic matrix. Further analyses confirm that apart from porosity, grain
boundary resistance and interface thermal resistance influence the effective
value of thermal diffusivity of the samples under investigation.