Jinesh, Mathew; Thomas, K J; Nampoori, V P N; Radhakrishnan, P(IFSA, 2007)
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Abstract:
A comparative study of two biopolymer based fiber optic humidity sensors is presented in this paper. Sensing elements Agarose and Chitosan swells in the presence of water vapour and undergoes changes in refractive index and modulates the intensity of light propagating through a fiber with Agarose or Chitosan as cladding.
Pravitha, Ramanand; Nampoori, V P N; Sreenivasan, R(Imperial College Press, 2004)
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Abstract:
n this paper, a time series complexity analysis of dense array electroencephalogram signals is carried out using the recently introduced Sample Entropy (SampEn) measure. This statistic quantifies the regularity in signals recorded from systems that can vary from the purely deterministic to purely stochastic realm. The present analysis is conducted with an objective of gaining insight into complexity variations related to changing brain dynamics for EEG recorded from the three cases of passive, eyes closed condition, a mental arithmetic task and the same mental task carried out after a physical exertion task. It is observed that the statistic is a robust quantifier of complexity suited for short physiological signals such as the EEG and it points to the specific brain regions that exhibit lowered complexity during the mental task state as compared to a passive, relaxed state. In the case of mental tasks carried out before and after the performance of a physical exercise, the statistic can detect the variations brought in by the intermediate fatigue inducing exercise period. This enhances its utility in detecting subtle changes in the brain state that can find wider scope for applications in EEG based brain studies.
Rajesh, S; Nandakumaran, V M(Elsevier, January , 2006)
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Abstract:
We show numerically that direct delayed optoelectronic feedback can suppress hysteresis and bistability in a directly modulated semiconductor laser. The simulation of a laser with feedback is performed for a considerable range of feedback strengths and delays and the corresponding values for the areas of the hysteresis loops are calculated. It is shown that the hysteresis loop completely vanishes for certain combinations of these parameters. The regimes for the disappearance of bistability are classified globally. Different dynamical states of the laser are characterized using bifurcation diagrams and time series plots.
Rajasree, K; Ravi Kumar, A V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Springer, April , 1992)
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Abstract:
Photothermal deflection technique was used for determining the laser damage threshold of polymer samples of teflon (PTFE) and nylon. The experiment was conducted using a Q-switched Nd-YAG laser operating at its fundamental wavelength (1-06μm, pulse width 10 nS FWHM) as irradiation source and a He-Ne laser as the probe beam, along with a position sensitive detector. The damage threshold values determined by photothermal deflection method were in good agreement with those determined by other methods.
Syamaprasad, U; Vallabhan, C P G(Institute of Physics, 1981)
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Abstract:
Four distinct peaks are observed at 140, -26, -132 and -140°C in the sigma x* against T-1 plot between 200 and - 196°C for (NH4)3H(SO4)2, corresponding to four different phase
transitions of which the one at -26°C is reported here for the first time. Data on doped
samples reveal the charge transport mechanism in the crystal.
Unnikrishnan, K P; Jayan,Thomas; Nampoori, V P N; Vallabhan, C P G(Elsevier, June , 2002)
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Abstract:
Here we report measurements of third-order susceptibility χ(3), figure of merit F defined as χ(3)/α (where α is the absorption coefficient) and second hyperpolarizability 〈γ〉 of some metal substituted phthalocyanines and a naphthalocyanine in solutions of dimethyl formamide using degenerate four wave mixing at 532 nm under nanosecond excitation. It was found that among samples investigated, bis-naphthalocyanine possessed the highest value of 〈γ〉 followed by the bis-phthalocyanine. This observation is explained on the basis that bis-naphthalocyanine followed by bis-phthalocyanine has higher degree of π electron conjugation.
Suresh, Vettoor V; Nandakumaran, V M(Physica C,Elsevier Science, 1992)
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Abstract:
The density of states and the low temperature specific heat of higb-Tc superconductors are calculated in a functional integral formalism using the slave boson technique. The manybody calculation in a saddle point approximation shows that the Iow energy
sector is dominated by 3 single band. The calculated values of density of states are in good agreement with experimental results.
Suresh, Kumar P; Thomas Lee, S; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(Elsevier, 2002)
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Abstract:
The design and development of a cost-effective, simple, sensitive and portable LED based fiber optic evanescent wave sensor for simultaneously detecting trace amounts of chromium and nitrite in water are presented. In order to obtain the desired performance, the middle portions of two multimode plastic clad silica fibers are unclad and are used as the sensing elements in the two arms of the sensor. Each of the sensor arms is sourced by separate super bright green LEDs, which are modulated in a time-sharing manner and a single photo detector is employed for detecting these light signals. The performance and characteristics of this system clearly establish the usefulness of the technique for detecting very low concentrations of the dissolved contaminants.
Rajasree, K; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
A pulsed Nd-YAG laser beam is used to produce a transient refractive index gradient in air adjoining the plane surface of the sample material. This refractive index gradient is probed by a continuous He-Ne laser beam propagating parallel to the sample surface. The observed deflection signals produced by the probe beam exhibit drastic variations when the pump laser energy density crosses the damage threshold for the sample. The measurements are used to estimate the damage threshold for a few polymer samples. The present values are found to be in good agreement with those determined by other methods.
Jayaraj, M K; Vallabhan, C P G(Elsevier, October , 1989)
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Abstract:
The dielectric properties of vacuum-deposited europium oxide films have been investigated in the frequency range from 1 kHz to 1 MHz at various temperatures (300-543 K). The dielectric constant is found to depend on film thickness and it attains a constant value beyond 1000 Å. Films deposited at higher substrate temperatures (above 423 K) exhibit improved dielectric properties owing to the recovery of stoichiometry. The frequency variation of the loss factor exhibits a minimum which increases with rise in temperature. The breakdown field strength (about 106V cm-1) is found to be thickness dependent and it varies in accordance with the Forlani-Minnaja relation. The films exhibit ohmic conduction with an activation energy of 0.86 eV at low electric fields but at higher fields the conductivity becomes space charge limited. X-ray studies show that the films are amorphous in nature. The a.c. conductivity is proportional to ω at low frequency, whereas a square law dependence is observed at higher frequencies. The optical constants n, α and k and optical band gap are calculated from the UV-visible-near-IR spectra.
Jayaraj, M K; Vallabhan, C P G(Thin Solid Films, 1991)
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Abstract:
The dielectric properties of electron beam evaporated Sm2O3 films have been
investigated in the frequency range from 1 kHz to 1 MHz at various temperatures
(300 K-453 K). The dielectric constant is found to depend on film thickness and it
attains a constant value beyond 1000 A. The present electron beam evaporated
Sm2O3 films have a high dielectric constant of 43. The frequency dependence of
and tan teeta at various temperatures is also studied.
Radhakrishnan, P; Nampoori, V P N; 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.
Nampoori, V P N; Radhakrishnan, P; 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.
Kailasnath, M; Nampoori, V P N; Radhakrishnan, P; Nishant, Kumar; Bikas, Ranjan(PHOTONICS-2008: International Conference on Fiber Optics and Photonics IIT Delhi, India, 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.
Parvathi, R; Indic, P; Nampoori, V P N(Elsevier, February 11, 2002)
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Abstract:
Nonlinear time series analysis is employed to study the complex behaviour exhibited by a coupled pair of Rossler systems. Dimensional analysis with emphasis on the topological correlation dimension and the Kolmogorov entropy of the system is carried out in the coupling parameter space. The regime of phase synchronization is identified and the extent of synchronization between the systems constituting the coupled system is quantified by the phase synchronization index. The effect of noise on the coupling between the systems is also investigated. An exhaustive study of the topological, dynamical and synchronization properties of the nonlinear system under consideration in its characteristic parameter space is attempted.
Riju, C Issac; Vasudevan Pillai, K; Harilal, S S; Geetha K, Varier; Bindhu, C V; Pramod, Gopinath; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Elsevier Science, 1998)
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Abstract:
Laser produced plasma from silver is generated using a Q-switched Nd:YAG laser. Optical emission spectroscopy is used
to carry out time of flight (TOF) analysis of atomic particles. An anomalous double peak profile in the TOF distribution is
observed at low pressure. A collection of slower species emerge at reduced pressure below 4 X lO-3 mbar and this species
has a greater velocity spread. At high pressure the plasma expansion follows the shockwave model with cylindrical
symmetry whereas at reduced pressure it shows unsteady adiabatic expansion (UAE). During UAE the species show a
parabolic increases in the expansion time with radial distance whereas during shock wave expansion the exponent is less
than one. The angular distribution of the ablated species in the plume is obtained from the measurement of optical density of
thin films deposited on to glass substrates kept perpendicular to the plume. There is a sharp variation in the film thickness
away from the film centre due to asymmetries in the plume.
Saratchandran, P P; Nandakumaran, V M; Ambika, G(Springer, November , 1996)
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Abstract:
By introducing a periodic perturbation in the control parameter of the logistic map we have investigated the period locking properties of the map. The map then gets locked onto the periodicity of the perturbation for a wide range of values of the parameter and hence can lead to a control of the chaotic regime. This parametrically perturbed map exhibits many other interesting features like the presence of bubble structures, repeated reappearance of periodic cycles beyond the chaotic regime, dependence of the escape parameter on the seed value and also on the initial phase of the perturbation etc.
Thomas, Kuruvilla; Nandakumaran, V M(Indian Academy of Sciences, March , 2000)
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Abstract:
The effect of coupling two chaotic Nd:YAG lasers with intracavity KTP crystal for frequency doubling is numerically studied for the case of the laser operating in three longitudinal modes. It is seen that the system goes from chaotic to periodic and then to steady state as the coupling constant is increased. The intensity time series and phase diagrams are drawn and the Lyapunov characteristic exponent is calculated to characterize the chaotic and periodic regions.
Manu P, John; Jijo, P U; Nandakumaran, V M(Indian Academy of Sciences, March , 2009)
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Abstract:
We study the effect of parameter fluctuations and the resultant multiplicative
noise on the synchronization of coupled chaotic systems. We introduce a new quantity,
the fluctuation rate Ф as the number of perturbations occurring to the parameter in unit
time. It is shown that ϕ is the most significant quantity that determines the quality of
synchronization. It is found that parameter fluctuations with high fluctuation rates do not
destroy synchronization, irrespective of the statistical features of the fluctuations. We also
present a quasi-analytic explanation to the relation between ϕ and the error in synchrony.
Sony, George; Ajai, Kumar; Singh, R K; Nampoori, V P N(Springer, 2010)
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Abstract:
A study has been carried out to understand the
influence of ambient gases on the dynamics of laser-blow-off
plumes of multi-layered LiF–C thin film. Plume images
at various time intervals ranging from 100 to 3000 ns have
been recorded using an intensified CCD camera. Enhancement
in the plume intensity and change in size and shape
occurs on introducing ambient gases and these changes are
highly dependent on the nature and composition of the ambient
gas used. Velocity of the plume was found to be
higher in helium ambient whereas intensity enhancement is
greater in argon environment. The plume shapes have maximum
size at 10−2 and 10−1 Torr of Ar and He pressures,
respectively. As the background pressure increases further
(>10−2 Torr: depending on the nature of gas), the plume
gets compressed/focused in the lateral direction. Internal
structure formation and turbulences are observed at higher
pressures (>10−1 Torr) in both ambient gases.