Radhakrishnan, P; Nampoori, V P N; Girijavallabhan, C P; Ritty J, Nedumpara; Geetha, K; Dann, V J(Journal of Optics A: Pure and Applied Optics, Institute of Physics Publishing, January 9, 2007)
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Abstract:
We report unusual spectral narrowing and laser emission from polymer thin
films doped with Coumarin 540 dye. The laser emission from the polymer
films is found to be highly dependent upon the excitation length of the
medium. Even a short length of 1.75 mm of the dye doped film gave rise to
laser emission with FWHM of 0.3 nm for a pump intensity of 825 kW cm−2.
The partial reflections from the broad lateral surfaces of the free standing
films provided the optical feedback for the laser emission. Occurrence of
well-resolved equally spaced resonant modes confirmed the effect of a
Fabry–Perot-like optical cavity between the film surfaces
Ritty J, Nedumpara; Geetha, K; Dann, V J; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(IOP Publishing, 2007)
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Abstract:
We report unusual spectral narrowing and laser emission from polymer thin
films doped with Coumarin 540 dye. The laser emission from the polymer
films is found to be highly dependent upon the excitation length of the
medium. Even a short length of 1.75 mm of the dye doped film gave rise to
laser emission with FWHM of 0.3 nm for a pump intensity of 825 kW cm−2.
The partial reflections from the broad lateral surfaces of the free standing
films provided the optical feedback for the laser emission. Occurrence of
well-resolved equally spaced resonant modes confirmed the effect of a
Fabry–Perot-like optical cavity between the film surfaces.
Litty, Irimpan; Bindu, Krishnan; Nampoori, V P N; Radhakrishnan, P(Optical Society of America, August 20, 2008)
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Abstract:
We present the spectral and nonlinear optical properties of ZnO-SiO2 nanocomposites prepared by colloidal chemical synthesis. Obvious enhancement of ultraviolet (UV) emission of the samples is observed, and the strongest UV emission of a typical ZnO-SiO2 nanocomposite is over three times stronger than that of pure ZnO. The nonlinearity of the silica colloid is low, and its nonlinear response can be improved by making composites with ZnO. These nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behavior. The observed nonlinear absorption is explained through two photon absorption followed by weak free carrier absorption and nonlinear scattering. The nonlinear refractive index and the nonlinear absorption increase with increasing ZnO volume fraction and can be attributed to the enhancement of exciton oscillator strength. ZnO-SiO2 is a potential nanocomposite material for the UV light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.
Thomas Lee, S; Dinesh Kumar, S; Suresh, Kumar P; Radhakrishnan, P; Vallabhan, C P G; Nampoori, V P N(Elsevier, 2003)
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Abstract:
We propose and demonstrate a new technique for evanescent wave chemical sensing by writing long period gratings in a bare multimode plastic clad silica fiber. The sensing length of the present sensor is only 10 mm, but is as sensitive as a conventional unclad evanescent wave sensor having about 100 mm sensing length. The minimum measurable concentration of the sensor reported here is 10 nmol/l and the operating range is more than 4 orders of magnitude. Moreover, the detection is carried out in two independent detection configurations viz., bright field detection scheme that detects the core-mode power and dark field detection scheme that detects the cladding mode power. The use of such a double detection scheme definitely enhances the reliability and accuracy of the results. Furthermore, the cladding of the present fiber need not be removed as done in conventional evanescent wave fiber sensors.
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Geetha, K; Rajesh, M(Journal of Optics A: Pure and Applied Optics, Institute of Physics Publishing, February 24, 2004)
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Abstract:
We report the position dependent tuning of fluorescence emission from
rhodamine 6G doped polymethylmethacrylate film waveguide using a side
illumination technique. The transmitted fluorescence as a function of the
distance from the point of illumination is measured by translating the
waveguide horizontally across a monochromatic light source. This technique
has been utilized to characterize the optical loss in dye doped waveguides.
We observe that the optical loss coefficients for shorter and longer distances
of propagation through the dye doped waveguide are different. At longer
distance of propagation a decrease in optical loss coefficient is observed
Thomas, Baby; Ramachandran, T; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP Publishing, 1991)
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Abstract:
A low inductance, triggered spark gap switch suitable for a high-current fast discharge system has been developed. The details of the design and fabrication of this pressurized spark gap, which uses only commonly available materials are described. A transverse discharge Blumlein-driven N2 laser incorporating this device gives a peak output power of 700 kW with a FWHM of 3 ns and an efficiency of 0.51%, which is remarkably high for a pulsed nitrogen laser system.
Moosad, K P B; Abdul Rasheed, T M; Nampoori, V P N; Sathianandan, K(Optical Society of America, 1990)
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Abstract:
Low power optical phase conjugation in polyvinyl alcohol
films embedded with saturable dyes is reported. Phase
conjugate reflectivity achieved is higher than that obtained
in the case of similar gelatin films.
Jayaraj, M K; Vallabhan, C P G(IOP Publishing, 1990)
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Abstract:
AC thin film electroluminescent devices of MIS and MISIM have been fabricated with a novel dielectric layer of Eu2O3 as an insulator. The threshold voltage for light emission is found to depend strongly on the frequency of excitation source in these devices. These devices are fabricated with an active layer of ZnS:Mn and a novel dielectric layer of Eu2O3 as an insulator. The observed frequency dependence of brightness-voltage characteristics has been explained on the basis of the loss characteristic of the insulator layer. Changes in the threshold voltage and brightness with variation in emitting or insulating film thickness have been investigated in metal-insulator-semiconductor (MIS) structures. It has been found that the decrease in brightness occurring with decreasing ZnS layer thickness can be compensated by an increase in brightness obtained by reducing the insulator thickness. The optimal condition for low threshold voltage and higher stability has been shown to occur when the active layer to insulator thickness ratio lies between one and two.
Nampoori, V P N; Litty, Irimpan; Bindu, Krishnan; Radhakrishnan, P(Elsevier Journal of Colloid and Interface Science, April 29, 2008)
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Abstract:
In this article we present the spectral and nonlinear optical properties of ZnO–TiO2 nanocomposites prepared
by colloidal chemical synthesis. Emission peaks of ZnO–TiO2 nanocomposites change from 340 nm
to 385 nm almost in proportion to changes in Eg. The nanocomposites show self-defocusing nonlinearity
and good nonlinear absorption behaviour. The nonlinear refractive index and the nonlinear absorption
increase with increasing TiO2 volume fraction at 532 nm and can be attributed to the enhancement of
exciton oscillator strength. ZnO–TiO2 is a potential nanocomposite material for the tunable light emission
and for the development of nonlinear optical devices with a relatively small limiting threshold
Swapna, Nair S; Rajesh, S; Abraham, V S; Anantharaman, M R; Nampoori, V P N(Elsevier, 2006)
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Abstract:
Fine magnetic particles (size≅100 Å) belonging to the series ZnxFe1−xFe2O4 were synthesized by cold co-precipitation methods and their structural properties were evaluated using X-ray diffraction. Magnetization studies have been carried out using vibrating sample magnetometry (VSM) showing near-zero loss loop characteristics. Ferrofluids were then prepared employing these fine magnetic powders using oleic acid as surfactant and kerosene as carrier liquid by modifying the usually reported synthesis technique in order to induce anisotropy and enhance the magneto-optical signals. Liquid thin films of these fluids were prepared and field-induced laser transmission through these films was studied. The transmitted light intensity decreases at the centre with applied magnetic field in a linear fashion when subjected to low magnetic fields and saturate at higher fields. This is in accordance with the saturation in cluster formation. The pattern exhibited by these films in the presence of different magnetic fields was observed with the help of a CCD camera and was recorded photographically.
Rajasree, K; Vidyalal, V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, August 1, 1993)
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Abstract:
Laser‐induced damage and ablation thresholds of bulk superconducting samples of Bi2(SrCa)xCu3Oy(x=2, 2.2, 2.6, 2.8, 3) and Bi1.6 (Pb)xSr2Ca2Cu3 Oy (x=0, 0.1, 0.2, 0.3, 0.4) for irradiation with a 1.06 μm beam from a Nd‐YAG laser have been determined as a function of x by the pulsed photothermal deflection technique. The threshold values of power density for ablation as well as damage are found to increase with increasing values of x in both systems while in the Pb‐doped system the threshold values decrease above a specific value of x, coinciding with the point at which the Tc also begins to fall.
Bindhu, C V; Harilal, S S; Geetha K, Varier; Issac, Riju C; Nampoori, V P N; Vallabhan, C P G(IOP, 1996)
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Abstract:
The dual-beam thermal lens technique has been found to be very
effective for the measurement of fluorescence quantum yields of dye solutions. The
concentration-dependence of the quantum yield of rhodamine B in methanol is
studied here using this technique. The observed results are in line with the
conclusion that the reduction in the quantum yield in the quenching region is
essentially due to the non-radiative relaxation of the absorbed energy. The thermal
lens has been found to become abberated above 40 mW of pump laser power.
This low value for the upper limit of pump power is due to the fact that the medium
is a resonantly absorbing one.
Vidyalal, V; Rajasree, K; Vallabhan, C P G(World Scientific Publishing Company, 1996)
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Abstract:
A simple experimental set-up is described to measure the electromagnetic shielding property of high Tc superconducting samples. Measurements were performed using HTSC materials in the form of laser ablated thin films, powders and sintered pellets. Samples used were Gd-123 in pure and doped form as well as a few Bi-based superconducting ceramics. For comparison, similar measurements were carried out on metals like aluminium, copper and μ metal. Very effective shielding was observed for HTSC materials compared to the conventional materials mentioned above. However it also depended on the sample types and poor shielding was observed for powdered HTSC material in comparison to thin films prepared by laser ablation.
Ambika, G; Nandakumaran, V M; Babu Joseph,K(World Scientific Publishing Company, 1991)
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Abstract:
We consider a resistively shunted Josephson junction with a resistance that depends inversely on voltage. It is shown that such a junction in the underdamped case can give rise to extremely long-lived metastable states even in the absence of external noise. We investigate numerically this metastable state and its transition to a chaotic state. The junction voltages corresponding to these states are studied.
Nampoori, V P N; Senoy, Thomas; Jinesh, Mathew; Radhakrishnan, P; George, A K; Al-Harthi,R V; Ramanujan, R V; Anantharaman, M R(Sensors and Actuators: A Physical, May 11, 2010)
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Abstract:
Metallic glass alloy Metglas 2826 MB based amorphous magnetic thin films were
fabricated by the thermal evaporation technique. Transmission electron micrographs and
electron diffraction pattern showed the amorphous nature of the films. Composition of
the films was analyzed employing x-ray photoelectron spectroscopy and energy
dispersive x-ray spectroscopy techniques. The film was integrated to a long period fibre
grating. It was observed that the resonance wavelength of the fibre grating decreased with
an increase in the magnetic field. Change in the resonance wavelength was minimal at
higher magnetic fields. Field dependent magnetostriction values revealed the potential
application of these films in magnetostrictive sensor devices
Thomas Lee, S; Aneeshkumar, B; Radhakrishnan, P; Vallabhan, C P G; Nampoori, V P N(Elsevier, May , 2002)
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Abstract:
Optical fiber sensors developed for measuring pH values usually employ an unclad and unstrained section of the fiber. In this paper, we describe the design and fabrication of a microbent fiber optic sensor that can be used for pH sensing. In order to obtain the desired performance, a permanently microbent portion of a plastic optic fiber is coated with a thin film of dye impregnated sol–gel material. The measurements are simultaneously carried out in two independent detection schemes viz., the bright field detection configuration for detecting the core modes and dark field detection configuration, for detecting the cladding modes. The results of measurements of core mode-power and cladding mode-power variation with change in pH of a solution surrounding the coated portion of the fiber is presented. This paper thus demonstrates how a bare plastic fiber can be modified for pH sensing in a simple and cost effective manner.
Thomas Lee, S; Nampoori, V P N; Vallabhan, C P G; Radhakrishnan, P; Geetha, K(Society of Photo-Optical Instrumentation Engineers, 2002)
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Abstract:
Microbent optical fibers are potential candidates for evanescent wave sensing. We investigate the behavior of a permanently microbent fiber optic sensor when it is immersed in an absorbing medium. Two distinct detection schemes, namely, bright-field and dark-field detection configuration, are employed for the measurements. The optical power propagating through the sensor is found to vary in a logarithmic fashion with the concentration of the absorbing species in the surrounding medium. We observe that the sensitivity of the setup is dependent on the bending amplitude and length of the microbend region for the bright-field detection scheme, while it is relatively independent of both for the dark-field detection configuration. This feature can be exploited in compact sensor designs where reduction of the sensing region length is possible without sacrificing sensitivity.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(Springer, May , 1994)
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Abstract:
The changes in emission characteristics of a neon hollow cathode discharge by resonant laser excitation of 1s 5→2p 2 and 1s 5→2p 4 transition have been studied by simultaneously monitoring the optogalvanic effect and the laser induced fluorescence. It has been observed that resonant excitation causes substantial variation in the relative intensities of lines in the emission spectrum of neon discharge.
Nampoori, V P N; Samuel, Varghese; Suresh, Nair; Girijavallabhan, C P(Elsevier, Journal of Optics Communications, November 26, 2008)
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Abstract:
A new method for the fabrication of high uniformity monolithic 1 x 4 single mode fused coupler is
described together with details of its performance in terms of coupling ratio, spectral response and uniformity.
The fabricated device exhibits ultra-broadband performance with a port-to-port uniformity of
0.4 dB. The reliability of such couplers is also evaluated and found to have good stability. Moreover, by
controlling the process parameters it is possible to control the power remaining in the through put port
of the device, which can be used for dedicated non-intrusive network health monitoring.