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.
Girijavallabhan, C P; Nampoori, V P N; Suresh, Nair; Samuel, Varghese(Elsevier, Journal of Optics Communications, November 26, 2008)
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Abstract:
A new method for the fabrication of high uniformity monolithic 1 x4 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
Sheeba, Mavila; Thomas, Kannampuzha J; Rajesh, Mandamparambil; Nampoori, Vadakkedathu P N; Vallabhan, Chakkalakkal P G; Radhakrishnan, Padmanabhan(Optical Society of America, 2007)
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Abstract:
Multimode laser emission is observed in a polymer optical fiber doped with a mixture of Rhodamine 6G (Rh 6G) and Rhodamine B (Rh B) dyes. Tuning of laser emission is achieved by using the mixture of dyes due to the energy transfer occurring from donor molecule (Rh 6G) to acceptor molecule (Rh B). The dye doped poly(methyl methacrylate)-based polymer optical fiber is pumped axially at one end of the fiber using a 532 nm pulsed laser beam from a Nd:YAG laser and the fluorescence emission is collected from the other end. At low pump energy levels, fluorescence emission is observed. When the energy is increased beyond a threshold value, laser emission occurs with a multimode structure. The optical feedback for the gain medium is provided by the cylindrical surface of the optical fiber, which acts as a cavity. This fact is confirmed by the mode spacing dependence on the diameter of the fiber.
Bindu M, Krishna; Manu P, John; Nandakumaran, V M(Elsevier, April , 2010)
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Abstract:
Isochronal synchronisation between the elements of an array of three mutually coupled directly modulated semiconductor lasers is utilized for the purpose of simultaneous bidirectional secure communication. Chaotic synchronisation is achieved by adding the coupling signal to the self feedback signal provided to each element of the array. A symmetric coupling is effective in inducing synchronisation between the elements of the array. This coupling scheme provides a direct link between every pair of elements thus making the method suitable for simultaneous bidirectional communication between them. Both analog and digital messages are successfully encrypted and decrypted simultaneously by each element of the array.
Aneeshkumar, B; Pramod, Gopinath; Jayan,Thomas; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(Elsevier, June , 2004)
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Abstract:
The nanosecond optical limiting characteristics of sandwich-type neodymium diphthalocyanine in a co-polymer matrix of polymethyl methacrylate (PMMA) and methyl-2-cyanoacrylate have been studied for the first time. The measurements were performed using 9 ns laser pulses generated from a frequency-doubled Nd:YAG laser at 532 nm wavelength. The optical limiting performance of neodymium diphthalocyanine in co-polymer host was studied at different linear transmission. Laser damage threshold was also measured for the doped and undoped co-polymer samples. The optical limiting response is attributed to reverse saturable absorption which is due to excited-state absorption.
Kumar, G A; Gin, Jose; Vinoy, Thomas; Unnikrishnan, N V; Nampoori, V P N(Elsevier, 2003)
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Abstract:
Optical absorption studies of phthalocyanines (Pc-s) in borate glass matrix have been reported for the first time. Measurements have been done corresponding to photon energies between 1.1 and 6.2 eV for free base, manganese, iron, nickel, molybdenum, cobalt and copper phthalocyanines. Several new discrete transitions are observed in the UV–vis region of the spectra in addition to a strong continuum component of absorption in the IR region. Values of some of the important optical constants viz. absorption coefficient (α), molar extinction coefficient (ε), absorption cross-section (σa), band width (Δλ), electric dipole strength (q2) and oscillator strength (f) for the relevant electronic transitions are also presented. All the data reported for Pc-s in the new matrix have been compared with those corresponding to solution, vapor and thin film media.
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.
Lalaja, Varghese; Nampoori, V P N; Pratap, R(Current Science, October 20, 1987)
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Abstract:
A mathematical analysis of an electroencephalogram of a human Brain during an epileptic
seizure shows that the K2 entropy decreases as compared to a clinically normal brain while the
dimension of the attractor does not show significant deviation.
Unnikrishnan, K P; Nampoori, V P N; Ramakrishnan, V; Umadevi, M; Vallabhan, C P G(Institute of Physics Publishing, 2003)
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Abstract:
Nonlinear optical absorption in silver nanosol was investigated at selected wavelengths (456 nm, 477 nm and 532 nm) using open aperture Z-scan technique. It was observed that nature of nonlinear absorption is sensitively dependent on input fluence as well as on excitation wavelength. Besides, the present sample was found to exhibit reverse saturable absorption (RSA) and saturable absorption (SA) at these wavelengths depending on excitation fluence. RSA is attributed to enhanced absorption resulting from photochemical changes. SA observed for fluence values lower and higher than those corresponding to RSA are, respectively, attributed to plasmon bleach and saturation of RSA.
Litty, Irimpan; Bindu, Krishnan; Nampoori, V P N; Radhakrishnan, P(Elsevier, 2008)
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Abstract:
In this article we present the nonlinear optical properties of ZnO–TiO2–SiO2 nanocomposites prepared by colloidal chemical synthesis. Nonlinear optical response of these samples is studied using nanosecond laser pulses at an off-resonance wavelength. The nonlinearity of the silica colloid is low and its nonlinear response can be improved by making composites with ZnO and TiO2. These nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behaviour. The nonlinear refractive index and the nonlinear absorption increases with increasing ZnO volume fraction. The observed nonlinear absorption is explained by two photon absorption followed by weak free carrier absorption and nonlinear scattering. ZnO–TiO2–SiO2 is a potential nanocomposite material for the development of nonlinear optical devices with a relatively small limiting threshold.