Santhakumari, N C; Vallabhan, C P G(Elsevier, May , 1992)
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Abstract:
D.C. and a.c. electrical conductivities, dielectric constant and dielectric loss factor in single crystals of ethylenediammonium sulphate, (H3NCH2CH2NH3)(SO4), have been measured axiswise as a function of temperature. Anomalous variations in all the above properties at 480 K indicate the occurrence of a phase transition in the above material at this temperature. The existence of such a phase transition is also confirmed by DSC measurements. Electrical conductivity results are analysed and the activation energies of conduction at different temperature regions have been evaluated from the logσ vs 103T−1 plot. Possible mechanisms for the electrical conduction process are discussed, the available results being in favour of a proton transport model.
Santhakumari, N C; Vallabhan, C P G(Elsevier, April , 1991)
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dc and ac electrical conductivities, dielectric constant and dielectric loss factor in single crystals of ethylenediammonium dinitrate (EDN) have been measured axiswise as a function of temperature. All the above properties exhibit anomalous variations at 404 K thereby confirming the occurence of a phase transition in EDN at this temperature. Electrical conductivity parameters have been evaluated and possible conduction mechanisms are discussed. The role of protons in electrical trasport phenomenon is established by chemical analysis.
Harilal, S S; Bindhu, C V; Issac, Riju C; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, September 1, 1997)
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Abstract:
Plasma generated by fundamental radiation from a Nd:YAG laser focused onto a graphite target is studied spectroscopically. Measured line profiles of several ionic species were used to infer electron temperature and density at several sections located in front of the target surface. Line intensities of successive ionization states of carbon were used for electron temperature calculations. Stark broadened profiles of singly ionized species have been utilized for electron density measurements. Electron density as well as electron temperature were studied as functions of laser irradiance and time elapsed after the incidence of laser pulse. The validity of the assumption of local thermodynamic equilibrium is discussed in light of the results obtained.
Harilal, S S; Issac, Riju C; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, April , 1997)
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Abstract:
The emission features of laser ablated graphite plume generated in a helium ambient atmosphere have been investigated with time and space resolved plasma diagnostic technique. Time resolved optical emission spectroscopy is employed to reveal the velocity distribution of different species ejected during ablation. At lower values of laser fluences only a slowly propagating component of C2 is seen. At high fluences emission from C2 shows a twin peak distribution in time. The formation of an emission peak with diminished time delay giving an energetic peak at higher laser fluences is attributed to many body recombination. It is also observed that these double peaks get modified into triple peak time of flight distribution at distances greater than 16 mm from the target. The occurrence of multiple peaks in the C2 emission is mainly due to the delays caused from the different formation mechanism of C2 species. The velocity distribution of the faster peak exhibits an oscillating character with distance from the target surface.
Vallabhan, C P G; Thomas, J; Pillai, V N S; Nampoori, V P N(Springer, 1999)
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Abstract:
The electrical conductivity and thermal diffusivity of pristine and iodine doped vanadyl naphthalocyanine (VONc) were studied. In the pristine sample, the temperature dependence was very weak below 300 K. The increase in conductivity at higher temperature must be due to an enhancement in carrier density with increase in thermal energy. The electrical conductivity of VONc increased when doped with iodine. The behavior of VONcI indicated that considerable changes have occurred in the electronic environment of the molecule as a result of doping. Iodine doping enhanced the thermal diffusivity of VONc. The increase in thermal diffusivity of the iodine doped sample may be due to the disorder of iodine atoms occupying the channels in one dimensional lattices.
Ravi Kumar, A V; Padmaja, G; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Springer, October , 1991)
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Abstract:
The acoustic signals generated in solids due to interaction with pulsed laser beam is used to determine the ablation threshold of bulk polymer samples of teflon (polytetrafluoroethylene) and nylon under the irradiation from a Q-switched Nd:YAG laser at 1.06µm wavelength. A suitably designed piezoelectric transducer is employed for the detection of photoacoustic (PA) signals generated in this process. It has been observed that an abrupt increase in the amplitude of the PA signal occurs at the ablation threshold. Also there exist distinct values for the threshold corresponding to different mechanisms operative in producing damages like surface morphology, bond breaking and melting processes at different laser energy densities.
Girijavallabhan, C P; Nampoori, V P N; Radhakrishnan, P; Thomas Lee, S; Suresh, Kumar P; Unnikrishnan, K P; Sugunan, S(INSTITUTE OF PHYSICS PUBLISHING, April 24, 2003)
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Abstract:
In this communication, we discuss the details of fabricating an off-line fibre
optic sensor (FOS) based on evanescent wave absorption for detecting trace
amounts of Fe3+ in water. Two types of FOS are developed; one type uses
the unclad portion of a multimode silica fibre as the sensing region whereas
the other employs the microbent portion of a multimode plastic fibre as the
sensing region. Sensing is performed by measuring the absorption of the
evanescent wave in a reagent medium surrounding the sensing region. To
evaluate the relative merits of the two types of FOS in Fe3+ sensing, a
comparative study of the sensors is made, which reveals the superiority of
the latter in many respects, such as smaller sensing length, use of a double
detection scheme (for detecting both core and cladding modes) and higher
sensitivity of cladding mode detection at an intermediate range of
concentration along with the added advantage that plastic fibres are
inexpensive. A detection limit of 1 ppb is observed in both types of fibre and
the range of detection can be as large as 1 ppb–50 ppm. All the
measurements are carried out using a LabVIEW set-up.
Kailasnath, M; Nishant, Kumar; Nampoori, V P N; Vallabhan, C P G; Radhakrishnan, P(Elsevier, 2008)
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Abstract:
Dependence of energy transfer parameters on excitation wavelength has been investigated in poly (methyl methacrylate) (PMMA) optical fibre preforms doped with C 540:Rh B dye mixture by studying the fluorescence intensity and the lifetime variations. A fluorescence spectrophotometer was used to record the excitation spectra of the samples for the emission wavelengths 495 and 580 nm. The fluorescence emission from the polymer rods was studied at four specific excitation wavelengths viz; 445, 465, 488 and 532 nm. The fluorescence lifetime of the donor molecule was experimentally measured in polymer matrix by time correlated single photon counting technique. The energy transfer rate constants and transfer efficiencies were calculated and their dependence on the acceptor concentration was analysed for three excitation wavelengths. It was found that any change in the excitation wavelength leads to significant variations in the quenching characteristics, which in turn affect the calculated energy transfer parameters.
Bindhu, C V; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Springer-Verlag, 2000)
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Abstract:
Optical limiting and thermal lensing studies are
carried out in C70–toluene solutions. The measurements are
performed using 9-ns pulses generated from a frequencydoubled
Nd:YAG laser at 532 nm. Optical limiting studies in
fullerene molecules lead to the conclusion that reverse saturable
absorption is the major mechanism for limiting. Analysis
of thermal lensing measurements showed a quadratic
dependence of thermal lens signal on incident laser energy,
which also supports the view that optical limiting in C70 arises
due to sequential two-photon absorption via excited triplet
state (reverse saturable absorption).
Rajesh, Mandamparambil; Sheeba, Mavila; Geetha, Karinjamanna; Vallabhan, Chakkalakkal P G; Radhakrishnan, Padmanabhan; Nampoori, Vadakkedathu P N(Optical Society of America, January , 2007)
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Abstract:
The fabrication and characterization of a Rhodamine 6G-doped polymer optical fiber amplifier have been carried out. Two different schemes were employed to characterize the optical fiber: the stripe illumination technique to study the fiber as a gain medium and another technique to study its performance as an amplifier. We observed a spectral narrowing from 42 to 7 nm when the pump energy was increased to 6 mJ in the stripe illumination geometry. A gain of 18 dB was obtained in the amplifier configuration. The effects of pump power and dye concentration on the performance of the fiber as an amplifier were also studied.
Sheeba, M; Rajesh, M; Nampoori, V P N; Radhakrishnan, P(Optical Society of America, 2008)
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Rhodamine 6G and Rhodamine B dye mixture doped polymer optical fiber amplifier (POFA), which can operate in a broad wavelength region (60 nm), has been successfully fabricated and tested. Tunable operation of the amplifier over a broad wavelength region is achieved by mixing different ratios of the dyes. The dye doped POFA is pumped axially using 532 nm, 10 ns laser pulses from a frequency doubled Q-switched Nd: YAG laser and the signals are taken from an optical parametric oscillator. A maximum gain of 22.3 dB at 617 nm wavelength has been obtained for a 7 cm long dye mixture doped POFA. The effects of pump energy and length of the fiber on the performance of the fiber amplifier are also studied. There exists an optimum length for which the amplifier gain is at a maximum value.
Rajesh, M; Geetha, K; Sheeba, M; Radhakrishnan, P; Vallabhan, C P G; Nampoori, V P N(Elsevier, May , 2006)
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Abstract:
A simple, effective and inexpensive fiber optic sensor for investigating the setting characteristics of various grades of cement is described. A finite length of unsheathed multimode optical fiber laid inside the cement mix, is subjected to stress during the setting process. The microbends created on the fiber due to this stress directly influence the intensity of light propagating through the fiber. Continuous monitoring of such variations in the light output transmitted through the fiber gives a clear measure of the setting characteristics of the cement mix, thus providing a simple and elegant technique of great practical importance in the field of civil engineering. The smart fiber optic sensor described above can be incorporated into a building during the construction process itself so that continuous monitoring of the deterioration process for the entire life time of the building can be carried out.
Girijavallabhan, C P; Nampoori, V P N; Radhakrishnan, P; Sheeba, M; Rajesh, M(Measurement Science and Technology, Institute of Physics Publishing, October 4, 2005)
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Abstract:
The design and development of a fibre optic evanescent wave refractometer
for the detection of trace amounts of paraffin oil and palm oil in coconut oil
is presented. This sensor is based on a side-polished plastic optical fibre. At
the sensing region, the cladding and a small portion of the core are removed
and the fibre nicely polished. The sensing region is fabricated in such a
manner that it sits perfectly within a bent mould. This bending of the
sensing region enhances its sensitivity. The oil mixture of different mix
ratios is introduced into the sensing region and we observed a sharp decrease
in the output intensity. The observed variation in the intensity is found to be
linear and the detection limit is 2% (by volume) paraffin oil/palm oil in
coconut oil. The resolution of this refractometric sensor is of the order of
10−3. Since coconut oil is consumed in large volumes as edible oil in south
India, this fibre optic sensor finds great relevance for the detection of
adulterants such as paraffin oil or palm oil which are readily miscible in
coconut oil. The advantage of this type of sensor is that it is inexpensive and
easy to set up. Another attraction of the side-polished fibre is that only a
very small amount of analyte is needed and its response time is only 7 s.
Harilal, S S; Issac, Riju C; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Pramana, September , 1997)
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Abstract:
Time resolved optical emission spectroscopy is employed to study the expansion
dynamics of C2 species in a graphite plasma produced during the Nd : YAG ablation. At low laser
fluences a single peak distribution with low kinetic energy is observed. At higher fluences a twin
peak distribution is found. It has been noted that these double peak time of flight distribution splits
into a triple peak structure at distances >_ 17mm from the target surface. The reason for the
occurrence of multiple peak is due to different formation mechanisms of C2 species
Sathy, P; Reji, Philip; Nampoori, V P N; Vallabhan, C P G(Springer, June , 1990)
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Abstract:
Experimental method for measuring photoacoustic(PA) signals generated by a pulsed laser beam in liquids is described. The pulsed PA technique is found to be a convenient and accurate method for determination of quantum yield in fluorescent dye solutions. Concentration dependence of quantum yield of rhodamine 6G in water is studied using the above method. The results indicate that the quantum yield decreases with increase in concentration in the quenching region in agreement with the existing reports based on radiometric measurements.
Thomas, Baby; Sathianandan, K; Nampoori, V P N; Vallabhan, C P G; Ramachandran, T(American Institute of Physics, September , 1991)
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Abstract:
A high power Nz laser of the double-Blumlein type having a modified gas flow system,
electrode configuration, and discharge geometry with minimum inductance is described. By
incorporating a triggere’d-pressurized spark gap switch, arc-free operation was achieved
for a wide E/P range. The device gives a peak power in excess 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. The dependence of output power on parameters such as operating pressure, voltage,
and repetition rate are discussed.
Sasi Kumar, P R; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Pramana, March , 1994)
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Abstract:
High resolution optogalvanic spectrum of the (11, 7) band in the first positive system
of nitrogen molecule has been recorded from 17179 to 17376 cm- 1. Assignment of 432 rotational
lines belonging to the 27 branches of this band has been carried out.
Subhadra, V K; Syamaprasad, U; Vallabhan, C P G(American Institute of Physics, 1983)
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Abstract:
Results of axiswise measurements of the electrical conductivity (dc and ac) and dielectric constant of NH4H2PO4 confirm the occurrence of the recently suggested high‐temperature phase transition in this crystal (at 133 °C). The corresponding transition in ND4D2PO4 observed here for the first time takes place at 141.5 °C. The mechanism involved in these transitions and those associated with the electrical conduction and dielectric anomalies are explained on the basis of the motional effects of the ammonium ions in these crystals. Conductivity values for deuterated crystals give direct evidence for the predominance of protonic conduction throughout the entire range of temperatures studied (30–260 °C).
Unnikrishna Pillai, P R; Padma, Nambisan; Nampoori, V P N; Vallabhan, C P G(HEREDITAS ( Beijing ), 1997)
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Abstract:
Laser irradiation at wavelength 514 nm was used to study the effect, of lasers in inducing
chromosomal aberrations at mitosis. This study offers a new radiation system which could be used for the
induction of mutations. Results are compared with those obtained from studies using y-rays as irradiation
source.