Jinesh, Mathew; Mahesh, V V; Radhakrishnan, P(International Frequency Sensor Association, August , 2008)
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Abstract:
The design and fabrication of fiber based ammonia sensors employing Bromothymol blue
and Chitosan as sensing elements are presented in this paper. In the presence of ammonia gas the
absorption of Bromothymol blue changes while in the case of Chitosan the refractive index changes
which in turn modulates the intensity of light propagating through a fiber.
Radhakrishnan, P; V P N Nampoori; 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.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan C 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.
Radhakrishnan, P; V P N Nampoori; Litty, Irimpan; Bindu, Krishnan; Deepthy, A(Journal of Physics D : Applied Physics, August 30, 2007)
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Abstract:
In this paper, the fluorescence behaviour of nano colloids of ZnO has been
studied as a function of the excitation wavelength. We have found that
excitation at the tail of the absorption band gives rise to an emission that
shifts with the change of the excitation wavelength. The excitation
wavelength dependent shift of the fluorescence maximum is measured to be
between 60 and 100 nm. This kind of excitation wavelength dependent
fluorescence behaviour, which may appear to be in violation of Kasha’s rule
of excitation wavelength independence of the emission spectrum, has been
observed for nano ZnO colloids prepared by two different chemical routes
and different capping agents. It is shown that the existence of a distribution
of energetically different molecules in the ground state coupled with a low
rate of the excited state relaxation processes, namely, solvation and energy
transfer, are responsible for the excitation wavelength dependent
fluorescence behaviour of the systems.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Shelly John, M(Measurement Science and Technology, UK, November 12, 1998)
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Abstract:
A novel sensing technique for the in situ monitoring of the rate of pulsed laser
deposition (PLD) of metal thin films has been developed. This optical fibre based sensor
works on the principle of the evanescent wave penetration of waveguide modes into the
uncladded portion of a multimode fibre. The utility of this optical fibre sensor is
demonstrated in the case of PLD of silver thin films obtained by a Q-switched Nd:YAG laser
which is used to irradiate a silver target at the required conditions for the preparation of thin
films. This paper describes the performance and characteristics of the sensor and shows how
the device can be used as an effective tool for the monitoring of the deposition rate of silver
thin films. The fibre optic sensor is very simple, inexpensive and highly sensitive compared
with existing techniques for thin film deposition rate measurements
Radhakrishnan, P; Girijavallabhan, C P; V P N Nampoori; Sivsankara Pillai, V N; Suresh Kumar, P(Journal of Optics : A Pure and Applied Optics, March 8, 2002)
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Abstract:
A fibre optic technique for detecting trace amounts of nitrite compounds in
water is described. The off-line fibre optic sensor outlined here is based on
evanescent field absorption in a test solution formed by the reaction of nitrite
compounds in water with suitable chemical reagents. A short unclad portion
of a plastic clad silica fibre acts as the sensing region. The experimental
results clearly establish the usefulness of the present technique for detecting
very low concentrations of the order of 1 ppb (parts per billion) of nitrite
compounds with a large dynamic range of 1–1000 ppb. Such a high
sensitivity enables the present device to be used for measuring the nitrite
content in drinking water.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C 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.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Geetha, K; Rajesh, M(Journal of Optics A: Pure and Applied Optics, Institute of Physics Publishing, January 13, 2006)
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Abstract:
We present a compact solid-state laser based on leaky mode propagation
from a dye-doped polymer free-standing film waveguide. The edge emitted
spectrum clearly indicated the existence of periodic resonant modes. The
reflections from the lateral faces of the free-standing film provided the
optical feedback thus giving rise to a Fabry–Perot like optical cavity. This
together with the guidance through the gain medium gave rise to intense
narrow emission lines. For a pump energy of 1.82 mJ/pulse, an intense line
with FWHM ∼0.4 nmwas observed at 576.5 nm.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Sajan, D George; Aji, A Anapara; K G K Warrier(Proceedings of SPIE, 2003)
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Abstract:
In this paper, we report the measurements of thermal diffusivity of nano Ag metal dispersed ceramic alumina
matrix sintered at different temperatures using laser induced non-destructive photoacoustic technique. Measurements of
thermal diffusivity also have been carried out on specimens with various concentration of nano metal. Analysis of the
data is done on the basis of one-dimensional model of Rosencwaig and Gersho. The present measurements on the
thermal diffusivity of nano metal dispersed ceramic alumina shows that porosity has a great influence on the heat
transport and the thermal diffusivity value. The present analysis also shows that the inclusion of nano metal into
ceramic matrix increases its interconnectivity and hence the thermal diffusivity value. The present study on the samples
sintered at different temperature shows that the porosity of the ceramics varies considerably with the change in sintering
temperature. The results are interpreted in terms of phonon assisted heat transfer mechanism and the exclusion of pores
with the increase in sintering temperature
Radhakrishnan, P; V P N Nampoori; 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
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; 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
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.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Nibu, A George; George, A K(Journal of Physica: Condensed Matter, Institute of Physics Publishing, November 28, 2000)
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Abstract:
The thermal effusivity values in the isotropic phase of certain comb-shaped
polymers have been evaluated for the first time using an open photoacoustic
cell configuration. The compounds investigated have siloxane and acrylate
backbone and they carry mesogenic groups in their side chain. The results
indicate that the polymer chain length as well as the side chain length have
pronounced influence on the thermal effusivity values in liquid crystalline
polymers
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; George, N A; Paul, T; Sebastian, M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Sajan, D George; George, A K(Smart Materials and Structures, July 5, 2007)
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Abstract:
Complete thermal characterization of liquid crystal mixtures in the smectic
phase consisting of various relative volume fractions of cholesterol and
1-hexadecanol have been carried out using the photoacoustic technique.
Thermal diffusivity values of these liquid crystal mixtures are evaluated using
the open cell photoacoustic technique whereas the thermal effusivity value is
measured using the conventional photoacoustic technique. From the
measured values of these transient thermophysical parameters, the thermal
conductivity and heat capacity of the sample under investigation are
calculated. Analyses of the results show that all the thermophysical
parameters depend strongly on the volume fraction of the constituents.
Results are interpreted in terms of enhanced hydrogen bonding and the
consequent enhancement in cohesive thermal energy transport with
increasing volume fraction of 1-hexadecanol
Radhakrishnan, P; Girijavallabhan, C P; Nibu, A George; Aneeshkumar, B(Journal of Physics D : Applied Physics, April 7, 1999)
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Abstract:
The photobleaching of the lasing dye Rhodamine 6G embedded in the solid
matrix poly(methyl methacrylate) was investigated using a photoacoustic technique.
Chopped laser radiation from an argon ion laser at four different wavelengths was used for
the study. Experimental results indicate that the photobleaching rate is directly proportional
to the incident laser power while it decreases with increase in concentration of the dye
molecules. In the present case we have not observed any dependence of photobleaching on
the chopping frequency. One-photon absorption is found to be responsible for the
photobleaching of the dye within the selected range of laser power.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Sajan, D George(Physical Review, American Physical Society, 2003)
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Abstract:
In this paper, we report the in-plane and cross-plane measurements of the thermal diffusivity of double
epitaxial layers of n-type GaAs doped with various concentrations of Si and a p-type Be-doped GaAs layer
grown on a GaAs substrate by the molecular beam epitaxial method, using the laser-induced nondestructive
photothermal deflection technique. The thermal diffusivity value is evaluated from the slope of the graph of the
phase of the photothermal deflection signal as a function of pump-probe offset. Analysis of the data shows that
the cross-plane thermal diffusivity is less than that of the in-plane thermal diffusivity. It is also seen that the
doping concentration has a great influence on the thermal diffusivity value. Measurement of p-type Be-doped
samples shows that the nature of the dopant also influences the effective thermal diffusivity value. The results
are interpreted in terms of a phonon-assisted heat transfer mechanism and the various scattering process
involved in the propagation of phonons
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Kumar, G A; Thomas, J; Unnikrishnan, N V; George, N(Journal of Materials Science ,Kluwer Academic Publishers, 2000)
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Abstract:
Physical and optical properties of various free base and metallic phthalocyanine (Pc) doped
glass matrix are reported for the first time. Absorption spectral measurements of H2Pc,
MnPc, NiPc, CoPc, CuPc, MoOPc, ZnPc and FePc doped borate glass matrix have been
made in the 200–1100 nm region and the spectra obtained are analyzed in the 2.1–6.2 eV
region to obtain the optical band gap (Eg) and the width of the band tail (Et). Other
important optical and physical parameters viz. refractive index (n), molar extinction
coefficient ("), density (½), glass transition temperature (Tg), molecular concentration (N ),
polaron radius (rp), intermolecular separation (R), molar refractivity (Rm) are also reported.
Radhakrishnan, P; Girijavallabhan, C P; V P N Nampoori; Thomas, K J; Sheeba, M(Journal of Optics A: Pure and Applied Optics, March 27, 2008)
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Abstract:
Polymethyl methacrylate (PMMA) optical fibres are fabricated by a preform drawing process.
The Raman spectra of PMMA fibres are recorded using a diode pumped solid state laser
emitting at 532 nm and a CCD-spectrograph in the 400–3800 cm−1 range. The variation of the
Raman intensity with the length of the optical fibre is studied. Investigations are carried out on
the variation of FWHM of the Raman peak at 2957 cm−1 with the length of the optical fibre and
pump power. The differential scattering cross section and gain coefficient of the Raman peak at
2957 cm−1 in PMMA are calculated in relation to that of toluene.
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Thomas Lee, S; Jose, Gin; Unnikrishnan, N V(Journal of Optics A: Pure and Applied Optics, July 12, 2001)
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Abstract:
The fabrication and characterization of a fibre optic pH sensor based on
evanescent wave absorption is presented. The unclad portion of a
multi-mode optical fibre is coated with a pH sensitive dye, which is
immobilized by the sol–gel route. The sensitivity of the device has been
found to increase when multiple sol–gel coatings are used as the sensing
region. The dynamic range and the temporal response of the sensor are
investigated for two different dyes, namely bromocresol purple and
bromocresol green. The performance of the device is evaluated in terms of
the results obtained during actual measurements