Sajan, D George; Rajesh, K; Warrier, K G K; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(October 29, 2011)
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Abstract:
The laser induced non-destructive photoacoustic technique has been employed
to measure the thermal diffusivity of lanthanum phosphate ceramics prepared
by the sol–gel route. The thermal diffusivity value was evaluated by knowing
the transition frequency between the thermally thin to thermally thick region
from the log–log plot of photoacoustic amplitude versus chopping frequency.
Analysis of the data was carried out on the basis of the one-dimensional
model of Rosencwaig and Gersho. The present investigation reveals that the
sintering temperature has great influence on the propagation of heat carriers
and hence on the thermal diffusivity value. The results were interpreted
in terms of variations in porosity with sintering temperature as well as with
changes in grain size.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(IOP, 1994)
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Abstract:
Laser-induced photoelectric and photoemission optogalvanic effects in a Ne-Nd hollow cathode discharge have been studied using a continuous wave laser source. The potential barrier for photoinduced electron emission from the cathode decreases as the applied voltage is increased. Owing to secondary electron emission in the plasma, the photocurrent is greater than that without discharge. The multiplication of secondary electrons and the quantum efficiency are also investigated.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(Elsevier, August 1, 1995)
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Abstract:
The photoemission optogalvanic (POG) effect has been investigated in a neon-neodymium hollow cathode discharge using
cw laser excitation. Both positive and negative effects were observed. It was found that the amplitude of the POG signal was
unstable near the instability region of the discharge.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
A two-photon induced photoemission optogalvanic effect which brings about a change in the discharge voltage when a pulsed dye laser beam is focused on a tungsten electrode has been described. The experiment is performed with N2, NO2 and Ar discharges. The magnitude of the signal voltage is studied as a function of laser energy and discharge current. The effective quantum efficiency in the discharge is found to be larger than that in the vacuum condition.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
A two-photon induced photoemission optogalvanic effect which brings about a change in the discharge voltage when a pulsed dye laser beam is focused on a tungsten electrode has been described. The experiment is performed with N2, NO2 and Ar discharges. The magnitude of the signal voltage is studied as a function of laser energy and discharge current. The effective quantum efficiency in the discharge is found to be larger than that in the vacuum condition.
Ajith Prasad, K C; Nampoori, V P N; Vallabhan, C P G(Elsevier, 1996)
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Abstract:
Photoemission optogalvanaic (POG) effect has been observed by irradiating copper target electrode, in a nitrogen discharge cell using 1.06 μm and frequency doubled 532 nm Nd:YAG laser pulse. Measurement of the nature of the variation of POG signal strength with 532 nm laser fluence confirms the two photon induced photoelectric emission from copper. However, using 1.06 μm laser pulses thermally assisted photoemission is observed.
Lyjo, Joseph K; Dayas, K R; Soniya, Damodar; Bindu, Krishnan; Krishnankutty, K; Nampoori, V P N; Radhakrishnan, P(Elsevier, December 15, 2008)
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Abstract:
The laser-induced luminescence studies of the rare earth titanates (R2Ti2O7) (R = La, Nd and Gd) using 355 nm radiation from an Nd:YAG laser are presented. These samples with submicron or nanometer size are prepared by the self-propagating high temperature synthesis (SHS) method and there is no known fluorescence shown by these rare earths in the visible region. Hence, the luminescence transitions shown by the La2Ti2O7 near 610 nm and Gd2Ti2O7 near 767 nm are quite interesting. Though La3+ ions with no 4f electrons have no electronic energy levels that can induce excitation and luminescence processes in the visible region, the presence of the Ti3+ ions leads to luminescence in this region.
Annieta, Philip K; Lyjo, Joseph K; Litty, Irimpan; Radhakrishnan, P; Nampoori, V P N(Department of Photonics, 2004)
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Abstract:
Laser induced photoacoustic (PA) technique is used in the study of photostability of polymethyl
methacrylate (PMMA) films doped with Rhodamine 6G -Rhodamine B dye system. Energy transfer from a donor
molecule to an acceptor molecule in a dye mixture affects the output of the dye system. Details of investigations on
the role of laser power, modulation frequency and the irradiation wavelength on the photosensitivity of the dye
mixture doped PMMA films are presented.
We have performed thermal diffusion measurements
of nanofluid containing gold and rhodamine 6G dye
in various ratios. At certain concentrations, gold is nearly
four times more efficient than water in dissipating small
temperature fluctuations in a medium, and therefore it will
find applications as heat transfer fluids. We have employed
dual-beam mode-matched thermal lens technique for the
present investigation. It is a sensitive technique in measuring
photothermal parameters because of the use of a lowpower,
stabilized laser source as the probe. We also present
the results of fluorescence measurements of the dye in the
nanogold environment.
V P N Nampoori; Radhakrishnan, P; 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
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Sajan, D George(Physical Review, American Physical Society, October 16, 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; 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
Nibu, A George; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(Optical Society of America, August , 2002)
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Abstract:
Photothermal beam deflection studies were carried out with GaAs epitaxial double layers grown on semi-insulating GaAs substrates. The impurity densities in thin epitaxial layers were found to influence the effective thermal diffusivity of the entire structure.
George, S D; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Springer-Verlag, 2003)
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Abstract:
Laser induced transverse photothermal deflection
technique has been employed to determine the thermal parameters
of InP doped with Sn, S and Fe as well as intrinsic
InP. The thermal diffusivity values of these various samples
are evaluated from the slope of the curve plotted between the
phase of photothermal deflection signal and pump-probe offset.
Analysis of the data shows that heat transport and hence
the thermal diffusivity value, is greatly affected by the introduction
of dopant. It is also seen that the direction of heat flow with
respect to the plane of cleavage of semiconductor wafers influences
the thermal diffusivity value. The results are explained in
terms of dominating phonon assisted heat transfer mechanism in semiconductors.
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; 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
Nampoori, V P N; Radhakrishnan, P; 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; Nampoori, V P N; 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.
Sreeramana Aithal, P; Nagaraja, H S; Mohan Rao, P; Nampoori, V P N; Vallabhan, C P G; Avasthi, D K(Elsevier, 1997)
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Abstract:
Organic nonlinear optical single crystals of Methyl para-Hydroxy Benzoate (MHB) have been grown using gel-solution technique. These crystals are cut along z-axis and are bombarded with Ag14+ ions of energy 100 MeV. The results show an increase in refractive index at the ion irradiated region. The dielectric constant of the irradiated crystal is increased more than 15 times compared to that of a nonirradiated crystal. The result of these changes and comparative study of second harmonic generation (SHG) efficiency before and after irradiation is discussed.
Issac, R C; Varier, G K; Gopinath, P; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Springer Berlin / Heidelberg, 1998)
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Abstract:
A silver target kept under partial vacuum conditions
was irradiated with focused nanosecond pulses at
1:06 mm from a Nd:YAG laser. The electron emission monitored
with a Langmuir probe shows a clear twin-peak distribution.
The first peak which is very sharp has only a small delay
and it indicates prompt electron emission with energy as
much as 60 5 eV. Also the prompt electron emission shows
a temporal profile with a width that is same as that for the
laser pulse whereas the second peak is broader, covers several
microseconds, and represents the low-energy electrons
(2 0:5 eV) associated with the laser-induced silver plasma
as revealed by time-of-flight measurements. It has been found
that prompt electrons ejected from the target collisionally
excite and ionize ambient gas molecules. Clearly resolved
rotational structure is observed in the emission spectra of
ambient nitrogen molecules. Combined with time-resolved
spectroscopy, the prompt electrons can be used as excitation
sources for various collisional excitation–relaxation experiments.
The electron density corresponding to the first peak is
estimated to be of the order of 1017 cm?--3 and it is found that
the density increases as a function of distance away from the
target. Dependence of probe current on laser intensity shows
plasma shielding at high laser intensities.
Geetha, K; Rajesh, M; Nampoori, V P N; Vallabhan, C P G; Radhakrishnan, P(Optical Society of America, 2006)
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Abstract:
The propagation characteristics of amplified spontaneous emission (ASE) through a rhodamine 6 G-doped polymethyl methacrylate freestanding film waveguide were studied. This was done by shifting the excitation stripe horizontally along a transversely pumped waveguide. By this method, we could tune the ASE wavelength. The maximum tunability thus obtained was ~18 nm with a pump stripe length of 6 mm.