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.
Bindhu, C V; Harilal, S S; Riju, C Issac; Nampoori, V P N; Vallabhan, C P G(World Scientific Puhlishing Company, 1996)
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Abstract:
Pulsed photoacoustic technique which is found to be a very convenient and accurate method, is used for the determination of absolute fluorescence quantum yield of laser dye rhodamine B. Concentration and power dependence of quantum yield of rhodamine B in methanol for excitation at 532 nm is reported here. Results show that a rapid decrease in quantum yield as the concentration is increased and finally it reaches the limit corresponding to fluorescence quenching.
Bindhu, C V; Harilal, S S; Issac, Riju C; Geetha K, Varier; Nampoori, V P N; Vallabhan, C P G(Pramana, March , 1995)
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Abstract:
Pulsed photoacoustic measurements have been carried out in toluene at 532 nm
wavelength using a Q-switched frequency doubled Nd:YAG laser. The variation of photoacoustic
signal amplitude with incident laser power indicates that at lower laser powers one
photon absorption takes place at this wavelength while a clear two photon absorption occurs in
this liquid at higher laser powers. The studies made here demonstrate that pulsed photoacoustic
technique is simple and effective for the investigation of multiphoton processes in
liquids.
A dynamical system with a damping that is quadratic in velocity is converted into the Hamiltonian format using a nonlinear transformation. Its quantum mechanical behaviour is then analysed by invoking the Gaussian effective potential technique. The method is worked out explicitly for the Duffing oscillator potential.
Nampoori, V P N; Girijavallabhan, C P; Radhakrishnan, P; 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; Girijavallabhan, C P; Nampoori, V P N; 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.
Achamma, Kurian; Unnikrishnan, K P; Thomas Lee, S; Nampoori, V P N; Vallabhan, C P G(Taylor & Francis, 2002)
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Abstract:
A sensitive method based on the principle of photothermal phenomena to realize optical
logic gates is presented. A dual beam thermal lens method using low power cw lasers in a
dye-doped polymer can be very effectively used as an alternate technique to perform the
logical function such as NAND, AND and OR.