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; 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.
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.
Thomas Lee, S; Jose, Gin; Nampoori, V P N; Vallabhan, C P G; Unnikrishnan, N V; Radhakrishnan, P(IOP Publishing, 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.
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; 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
Sheeba, M; Rajesh, M; Mathew, S; Nampoori, V P N; Vallabhan, C P G; Radhakrishnan, P(Optical Society of America, April , 2008)
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Abstract:
Two-photon excited (TPE) side illumination fluorescence studies in a Rh6G-RhB dye mixture doped polymer optical fiber (POF) and the effect of energy transfer on the attenuation coefficient is reported. The dye doped POF is pumped sideways using 800 nm, 70 fs laser pulses from a Ti:sapphire laser, and the TPE fluorescence emission is collected from the end of the fiber for different propagation distances. The fluorescence intensity of RhB doped POF is enhanced in the presence of Rh6G as a result of energy transfer from Rh6G to RhB. Because of the reabsorption and reemission process in dye molecules, an effective energy transfer is observed from the shorter wavelength part of the fluorescence spectrum to the longer wavelength part as the propagation distance is increased in dye doped POF. An energy transfer coefficient is found to be higher at shorter propagation distances compared to longer distances. A TPE fluorescence signal is used to characterize the optical attenuation coefficient in dye doped POF. The attenuation coefficient decreases at longer propagation distances due to the reabsorption and reemission process taking place within the dye doped fiber as the propagation distance is increased.
Wilson,K J; Vallabhan, C P G(IOP Publishing, 1989)
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Abstract:
A simple and inexpensive linear magnetic field sweep generating system suitable for magnetic resonance experiments is described. The circuit, utilising a modified IC bootstrap configuration, generates field sweep over a wide range of sweep durations with excellent sweep linearity.
Bindhu, C V; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(World Scientific, 1999)
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Abstract:
Dual beam thermal lens tecbnique is successfully employed for the determination of absolute
Fluorescence quantum yield of rhodamine 6G lnser dye in different solvents. A
532 nm radiation from a Q-switched Nd:YAG laser was used for the excitation purpose.
The fluorescence quantum yield values are found to be strongly influenced by environmental
effects. It has been observed that fluorescence yield is greater for rhodamine 6G
in ethylene glycol system than in water or in methanol. Our results also indicate that
parameters like concentration of the dye solution, aggregate formation and excited state
absorption affect the absolute values of fluorescence yield significantly.
Harilal, S S; Issac, Riju C; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(IOP, 1997)
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Abstract:
Time and space resolved spectroscopic studies of the molecular band
emission from C2 are performed in the plasma produced by irradiating a graphite
target with 1:06 m radiation from a Q-switched Nd:YAG laser. High-resolution
spectra are recorded from points located at distances up to 15 mm from the target
in the presence of ambient helium gas pressure. Depending on the laser
irradiance, time of observation and position of the sampled volume of the plasma
the features of the emission spectrum are found to change drastically. The
vibrational temperature and population distribution in the different vibrational levels
of C2 molecules have been evaluated as a function of distance for different time
delays and laser irradiance. It is also found that the vibrational temperature of C2
molecules decreases with increasing helium pressure.
Padmaja, G; Ravi Kumar, A V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
Spatial and temporal analyses of the spectra of the laser induced plasma from a polytetrafluroethylene (PTFE) target obtained with the 1.06 mu m radiation from a Q-switched Nd:YAG laser have been carried out. The spatially resolved spectra of the plasma emission show that molecular bands of C2 (Swan bands) and CN are very intense in the outer regions of the plasma, whereas higher ionized states of carbon are predominant in the core region of the plasma emission. The vibrational temperature and population distribution in the different vibrational levels have been studied as a function of laser energy. From the time resolved studies, it has been observed that there exist fairly large time delays for the onset of emission from all the species in the outer region of the plasma. The molecular bands in each region exhibit much larger time delays in comparison to the ionic lines in the plasma.
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; Nandakumaran, V M; Dann, V J; Mathew, M V(Plasma Science and Technology, August , 2007)
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Abstract:
Optical emission from TiO2 plasma, generated by a nanosecond laser is spectroscopically
analysed. The main chemical species are identified and the spatio-temporal distribution of
the plasma parameters such as electron temperature and density are characterized based on the
study of spectral distribution of the line intensities and their broadening characteristics. The parameters
of laser induced plasma vary quickly owing to its expansion at low background pressure
and the possible deviations from local thermodynamic equilibrium conditions are tested to show
its validity.
Padmaja, G; Ravi Kumar, A V; Vidyalal, V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Springer, May , 1989)
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Abstract:
Laser induced plasma emission spectra from highT c superconducting samples of YBa2Cu3O7 and GdBa2Cu3O7 obtained with 1.06µm radiation from a Q switched Nd:YAG laser beam has been analysed. The results clearly show the presence of diatomic oxides in addition to ionized species of the constituent metals in the plasma thus produced.
Kumar, G A; Thomas, J; George, N; Nampoori, V P N; Radhakrishnan, P; Vallabhan, C P G(Society of Glass Technology, 2000)
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Abstract:
Optical absorption and emission spectral studies of free and metal naphthalocyanine doped borate glass matrix are reported for the first time. Absorption spectra recorded in the UV- VIS-NIR region show the characteristic absorption bands, namely, the B-band and Q-band of the naphthalocyanine (Nc) molecule. Some of the important spectral parameters, namely, the optical absorption coefficient (α), molar extinction coefficient (ε) and absorption cross section (σa) of the principal absorption transitions are determined. Optical band gap (Eg) of the materials evaluated from the functional dependence of absorption coefficient on photon energy lies in the range 1.6 eV≤Eg≤2.1 eV. All fluorescence spectra except that of EuNc consist of an intense band in the 765 nm region corresponding to the excitation of Q-band. In EuNc the maximum fluorescence intensity band is observed at 824 nm. The intensity of the principal fluorescence band is maximum in ZnNc, whereas it is minimum in H2Nc. Radiative parameters of the principal fluorescence transitions corresponding to the Q-band excitation are also reported for the naphthalocyanine and phthalocyanine based matrices.
Bindhu, C V; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Journal of Physics D: Applied Physics, 1999)
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Abstract:
Dual-beam transient thermal lens studies were carried out in aqueous solutions of
rhodamine 6G using 532 nm pulses from a frequency-doubled Nd:YAG laser. The analysis of
the observed data showed that the thermal lens method can effectively be utilized to study the
nonlinear absorption and aggregation which are taking place in a dye medium.
Achamma, Kurian; Nibu, A George; Binoy, Paul; Nampoori, V P N; Vallabhan, C P G(Taylor & Francis, 2002)
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Abstract:
In this paper we report the use of the dual beam thermal lens technique as a quantitative
method to determine absolute fluorescence quantum efficiency and concentration
quenching of fluorescence emission from rhodamine 6G doped Poly(methyl
methacrylate) (PMMA), prepared with different concentrations of the dye. A
comparison of the present data with that reported in the literature indicates that the
observed variation of fluorescence quantum yield with respect to the dye
concentration follows a similar profile as in the earlier reported observations on
rhodamine 6G in solution. The photodegradation of the dye molecules under cw laser
excitation is also studied using the present method.
Achamma, Kurian; Thomas Lee, S; Unnikrishnan, K P; Sajan, D George; Nampoori, V P N; Vallabhan, C P G(World Scientific, 2003)
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Abstract:
Two-photon absorption spectrum of aniline is recorded using thermal lens effect with
optical parametric oscillator as the pump source. Studies show that the two-photon
absorption spectrum observed corresponds to I Al -. I B2 transition of aniline.
Jayaraj, M K; Vallabhan, C P G(IOP Publishing, 1989)
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Abstract:
The method of preparation of ZnS phosphors doped with praseodymium and copper is given. The electroluminescence (EL) spectrum of ZnS:Pr,Cl has two broad bands at 470 and 570 nm. ZnS:Cu,Pr,Cl gives white emission with spectral peaks at 470, 520, 570 and 640 nm. The EL spectra of both types of phosphor exhibit a conspicuous colour shift as the frequency of the excitation voltage is varied. Detailed investigations show that the relative intensities of spectral peaks are strongly dependent on the frequency of the excitation voltage. The colour shift is explained on the basis of the Schon-Klasens model.
Achamma, Kurian; Sajan, D George; Nampoori, V P N; Vallabhan, C P G(Elsevier, 2005)
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Abstract:
A sensitive method based on the principle of photothermal phenomena to study the energy transfer processes in organic dye mixtures is presented. A dual beam thermal lens method can be very effectively used as an alternate technique to determine the molecular distance between donor and acceptor in fluorescein–rhodamine B mixture using optical parametric oscillator.
Harilal, S S; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Applied Spectroscopy, 1998)
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Abstract:
Spectroscopic studies of laser -induced plasma from a high-temperature
superconducting material, viz., YBa2Cu3O7 (YBCO), have
been carried out. Electron temperature and electron density measurements
were made from spectral data. The Stark broad ening of
emission lines was used to determine the electron density, and the
ratio of line in tensities was exploited for the determination of electron
temperature. An initial electron temperature of 2.35 eV and
electron density of 2.5 3 1017 cm2 3 were observed. The dependence
on electron temperature and density on different experimental parameters
such as distance from the target, delay time after the in itiation
of the plasm a, and laser irradiance is also discussed in detail.
Index Headings: Laser -plasma spectroscopy; Plasma diagnostics;
Emission spectroscop y; YBa2Cu3O7.