Bindhu, C V; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Society of Photo-Optical Instrumentation Engineers, 1998)
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Abstract:
Thermal diffusivity measurements are carried out in certain organic liquids using the pulsed dual beam thermal lens technique. The 532 nm pulses from a frequency doubled Q-switched Nd:YAG laser are used as the heating source and an intensity stabilized He-Ne laser serves as the probe beam. Experimental determination of the characteristic time constant of the transient thermal lens signal is verified theoretically. Measured thermal diffusivity values are in excellent agreement with literature values.
V P N Nampoori; Santhi Ani, Joseph; Misha, Hari; Mathew, S; Gaurav, Sharma; Soumya; Hadiya, V M; Radhakrishnan, P(Elsevier, Journal of Optics Communications, October 6, 2009)
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Abstract:
Dual beam mode-matched thermal lens method has been employed to measure the heat diffusion in
nanofluid of silver with various volumes of rhodamine 6G, both dispersed in water. The important observation
is an indication of temperature dependent diffusivity and that the overall heat diffusion is slower
in the chemically prepared Ag sol compared to that of water. The experimental results can be explained
assuming that Brownian motion is the main mechanism of heat transfer under the present experimental
conditions. Light induced aggregation of the nanoparticles can also result in an anomalous diffusion
behavior.
Sankara Raman, S; Nampoori, V P N; Vallabhan, C P G; Saravanan, N; Mohammed Yusuff, K K(Springer, 1999)
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Abstract:
The thermal diffusivities of some polystyrene supported Schiff complexes of Co(II) and Cu(II) were determined by the laser induced photoacoustic technique. The effect of metal as well as the halogen part on thermal diffusivity of polymer supported complexes was studied. The thermal diffusivity of Co complexes increases while it decreases in Cu complexes with Cl, Br and I substitutions, respectively.
Achamma, Kurian; Unnikrishnan, K P; Sajan, D George; Pramod, Gopinath; Nampoori, V P N; Vallabhan, C P G(Elsevier, 2003)
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Abstract:
The wavelength dependence of thermal lens signal from organic dyes are studied using dual beam thermal lens technique. It is found that the profile of thermal lens spectrum widely differ from the conventional absorption spectrum in the case of rhodamine B unlike in the case of crystal violet. This is explained on the basis of varying contribution of nonradiative relaxations from the excited vibronic levels.
Achamma, Kurian; Unnikrishnan, K P; Sajan, D George; Pramod, Gopinath; Nampoori, V P N; Vallabhan, C P G(Elsevier, 2003)
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Abstract:
The wavelength dependence of thermal lens signal from organic dyes are studied using dual beam thermal lens
technique. It is found that the profile of thermal lens spectrum widely differ from the conventional absorption spectrum
in the case of rhodamine B unlike in the case of crystal violet. This is explained on the basis of varying contribution of
nonradiative relaxations from the excited vibronic levels.
Santhi, A; Kala, U L; Nedumpara, R J; Kurian, A; Kurup, M R P; Radhakrishnan, P; Nampoori, V P N(Springer, 2004)
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Abstract:
The fluorescence spectrum of the schiff base obtained from salicylaldehyde and 2-aminophenol is studied using an argon-ion laser as the excitation source and its fluorescence quantum yield (Qf) is determined using a thermal lens method. This is a nondestructive technique that gives the absolute value of Qf without the need for a fluorescence standard. The quantum-yield values are calculated for various concentrations of the solution in chloroform and also for various excitation wavelengths. The value of Qf is relatively high, and is concentration dependent. The maximum value of Qf obtained is nearly 0.78. The high value of the fluorescence quantum yield will render the schiff base useful as a fluorescent marker for biological applications. Photostability and gain studies will assess its suitability as a laser dye.
Achamma, Kurian; Sajan, D George; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Elsevier, 2007)
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Abstract:
The effect of pH on the fluorescence efficiency of fluorescein is evaluated using thermal lens technique. Fluorescence efficiency increases as the
sample becomes more and more alkaline. But when fluorescein is mixed with rhodamine B fluorescence quenching of fluorescein takes place with
the excitation of rhodamine B. The electronic energy transfer in this mixture is investigated using Optical Parametric Oscillator as the excitation
source. The effect of pH on the efficiency of energy transfer in fluorescein–rhodamine B mixture is presented.
Unnikrishnan, K P; Jayan,Thomas; Nampoori, V P N; Vallabhan, C P G(Elsevier, April 1, 2002)
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Abstract:
Third order nonlinear susceptibility χ(3) and second hyperpolarizability (γ) of a bis-naphthalocyanine viz. europium naphthalocyanines, Eu(Nc)2, were measured in dimethyl formamide solution using degenerate four wave mixing at 532 nm under nanosecond pulse excitation. Effective nonlinear absorption coefficient, βeff and imaginary part of nonlinear susceptibility, Im(χ(3)) were obtained using open aperture /Z-scan technique at the same wavelength. Optical limiting property of the sample was also investigated. The role of excited state absorption in deciding the nonlinear properties of this material is discussed.
Padmaja, G; Ravi Kumar, A V; Vidyalal, V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP Publishing, 1989)
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Abstract:
In order to characterise the laser ablation process from high-Tc superconductors, the time evolution of plasma produced by a Q-switching Nd:YAG laser from a GdBa2Cu3O7 superconducting sample has been studied using spectroscopic and ion-probe techniques. It has been observed that there is a fairly large delay for the onset of the emission from oxide species in comparison with those from atoms and ions of the constituent elements present in the plasma. Faster decay occurs for emission from oxides and ions compared with that from neutral atoms. These observations support the view that oxides are not directly produced from the target, but are formed by the recombination process while the plasma cools down. Plasma parameters such as temperature and velocity are also evaluated.
Harilal, S S; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Springer Berlin / Heidelberg, 1998)
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Abstract:
Laser radiation at 1.06 µm from a pulsed Nd:YAG
laser was focused onto a multielement YBa2Cu3O7 target in
vacuum and the plasma thus generated was studied using
time-resolved spectroscopic techniques. Line broadening of
the Ba I emission line at 553.5 nm was monitored as a function
of time elapsed after the incidence of a laser pulse on the
target. Measured line profiles of barium species were used to
infer the electron density and temperature, and the time evolution
of these important plasma parameters has been worked out.
Harilal, S S; Issac, Riju C; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Jpn. J. Appl. Phys, 1997)
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Abstract:
We report time resolved study of C2 emission from laser produced carbon plasma in presence of ambient helium
gas. The 1.06µm: radiation from a Nd:YAG laser was focused onto a graphite target where it·produced a transient
plasma. We observed double peak structure in the time profile of C2 species. The twin peaks were observed only
after a threshold laser fluence. It is proposed that the faster velocity component in the temporal profiles originates
mainly due to recombination processes. The laser fluence and ambient gas dependence of the double peak intensity
distribution is also reported.
Harilal, S S; Issac, Riju C; Bindhu, C V; Pramod, Gopinath; Nampoori, V P N; Vallabhan, C P G(Elsevier, 1997)
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Abstract:
Time and space resolved studies of emission from CN molecules have been carried out in the plasma produced from
graphite target by 1.06 urn pulses from a Q-switched Nd:YAG laser. Depending on the laser pulse energy, 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 have
been studied as functions of distance, time, laser energy and ambient gas pressure. Evidence for nonlinear effects of
the plasma medium such as self focusing which exhibits threshold-like behaviour are also obtained. Temperature and
electron density of the plasma have been evaluated using the relative line intensities of successive ionization stages of
carbon atom. These electron density measurements are verified by using Stark broadening method.
Riju, C Issac; Pramod, Gopinath; Geetha K, Varier; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, July 13, 1998)
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Abstract:
Laser-induced plasma generated from a silver target under partial vacuum conditions using the
fundamental output of nanosecond duration from a pulsed Nd:yttrium aluminum garnet laser is
studied using a Langmuir probe. The time of flight measurements show a clear twin peak
distribution in the temporal profile of electron emission. The first peak has almost the same duration
as the laser pulse while the second lasts for several microseconds. The prompt electrons are
energetic enough ('60 eV) to ionize the ambient gas molecules or atoms. The use of prompt
electron pulses as sources for electron impact excitation is demonstrated by taking nitrogen, carbon
dioxide, and argon as ambient gases.
Bindhu, C V; Harilal, S S; Achamma, Kurian; Nampoori, V P N; Vallabhan, C P G(World Scientific Publishing Company, 1998)
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Abstract:
Dual beam transient thermal lens studies were carried out in rhodamine 6G methanol solutions using 532 nm pulses from a frequency doubled Nd:YAG laser. Analysis of thermal lens signal shows the existence of different nonlinear processes like two photon absorption and three photon absorption phenomena along with one photon absorption. Concentration of the dye in the solution has been found to influence the occurrence of the different processes in a significant way.
Sreekumar, J; Nandakumaran, V M(World Scientific Publishing Company, 1990)
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Abstract:
Large amplitude local density fluctuations in a thin superfluid He film is considered. It is shown that these large amplitude fluctuations travel and behave like "quasi-solitons" under collision, even when the full nonlinearity arising from the Van der Waals potential is taken into account.
Harikrishnan, K P; Nandakumaran, V M(Elsevier, December 7, 1987)
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Abstract:
We study the period-doubling bifurcations to chaos in a logistic map with a nonlinearly modulated parameter and show that the bifurcation structure is modified significantly. Using the renormalisation method due to Derrida et al. we establish the universal behaviour of the system at the onset of chaos.
V P N Nampoori; Radhakrishnan, P; Girijavallabhan, C P; Nibu, A George; George, A K(Applied Physics B Lasers and Optics, June 18, 2001)
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Abstract:
In this paper, we describe the use of an open cell
photoacoustic configuration for the evaluation of the thermal
effusivity of liquid crystals. The feasibility, precision and reliability
of the method are initially established by measuring
the thermal effusivities of water and glycerol, for which the
effusivity values are known accurately. In order to demonstrate
the use of the present method in the thermal characterization
of liquid crystals, we have measured the thermal
effusivity values in various mesophases of 4-cyano-4 -
octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl
(7OCB) liquid crystals using a variable temperature open
photoacoustic cell. A comparison of the measured values for
the two liquid crystals shows that the thermal effusivities of
7OCB in the nematic and isotropic phases are slightly less
than those of 8OCB in the corresponding phases
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Nibu, A George; George, A K(Applied Physics B Lasers and Optics, July 18, 2001)
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Abstract:
In this paper, we describe the use of an open cell
photoacoustic configuration for the evaluation of the thermal
effusivity of liquid crystals. The feasibility, precision and reliability
of the method are initially established by measuring
the thermal effusivities of water and glycerol, for which the
effusivity values are known accurately. In order to demonstrate
the use of the present method in the thermal characterization
of liquid crystals, we have measured the thermal
effusivity values in various mesophases of 4-cyano-4 -
octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl
(7OCB) liquid crystals using a variable temperature open
photoacoustic cell. A comparison of the measured values for
the two liquid crystals shows that the thermal effusivities of
7OCB in the nematic and isotropic phases are slightly less
than those of 8OCB in the corresponding phases
Radhakrishnan, P; Nampoori, V P N; Girijavallabhan, C P; Nibu, A George; George, A K(Applied Physics B Lasers and Optics, July 18, 2001)
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Abstract:
In this paper, we describe the use of an open cell
photoacoustic configuration for the evaluation of the thermal
effusivity of liquid crystals. The feasibility, precision and reliability
of the method are initially established by measuring
the thermal effusivities of water and glycerol, for which the
effusivity values are known accurately. In order to demonstrate
the use of the present method in the thermal characterization
of liquid crystals, we have measured the thermal
effusivity values in various mesophases of 4-cyano-4 -
octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl
(7OCB) liquid crystals using a variable temperature open
photoacoustic cell. A comparison of the measured values for
the two liquid crystals shows that the thermal effusivities of
7OCB in the nematic and isotropic phases are slightly less
than those of 8OCB in the corresponding phases
Nampoori, V P N; Vallabhan, C P G; George, A K; Radhakrishnan, P; Rajasree, K; Vidyalal, V(Elsevier, July , 1998)
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Abstract:
he phenomenon of single beam mirage effect, otherwise known as photothermal deflection (PTD) effect using a He–Ne laser beam has been employed to detect phase transitions in some liquid crystals. It has been observed that anomalous changes in amplitude occur in the PTD signal level near the transition temperature. The experimental details and the results of measurements made in liquid crystals E8, M21 and M24 are given in this paper.