Pramod Gopinath

Synthesis of reduced graphene oxide–ZnO hybrid with enhanced optical limiting properties

Reduced graphene oxide–ZnO (rGO–ZnO) is synthesized by in situ growth of ZnO on the surface of graphene oxide (GO) by facile solution methods; namely, a solution precipitation method and a hydrothermal method. The characterization shows that GO is reduced and exfoliated to graphene sheets and ZnO is partially covered by graphene layers. A formation mechanism of rGO–ZnO hybrid proposed is well suited to the experimental results. Structural analysis indicates that the graphene possesses a greater extent of π-conjugation in the hydrothermally prepared rGO–ZnO hybrid. These hybrids exhibit an intense and broad absorption in the visible region, in addition to the sharp UV absorption peak. These rGO–ZnO hybrids also show fluorescence quenching properties. The nonlinear absorption coefficient and optical limiting properties of rGO–ZnO hybrid is evaluated using an open aperture Z-scan technique. More interestingly, we found that there is a 15 times enhancement in the nonlinear absorption coefficient of the rGO–ZnO hybrid compared to the bare ZnO. The hybrid shows excellent optical limiting properties compared to its individual counterparts, implying the usability of these hybrids in optoelectronics.

Time resolved study of CN band emission from plasma generated by laser irradiation of graphite

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. 0 1997 Elsevier Science B.V.

Optical-limiting response of rare-earth metallo-phthalocyanine-doped copolymer matrix

The nanosecond optical-limiting characteristics (at 532 nm) of some rare-earth metallo-phthalocyanines (Sm(Pc)2,Eu(Pc)2, and LaPc) doped in a copolymer matrix of poly(methyl methacrylate) and methyl-2-cyanoacrylate have been studied for the first time to our knowledge. The optical-limiting response is attributed to reverse saturable absorption due to excited-state absorption. The performance of LaPc in a copolymer host is studied at different linear transmissions. The laser damage thresholds of all the samples are also reported.

Optical limiting studies of ZnO nanotops and its polymer nanocomposite films

In the present work, we have investigated the optical limiting property of zinc oxide (ZnO) nanotop colloids and nanocomposite films of polymer-ZnO nanotops, using Z-scan technique. ZnO nanotops are prepared using two different capping agents. Films of polymethyl methacrylate-ZnO nanotop composite are fabricated for two concentrations of ZnO using spin coating technique. On exposing the nanocolloids to a pulsed nanosecond laser, the two photon absorption coefficient is enhanced with increase in applied fluence and particle size. The efficiency of limiting is found to be enhanced for the composite films, opening a way towards optoelectronic device fabrication.

Twin peak distribution of electron emission profile and impact ionization of ambient molecules during laser ablation of silver target

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.

Thermal lens spectrum of organic dyes using optical parametric oscillator

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.

Influence of magnetic field on laser-produced barium plasmas: Spectral and dynamic behaviour of neutral and ionic species

The expansion dynamics and spectral behaviour of plasma produced by a Nd:YAG laser (λ = 1.064 μm, pulse width: 8 ns) from barium target and expanding in 0.45 T transverse magnetic field in vacuum (10−5 Torr pressure) are investigated using time-of-flight optical emission spectroscopy. The experiments are carried out at various laser fluences from 12 to 31 J/cm2. The temporal profiles of neutral (Ba I 553.5 and 577.7 nm) lines are temporally broadened, while that of ionic (Ba II 413.0 and 455.4 nm) lines show strong confinement in the presence of a magnetic field. In the absence of magnetic field, the temporal profile of Ba I 553.5 nm is exactly reproduced by fitting with two Shifted Maxwell Boltzmann (SMB) Distribution components, while in the presence of a magnetic field the profile could only be fitted with three components. The field enhanced and field induced SMB components of neutral profile are correlated with populations of ground state, metastable states, and long-lived Rydberg states present in t...

NONLINEAR ABSORPTION AND OPTICAL LIMITING IN SOLUTIONS OF SOME RARE EARTH SUBSTITUTED PHTHALOCYANINES

Effective nonlinear absorption coefficient, βeff, of solutions of some rare earth substituted phthalocyanines (viz: Sm(Pc)2, Eu(Pc)2 and LaPc) was measured using open aperture Z-scan technique under nanosecond pulse excitation at 532 nm. The effect of nonlinear absorption on optical limiting in these samples was also investigated. Both nonlinear absorption and optical limiting phenomena in these samples are attributed to sequential two-photon absorption (STPA) process.

Phenylenediamine functionalized reduced graphene oxide/polyaniline hybrid: synthesis, characterization, improved conductivity and photocurrent generation

Phenylenediamine functionalized reduced graphene oxide (GONH2) is designed and synthesized for the preparation of a polyaniline/reduced graphene oxide hybrid having better interactions than that of the unfunctionalized graphite oxide/polyaniline hybrid. Hybrids are synthesized by the in situ polymerization of aniline in the presence of GONH2. Defects generated in GONH2 during functionalization are repaired during hybrid formation. Due to the presence of free amino groups in GONH2, it will participate in the polymerization of aniline and covalent bonds will be formed between polyaniline and GONH2, resulting in the increased room temperature conductivity and photocurrent generation compared to pure polyaniline and polyaniline/graphite oxide hybrid synthesized in the same experimental conditions. Significant reduction and exfoliation of graphite oxide after functionalization with phenylenediamine, formation of direct conducting pathways by covalent connections and the photoinduced electron transfer between polyaniline and GONH2 plays a major role for the improved electrical conductivity and photocurrent generation.

STUDY OF ENERGY TRANSFER IN ORGANIC DYE PAIRS USING THERMAL LENS TECHNIQUE

It is demonstrated that the dual beam thermal lens technique can be very effectively used for the study of energy transfer processes in organic dye mixtures. The energy transfer rate and critical transfer radius in Rhodamine 6G — Rhodamine B are calculated using this technique. The observed results support Forster type resonance transfer due to long-range dipole–dipole interaction as reported by earlier workers using other techniques.