Sucharita Sinha

Solvent dependent nonlinear refraction in organic dye solution

Nonlinear refraction in rhodamine-B (Rh-B) organic dye solution at 510.6 nm was studied using the Z-scan technique with 40 ns laser pulses from a copper vapor laser. The observed nonlinear refraction is of thermal origin resulting from nonradiative energy transfer from the dye molecules to the solvent molecules and is therefore strongly dependent on the thermal properties of the solvent. Using this scheme, the thermal properties of an aqueous dye solution were compared, with that using water-surfactant mixture as the solvent.

3D Hetero-architecture of GdB6 nanoparticles on lessened cubic Cu2O nanowires: enhanced field emission behaviour

The field emission properties (FE) of a heteroarchitecture comprised of gadolinium hexaboride nanoparticles uniformly decorated over Cu2O nanoneedles (GdB6/Cu2O) have been investigated at the base pressure of ~1 × 10−8 mbar. Under the optimized pulsed laser deposition (PLD), a well-adhered coating of GdB6 nanoparticles on chemically synthesized cuprous oxide (Cu2O) nanoneedles has been obtained. A plausible growth mechanism of the hierarchical assembly of GdB6 nanoparticles on the Cu2O nanoneedles is explained on the basis of structural analysis carried out using SEM and TEM. A low turn-on field of ~2.3 V μm−1 (to draw an emission current density ~1 μA cm−2) is observed along with stable emission current at the preset value of ~2 μA over 4 h. The enhanced emission behaviour of the GdB6/Cu2O heteroarchitecture, in contrast to the pristine Cu2O nanoneedles, is attributed to its high aspect ratio and low work function. The observed FE results demonstrate GdB6/Cu2O heteroarchitecture as a potential candidate for application in various vacuum nano/microelectronic devices.

Spectral characteristics of a binary dye-mixture laser

We have investigated laser action in a binary mixture of dyes, Rh-6G and DCM, resulting in tunable laser emission over an extended frequency region. The two dyes absorb the same pump radiation but fluoresce over frequency ranges that are shifted with respect to each other, thereby resulting in extended tunability. Following a time-dependent analysis of a rate-equation model that describe the operation of such a laser, theoretical estimates for optimum dye concentrations and the corresponding extension of the laser tuning range have been obtained.

Laser emission from self-assembled active photonic crystal matrix

Three-dimensionally ordered photonic crystals were grown using self-assembly technique from Rhodamine-B dye doped polystyrene micro-spheres resulting in a stop band at 611 nm overlapping the emission spectrum of the dye. When excited at a wavelength away from the stop band, using a frequency-doubled Nd:YAG laser, the crystal showed angle- dependent suppression of spontaneous emission of the dye in the wavelength range of the photonic stop band and enhancement at the band edge, in reflection and transmission geometries. Spectral narrowing, a sharp threshold and a highly directional emission, all indicative of stimulated emission, were observed from the active photonic crystal matrix.

Analysis of Lasing in Dye-Doped Photonic Crystals

Recently, we experimentally demonstrated room-temperature lasing of self-assembled opal photonic crystal (PhC) made of rhodamine-B-doped polystyrene colloids. Here, we explain this experimental observations by analyzing the phenomenon of light amplification in dye-activated PhCs via a complex-valued permittivity of the colloids. We show that the lasing is facilitated by the enhanced distributed feedback due to the reduced group velocity in the vicinity of the photonic band edge. This simple approach to the analysis of PhC lasing behavior allows us to calculate the lasing wavelength in close agreement with the experimental value. It also enables the estimation of gain coefficient required for lasing and may prove useful in design of compact PhC-based lasers.

Role of the stimulated-emission rate in the photostability of solid-state dye lasers

The lasing and photostability characteristics of Rhodamine 6G and Pyrromethene 567 dyes dispersed in polymeric host materials have been investigated as a function of the intensities of incident pump and signal beams in a longitudinally pumped dye laser in an oscillator-amplifier configuration. A substantial reduction in the rate of photodegradation was observed under lasing conditions and with increasing signal intensity in a dye amplifier, establishing that the service lives of these materials improve with an increase in the rate of stimulated emission. We observed approximately 62% amplifier efficiency at 2 Hz operation and 10% reduction in amplifier efficiency at 10 Hz operation after exposure of 72,000 pulses by use of a Pyrromethene disk.

High-repetition-rate, narrow-band dye lasers with water as a solvent for dyes.

The performance of a copper vapor laser-pumped narrow-band dye laser in oscillator-amplifier configuration with water-based binary mixture solvents is described. Although oscillator efficiency in water-surfactant (sodium lauryl sulfate) solvent was comparable with that that employed pure ethanolic solvent, amplifier efficiency was found to be lower. Experiments that were carried out with vertically polarized pump beams and either horizontally or vertically polarized signal beams show that, in case of both the pump and signal having orthogonal polarization (horizontal) and same polarization (vertical), the extraction efficiency for both ethanolic and water-micelle media increased substantially from 15.7% to 18.5% and from 10% to 12.5%, respectively. However, the relative difference remained nearly the same, indicating that a slower orientational diffusion of excited dye molecules in a micellar medium is not responsible for a decrease in amplifier efficiency. Amplifier efficiency comparable with that containing ethanolic dye solutions could be obtained with a binary solvent that comprises a mixture of water and about 30% n-propanol. The performances of two efficient dyes, Rhodamine-6G and Kiton Red S, using water-based solvents were studied.

Longitudinal mode selection in hybrid CO2 laser for subthreshold operation of cw discharge section

The relative mode intensities in the hybrid CO2 laser output are investigated for subthreshold operation of cw gain section. The rate equation model developed for the purpose provides estimates in very satisfactory agreement with experimental results. The model also predicts the existence of an optimum pressure and length for the cw discharge section and shows that very good longitudinal mode discrimination is possible even when cw discharge section gain is only a tenth of that required for oscillation.

Design and the operational characteristics of a 16 µ CF4 laser

The design and the operational characteristics of a CO2 laser pumped CF4 laser developed at BARC are reported. Output energies of up to 20 mJ have been obtained at 615 cm−1 with an absorbed energy conversion efficiency of 10%.

Pulsed laser deposition of lanthanum phosphate protective films

Pulsed laser deposition (PLD) technique has been successfully employed to generate thin film coating of lanthanum phosphate on various substrates. Sintered targets of lanthanum phosphate were ablated using pulsed Nd:YAG laser radiation. Although, coatings generated by PLD on polished stainless steel substrates, as well as on optically polished quartz samples were found to be uniform and having good homogeneity, the coatings had poor adhesion characteristics with the substrate surface. However, PLD based coating on surface treated stainless steel substrates (either sandblasted or having a pre-coating of NiCrAlY alloy) showed strong bonding with the substrate surface. Structural investigation of the deposited films under scanning electron microscope revealed high level of homogeneity and absence of porosity. PLD based thin film coatings of lanthanum phosphate were observed to be superior in terms of their microstructural characteristics in comparison to plasma based coatings that had a distinctly higher lev...