Sheenu Thomas

Preparation and optical characterization of novel Ge-Se-Sb/PVA composite films for optical limiting application

We report on the nonlinear optical characterization of a new amorphous composite based on Ge28Se60Sb12/PVA. Nonlinear optical characterization of these films was studied by the Z-scan technique using an Nd:YAG laser (532nm, 7ns, 10Hz).The studies show that the material is highly nonlinear. The films show reverse saturable absorption, which makes it useful for optical limiting applications. (Some figures in this article are in colour only in the electronic version)

Nonlinear optical studies on nanocolloidal Ga–Sb–Ge–Se chalcogenide glass

In this paper we report the preparation and the optical studies on nanocolloidal solutions of Ga5Sb10Ge25Se60 for the first time. The optical band gap of the material is found to be tunable depending on the cluster size. The cluster formation and the dependence of the cluster size with concentration were confirmed by the scanning electron microscope analysis. Nonlinear optical characterization of these solutions were studied by the Z-scan technique using an Nd:YAG laser (532 nm, 7 ns, 10 Hz).The studies show that the material is highly nonlinear. The solutions show reverse saturable absorption, which makes it useful for optical limiting applications.

Effect of betanin natural dye extracted from red beet root on the non linear optical properties ZnO nanoplates embedded in polymeric matrices

In this article, we have investigated the effect of betanin natural dye extracted from red beetroot on nonlinear optical properties of ZnO nanoplates embedded in polymeric matrices through the Z-scan technique using an Nd: YAG laser (532 nm, 7 ns, 10 Hz). We observed reverse saturable absorption (RSA) at 532 nm for dye and ZnO nanoplates. A strong influence on RSA behavior of nanoplates-PVA matrix was observed by adding betanin natural dye. The influence of betanin on the nonlinear character of ZnO-PVA system leads to saturable absorption and again to RSA on increasing input fluence. Such a change over in the sign of the nonlinearity is due to the interplay of the exciton bleaching and optical limiting mechanisms, and probably due the presence of sucrose. Theoretical analysis has been performed using a model based on nonlinear absorption coefficient and saturation intensity. The result of present study gives an additional mechanism for the gain enhancement in dye doped ZnO matrix.

Thermal diffusivity of solids by photoacoustic cell rotation and phase lag measurement

A single‐beam photoacoustic phase lag measurement is demonstrated for determining the thermal diffusivity α of solid samples. The method involves measurement of the phase difference of the photoacoustic signal between front and rear surface illuminations at a single chopping frequency with the same light beam. Results on representative samples, with widely different thermal properties, show very good agreement with literature values. Temperature variation of α can also be measured with the setup.

Impact of intermediate localized states on nonlinear optical absorption of Ga-Ge-Se nanocolloidal solutions

We present the linear and nonlinear optical studies on nanocolloidal solutions of Ga9Ge27Se64 glass with varying concentrations. Optical bandgap of the material is found to vary with respect to the concentration of the solute in the solution. An intermediate peak in the band tail of the absorption spectra is observed due to the presence of energy band in the forbidden gap. The existence of fluorescence emission confirms the above argument. Nonlinear absorption is studied using open aperture Z-scan technique. The mechanism behind nonlinear absorption is predicted as two photon as well as two step photon absorption. Nonlinearity increases with decrease in optical bandgap which in turn depends on the concentration of the nanocolloidal solutions.

OPTICAL LIMITING IN TeO2–ZnO GLASS FROM Z-SCAN TECHNIQUE

TeO2–ZnO glasses were prepared by conventional melting procedure, and their optical properties were determined. In the present work, we measured the optical non-linearity of the glass. The measurement was carried out using a single beam z-scan technique given by Sheike Bahe et al. We could observe that the material exhibits reverse saturable absorption, and the effective nonlinear absorption coefficients are numerically evaluated. This study is important in identifying the nonlinear material as a Reverse Saturable Absorption (RSA) based optical limiter.

Investigation of chemical and mechanical extrema in Ge-In-Se glasses using a photoacoustic technique

Twelve glass compositions, belonging to the GexIn5Se95-x family of semiconducting chalcogenides are prepared from high-purity constituent elements by a melt quenching technique. The variation of the optical band gap (Eg) and thermal diffusivity (α) with average coordination number (Z) are investigated using a photoacoustic (PA) technique. The Eg values obtained from PA measurements have been compared with the values obtained from UV-Vis-NIR absorption spectra. Extrema are seen in the α against Z curves at Z values of 2.41 and 2.67, which correspond to the topological thresholds, and a maximum in the Eg against Z curve is found at Z = 2.73, corresponding to the chemical threshold of this system of glasses. These results are interpreted in terms of the Phillips-Thorpe model of rigidity percolation based on constraints theory, Tanakas model of dimensional transition and the chemically ordered covalent network model for tetrahedrally-bonded glasses.

Concentration tuned bandgap and corresponding nonlinear refractive index dispersion in Ga-Ge-Se nanocolloids

Solution-processing of chalcogenide glass materials has attracted attention for the fabrication of photonic devices. We have carried out linear and nonlinear optical studies on Ga9Ge27Se64 nanocolloidal solutions with varying concentrations and witnessed the opportunity for engineering the optical properties of the material by simply changing the concentration of solute in the solvent. An enhancement of bandgap, deterioration, and blue shift in the band tail states with decrease in concentration were observed from the absorption spectra. Open and closed aperture Z-scan studies reveal tunability of nonlinear absorption coefficient (β) and nonlinear refractive index (n2). As the bandgap approaches the one photon absorption edge, the material shows very large negative nonlinear refraction and also “two step photon absorption” due to the presence of intermediate tail states. The radial phase shift (Δφ0) induced after travelling through the nanocolloidal solution with respect to bandgap is plotted and we predi...

Experimental verification of localized defect states in Ga-Ge-Se nano colloidal solutions

Linear optical studies on Ga9Ge27Se64 nanocolloidal solutions were carried out. Evidence for the existence of defect states in these nanoclusters are obtained from absorption spectra, and its occurrence is emphasized by analyzing the fluorescence and thermal diffusivity studies. An intermediate peak in the band tail of the absorption spectrum is observed at high concentration, and it is interpreted as the absorption due to localized defect states. Fluorescence emission corresponding to this peak confirms the same. The thermal diffusivity studies show that nanocolloidal solutions with less solute concentration have high thermal diffusivity. This indicates the existence of high defect states density which increases with the increase in cluster size, leads to scattering of phonons from the defect centers and in turn results in lowering of thermal diffusivity.

Two photon absorption in TeO2-ZnO glass at different laser irradiances

TeO2-ZnO glasses were prepared and their optical properties were determined. In the present work we measured the optical non-linearity of the glass using a single beam z-scan technique given by Sheike Bahe et al. The open aperture z-scan curve of TeO2-ZnO proves that nonlinear optical properties are highly fluence dependent.