S. Divya

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.

Electronic and optical properties of TiO2 and its polymorphs by Z-scan method

TiO2 is a material which has attracted considerable attention from the scientific community for its innumerable properties. TiO2 is known to exist in nature in three different crystalline structures: rutile, anatase, and brookite. Anatase and rutile TiO2 films have been widely characterized for their potential applications in solar cells, self-cleaning coatings, and photocatalysis. In the present report, the third-order nonlinear susceptibilities of TiO2 and its polymorphs, anatase, and rutile, prepared by the sol—gel technique followed by heat treatment are investigated using the Z-scan technique at a wavelength of 532 nm with a duration of 7 ns. Imaginary and real values of χ(3) for amorphous, anatase, and rutile are also calculated and found to be 5 × 10−19 m2/V2, 27 × 10−19 m2/V2, 19 × 10−19 m2/V2, respectively. It is found that the values of the optical constants of amorphous TiO2 after heat treatment vary considerably. It is assumed that this could be due to the variation in the electronic structure of TiO2 synchronous with the formation of its polymorphs, anatase, and rutile. Amorphous TiO2 is marked by the localization of the tail states near the band gap, whereas its crystalline counterparts are characterized by completely delocalized tail states.

Electrolyte/photoanode engineered performance of TiO2 based dye sensitised solar cells

The performance of dye sensitized solar cells (DSSCs) depends on the collective contribution from its constituents which include the nanoparticle film, dye, electrolyte, and the counter electrode. In this report, we have tried to elucidate the varying performance of the TiO2 based DSSCs standardised using N719 dye and Platinum as counter electrode with various electrolytes including quasi static electrolytes. We have also evaluated the photovoltaic characteristics of the cells employing different morphological structured TiO2 photoanode. The DSSC based on the hierarchical anatase TiO2 nanotree photoelectrode showed the highest light-to-electricity conversion efficiency of 10.2%.

Tailoring optical properties of TiO2 in silica glass for limiting applications

We present the linear and nonlinear optical studies on TiO2−SiO2 nanocomposites with varying compositions. Optical band gap of the material is found to vary with the amount of SiO2 in the composite. The phenomenon of two-photon absorption (TPA) in TiO2/SiO2 nanocomposites has been studied using open aperture Z-scan technique. The nanocomposites show better nonlinear optical properties than pure TiO2, which can be attributed to the surface states and weak dielectric confinement of TiO2 nanoparticles by SiO2 matrix. The nanocomposites are thermally treated and similar studies are performed. The anatase form of TiO2 in the nanocomposites shows superior properties relative to the amorphous and rutile counterpart. The involved mechanism is explained by rendering the dominant role played by the excitons in the TiO2 nanoparticles.

Intensity and composition-dependent sign reversal of non-linearity in TiO2/CeO2 nanocomposites

CeO2/TiO2 composite nanoparticles with different Ce/Ti molar ratios have been successfully synthesized via sol—gel method. It was found that the band gap of the nanocomposite is tunable by varying Ce/Ti content. The nonlinear response of the sample was studied by using the nanosecond laser pulses from a Q switched Nd:Yag laser employing the Z-scan method. Open aperture Z-scan experiment revealed that with the increase in the CeO2 amount in the nanocomposite, the non-linearity of the composite increases, and it was assumed that this could be due to the modification of TiO2 dipole symmetry by the addition of CeO2. Closed aperture Z-scan experiment showed that when the CeO2 amount increases, positive nonlinear refraction decreases, and this could be attributed to the increase in the two photon absorption which subsequently suppresses the nonlinear refraction.

Investigation on nonlinear properties of Ga-Ge-Se nanocolloidal solutions

We present the preparation and nonlinear optical characterization of nanocolloidal solutions of Ge27Ga9Se64 glass with different concentrations. Their nonlinear absorption is studied using open aperture Z-scan technique. Optical band gap of the material is be tuned by changing the concentration of the solute in the solution. Nonlinearity increases with decrease in optical band gap which in turn depends on the concentrations of the nanocolloidal solutions.

Power and composition dependent non linear optical switching of TiO 2 -SiO 2 nano composites

Linear as well as nonlinear absorption of TiO2-SiO2 nanocomposites prepared by sol gel process has been reported. To investigate the nonlinear properties, Z Scan technique was employed with 532 nm as the excitation wavelength followed by the calculation of the nonlinear optical constants. Z-scan experiments showed that when the excitation energy increased, the nonlinear absorption of the nanocomposites changed from reverse-saturable absorption to saturable absorption and then again back to reverse saturable absorption. Such a changeover in the sign of the nonlinearity is related to the interplay of exciton bleach and optical limiting mechanisms. The observed nonlinear absorption is found to be due to two photon absorption. The enhancement of the third-order nonlinearity in the composites can be due to the concentration of exciton oscillator strength.