Bhanu P. Singh

Effects of Metal Substitution on Third-order Optical Non-linearity of Porphyrin Macrocycle

The effects of axial ligands in Sn(IV)-substituted 5,10,15,20-tetrakis [4-(carboethoxymethyleneoxy)phenyl]porphyrin and divalent transition metal substitution in 5,10,15,20-tetraphenylporphyrin on the second molecular hyperpolarizability (γ) at 802 nm were studied using the Z-scan technique. A significant increase in γ for divalent metal ions is observed with decreasing d-shell occupancy. In the case of the tetravalent metal-substituted porphyrin a dramatic enhancement of γ is seen when a strongly electronegative axial ligand such as I2 is attached to the metal ion. A plausible explanation for the observed trend of γ has been sought in terms of metal-ligand interaction.

Optical phase conjugation in Rhodamine-6G doped boric acid glass

Abstract Optical phase conjugation has been demonstrated in Rhodamine-6G doped boric acid glass using a cw argon ion laser. The dependence of the phase conjugated signal on the intensity and wavelength of the pump beam is studied The role of amplitude and phase grating contributions to the phase conjugated signal is discussed.

Polarization-based optical logic using laser-excited gratings.

We present a novel way of performing optical logic. The proposed logic device is based on the polarization switching observed in the first-order diffraction from a laser-excited grating in a dye-doped solid.

Origin of methyl torsional barrier in 1-methyl-2-(1H)-pyridone.

The laser induced fluorescence excitation and single vibronic excitation dispersed fluorescence spectra have been studied for supersonic jet cooled 1-methyl-2(1h)-pyridone. The methyl torsional bands and some low frequency vibrational transitions were assigned for both ground and excited states. The torsional parameters V(3)=244 cm(-1) and V(6)=15 cm(-1) for the ground state and V(3)=164 cm(-1) and V(6)=40 cm(-1) for the excited state were obtained. To get the insight into the methyl torsional barrier, ab initio calculations were performed and compared with the experimental results. Origin of potential barrier was traced by partitioning the barrier energy into changes in bond-antibond interaction, structural, and steric energies accompanying methyl rotation using natural bond orbital analysis. The role of local interactions in ascertaining the barrier potential reveals that its nature cannot be understood without considering the molecular flexing. The hyperconjugation between CHsigma(*) and ring pi(*) observed in lowest unoccupied molecular orbital (LUMO) stabilizes the methyl group conformer that undergoes a 60 degrees rotation in the excited state with respect to that of the ground state, and it is the change in LUMO that plays important role in the excited state barrier formation.

Origin of methyl torsional potential barrier — An overview

This paper presents the evolution of views on methyl internal rotation potential barrier. Various mechanisms proposed for the origin of torsional barrier in ethane have been reviewed. Inadequacy of one dimensional description of internal rotation has been highlighted in small methyl conjugated molecules in the light of its multidimensional nature. The effect of skeletal flexing on the picture of barrier formation by dissecting the barrier energy into potential type, virial type and symmetry type is described. The role of π and σ electrons at different stages of molecular flexing is discussed. The analysis identifies the dominant contributions to barrier origin as π-bonding changes during rigid rotation and σ-bonding changes resulting from bond lengthening during methyl group rotation. The contribution of lone pair electrons in determining the preferred structure of the methyl group in imine compounds such as 1-methyl 2-(1H)-pyridinimine is presented.

Strictly monolayer large continuous MoS2 films on diverse substrates and their luminescence properties

Despite a tremendous interest on molybdenum disulfide as a thinnest direct band gap semiconductor, single step synthesis of a large area purely monolayer MoS2 film has not yet been reported. Here, we report a CVD route to synthesize a continuous film of strictly monolayer MoS2 covering an area as large as a few cm2 on a variety of different substrates without using any seeding material or any elaborate pretreatment of the substrate. This is achieved by allowing the growth to take place in the naturally formed gap between a piece of SiO2 coated Si wafer and the substrate, when the latter is placed on top of the former inside a CVD reactor. We propose a qualitative model to explain why the MoS2 films are always strictly monolayer in this method. The photoluminescence study of these monolayers shows the characteristic excitonic and trionic features associated with monolayer MoS2. In addition, a broad defect related luminescence band appears at ∼1.7 eV. As temperature decreases, the intensity of this broad fe...

Fast and reversible excited state absorption in II-VI-based nanocomposite thin films

Nanocomposite CdS-ZnO thin films deposited directly on quartz substrates by high-pressure magnetron sputtering show a completely reversible photodarkening at a very low threshold intensity (∼1kWcm−2), and a moderately fast recovery time (<1ms). This makes them ideal in optical limiting applications for both continuous wave as well as high rep-rate pulsed lasers. The same system also shows an intensity-dependent quenching of the photoluminescence. Using a pump-probe experiment, we show that photodarkening in such a quantum-dot thin film originates from excited state absorption.

Green synthesis of gold nanoparticles using extracts of Artocarpus Lakoocha fruit and its leaves, and Eriobotrya Japonica leaves

Gold nanoparticles (AuNPs) synthesis is demonstrated successfully using fresh young leaves of Artocarpus Lakoocha (A. Lakoocha), fruit pulp of A. Lakoocha and loquat (Eriobotrya Japonica) leaves. We have also compared green synthesis with chemical assisted tri-n-octyl-phosphine (TOP) stabilized gold nanoparticles. Samples were characterized with transmission electron microscopy (TEM), Fourier transform infrared spectroscopy and UV-Visible spectroscopy. TEM images have shown that the average size of the particles is 15.06, 36.8 and 25.08 nm for A. Lakoocha fruits, A. Lakoocha leaves and loquat leaves assisted gold nanoparticles, respectively. Hydrogen tetrachloroaurate is reduced and AuNPs are stabilized by phenols, hydroxyls and carboxyls groups such as terpenoids, flavonoids, tannins etc, present in young leaves and fruit extracts. It was observed that green synthesis using botanical extracts is a cost effective and non- toxic way for nanoparticle preparation.

Origin of methyl torsional barrier in 1-methyl-2(1H)-pyridinimine and 3-methyl-2(1H)-pyridone: II. Ground state

To get the insight into the electronic structure-methyl torsion correlation in nitrogen heterocyclic molecules, a comparative study on torsion of the methyl group in 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) was carried out using ab initio calculations. To understand the barrier forming mechanism in the ground state and its consequence on the molecular structure, the ground state torsional potential has been investigated by partitioning the barrier energy using the natural bond orbital (NBO) theoretical framework. The NBO analysis reveals that the delocalization energy is the barrier forming term whereas the Lewis energy is always antibarrier for all these molecules. To get further insight into the effect of local electronic structure on the methyl torsional barrier, the individual bond-antibond interactions and structural energy contributions have been investigated. It was found that when the bond order difference between the vicinal bonds does not change appreciably during the course of methyl rotation, the local electronic interactions with the methyl group do not play any decisive role in barrier formation as observed in the case of 1MPY and 1MPI. In these cases, it is the skeletal relaxation during methyl rotation that plays an important role in determining the barrier. On the other hand, if the bond order change is appreciable as is the case for 3MPY, the local interactions alone suffice to describe the origin of the torsional barrier of the methyl group.

Morphology and optical study of dye-doped TiO2-SiO2 thin films

In the present work, we have prepared functional dye doped TiO2-SiO2 thin films by vertical sedimentation technique. Thin film samples are annealed at different temperature from 50oC to 850oC. Morphology and chemical bonding information is analysed using atomic force microscopy (AFM), and Fourier transform infrared spectroscopy (FTIR) respectively. Optical properties are characterized by using UV-visible spectroscopy. Effect of annealing temperature on the photonic forbidden band gap is also presented. The experimental measured values are compared with theoretical estimated results.