Pratima Sen

Synthesis and characterization of a novel polyazomethine ether for NLO application

Polymer containing azomethine and ether linking groups between phenylene rings has been prepared. It is a polycondensation reaction of a single monomer containing nitro as well as hydroxyl group in p,p′ position from 4-nitroaniline and 4-hydroxy benzaldehyde. The polymer has been characterized by elemental analysis, IR, UV and NMR spectroscopies, viscometry and thermal studies. The thermotropic liquid crystalline behaviour of the polymer is investigated by optical polarizing microscopy. The crystalline behaviour of the polymer is confirmed by XRD study. The polymer shows good second harmonic generation for non-linear optical application.

Effect of shell thickness on exciton and biexciton binding energies in ZnSe/ZnS core/shell quantum dot

The exciton and biexciton binding energies are calculated for a ZnSe/ZnS core/shell quantum dot incorporating WKB approximation. With the variations in shell thickness, the exciton binding energy shows nonlinear behavior. A maximum value of exciton binding energy is expected to occur for shell thickness equals to the core radius. Similar results are reported for biexcitons where for thicker shells the biexciton attains antibonding. Corresponding to an increase in binding energy, a red shift in the PL spectra is expected which is compatible with the experimental observations.

Semiconductor core-shell quantum dot: A low temperature nano-sensor material

This paper presents an analytical study of temperature dependent photoluminescence (PL) in core-shell quantum dots (CSQDs) made of most frequently used II-VI semiconducting materials. The analysis incorporates the temperature dependent radiative recombination processes in the calculation of the integrated PL intensity. The PL intensity has been derived using semiclassical density matrix formalism for the CSQDs exhibiting excitonic and biexcitonic features. The numerical estimates show that the PL intensity response and PL peak shifts are non-trivial at low temperature in such CSQDs and can be useful in the design of a temperature sensor.

Phase conjugation in magnetized semiconductors by stimulated Brillouin scattering

Based upon the density matrix formulations of nonlinear optical susceptibility, optical phase conjugation via stimulated Brillouin scattering (OPC-SPS) in the presence of a moderately high magnetostatic field is studied in centrosymmetric semiconducting crystals under a parabolic approximation. The magnetostatic field B has a twofold contribution to the coherent radiation - semiconductor interaction through the formation of discrete Landau levels and the sharpening of the density-of-states. Numerical estimations made for an InSb sample at low temperature irradiated by a 5.5 to 6.0 m CO laser reveal enhancement in the value of Brillouin susceptibility in the presence of the magnetostatic field B. This in turn lowers the threshold intensity required for the onset of OPC-SBS. A considerably high reflectivity is obtained when B is increased from 1 to 4.02 T. For experimental observation of OPC-SBS as described in the paper, it may be stressed that the polarization of the laser should be properly determined so that the crystal is effectively centrosymmetric.

Steady state optical gain in small semiconductor quantum dots

The optical gain phenomena in a single semiconductor quantum dot as well as in an inhomogeneous distribution of quantum dots have been studied. The analytical results obtained using the density matrix technique and numerical analysis for CdS quantum dots show an enhancement of gain in the presence of biexcitons. The numerical results show larger gain in bigger quantum dots. A redshift of gain maxima occurs when the distribution width is large. The realization of high gain quantum dot lasers and photodetectors appears to be a distinct possibility.

Type I and type II band alignments in ZnO/MgZnO bilayer films

We report the change in the type of band alignments due to an increase in the dopant (Mg) concentration in pulsed laser deposited ZnO/MgZnO bilayer film. The band offset measurements were carried out from the core level shifts as well as valence band maxima in the single as well as the bilayer films. The change in the type of band alignment is attributed to the surface enrichment of Mg at the heterojunction.

Transport and magnetotransport study of Mg doped ZnO thin films

We report negative magnetoresistance in pulsed laser deposited single phase ZnO and Mg0.268Zn0.732O films and attribute it to the presence of oxygen interstitials (Oi) and zinc interstitials (Zni) as observed in the X-ray photoelectron spectra of the films. An interesting feature of reduction of negative magnetoresistance at low temperatures and large fields in Mg0.268Zn0.732O film is observed and is explained by taking into account the localized scattering.

Second harmonic generation in ZnO nanorods

A comparative study of the second harmonic generation in ZnO micro particles and nanorods has been experimentally studied by Kurtz technique using nanosecond pulsed Nd:YAG laser. The results have been theoretically explained by taking into account the quadrupole moment as well as surface nonlinearity. It was observed that the nanorods yield polarized second harmonic signal while the second harmonic signal obtained from the micro particles was unpolarized.

Optical coherent transient effects in magnetoactive GaAs/AlGaAs quantum well structure

The effect of an external magnetic field on optical coherent transient phenomena has been analyzed for semiconductor quantum well structures (QWS). The Luttinger Hamiltonian is used to incorporate the complex valence band structure which is modified due to the application of the magnetic field. We have considered a GaAs/AlGaAs QWS with magnetic field applied in the direction perpendicular to the plane of the well. We get two quantum levels due to confinement, and doubly degenerate heavy-hole and light-hole bands corresponding to each quantum level, thus leading to eight hole subbands. This results in an 8×8 Hamiltonian in an appropriately chosen basis. Energy eigenvalues and eigenvectors have been calculated by numerically diagonalizing the above Hamiltonian. The absorption characteristics and the optical coherent transient phenomena have been studied by solving the effective semiconductor Bloch equations in the weak excitation regime. The polarization induced in conjunction with the results obtained by s...

Nonlinear absorption in semiconductor quantum dots

The density matrix approach has been employed to understand the mechanism of nonlinear absorption / gain in a single semiconductor quantum dot as well as in an inhomogeneous distribution of quantum dots. The analytical results show an enhancement of excitonic gain in the presence of biexcitons. The same has been confirmed by the numerical analysis carried out for a realistic system of semiconductor quantum dots of CdS embedded in a glass matrix. The numerical study exhibits a red shift of maximum gain at large distribution widths.