Arup Neogi

Refractive Index Change Due to Volume-Phase Transition in Polyacrylamide Gel Nanospheres for Optoelectronics and Bio-photonics

Poly(N-isopropylacrylamide) (PNIPAM) hydrogel nanospheres response to global temperature stimuli across the low critical solution temperature is studied as the water content in the polymer network is modified. The refractive index of gel nanoparticles was measured as a function of temperature using spectroscopic ellipsometry. The volume of the nanospheres reduces by 40% resulting in a modification of light scattering properties of the medium. A change in temperature from 33 to 34 °C results in a volume contraction of nanospheres which is accompanied by 8–10% enhancement in the refractive index of the gel network.

1.45 µm Intersubband Absorption in InGaAs/AlAsSb Grown by Molecular Beam Epitaxy

InGaAs/AlAsSb multi-quantum wells lattice matched to InP substrates have been grown by molecular beam epitaxy, and the intersubband transitions in doped quantum wells have been studied. By precisely controlling the As to Sb flux ratio and substrate temperature, fairly abrupt interfaces with 1–2 monolayers of compositionary varying layers in both heterointerfaces have been achieved as confirmed from the high-resolution transmission electron microscopy (HRTEM) lattice images. Polarization-resolved absorption spectra were measured either with a conventional optical absorption measurement system or a Fourier-transform infrared spectrometer. We have observed an intersubband transition as short as 1.45 µm (0.85 eV) in 2.0-nm-thick InGaAs/AlAsSb quantum wells. This is the shortest quantum well intersubband transition ever reported in any materials system.

Oscillating magnetic field-actuated microvalves for micro- and nanofluidics

The feasibility of using tunable magnetic nanoparticles embedded in cylindrical hydrogel materials as a flow regulator via thermo-mechanical gating is studied within microfluidic channels. Ferromagnetic nanoparticles (Fe3O4) encapsulated within a thermo-sensitive polymer network (-poly(N-isopropylacrylamide) (PNIPAM)) was polymerized inside 300 µm diameter micro-capillary tubes. An oscillating magnetic field range 20–125 Oe, (100–1000 kHz) was used to induce heat and control the valving action. Valving action was effectively regulated by modulating the magnetically responsive PNIPAM networks (MPNIPAM) and thereby physically regulating the harmonics (swelling and shrinking) of the polymer monolith inside the microchannel. Magnetic properties in terms of saturation magnetization, remanence and coercivity of the designed system have been extracted for data accuracy. The optimum concentration of NIPAM monomer in the polymer matrix and the embedded nanoparticles yield ~80% volume shrinkage inside the microchannel, which is close to the undoped PNIPAM system, without compromising the oscillating field induced heating. Very importantly, the oscillating field-actuated de-swelling response time is ~3 s, which is significantly faster than the thermal actuation, and in addition the microvalve exhibits a faster response time compared with the macrovalve (MPNIPAM monolith inside 1500 µm diameter channel). The enhanced shrinkage rate and the actuation efficiency might be ideal for many biomedical applications, including synergistic application of heat and sustained releasing capability of chemotherapeutic agents.

Surface plasmon enhanced UV emission in AlGaN/GaN quantum well

The surface plasmon (SP) energy for resonant enhancement of light has shown to be modified by the epitaxial substrate and the overlying metal thin film. The modification of SP energy in AlGaN/GaN epitaxial layers is studied using spectroscopic ellipsometry for enhanced UV-light emission. Silver induced SP can be extended to the UV wavelength range by increasing the aluminum concentration in AlxGa1−xN epilayer. A threefold increase in the UV-light emission is observed from AlGaN/GaN quantum well due to silver induced SP. Photoluminescence lifetime measurements confirm the resonant plasmon induced increase in Purcell factor as observed from the PL intensity measurements.

Optimization of nonlinear optical properties of ZnO micro and nanocrystals for biophotonics

The defect and impurity states in ZnO nanocrystals synthesized using the plasma arc technique can be modified to optimize the nonlinear optical properties for optoelectronic and biophotonic applications. Highly efficient second harmonic signals over a wide range of near-infrared wavelengths, spanning from 735 nm-980 nm, has been observed and can be used in biological imaging. The use of further high energy excitation ranging from 700 nm-755 nm leads to two-photon absorption and yields broadband two photon emission extending from the 370 nm-450 nm wavelength regime which can be useful for therapeutic applications.

Electric field enhanced photoluminescence of CdTe quantum dots encapsulated in poly (N-isopropylacrylamide) nano-spheres

Photoluminescence from CdTe quantum dots encapsulated within hydrogel nanospheres can be controlled by the application an external dc electric field. Dynamic light scattering measurement of hydrogel placed under an electric field shows the collapse of the hydrogel sphere from 312 to 180 nm due to volume phase transition. Distances between quantum dots placed within the hydrogel sphere can be controlled by the applied field. A 50% enhancement in the PL intensity is observed under the influence of a dc field less than 5 V/cm. A red-shift in the peak PL intensity and emission from larger sized dots indicate energy transfer between the quantum dots. The collapse of gels is reversible and therefore has potential application in non-volatile memory devices.

Polarization-dependent nonlinear gain in semiconductor lasers

We have numerically studied the nonlinear gain coefficients in terms of spectral hole burning for the optical fields in parallel and orthogonal polarizations in semiconductor lasers by solving the equation of motion for the density matrix in perturbation series. The electronic band structures and the transition matrix elements used in the calculations are obtained by diagonalizing Luttingers Hamiltonian. In the present analysis for InGaAsP lasers, the cross-saturation coefficient for the parallel polarizations is twice as large as the self-saturation. Also, the cross-saturation coefficient for the orthogonal polarizations, which affects the polarization switching and polarization bistable operations of the laser, rests between the two. The relative magnitude of self-saturation coefficients and cross-saturation coefficients for orthogonal polarizations satisfies the condition for polarization bistable operations. We also discuss the effect of carrier heating on gain saturation coefficients.

Coupling of spontaneous emission from GaN–AlN quantum dots into silver surface plasmons

We have demonstrated the decay of spontaneous emission (SE) from AlN-GaN quantum dots (QDs) into silver surface plasmon (SP) modes in the ultraviolet at approximately 375-380 nm. Using time-resolved photoluminescence (PL), we show that the electron-hole recombination rate in AlN-GaN QDs is enhanced when SE is resonantly coupled to a metal SP mode, corresponding to the dip in the continuous-wave PL spectrum. Exciton recombination by means of silver SP modes is as much as 3-7 times faster than in normal QD SE and depends strongly on emission wavelength and thickness of the silver.

Intersubband transitions and ultrafast all-optical modulation using multiple InGaAs-AlAsSb-InP coupled double-quantum-well structures

InP-based InGaAs-AlAs-AlAsSb coupled double-quantum-well structures have been optimized using wave-function engineering techniques to achieve near-infrared intersubband (ISB) transitions. Intersubband transitions at communication wavelengths of 1.3 and 1.55 /spl mu/m can be achieved in both near-symmetric and asymmetric coupled quantum-well structures by tailoring the well width, the central barrier width, and the carrier population in the conduction subband states by controlling either the doping level or the carrier temperature. We demonstrate ultrafast all-optical modulation of interband-resonant light at 1.3 /spl mu/m using intersubband-resonant light pulses at 1.55 /spl mu/m. An ultrafast absorption recovery time of 1.3 ps has been observed at 1.3 /spl mu/m, which can be reduced to 800 fs by probing at a higher energy above the Fermi level in the conduction band.

Self-assembled deoxyguanosine based molecular electronic device on GaN substrates

Nanoscale hybrid molecular organic photodetectors based on self-assembled guanosine molecules conjugated to wide-bandgap GaN semiconductors has been realized in the ultraviolet wavelength regime. Metal-semiconductor-metal based photodetector is fabricated using ordering of modified guanosine based semiconductor nanowires which exhibit I-V characteristics with high current response and higher rectification ratio compared to Si based hybrid photodetectors. Photocurrent response of a two-terminal device shows the typical characteristics of a semiconductor photodiode with a cutoff wavelength at ∼325nm. The I-V characteristics have been elucidated using the induced polarization properties of self-assembled guanosine semiconductor.