Joseph Thomas Andrews

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.

Polarization sensitive optical low-coherence reflectometry for blood glucose monitoring in human subjects

A device based on polarization sensitive optical low-coherence reflectometry is developed to monitor blood glucose levels in human subjects. The device was initially tested with tissue phantom. The measurements with human subjects for various glucose concentration levels are found to be linearly dependent on the ellipticity obtainable from the home-made phase-sensitive optical low-coherence reflectometry device. The linearity obtained between glucose concentration and ellipticity are explained with theoretical calculations using Mie theory. A comparison of results with standard clinical methods establishes the utility of the present device for non-invasive glucose monitoring.

Negative thermo-optic coefficients and optical limiting response in pulsed laser deposited Mg-doped ZnO thin films

Thermo-optic properties and optical limiting characteristics are investigated in pulsed laser deposited Mg-doped ZnO thin films by performing a Z-scan experiment in the off-resonant regime using a CW He–Ne laser source. Optical limiting studies in these thin films lead to a conclusion that reverse saturable absorption is the major mechanism for optical limiting properties in these films and nonlinear refraction is caused due to the thermal lensing effect. Negative thermo-optic coefficients were observed, indicating potentiality for athermalization, required to counter the effect of thermal expansion. Sufficiently large thermo-optic coefficients and good optical limiting response suggest the utility of MgZnO films for thermo-optic switches and optical limiting device applications.

Effect of the polarization state on squeezed-state generation in GaAs

We have theoretically analysed the effect of polarization of the initial state on squeezed-state generation. The analysis has been applied to a sample of GaAs irradiated by a KCl:Na laser tunable over the range 1.62-1.91 µm. The ellipticity of error ellipse and time and power dependence of quadrature variance suggest that the circular polarization state is most suitable for squeezed-state generation. We have also extended our analysis to a homodyne detection technique and have studied the power dependence of signal-to-noise ratio.

Effect of Shell and Shell Thickness on Photoluminescence (PL) of a CdSe/ZnS Core – Shell Quantum Dot

The Photoluminescence (PL) intensity is theoretically calculated for a CdSe/ZnS Core – Shell Quantum Dot incorporating WKB approximation. Coating CdSe bare quantum dots with a ZnS layer indicates an enhancement in the PL intensity remarkably with a red-shift in spectra. The PL intensity increases with initial shell growth and then it starts to decrease if outer shell growths continue. Smaller Core – Shell Quantum Dots shows greater PL intensity with more red-sift in PL spectra.

Cyclic Correlation of Diffuse Reflected Signal with Glucose Concentration and Scatterer Size

The utility of optical coherence tomography signal intensi ty for measurement of glucose concentration has been analys ed in tissue phantom and blood samples from human subjects. The di ffusion equation based calculations as well as in-vivo OCT s ignal measurements confirms the cyclic correlation of signal inte nsi y with glucose concentration and scatterer size. I. I NTRODUCTION Everyday, almost 150 million people world wide face the prob lem of diabetic metabolic control. Both the hypoand hyperglycemic conditions of patients have fatal consequences an d warrant blood glucose monitoring at regular interval. Exi sting monitors of blood glucose can be widely classified into three classes viz., invasive, minimally invasive, and noninvasi e. Invasive monitoring require small volume of blood and they are inappr o riate for continuous monitoring of blood glucose. Minima lly invasive monitors analyze tissue fluid, and skin injury is mi ni ally. On the other hand, noninvasive devices are painles s and skin injury is absent. Many research groups are working on to develop a real time non invasive tool for monitoring blood glucose at clinical level. Recent review by Bazaev and Selishchev [1] discuss va rious noninvasive methods for blood glucose measurement and monitoring. Existing non-invasive blood glucose monit ring work on various modalities such as absorption spectro s opy [2], optical activity and polarimetry [3], [4], Optical Coh erence Tomography (OCT) [5], [6], bioimpedance spectrosco py [2], fluorescence [7], etc. The other optical techniques include Raman spectroscopy [8] and reflectance spectroscopy [9]. Ho wever, due to optical interference, poor signal strength, and cali br tion issues, optical methods still face many challenges [10]. The measurement of blood glucose level using OCT technique e xhibits large fluctuations due to motion artifacts or other physiological and environmental conditions. [5], [6]. Optical coherence tomography is a nondestructive techniqu e that examines the internal structure of superficial layers of biological tissues. It is based on interferometric recordi ng of near-infrared light backscattered from the point of st udy, which could carry information. Conventionally, in OCT backscatt ered light is collected, measured, and integrated to assemb le i ages [11]. The impetus for the present paper comes from two reasons. Fir stly, to correlate the glucose concentration with experime ntally observed diffuse reflectance signal from OCT. Secondly to ex amine the usability of OCT signal intensity for measurement of glucose concentration. Accordingly, the paper has been d ivided into three parts. Section two deals with the theoreti cal analysis where Mie scattering theory and diffusion equatio n are used to understand the dependence of solute concentrat ion on OCT signal intensity in a turbid medium. In the third secti on, experimental results are exhibited for various samples of aqueous solutions of glucose dissolved in tissue phantom as the catterers. The experiment was also performed with bloo d samples from voluntary blood donors. In the fourth section, the numerical calculations were carried out for various glu cose concentrations in aqueous solution having different parti cle sizes as scatterers using the theoretical analysis. It i s observed that a cyclic correlation of diffuse reflected signal exist as a fu nction of the glucose concentration and size of the scattere rs. To support the theoretical results, experimental observatio ns are also presented. II. T HEORETICAL FORMULATIONS Blood is a turbid medium consisting of extracellular fluid (E CF) and various scatterers e.g. red blood cells (RBC), white blood cell (WBC), etc. Multiple scattering from RBC gives rise to d iffusion of light from blood. The average diffuse intensity (Ud) is related to the scattering cross section via the relation [ 6] Ud(r) = Ud(0) exp(−kdr). (1) kd is the spatial decay constant and is given by kd ( = ρ √ 3σaσtr ) where,ρ is the number density of scatterers in the medium, σtr is the transport cross section and is given by σtr [= σs(1 − g) + σa] with σs being the scattering coefficient, g is the anisotropy parameter and σa is the absorption coefficient.

All Optical Quantum CNOT Gate in Semiconductor Quantum Dots

Using density matrix approach the possibility of quantum logic operation in a single semiconductor quantum dot (QD) has been explored. The two photon excitation of the QD creates population in the biexciton state which is subsequently depopulated by the second pulse resonant with the biexciton-exciton transition. The theoretical analysis suggest that CNOT gate can be formed by choosing the pulses of appropriate pulse area.

MOEMS optical delay line for optical coherence tomography

Micro-Opto-Electro-Mechanical optical coherence tomography, a lab-on-chip for biomedical applications is designed, studied, fabricated and characterized. To fabricate the device standard PolyMUMPS processes is adopted. We report the utilization of electro-optic modulator for a fast scanning optical delay line for time domain optical coherence tomography. Design optimization are performed using Tanner EDA while simulations are performed using COMSOL. The paper summarizes various results and fabrication methodology adopted. The success of the device promises a future hand-held or endoscopic optical coherence tomography for biomedical applications.

Pump-probe absorption spectra of small quantum dots

Density matrix approach has been employed to analyze the pump-probe spectroscopic absorption spectra of small semiconductor quantum dots (QDs) under strong confinement regime (SCR) with sizes smaller than the bulk exciton Bohr radius such that the Columbic interaction energy becomes negligible in comparison to the confinement energy. The average time rate of absorption has been obtained by incorporating the radiative and nonradiative decay processes as well as the inhomogeneous broadening arising due to nonuniform QD sizes. The analytical results are obtained for QDs duly irradiated by a strong near resonant pump and broadband weak probe. Numerical estimations have been made for (i) isolated QDs and (ii) QD-arrays of GaAs and CdS. The results agree very well with the available experimental observations in CdS QDs. The results in case of GaAs QDs can lead one to experimentally estimate absorption/gain spectra in the important III-V semiconducting mesoscopic structures.