Sumitra Singh

Automated small tilt-angle measurement using Lau interferometry

A technique for a tilt-angle measurement of reflecting objects based on the Lau interferometry coupled with the moire readout has been proposed. A white-light incoherent source illuminates a set of two gratings, resulting in the generation of the Fresnel image due to the Lau effect. The Fresnel image is projected onto a reflecting object. The image reflected from the object is superimposed onto an identical grating, which results in the formation of a moiré fringe pattern. The inclination angle of moiré fringes is a function of tilt angle of the object. Theory and experimental arrangement of the proposed technique is presented and results of the investigation are reported.

Setting sensitivity in collimation testing using Lau interferometry

An improved method for testing the collimation of an incoherent optical beam using Lau interferometry is presented. The experimental setup consists of a white light source and a set of three identical gratings. A source grating G1 is placed in front of a collimating lens. Beyond the collimating lens gratings G2 and G3 are so aligned that the lines of the gratings make a small but equal and opposite angle with the vertical. The self-image of grating G2 is superimposed on the grating G3, resulting in the generation of moire fringes. Decollimation of the optical beam results in a change in the dimensions of the self-image. This change is detected using the magnification effect of the moire method. The horizontal moire fringes are indicative of collimation of the optical beam. Deviation from the horizontal inclination of the moire fringes indicates setting in of decollimation. Theoretical and experimental investigations to determine the sensitivity achievable in this collimation testing technique are undertaken and results of the investigations are reported. Experimental results are in close agreement with the theoretical predictions.

Fabrication of Photonic Crystal Line Defect Waveguides by Use of Optical Lithography and Focused Ion Beam

A fabrication strategy for realizing photonic crystal structures with long interfacing waveguide has been reported. The technique utilizes a combination of optical lithography and focused ion beam milling to fabricate the device.

Improved light extraction efficiency of InGaN/GaN blue LED by patterning free surfaces

The light extraction efficiency of GaN/InGaN based blue LED is improved by using hexagonal facets on n- and p- free surfaces and on substrate interfaces. Light interaction with patterned surfaces having hexagonal facets significantly improved the light extraction efficiency. The simulated results show that the light extraction efficiency is drastically improved from 18.08% to 89.27% by patterning the free surfaces.

Realization of large-scale photonic crystal cavity-based devices

Abstract. This paper demonstrates an approach for fabricating large-scale photonic crystal (PhC)-based devices using a combination of optical and focused ion beam (FIB) lithography techniques. Optical lithography along with reactive ion etching parameters is optimized to realize the layout of device structure and thereafter FIB milling is optimized to realize the designed PhC structure at those identified locations. At first, with the help of a specially designed mask and using optical lithography along with reactive ion etching, a number of rectangular areas of dimension of 10  μm×20  μm along with input and output waveguides of width ∼700  nm and thickness of ∼250  nm have been fabricated. Subsequently, use of FIB milling, a periodic PhC structure of lattice constant of 600 nm, having a hole diameter of ∼480  nm along with a defect hole diameter of ∼250  nm have been realized successfully on the selected areas. This method shows a promising application in fabricating PhC structure with device size >1  cm2 at large scale, eliminating the problems of standard nanolithography techniques.

Top- and Bottom-Patterned GaN/InGaN Violet Light Emitting Diode Structure for Enhanced Light Extraction

Micro-patterning is carried out on top-surface of GaN/InGaN violet LEDs grown on patterned sapphire substrates (PSS). Electro-luminescence/ light-output of hexagonal hole-type and grating-type LEDs are found 1.8/2.0 and 1.4/1.5 times higher compared to conventional LED respectively.

Theoretical analysis of blue to white down conversion for light-emitting diode light with yttrium aluminum garnet phosphor

Abstract. The down conversion of blue to white light with yttrium aluminum garnet (YAG) phosphor is analyzed theoretically for GaN/InGaN light emitting diodes. A cerium-doped YAG phosphor with particle size of ∼10  μm having peak emission wavelength of 560 nm is considered in this study. Effects of phosphor concentration, thickness of the conversion medium, excitation spectrum, and driving current are studied in terms of luminous efficacy and the quality of white light emission. It has been observed that the above parameters have a significant effect on chromaticity coordinates. Ray-tracing simulation results show that the luminous efficacy of down-converted white light is found to be 3.25 times the blue light excitation with phosphor concentration of 2.10E7  cm−3 and thickness of 30 μm for an injection current density of 10  A/cm2. A stable cool white light having correlated color temperature in a range of 5500–5600 K is achieved for the proposed optimized design for the variation of ambient temperatures from −25°C to 150°C.