Shanti Bhattacharya

GaN light emitting diodes (LEDs) on sapphire substrates can be improved by micro-patterning substrate to perform epitaxial over-growth which drastically reduces defects' density in the light emitting region. We patterned Al 2O3 with focused ion beam and show a successful overgrowth of GaN. The exact shape of pattern milled into Al2O3 was replicated into a 0.4-mm-thick shim of Ni by electroplating. The surface roughness of Ni was ∼5:5 ± 2 nm and is applicable for the most demanding replication of nano-rough surfaces. This technique can be used to replicate at micro-optical elements Fresnel-axicons defined by electron beam lithography made on sub-1 mm areas without stitching errors (Raith EBL). Shimming of macro-optical elements such as car back- reflectors is also demonstrated. Ni-shimming opens possibility to make replicas of nano-textured small and large area patterns and use them for thermal embossing and molding of optically-functionalized micro-fluidic chips and macro-optical elements. © 2013 Copyright SPIE.

A wide variety of MEMS micro-mirrors are being developed for various optical applications. One such application is Fourier Transform Spectrometry (FTS). The design, process optimization and fabrication of a micro-mirror for this application is presented. Large, non-tilting displacements of mirrors are required to achieve high FTS resolution. In order to obtain this without using Deep Reactive Ion Etching (DRIE), the micro-mirrors were fabricated on silicon using bulk micromachining techniques. This paper will present the process developed for fabrication of the mirror with the required specifications. In addition, results of the FTS experiments conducted with the micro-mirror will also be presented. © 2010 Copyright SPIE - The International Society for Optical Engineering.

The design and fabrication of transmission subwavelength binary gratings for operation as polarizing beam splitters (PBSs) at 1550 nm is presented in this paper. An analytical method called the modal method was used for the design as well as to predict the efficiencies of the polarization components in each order. Electron beam lithography has been employed to fabricate the subwavelength grating structures on poly methyl methacrylate (PMMA). The performance of the fabricated PBS has been evaluated by optical testing.

We describe a systematic approach to design, optimize, and characterize a Fourier-domain mode-locked (FDML) laser with an erbium-doped fiber amplifier (EDFA) as the optical gain medium. A highly stable temporal intensity profile is obtained by minimizing chromatic dispersion and polarization fluctuations. The obtained bandwidth of 21 nm, tuning speed of 50 kHz, and output power of 5 mW are the highest reported so far with an EDFA-based FDML laser.

It has been reported that a double wedge plate interferometer and Michelson interferometers in tandem can be used to produce a cross pattern that can serve as an array of equal intensity light spots. In this paper we report on the holographic recording of this cross pattern. The phase hologram so produced serves as an array generator with some interesting characteristics. Theory and experimental results are presented. © 1995 Taylor and Francis Ltd.

The realization of spiral phase optical elements on the cleaved end of an optical fiber by focused ion beam milling is presented. A focused Ga ion beam with an acceleration voltage of 30 keV is used to etch continuous spiral phase plates and fork gratings directly on the tip of the fiber. The phase characteristics of the output beam generated by the fabricated structures measured via an interference experiment confirmed the presence of phase singularity in the output beam. The devices are expected to be promising candidates for all-fiber beam shaping and optical trapping applications.

A binary Fresnel Zone Axilens (FZA) is designed for the infinite conjugate mode and the phase profile of a refractive axicon is combined with it to generate a composite Diffractive Optical Element (DOE). The FZA designed for two focal lengths generates a line focus along the propagation direction extending between the two focal planes. The ring pattern generated by the axicon is focused through this distance and the radius of the ring depends on the propagation distance. Hence, the radius of the focused ring pattern can be tuned, during the design process, within the two focal planes. The integration of the two functions was carried out by shifting the location of zones of FZA with respect to the phase profile of the refractive axicon resulting in a binary composite DOE. The FZAs and axicons were designed for different focal depth values and base angles respectively, in order to achieve different ring radii within the focal depth of each element. The elements were simulated using scalar diffraction formula and their focusing characteristics were analyzed. The DOEs were fabricated using electron beam direct writing and evaluated using a fiber coupled diode laser. The tunable ring patterns generated by the DOEs have prospective applications in microdrilling as well as microfabrication of circular diffractive and refractive optical elements.

Bessel beams have found important applications due to their propagation invariant nature. However, the presence of sidelobes has proven a hindrance in key imaging and biophotonics applications. We describe the design and generation of sidelobe-suppressed Bessel-like beams (SSBB) that provide enhanced contrast for light-sheet imaging. The sidelobe suppression is achieved by the interference of two Bessel beams with slightly different wavevectors. Axicon phase functions for each Bessel beam are combined into a single phase function using the random multiplexing technique. This phase function is realised using a spatial light modulator to generate a SSBB. The generated beam at 633 nm has a 1/e2 radius of 44 μm and a propagation invariant distance of 39 mm which is more than four times that of the Rayleigh range of a Gaussian beam with the same 1/e2 radius. Within this distance, the overall peak intensity of the sidelobes of the SSBB is less than 10% that of the main lobe peak intensity. In addition, through numerical simulation for the recovery of spatial frequencies, we show that the SSBB improves image contrast compared to a Bessel beam for light-sheet imaging. We also show that the designed phase function can be realised using a meta-optical element.

The Hilbert transform (HT) method is a technique to perform optical phase retrieval from a single off-axis interference pattern. However, one of the significant issues with the HT method is the error generated due to noise in the recorded interference pattern, which can be avoided using a proper pre-filtering step. The importance of pre-filtering of the interference patterns is studied in this study. The observations are verified experimentally by performing quantitative phase imaging of a white blood cell (WBC) using a Mach-Zehnder interferometer configuration.

Design and fabrication of a MEMS device for FTIR application is reported. The design delivers an out-of-plane resonant displacement of 98.5 μm at 30V AC actuation in finite element simulation. Device fabrication is carried out and the characterization is in progress.