Study of subwavelength structures for optical beam focusing and laser cavity.
Mote, Rakesh Ganpat.
Date of Issue2011
School of Mechanical and Aerospace Engineering
Precision Engineering and Nanotechnology Centre
With the rapid development of microelectronic industry of data processing, data storage/reading, and various optoelectronic devices, the demand for small-sized optical components and optical systems is increasing. Diffractive optics has a potential to improve optical systems by increasing their reliability, robustness, and functional integration, while reducing their sizes. The present work investigated diffractive optical elements with subwavelength features for near-field focusing and laser cavity. Fresnel zone plates (FZPs) in the visible wavelengths were studied for near-field focusing. In addition, for laser cavity reflectors, 2-dimensional grating based on high-index-contrast materials was investigated. Near-field focusing properties of FZPs were investigated in the visible regime by a 3-dimensional finite-difference time-domain (FDTD) method. It was proposed to use phase zone plate structured on a glass to improve the diffraction efficiency of subwavelength focusing. Furthermore, a simple analytical model was used to show that high numerical aperture phase FZP under the linearly polarized illumination produces a rotationally asymmetric focal spot. With a radially polarized illumination, rotationally symmetric focal spot with a minimum beamwidth of 0.39l is obtained. Focusing behavior of phase FZPs fabricated using Focused ion beam (FIB) was characterized by near-field scanning optical microscope (NSOM) and shown to be in agreement with the simulation results. The use of a 2-dimensional high-index-contrast grating (HCG) with a square periodic lattice is proposed to realize surface-emitting lasers. With a suitable design of the 2-dimensional HCGs, Q factor as high as 1032 was achieved.