Optical Properties of Multi-Microgratings, Their Replication and Applications
Rothenbach, Christian, Electrical Engineering - School of Engineering and Applied Science, University of Virginia
Gupta, Mool, Department of Electrical and Computer Engineering, University of Virginia
Multi-microgratings are defined as arrays of one-dimensional diffraction gratings inscribed inside similarly shaped cells and then arrayed in a two-dimensional periodic fashion. Due to the high number of spatial periodicities, these structures form intricate diffraction patterns. A meticulous understanding on how the diffraction patterns form and what their optical properties are has not previously been studied and it is necessary in order to be able to exploit these properties in a variety of applications. A theoretical model of these properties was formulated through analytical and graphical methods based on Fraunhofer diffraction theory, intensity distribution functions, Fourier Transforms and finite-difference time-domain (FDTD). The diffraction pattern was found to be formed by the individual contributions of the periodic elements in multi-microgratings and their interactions. To validate the theoretical model, multi-micrograting samples with 0.5 and 2 µm periods and 10 and 20 µm sides, arrays of hexagonal apertures and other structures were fabricated via electron beam lithographic method on silicon substrates. A polymer based replication method was demonstrated and PDMS replicas were fabricated from silicon masters. Optical properties of the fabricated structures and their replicas were characterized, their diffraction patterns were measured and explained. The optical diffraction efficiency of these samples was measured to be 32.1%. Finally, a brief study of possible applications of multi-microgratings was carried out in the context of a temperature measurement sensor. Diffracted beam spots were characterized for thermally induced changes in the diffraction angles and intensity. An optical interferometric regime was devised that allowed for a high dynamic range and high resolution temperature measurement that had a theoretical resolution of 300 times better (capable of resolving <0.1°C) than using one dimensional gratings. Alternative applications of multi-microgratings are proposed as well as future areas of research.
PHD (Doctor of Philosophy)
multi-micrograting, diffraction, FDTD, Fourier Transform