| Microelectronics fabrication techniques have now reached the point where gratings with sub-micron periodicity can be made reliably; this trend will continue, with lithographic critical dimensions shrinking to 70nm by 2010. Gratings with such small periods generally have all diffracted orders cut off for visible and IR radiation, and are labeled "subwavelength gratings". Subwavelength gratings exhibit several properties of potential use to the diffractive optics designer. Firstly, subwavelength gratings typically behave as layers of uniaxial material, with designable principal indices of refraction; this property allows for the design of broadband wide field-of-view matching layers and wave-retarders for high-index and lossy materials. Secondly, properly designed subwavelength gratings will exhibit abrupt reflectance/transmittance scattering resonances which have been proposed for use as narrowband and broadband frequency selective surfaces.; The thesis begins with a presentation of modeling and design techniques for subwavelength gratings. Modeling of subwavelength gratings via effective medium theories is examined, and the efficacies of such theories are determined by comparison to rigorous electromagnetic theory. One of these effective medium theories is then used as a tool to design antireflection surfaces and phase retarders in semiconductor materials. Next, the physical mechanism underlying the resonance phenomenon in subwavelength gratings is explored, with attention paid to the poorly understood normal incidence case; in the case of strongly modulated gratings, new and useful resonant behaviors are demonstrated. Following this, a fabrication process is described that can be used to produce semiconductor gratings with deep submicron periodicity. Finally, measurements of a silicon antireflection grating provide experimental verification of the subwavelength grating effective medium property. |
