Subwavelength antireflection and polarization grating elements: Analysis and fabrication

Abstract

Surface-relief, submicron period transmission gratings are fabricated in fused silica. A rigorous vector diffraction code, based on the coupled-wave analysis technique, is used to design and analyze the surface relief gratings. When light with wavelength greater than the grating period encounters such element, only zeroth order transmitted and reflected beams propagate, all other diffracted orders are evanescent. These surface-relief gratings act as homogeneous thin film layers of equivalent refractive indices. The equivalent refractive indices depend on grating characteristics, angle of incidence, and incident light polarization. These gratings can be used as equivalent anti-reflection coatings and as polarization elements. Since the grating structures are etched into the substrate material, these optical elements are durable and chemically resistant compared to resist gratings. Subwavelength elements may play a critical role in high power laser systems where damage resistant antireflection and birefringent materials may not exist. By gaining an understanding and being able to control the many variables involved in the grating fabrication process, one-dimensional and two-dimensional submicron period surface-relief resist gratings with rectangular profiles and precisely controlled dimensions are generated. Subsequent pattern transfer etch into underlying substrate layer resulted in one and two-dimensional gratings in fused silica. One-dimensional gratings fabricated in fused silica behaves as polarization elements, giving a maximum measured phase retardation of 50 degrees. To create a polarization insensitive antireflection structure, two-dimensional surface relief gratings are fabricated. These elements exhibited reflectivities near zero percent. The AR structures also showed broadband performance. Application of two-dimensional AR structures on a 16-level diffractive phase plate reduces the surface reflectance of the multilevel phase plate to 0.2%, from 3.3% of that of a bare fused silica surface. Subwavelength grating elements were found to damage when exposed to 45 mJ/cm² and 13 mJ/cm² of laser radiation at 1064 nm and 351 nm wavelength, respectively. The subwavelength gratings have laser damage thresholds comparable to that of bare fused silica. Initial effort on replicating the subwavelength grating structures in polymers yielded promising results, demonstrating the potential for mass production. Replicated elements exhibited no stress birefringence.