Abstract

Form birefringence is a well known effect of subwavelength periodic microstructures. The electric fields parallel to the grating grooves (TE polarization) and perpendicular to the grating grooves (TM polarization) need to satisfy different boundary conditions, resulting in different effective refractive indices for TE and TM polarized waves.¹. Many researchers have demonstrated this effect in the far infrared (IR) region.²,³ Recently, with the help of the advances in nanofabrication, 200 nm period gratings were fabricated in GaAs substrate showing strong form birefringence in near IR.⁴ Furthermore, these results were found to be in agreement with the numerical simulation results obtained using rigorous couple wave analysis (RCWA).⁵,⁶ Chen et. al. have demonstrated a polarization insensitive diffractive optical element using two dimensional subwavelength periodic microstructures.⁷ Due to its unique properties, form birefringent material is suitable in constructing novel birefringent computer generated holograms (BCGHs). Such BCGHs are general purpose diffractive optical elements (DOEs) that have independent impulse responses for the two orthogonal polarizations⁸,⁹. Aoyama et. al. have demonstrated a grating beam splitting polarizer using subwavelength grating fabricated in photo resist.¹⁰ The polarization contrast ratios, defined as the ratio of intensities obtained under two orthogonal polarizations at the designed diffraction order, were about 6:1 and 3:1 for the zeroth and first diffraction orders, respectively. In contrast, our approach is based on creating artificially a birefringent layer using a high spatial frequency grating (HSFG) which is then modulated by a large period diffractive structure similar to conventional DOEs. We call this approach form birefringent CGH (FBCGH). In our preliminary work¹¹ we have designed, fabricated and experimentally evaluated a novel FBCGH that implements a diffractive polarization beam splitter. In this manuscript we describe a general design method of FBCGH and discuss specific design examples.

© 1997 Optical Society of America