Polarization-selective computer-generated holograms made with form-birefringent nanostructures were designed, fabricated, and evaluated experimentally at 1.5 mm. The fabricated element showed a large polarization contrast ratio ͑.250: 1͒ and a high diffraction efficiency (.40% for a binary phase level element). The experimental evaluation was in good agreement with the design and modeling predictions. © 1996 Optical Society of America Polarization-selective phase-only birefringent computer-generated holograms (BCGH's) are generalpurpose diffractive elements that have independent impulse responses for orthogonal linear polarizations. Such elements are shown to be useful in many applications, including packing optoelectronic devices or systems, free-space optical interconnects, and image processing. The birefringence of the substrates in these conf igurations makes the elements sensitive to the polarization of the light. The twosubstrate approach is complicated to fabricate because it includes an assembly process of the two diffractive structures that requires high alignment accuracy. The single-substrate approach, on the other hand, is simpler to fabricate, but it is only an approximate solution. In this Letter we report a new approach for design and fabrication of BCGH elements. Our new approach involves creating a form-birefringent nanostructure and modulation of the refractive index as well as the birefringence at each pixel of the BCGH.Form birefrigence is a well-known effect of subwavelength periodic microstructures. The electric f ields 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.
5Many researchers have demonstrated this effect in the far-IR region. 6,7 Recently, with the help of the advances in nanofabrication, 200-nm period gratings were fabricated in a GaAs substrate that showed strong form birefringence in the near IR. Furthermore, these results were found to be in agreement with the numerical simulation results obtained by a rigorous coupled-wave analysis (RCWA).
9,10Design optimizations were performed for BCGH by form birefringence.
9Chen and Craighead demonstrated a polarization-insensitive diffractive optical element that uses two-dimensional subwavelength periodic microstructures.
11Aoyama and Yamashita demonstrated a grating beam splitting polarizer that uses a subwavelength grating fabricated in a photoresist.
12The polarization contrast ratios, def ined as the ratio of intensities obtained under two orthogonal polarizations at the designed diffraction order, were ϳ6:1 and ϳ 3: 1 for the 0th and 1st diffraction orders, respectively.In what follows, we report the design, fabrication, and experimental evaluation of a binary phase level BCGH element that uses form-birefrigent nanostructures [or form-birefringent computer-generated holograms (FBCGH's)] fabricated upon GaAs substrates for operation in the near...