Optimal design for antireflective tapered two-dimensional subwavelength grating structures

TLDR

The study investigates and develops techniques to design continuously tapered two‑dimensional subwavelength surface‑relief gratings for broadband antireflection using 2D effective‑medium theory. The authors employ a Klopfenstein taper to generate the optimum graded‑index profile with minimal depth, and verify its performance with both effective‑medium simulation and rigorous coupled‑wave analysis. The resulting Klopfenstein‑tapered 2D subwavelength grating reduces Fresnel reflections by 20 dB over a broad band at an air–substrate interface, maintains low reflectance over a field of view exceeding 110°, and outperforms the traditionally assumed pyramidal profile, which requires a significantly larger depth for comparable performance.

Abstract

Techniques for the design of continuously tapered two-dimensional (2D) subwavelength surface-relief grating structures for broadband antireflection surfaces are investigated. It has been determined that the Klopfenstein taper [ Proc. IRE44, 31 ( 1956)] produces the optimum graded-index profile with the smallest depth for any specified minimum reflectance. A technique is developed to design the equivalent tapered subwavelength surface-relief grating structure by use of 2D effective-medium theory. An optimal Klopfenstein tapered 2D subwavelength grating is designed to reduce the Fresnel reflections by 20 dB over a broad band from an air–substrate (ns = 3.0) interface. The performance is verified by use of both a 2D effective-medium-theory simulation algorithm and rigorous coupled-wave analysis. These structures are also shown to achieve this low reflectance over a wide field of view (θFOV > 110°). The pyramidal spatial profile, which has generally been assumed to produce the optimal broadband antireflection grating structure, is shown to require a significantly larger depth to achieve the same performance as a Klopfenstein-designed tapered antireflection grating structure.