Comparison between continuous and discrete subwavelength grating structures for antireflection surfaces

Both continuously tapered and discrete multilevel subwavelength grating structures are examined to determine the optimum method of designing antireflection surfaces. Continuously tapered gratings are designed with use of the optimal Klopfenstein graded index technique, while discrete multilevel stair-step gratings are designed with use of the Tschebyscheff quarter-wave synthesis technique. It is shown that a continuous design is always deeper than a discrete design. It is determined that since a subwavelength grating structure produces a bandpass surface, the high-pass (short-wave) performance of the continuous taper design cannot be realized. Therefore the discrete method of designing antireflection subwavelength gratings will always produce a shallower spatial profile or a smaller aspect ratio for any specified maximum reflection threshold level over a given passband.