New principle for unpolarized wideband reflectors

TL;DR

This paper introduces a novel unpolarized broadband reflector using a pair of orthogonally arranged polarization-sensitive gratings, achieving high reflectance over a wide spectral band in the near-infrared.

Contribution

It presents a new design of unpolarized broadband reflectors based on serially arranged polarized gratings optimized with numerical methods.

Findings

Achieves over 97% reflectance across a 500-nm wavelength band.

Provides an ultra-high-reflection band exceeding 99.9% over ~85 nm.

Demonstrates a simple, robust, and fabricable alternative to multilayer thin-film reflectors.

Abstract

There is immense scientific interest in the properties of resonant thin films embroidered with periodic nanoscale features. This device class possesses considerable innovation potential. Accordingly, we report unpolarized broadband reflectors enabled by a serial arrangement of a pair of polarized subwavelength gratings. Optimized with numerical methods, our elemental gratings consist of a partially-etched crystalline-silicon film on a quartz substrate. The resulting reflectors exhibit extremely wide spectral reflection bands in one polarization. By arranging two such reflectors sequentially with orthogonal periodicities, there results an unpolarized spectral band that exceeds those of the individual polarized bands. In the experiments reported herein, we achieve zero-order reflectance exceeding 97% under unpolarized light incidence over a 500-nm-wide wavelength band in the near-infrared domain. Moreover, the resonant unpolarized broadband accommodates an ultra-high-reflection band spanning ~85 nm and exceeding 99.9% in efficiency. The elemental polarization-sensitive reflectors based on one-dimensional resonant gratings have simple design, robust performance, and are straightforward to fabricate. Hence, this technology is a promising alternative to traditional multilayer thin-film reflectors especially at longer wavelengths of light where multilayer deposition may be infeasible or impractical.