Enhanced absorption per unit mass for infrared arrays using subwavelength metal-dielectric structures

TL;DR

This paper presents a novel design for infrared arrays that significantly enhances absorption efficiency per unit mass by utilizing subwavelength metal-dielectric structures and guided mode resonance, achieving broadband LWIR absorption.

Contribution

The study introduces a new pixel array design with subwavelength gratings and evanescent coupling, greatly improving absorption per mass in LWIR infrared arrays.

Findings

Achieved broadband LWIR absorption with up to 90% efficiency.

Enhanced absorption per unit mass by 1.33 to 7.33 times over previous structures.

Demonstrated effective evanescent field coupling in pixel arrays.

Abstract

The absorption to mass ratio of the infrared arrays is enhanced to approximately 1.33 to 7.33 times larger than the previously reported structures by incorporating two design characteristics: first, the coupling of evanescent fields in the air gaps around pixels to create effectively larger pixel sizes, and, second, the use of guided mode resonance (GMR) within the subwavelength metal-dielectric gratings. The bilayer Ti-Si3N4 gratings achieve broadband long-wave infrared (LWIR, 8 to 12 um) absorption by the combined effects of free carrier absorption by the thin Ti films and vibrational phonon absorption by the thick Si3N4 films. In the presence of GMR, this broadband absorption can be enormously enhanced even with low fill factor subwavelength grating cells. Further, the spacing and design of the cells can be modified to form a pixel array structure that couples the light falling in the air gaps via evanescent field coupling. Calculations are performed using the finite difference time domain (FDTD) technique. Excellent broadband absorption is observed for the optimized arrays, yielding maximum absorption of 90 percent across the LWIR and an average absorption-per-unit-mass (absorption/mass) per pixel of 3.45$\times$10^{13} kg^{-1}.