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

Abstract

The absorption-to-mass ratio of the infrared arrays is enhanced to ~1.33 ̶ 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 metaldielectric gratings. The bilayer Ti-Si3N4 gratings achieve broadband long-wave infrared (LWIR,  ~ 8 ̶ 12 m) 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% across the LWIR and an average absorption-per-unit-mass (absorption/mass) per pixel of 3.45×10 13 kg -1 .