  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}.
