Infrared Absorber Miniaturization: Exceptional Optical Phase Shift Induced by Ultrahigh Refractive Index Artificial Films

Abstract Fabry‐Pérot (FP) photonic absorbers have demonstrated their subwavelength thickness and integration convenience. However, without introducing abnormal phase shifts, it is hard to further miniaturize the FP absorbers. Herein, based on perfect optical phase‐matching, planar FP infrared (IR) absorbers can be more significantly thinned via ultrahigh refractive index (RI) artificial films, breaking the conventional quarter‐wavelength cavity limitation. Specifically, an ultrahigh RI (≈6.0) Ag nanowire‐Si metamaterial dielectric‐cavity covered by a giant RI (≈11.0) bismuth‐based absorbing layer is proposed to induce exceptional propagation phase accumulation and reflection phase shift. The resultant 155 nm FP absorber (not including the mirror layer) can support a resonance absorption at ≈4.4 µm. After vertically stacking, double‐cavity absorbers (below 310 nm) with multi‐resonant features are achieved, yielding broadband absorption from 1.4 to 14.4 µm. In addition, the whole FP absorbers are fabricated only through a sputtering process without any other procedures. In terms of optical phase management and refractive index engineering, this work creates a novel miniaturization scheme for FP infrared absorbers, offering a lot of opportunities applied in advanced nanophotonics.