Angle-Insensitive Defect-Mode Absorption in Photonic Crystals Containing Hyperbolic Metamaterials.

Omnidirectional optical devices are essential for photodetection, thermal radiation regulation, and solar energy harvesting. However, the photonic bandgaps and defect modes of conventional one-dimensional photonic crystals (1DPCs) are constrained by the Bragg scattering condition, leading to strong angular dependence that substantially limits their practical applications over wide angle ranges. In this work, we theoretically design and experimentally verify an angle-insensitive photonic crystal defect-mode absorber based on hyperbolic metamaterials (HMMs). Leveraging the unique isofrequency dispersion of HMMs, we introduce a phase compensation mechanism into a photonic crystal composed of alternating HMM and dielectric layers. Calculations show that inserting a metallic defect layer excites a highly localized defect mode within the bandgap, whose resonant wavelength remains almost unchanged with incident angle. To simplify fabrication and enhance absorption, we reduce the number of periods and design a heterostructure containing subwavelength Ag/TiO2 multilayers. Measurements under TM polarization over 0-70° show that the defect-mode peak shifts by only 3.5 nm, while the absorptance decreases from ~0.717 at normal incidence to ~0.292 at 70°. This study provides an effective strategy for designing and fabricating resonance wavelength angle-insensitive optical absorbers enabled by HMM-based phase compensation.