Selective excitation of hyperbolic phonon polaritons-induced broadband absorption via α-MoO₃ square pyramid arrays

Optical anisotropy of α-MoO₃ in its reststrahlen (RS) bands provides exciting opportunities for constructing the polarization-dependent devices. However, achieving broadband anisotropic absorptions through the same α-MoO₃ arrays is still challenging. In this study, we demonstrate that selective broadband absorption can be achieved by using the same α-MoO₃ square pyramid arrays (SPAs). For both the x and y polarizations, the absorption responses of the α-MoO₃ SPAs calculated by using the effective medium theory (EMT) agreed well with those of the FDTD, indicating the excellent selective broadband absorption of the α-MoO₃ SPAs are associated with the resonant hyperbolic phonon polaritons (HPhPs) modes assisted by the anisotropic gradient antireflection (AR) effect of the structure. The near-field distribution of the absorption wavelengths of the α-MoO₃ SPAs shows that the magnetic-field enhancement of the lager absorption wavelength tends to shift to the bottom of the α-MoO₃ SPAs due to the lateral Fabry-Pérot (F-P) resonance, and the electric-field distribution exhibits the ray-like light propagation trails due to the resonance nature of the HPhPs modes. In addition, broadband absorption of the α-MoO₃ SPAs can be maintained if the width of the bottom edge of the α-MoO₃ pyramid is large than 0.8 μm, and excellent anisotropic absorption performances are almost immune to the variations of the thickness of the spacer and the height of the α-MoO₃ pyramid.