Ultra-broadband mid-infrared absorber based on hyperbolic -MoO₃

Owing to their broad operational bandwidth, broadband absorbers are extensively used in various applications, such as solar cells, microbolometers, and light modulators. To enhance energy harvesting, it is essential to achieve insensitivity to wide incident angles and polarization-independent absorption. alpha-MoO3 is a natural polar van der Waals crystal that hosts phonon polaritons in the mid-infrared spectral region. These polaritons can manipulate the infrared light at the nanoscale and break the diffraction limit, offering a great opportunity for the design of broadband absorbers. In this study, we propose a strategy based on an array of crossed the permittivity of alpha-MoO3 in [100] and [001] crystallographic axes to expand its broadband absorption. Triangular one-dimensional grating pattern is initially considered, which shows an ultra-broadband and wide-angle insensitive absorption within the range of 10.23-18.07 mu m. Subsequently, square pyramidal two-dimensional arrays are investigated, which not only show ultra-broadband absorption and wide-angle insensitivity, but also exhibit polarization independence within the range of 10.46-18.32 mu m. This remarkable broadband absorption is elucidated by the effective medium theory and the local power dissipation density. These results demonstrate an effective strategy for constructing ultra-broadband absorbers based on alpha-MoO3 for applications in solar thermal and photovoltaic systems.