Seven-octave ultrabroadband metamaterial absorbers via quality-factor-weighted mode density modulation

Absorption is a crucial parameter in shaping wave propagation dynamics, yet achieving ultrabroadband absorption remains highly challenging, particularly in balancing a low-frequency and broad bandwidth. Here, we present a metamaterial absorber (MMA) capable of achieving simultaneous spectral coverage across a seven-octave range of near-perfect absorption from 100 to 12 800 Hz by engineering the quality-factor-weighted (<i>Q</i>-weighted) mode density. The <i>Q</i>-weighted mode density considers mode density, resonant frequencies, radiative loss and intrinsic loss of multiple resonant modes, providing a comprehensive approach to governing broadband absorption properties. By optimizing the number of resonant modes and managing intrinsic losses, our approach achieves an intensive <i>Q</i>-weighted mode density across an ultrawide bandwidth, enabling ultrabroadband absorption with high efficiency. These findings significantly advance the bandwidth capabilities of state-of-the-art MMAs and pave the way for the development of ultrabroadband metamaterial devices across various wave systems.