  Enabling perfect light absorption in ultrathin materials promises the
development of exotic photonic devices. Here we demonstrate new strategies that
can provide capabilities to rationally design ultrathin (thickness <
{\lambda}/10~{\lambda}/5) semiconductor perfect absorbers for arbitrary
wavelengths, including those at which the intrinsic absorption of the
semiconductor is weak, e.g. Si for near-IR wavelengths. This is in stark
contrast with the existing studies on ultrathin perfect absorbers, which have
focused on metallic materials or highly-absorptive semiconductors. Our design
strategies are built upon an intuitive model, coupled leaky mode theory that we
recently developed and can turn the design for perfect absorbers to the design
for leaky modes. The designed absorber is featured with extraordinary
absorption enhancement, miniaturized dimension, and high selectivity for the
wavelength, polarization, and angle of incident light. It can enable the
development of flexible, light-weight, high-performance, cost-effective, and
multifunctional optoelectronic devices that are difficult with current light
absorbers.
