  Enhancing absorption in optically thin semiconductors is the key in the
development of high-performance optical and optoelectronic devices. In this
paper, we resort to the concept of degenerate critical coupling and design an
ultra-thin semiconductor absorber composed of free-standing GaAs nanocylinder
metasurfaces in the near infrared. The numerical results show that perfect
absorption can be achieved through overlapping two Mie modes with opposite
symmetry, with each mode contributing a theoretical maximum of 50% in their
respective critical coupling state. The absorption also shows the
polarization-independent and angle-insensitive robustness. This work, together
with the design concept, opens up great opportunities for the realization of
high-efficiency metasurface devices, including optical emitters, modulators,
detectors, and sensors.
