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.
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