  We demonstrate that a self-complementary checkerboard-like metasurface works
as a broadband coherent perfect absorber (CPA) when symmetrically illuminated
by two counter-propagating incident waves. A theoretical analysis based on wave
interference and results of numerical simulations of the proposed metasurface
are provided. In addition, we experimentally demonstrate the proposed CPA in
the terahertz regime by using a time-domain spectroscopy technique. We observe
that the metasurface can work as a CPA below its lowest diffraction frequency.
The size of the absorptive areas of the proposed CPA can be much smaller than
the incident wavelength. Unlike conventional CPAs, the presented one
simultaneously achieves the broadband operation and energy concentration of
electromagnetic waves at the deep-subwavelength scale.

Optics