Conventional single- or multi-layer antireflection coatings to eliminate the undesirable interface reflection and enhance the transmission have strict requirements in the refractive index and film thickness of the materials being coated. Alternative approaches have been developed, for example, using surface relief structures. However, they pose difficulties and severe restrictions in device integration. Taking advantage of the tailored reflection/transmission and their dispersion, metasurfaces consisting of subwavelength planar metallic structures and combining with a dielectric spacer enable ultrathin antireflection coatings without much requirement in the refractive index of the spacer material. We have pioneered this concept particularly in the development of terahertz metamaterial antireflection coatings [1,2]. In this contribution, we will present our latest progress in antireflection metasurfaces for terahertz and mid-infrared. Particularly, in contrast to our previous metal-dielectric-metal structures enabling narrowband antireflection, we found that excellent antireflection can be accomplished using much simpler metal-dielectric structures - a combination of a single-layer metasurface and an ultrathin dielectric film, achieving a bandwidth comparable to quarter-wave antireflection. Further improvement of the bandwidth or achieving multi-band antireflection is possible by using more complex unit cells of the metasurfaces . We will present our numerical and experimental results, together with analytical analysis to elucidate the underlying principles .
 H.-T. Chen et al., “Antireflection coating using metamaterials and identification of its mechanism,” Phys. Rev. Lett. 105, 073901 (2010).  H.-T. Chen et al., “A numerical investigation of metamaterial antireflection coatings,” THz Sci. Technol. 3, 66 (2010).  N. K. Grady et al., “Terahertz metamaterials for linear polarization conversion and anomalous refraction,” Science 340, 1304 (2013).  H.-T. Chen, “Interference theory of metamaterial perfect absorbers,” Opt. Express 20, 7165 (2012).