Anomalous optical absorption in metallic gratings with subwavelength slits

The absorption in metallic gratings with subwavelength slits is theoretically investigated. Anomalous optical absorption occurs over a wide range of incident angles for both polarizations. In particular, a nearly perfect absorbance up to 99.5% with a significant bandwidth is attained for TM polarization with compound slits. Enhanced absorption is associated with extreme concentration of fields inside the structure. The respective field pattern depicts a special feature of surface plasmons excited on single interface only, which are identified as semibonding modes. The anomalous absorption is also achieved for TE polarization, when the compound grating is reduced to a simple grating. For this polarization, the nomalous absorption is attributed to the occurrence of trapped modes, with a slightly smaller absorbance (98.4%).