Unexpected Strong Optical Absorption in a Free-Standing Optical Thick Transmission Metallic Grating

In this paper, we predict unexpected optical absorption in an optical thick free-standing transmission metallic grating by introducing periodical back grooves into it. The absorption peak could be very high up to 90%. In the structure, the introduced periodical back grooves bring about different orders of transmission minima into the transmission spectrum, which are named as transmission minima introduced by back grooves (TMIBBGs) here. When TMIBBGs approach the Fabry-Pérot (FP)-like resonant transmission peaks supported in the slits, the latter will be strongly suppressed. However, the reflection dips are almost unaffected. As a result, absorption is greatly enhanced. More interesting, the positions of TMIBBGs can be tuned by modulating the structural parameters of back grooves to selectively inhibit different orders of FP-like resonant transmission peaks and realize corresponding enhanced absorption, meanwhile, without influence to other orders of FP-like resonant transmissions. Moreover, due to the asymmetry of the structure, the enhanced absorption is unidirectional. The physical mechanisms are qualitatively and quantitatively analyzed in this paper.