Perfect absorption in symmetrical single layer ultrathin metal grating with unilateral illumination

In symmetric environment, as the tangential electric field is almost constant across the ultrathin film, the absorption rate of the film is up to 0.5 under single side illumination. In this letter, we propose a symmetrical single layer ultrathin metal grating structure where perfect absorption (up to 0.99) can be achieved with unilateral illumination. We consider that this phenomenon of extraordinary optical absorption is caused by the coupling of the localized surface plasmon resonance and the long range surface plasmon. In particular, the electromagnetic response of the metal grating we proposed greatly resembles the electromagnetic induced absorption (EIA) which shows a narrow absorption peak within a broad absorption profile.