An ultra-thin optical absorber composed of an amorphous silicon slab and a silver grating

The upper limit of absorbance in an ultra-thin (<100 nm) a-silicon slab is about 50%. However, by embedding an ultra-thin silver grating into the back side of the a-silicon slab, the absorption can be highly enhanced and the fields would be strongly localized in the vicinity of the hybrid structure. Even though the structure is very thin (<100 nm), the absorbance could be very high up to ~98% with weak dependence on the light׳s polarization. Moreover, the angular tolerance of the absorption is very large, especially for TM polarized light with nearly 90° tolerance. Besides, the absorption spectrum can be designed to cover the visible and near infrared (NIR) region. The physical mechanism is ascribed to the high field concentration in the vicinity of the structure, which originates from the cavity-like resonance (for both polarizations) in the a-silicon slab as well as the excitation of semi-bonding SPs modes for TM polarization. These results indicate that the hybrid structure can act as an ultrathin optical absorber and may be integrated into photovoltaic devices because of its compact size.