Achieving near-unity absorption in planar semiconductor film on metal substrate

We theoretically studied a three-layer configuration composed of a semiconductor layer sandwiched between air and metal substrate. Using full-field calculations, it is shown that near-zero reflectivity and near-unity absorptivity occur at the same wavelength of 532 nm in a 12 nm thick a-Si on Ag substrate. The behind physical origin is that the reflection on the a-Si/Ag interface is modulated both on amplitude and phase by the thickness of a-Si, which can cancel out the reflection on the air/a-Si interface by suitable thickness choice. The polar diagrams and absorption spectra prove that near-unity absorption at any target wavelength and high absorption at wide wavelength range can be achieved by selecting appropriate semiconductor and metal substrate. Two application scenarios in photo-absorber and solar cell fields are proposed in the end. The devised concept can extremely save the raw materials (total thickness around 100 nm) and realize the component functionalities. Our findings serve as a guide for designing planar perfect absorbers for potential optical absorber and photovoltaic applications.