Double-channel absorption enhancement of graphene using narrow groove metal grating
Authors: Gao Jian  , Sang Tian  , Li Jun-Lang  , Wang La  ,   Department of Photoelectric Information Science and Engineering, School of Science, Jiangnan University, Wuxi 214122, China;,   Jiangsu Provincial Research Center of Light Industrial Optoelectronic Engineering and Technology, Jiangnan University, Wuxi 214122, China
Published: December 2020

A structure containing substrate/narrow groove metal grating/covering layer/graphene is constructed. The operational principle of the structure is based on the surface plasmon polariton (SPP) resonance excited by the metal grating and the Fabry-Prot (FP) resonance supported by the narrow grating groove. Double-channel absorption enhancement of monolayer graphene is realized in the visible range, and a simplified model is used to estimate the locations of the double-absorption channels. At the wavelengths of 462 nm and 768 nm, the light absorption efficiencies of graphene are 35.6% and 40.1%, respectively, which are more than 15.5 times the intrinsic light absorption of the monolayer graphene. Further analysis shows that the energy of the absorption peak at the short-wavelength position mainly concentrates on the surface of the metal grating, which has an obvious characteristic of the SPP mode. The resonant wavelength of SPP=476 nm, estimated by the simplified model, is basically consistent with the location of the short-wavelength absorption peak at 1=462 nm. The absorption characteristics are less affected by the thickness of the covering layer, the depth and width of the groove. For the long-wavelength absorption peak at 2=768 nm, the energy of the light field in the structure is mainly localized in the metal groove, which has a significant cavity resonance characteristic. Because the SPP resonance generates a strong electromagnetic coupling in the metal groove, the energy of the optical field is strongly confined by the grating groove. The localized light field energy gradually leaks out and is absorbed by the graphene layer above the groove, resulting in a significant increase in the light absorption efficiency of the graphene. The resonance position estimated by the FP cavity resonance model is 658 nm, which is larger than the actual absorption peak position 2=768 nm. This is because the exact length of the FP cavity is affected by the thickness of the SiO2 covering layer, and the presence of the SiO2 covering layer will enlarge the exact length of the FP cavity. To further increase the depth of the groove, the agreement between the estimated resonance position and the actual absorption peak will continue to increase. However, the increase of the thickness of the SiO2 covering layer will weaken the magnetic field enhancement effect in the groove, resulting in the decrease of light absorption efficiency of the structure and graphene. Since the absorption enhancement at the long-wavelength peak originates from the FP resonance in the narrow groove, it exhibits a good angle-insensitive absorption characteristic. The double-channel absorption enhancement of graphene based on the narrow grooved gratings may have potential applications in the fields of photodetection and solar cells.