利用窄刻槽金属光栅实现石墨烯双通道吸收增强

构建基底/窄刻槽金属光栅/覆盖层/石墨烯结构,利用金属光栅激发的表面等离子体激元共振和窄光栅刻槽支持的法布里-珀罗共振,在可见光波段实现单层石墨烯的双通道吸收增强,并结合简化模型估算出双吸收通道所在位置.在波长462和768 nm处,石墨烯的光吸收效率分别为35.6%和40.1%,相比石墨烯本征光吸收率的增强均超过15.5倍.进一步研究发现由于短波处吸收增强源于金属光栅的表面等离子体激元共振,其吸收特性受覆盖层厚度、刻槽深度和宽度变化的影响较小;而由于长波处吸收增强源于窄刻槽中的法布里-珀罗共振,因此呈现出良好的角度不敏感吸收特性.

Double-channel absorption enhancement of graphene using narrow groove metal grating

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-Pérot (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 λ_SPP=476 nm, estimated by the simplified model, is basically consistent with the location of the short-wavelength absorption peak at λ₁=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 λ₂=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 λ₂=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.