Narrow Band Total Absorber at Near-Infrared Wavelengths Using Monolayer Graphene and Sub-Wavelength Grating Based on Critical Coupling

Graphene has originally low absorption at telecommunication wavelengths. Therefore, it is necessary to overcome this hurdle to use graphene as an absorption media in many optoelectronic devices. In this paper, a narrow-bandwidth graphene-based total absorber is numerically and theoretically investigated. The proposed structure consists of a graphene sheet on a Si rectangular grating, which is placed on SiO
₂
spacer backed with the gold substrate. Due to the absence of plasmonic response of graphene in near infrared wavelengths, the method of critical coupling with guided mode resonance is employed to realize highly efficient absorption of light in the graphene sheet. The simulation results show that the maximum magnitude of absorption reaches near 100% (43-fold enhancement compared to isolated monolayer graphene absorption of 2.3%). Also, based on calculated results, the absorption of the proposed structure can be controlled by altering the geometrical parameters and chemical potential of graphene. In particular, the absorption spectrum is modulated by the external gate voltage. Moreover, we find that the operating wavelength is adjusted flexibly by an incident source angle. As a result, the proposed absorber has high directivity and can act similar to an antenna. The control ability of total absorption in the proposed structure provides potential applications for the realization of ultra-compact and high-performance optoelectronic devices, such as sensors and narrow-band filters.