Abstract
Graphene is an ideal ultrathin material for various optoelectronic devices, but poor light–graphene interaction limits its further applications particularly in the visible (Vis) to near-infrared (NIR) region. Despite tremendous efforts to improve light absorption in graphene, achieving highly efficient light absorption of monolayer graphene within a comparatively simple architecture is still urgently needed. Here, we demonstrate the interesting attribute of bound state in the continuum (BIC) for highly efficient light absorption of graphene by using a simple Si-based photonic crystal slab (PCS) with a slit. Near-perfect absorption of monolayer graphene can be realized due to high confinement of light and near-field enhancement in the Si-based PCS, where BIC turns into quasi-BIC due to the symmetry-breaking of the structure. Theoretical analysis based on the coupled mode theory (CMT) is proposed to evaluate the absorption performances of monolayer graphene integrated with the symmetry-broken PCS, which indicates that high absorption of graphene is feasible at critical coupling based on the destructive interference of transmission light. Moreover, the absorption spectra of the monolayer graphene are stable to the variations of the structural parameters, and the angular tolerances of classical incidence can be effectively improved via full conical incidence. By using the full conical incidence, the angular bandwidths for the peak absorptivity and for the central wavelength of graphene absorption can be enhanced more than five times and 2.92 times, respectively. When the Si-based PCS with graphene is used in refractive index sensors, excellent sensing performances with sensitivity of 604 nm/RIU and figure of merit (FoM) of 151 can be achieved.
Highlights
Graphene has attracted a high level of research interest due to its exceptional optical, electronical, chemical, and mechanical properties, combined with its unique electronic band structure [1,2]
Wang et al [38] showed that light absorption in graphene can be controlled by asymmetric parameter of suspended silicon nanodisk, and maximum absorption of 50% can be realized at critical coupling through quasi-bound state in the continuum (BIC) in the two-port system composed of graphene
Perfect absorption with A = 100% is realizable at critical coupling with γ = δ if t2 = 0, it is feasible to achieve highly efficient light absorption of graphene at critical coupling based on the destructive interference of transmission light within a comparatively simple structure composed of graphene
Summary
Graphene has attracted a high level of research interest due to its exceptional optical, electronical, chemical, and mechanical properties, combined with its unique electronic band structure [1,2]. Zhang et al [37] showed that optical absorption in graphene can be enhanced based on BIC using a sphere–graphene–slab structure, and ultrasensitive absorption of graphene can be obtained via the excitation of GSPPs. Later, Wang et al [38] showed that light absorption in graphene can be controlled by asymmetric parameter of suspended silicon nanodisk, and maximum absorption of 50% can be realized at critical coupling through quasi-BIC in the two-port system composed of graphene. Near-perfect absorption of the monolayer graphene is realized in the communication band by using the BIC based on the photonic crystal slab (PCS) of Si. The high confinement of light and near-field enhancement in the Si-based PCS with a slit contributes to the unexpected improvement of graphene absorption, where BIC turns into quasi-BIC due to the symmetry-breaking of the structure. Excellent sensing performances can be realized when the Si-based PCS with graphene is used in refractive index sensors
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