Highly Efficient Light Absorption of Monolayer Graphene by Quasi-Bound State in the Continuum

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.

Perfect Narrow‐Band Absorber of Monolayer Borophene in All‐Dielectric Grating Based on Quasi‐Bound State in the Continuum

“…be written as, [39,40] when the external leakage rate is the same as the inherent loss rate of borophene 𝛾 e = 𝛿 at the frequency of incident light 𝜔 = 𝜔 0 , i.e., the critical coupling condition is satisfied, A = 1 − |t| 2 ∕4. Of course, the perfect absorption can be realized when |t| 2 = 0.…”

“…The reflection and transmission coefficients of the structure are $$$$\begin{equation} \rho =r\frac{j\left(\omega -{\omega}_{0}\right)+\delta -{\gamma}_{e}}{j\left(\omega -{\omega}_{0}\right)+\delta +{\gamma}_{e}},\quad \xi =t\frac{j\left(\omega -{\omega}_{0}\right)+{\gamma}_{e}}{j\left(\omega -{\omega}_{0}\right)+\delta +{\gamma}_{e}} \end{equation}$$$$where ω is the frequency of the incident wave, ω 0 is the resonant frequency, 𝛿 is the intrinsic loss rate of borophene, and $${\gamma }_e$$ is the external leakage rate; r and t are the Fresnel reflection and transmission coefficients under off‐resonance condition, as no resonance is taking place in the structure. Therefore, the absorption of the device $$A = \ 1 - R - T = \ 1 - {| \rho |}^2 - {| \xi |}^2$$ can be written as, [ 39,40 ] …”

Low-Threshold and High-Extinction-Ratio Optical Bistability within a Graphene-Based Perfect Absorber

“…However, the interaction between light and monolayer graphene is very weak due to the ultrathin thickness of graphene, which seriously limits the applications of graphene in the optical field. In order to greatly enhance the light-graphene interaction, many kinds of graphene-based resonators [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ] and perfect absorbers [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ] have been proposed, and some applications of those structures have been demonstrated [ 35 , 36 , 37 , 38 , 39 ]. Until now, OB in graphene-based resonators has been theoretically and numerically studied [ 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 ].…”

High-quality-factor dual-band Fano resonances induced by dual bound states in the continuum using a planar nanohole slab

“…More specially, BICs can be perturbed via oblique incidence or symmetry-broken nanostructures, and the QBICs can be realized as the radiation channel between the eigenstates and the free space is opened [18,19]. However, most of the dielectric nanostructures used to excite QBICs with high Q-factor are complicated, such as asymmetrical nanocrosses [20], asymmetrical nanorings [21], asymmetrical nanobars [22][23][24] and asymmetrical nanorods [25][26][27][28], which are challenging in fabrication due to the requirement of inserting the deep subwavelength slits [20][21][22][23][24] or nanoholes [25][26][27][28] into the photonic structures. Other nanostructures such as the reshaped rectangular bars [29,30] have the increased sharp edges, making them more difficult to be accurately fabricated through conventional lithographic techniques, which reduces the Q-factor and the resonance lifetime of the devices due to the opening of additional leaky channels [31,32].…”