Ultrasensitive optical absorption in graphene based on bound states in the continuum

Abstract

We have designed a sphere-graphene-slab structure so that the electromagnetic wave can be well confined in the graphene due to the formation of a bound state in a continuum (BIC) of radiation modes. Based on such a bound state, we have realized strong optical absorption in the monolayer graphene. Such a strong optical absorption exhibits many advantages. It is ultrasensitive to the wavelength because the Q factor of the absorption peak can be more than 2000. By taking suitable BICs, the selective absorption for S and P waves has not only been realized, but also all-angle absorption for the S and P waves at the same time has been demonstrated. We have also found that ultrasensitive strong absorptions can appear at any wavelength from mid-infrared to far-infrared band. These phenomena are very beneficial to biosensing, perfect filters and waveguides.

Figure 1

(a) Diagram of the sphere-slab structure and coordinate. The spheres are arranged in a square lattice with the lattice constant a. The radii of spheres are 0.3a. The slab is placed next to the spheres and the thickness is 0.3a. (b) shows the absolute value of the electric field in one primitive cell at . The incident wave is along Z-axis normally to the XY-plane, the amplitude of the incident field is 1 and the polarization is along X-axis. Red coordinate represents the field intensity distribution along the X-axis at the interface between the spheres and the slab (Y = 0, Z = 1.1); Blue coordinate corresponds to the field intensity distribution along the Z-axis at X = 0.01 and Y = 0.01. (c) and (e) describe the reflectivity R as a function of the reduced wavelength and the component of wave vector k_x for S and P wave, respectively. Because the resonant peaks are too sharp to be displayed, we highlight the bound states with dashed lines. The boundary between the black and colored region is the light line. The corresponding reflectivity for the S and P waves at various incident angles are given in (d) and (f) as a function of the reduced wavelength .

Figure 2

(a) Schematic diagram of the sphere-graphene-slab structure. (b), (c) and (d) show the absorption as a function of wavelength λ under the normal incident wave. Here a is taken as 5.5185 μm. (b) Various thickness of the slab at r = 0.3a. (c) Various sizes of the sphere at D = 0.3a. (d) Different E_F at r = 0.3a and D = 0.3a. The other parameters are identical with those in Fig. 1.

Figure 3. Distributions of the electric field intensity in the sphere-graphene-slab structure at the resonant absorption case.

Here λ = 15 μm, a = 5.5185 μm, r = 0.3a and D = 0.3a. The other parameters are identical with those in Fig. 1. (a) Distributions of the electric field intensity in the XZ-plane at Y = 0. (b) Distributions of the electric field intensity in the XY-plane for one primitive cell with Z = 0.5, Z = 0.8, Z = 1.1 and Z = 1.39.

Figure 4. Absorption for the sphere-graphene-slab structure as a function of wavelength λ at various lattice constants under the normal incident wave.

The other parameters are identical with those in Fig. 1.

Figure 5. The absorption peaks as a function of the wavelength λ and the incident angle θ.

Here a = 5.5185 μm. The solid lines and circle dotted lines represent absorption peaks for the S wave and the P wave, respectively. The other parameters are identical with those in Fig. 1.