Tunable large resonant absorption in a midinfrared graphene Salisbury screen

The optical absorption properties of periodically patterned graphene plasmonic resonators are studied experimentally as the graphene sheet is placed near a metallic reflector. By varying the size and carrier density of the graphene, the parameters for achieving a surface impedance closely matched to free-space $(Z_0=377\Omega )$ are determined and shown to result in 24.5% total optical absorption in the graphene sheet. Theoretical analysis shows that complete absorption is achievable with higher doping or lower loss. This geometry, known as a Salisbury screen, provides an efficient means of light coupling to the highly confined graphene plasmonic modes for future optoelectronic applications.

Figure 1

(a) Schematic device structure of graphene Salisbury screen. The inset illustrates the device with the optical waves at the resonance condition. (b) dc resistance of graphene as a function of the gate voltage. The inset is an AFM image of 40 nm nanoresonators.

Figure 2

(a) The total absorption in the device for undoped (red dashed) and hole doped (blue solid) 40 nm nanoresonators. (b) The change in absorption with respect to the ab-sorption at the charge neutral point (CNP) in 40-nm-wide graphene nanoresonators at various doping levels. The solid black curve represents the absorption difference of bare (unpatterned) graphene. (c) Width dependence of the absorption difference with the carrier concentration of $1.42\times {10}^{13}{\mathrm{cm}}^{-2}$. The resonator width varies from 20 to 60 nm. The dashed curve shows the theoretical intensity of the surface parallel electric field at the ${\mathrm{SiN}}_x$ surface when graphene is absent.

Figure 3

(a) Peak frequency as a function of resonator width. Solid curves and the symbols plot the theoretical and measured peak frequencies, respectively. (b) Frequency dependence of the experimental (symbols) and theoretical (curves) maximum absorption differences with varying doping level. (c) Theoretical electric field profile of a 40 nm graphene nanoresonator with the highest achieved carrier density ($1.42\times {10}^{13}{\mathrm{cm}}^{-2}$).

Figure 4

(a) Dependence of normalized surface admittance $Y/Y_0$ of 40 nm graphene nanoribbon array on resonance (red) and the maximum absorption (blue) on the carrier mobility μ (intraband scattering rate $\Gamma =e{v}_F/\mu \sqrt{n\pi }$). The ${\mathrm{SiN}}_x$ thickness and the pitch are assumed to be 1 μm and 80 nm, respectively. (b) Maximum absorption in the device as a function of the ${\mathrm{SiN}}_x$ thickness and the mobility. Impedance matching condition $(Y=Y_0)$ is indicated as the gray dashed line. The red dotted curve indicates the condition for perfect absorption.