Dynamically tunable extraordinary light absorption in monolayer graphene

The high carrier mobility of graphene makes it an attractive material for electronics, however, graphene's application for optoelectronic systems is limited due to its low optical absorption. We present a cavity-coupled nanopatterned graphene absorber designed to sustain temporal and spatial overlap between localized surface plasmon resonance and cavity modes, thereby resulting in enhanced absorption up to an unprecedented value of theoretically $(60\%)$ and experimentally measured $(45\%)$ monolayer graphene in the technologically relevant 8–12-μm atmospheric transparent infrared imaging band. We demonstrate a wide electrostatic tunability of the absorption band $(\sim 2\mu \mathrm{m})$ by modifying the Fermi energy. The proposed device design allows enhanced absorption and dynamic tunability of chemical vapor deposition grown low carrier mobility graphene which provides a significant advantage over previous strategies where absorption enhancement was limited to exfoliated high carrier mobility graphene. We developed an analytical model that incorporates the coupling of the graphene electron and substrate phonons, providing valuable and instructive insights into the modified plasmon-phonon dispersion relation necessary to interpret the experimental observations. Such gate voltage and cavity tunable enhanced absorption in chemical vapor deposited large area monolayer graphene paves the path towards the scalable development of ultrasensitive infrared photodetectors, modulators, and other optoelectronic devices.

Figure 1

Extraordinary absorption in cavity-coupled nanomesh graphene and the effect of carrier mobility on absorption enhancement. (a) Left: FDTD and CDA predicted absorption of the patterned graphene. Right: FDTD prediction of absorption as a function of cavity thicknesses for the cavity-coupled case. The white solid and green dotted lines represent constructive and destructive cavity modes respectively. (b) FDTD predicted cavity length and wavelength dependent absorption for high (middle) and low mobility (right). The corresponding wavelength dependent absorption for two cavity thicknesses are shown on left.

Figure 2

Fabricated system and characterizations. (a) Schematic of the cavity-coupled patterned graphene. (b) Raman spectrum of grown pristine graphene. The presence of sharp and strong 2D peak proves monolayer graphene. (c) SEM image of the fabricated patterned graphene on dielectric slab (left) and Conductive AFM image of patterned graphene on copper foil (right).

Figure 3

Fabricated system and dynamic tunable response. Experimentally measured (solid) and theoretically predicted (dashed) mobility dependent tunable absorption spectra for a high (a) $(L=1.1\mu \mathrm{m})$ and low (b) $(L=1.6\mu \mathrm{m})$ mobility monolayer patterned graphene. Right figures showing the comparison of experimental, CDA, and theoretical results. Theoretical graphene plasmon frequency follows ${\omega }_{\mathrm{p}}\propto \surd E_F\propto n^{1/4}$.

Figure 4

Energy-loss dispersion. (a) Loss function for graphene with $E_F=1.0\mathrm{eV}$. $k_{\mathrm{spp}}$ is the plasmon wave number associated with the second mode. ${\omega }_0, {\omega }_1$, and ${\omega }_2$ represent the LSPR and the resonance propagating plasmon frequency without and with plasmon-phonon interaction, respectively. (b) Experimental and theoretical prediction of the plasmon excitation on patterned graphene with $\mathrm{period}=400\mathrm{nm}, \mathrm{diameter}=330\mathrm{nm}$, and $\mu =960\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ coupled to an optical cavity with cavity thickness of $1.1\mu \mathrm{m}$.