Coupling Light into Graphene Plasmons through Surface Acoustic Waves

We propose a scheme for coupling laser light into graphene plasmons with the help of electrically generated surface acoustic waves. The surface acoustic wave forms a diffraction grating which allows us to excite the long lived phononlike branch of the hybridized graphene plasmon-phonon dispersion with infrared laser light. Our approach avoids patterning the graphene sheet, does not rely on complicated optical near-field techniques, and allows us to electrically switch the coupling between far-field radiation and propagating graphene plasmons.

Figure 1

Sketch of the device. A high-frequency signal applied to an interdigital transducer (IDT) on a piezoelectric film generates a surface acoustic wave (SAW) propagating in the $x$ direction. The graphene sheet is deformed by the SAW, the periodic deformation acting as a refraction grid for the incident laser beam. A back gate allows tuning the Fermi energy of graphene.

Figure 2

(a), (c) Plasmon dispersion for graphene on AlN and on ZnO, respectively. The two hybridized plasmon branches are well described by the asymptotic expressions ${\omega }_{p\pm }(k)$ of Eq. (7) (white curves). The contour plot shows the imaginary part of the reflection coefficient $r^{\mathrm{TM}}(k,\omega )$; see Eq. (8a). Parameters are $E_F=0.4\mathrm{eV}$ and ${\tau }_e=2\times {10}^{-13}\mathrm{s}$ (corresponding to a mobility of $5000{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$). The white triangular area indicates the region of high intraband loss, $\omega <v_{F}k$. (b), (d) Plasmon lifetime ${\tau }_p$ along the two branches.

Figure 3

(a) Transmittance of AlN and graphene on AlN in the absence of surface corrugation, normal incident light. (b) Change in the transmittance of TM polarized light due to plasmon excitation in the presence of a SAW. SAW amplitude $\delta =4\mathrm{nm}$ and period ${\lambda }_g=0.25\mu \mathrm{m}$, graphene parameters $E_F$ and ${\tau }_e$ as in Fig. 2. (c) Same as (b) for various angles of incidence $\theta$. (d) Extinction spectrum of graphene on AlN in the presence of a SAW ($\delta =4\mathrm{nm}$ and ${\lambda }_g=0.25\mu \mathrm{m}$) for different values of $E_F$ and ${\tau }_e$. Peaks are due to excitation of plasmons in the ${\omega }_{p+}$ branch [see Eq. (7)]. (e) Same as (d) for excitation of plasmons in the ${\omega }_{p-}$ branch, $\delta =4\mathrm{nm}$ and ${\lambda }_g=1\mu \mathrm{m}$. (f) Extinction spectrum corresponding to (c).