Surface acoustic wave mediated coupling of free-space radiation into surface plasmon polaritons on plain metal films

We propose and demonstrate a surface acoustic wave-driven converter of light into surface plasmon polaritons. An otherwise unstructured thin metal film is deformed by traveling acoustic waves on a piezoelectric substrate underneath. This spatially periodic corrugation enables to bridge the momentum gap between free-space radiation and surface-bound modes. This principle is realized with plain gold films on a ${\text{LiNbO}}_3$ wafer where surface acoustic waves induce surface ripples in the metal. For near-infrared light we observe efficiencies of order ${10}^{-4}$ for exciting surface plasmon polaritons.

Figure 1

(Color online) (a) Scheme of a SAW driven plasmon launcher: an RF voltage is applied to an interdigital transducer and generates surface acoustic waves that deform a metal film on top. This grating enables to convert free-space radiation $\left({\vec{k}}_0\right)$ into propagating SPP modes $\left({\vec{k}}_{\text{SPP}}\right)$. (b) Difference of the normalized TM and TE reflectivity changes related to the SAW-induced grating. Solid line: theoretical prediction for the angular/spectral dependence of the resonance according to Eq. (2) and $m=1$.

Figure 2

(Color online) (a) Angular dependence of the SAW-induced normalized reflectivity change for TM- and TE-polarized light. The arrows pointing downwards (upwards) indicate the expected angular positions for the SPP (DE) resonances. (b) Angular dependence of the first-order diffraction efficiency ${\eta }_{-1}^{\ast }$. Dashed line: theoretical prediction for light of $\lambda =940\text{nm}$ wavelength.

Figure 3

(Color online) (a) Resonance angle for various frequencies and (b) exemplary angular scans for two different RF frequencies. Light is filtered via a bandpass at a central wavelength of $\lambda =940\text{nm}$ and $P=20\text{mW}$ RF power is applied.