We demonstrate here, that plasmonic excitations can be coupled efficiently to acoustic high-frequency waves (up to ~ 100 GHz). Such coupling is studied in a gold grating with a period of d = 400 nm deposited on a dielectric gadolinium gallium garnet substrate. The grating allows the optical excitation of surface plasmon polaritons (SPP’s) provided that the wavelength and the incidence angle of the light are selected in a way to match energy and quasi-momentum conservation laws. The feature of such matching is a dip in the optical reflectivity spectrum.
A broadband acoustic wavepacket is generated in form of an optically excited picosecond strain pulse. For that purpose, an Al transducer film deposited on the surface of the sample opposite to the plasmonic grating is illuminated by an 800 nm femtosecond laser pulse (pump pulse). The acoustic wavepacket propagates through the gadolinium gallium garnet substrate and reaches the grating, where the elastic waves undergo acoustic diffraction. This diffraction results in the generation of near-surface Longitudinal Acoustic (LA) waves in accordance with Bragg conditions: Phonon wavelength ( = 1, 2, 3...). The standing elastic waves are formed in the region near the surface where the electric field components of the SPP are essential.
The effect of the SPP interaction with those coherent near-surface acoustic waves is detected by probing the time-resolved reflectivity change of an optical pulse originating from the same laser and variably delayed relative to the pump pulse. An example of such a time-resolved reflectivity signal is shown in Fig.1a for the case where the incidence angle of the probe pulse corresponds to the SPP resonance. The corresponding Fourier spectrum in Fig.1b shows a sequence of equidistant peaks with frequencies up to 110 GHz. The peaks have maxima at frequencies which correspond to the near surface LA modes generated in accordance with Bragg conditions for phonon diffraction. The modulation of the stripe-gap spacings is the responsible mechanism for the SPP-phonon interaction.
The observed sub-THz acousto-plasmonic effects have high potential for applications in nanoplasmonic devices. In particular, such techniques could be employed in recently developed plasmon lasers (spasers).