Surface wave detection of hypersound in single plasmonic nanoantennas

RODRIGO BERTE; FABRICIO DELLA PICCA; YI LI; EMILIANO CORTÉS; STEFAN A. MAIER; ANDREA V. BRAGAS

Estrasburgo

Congreso; SPIE Photonics Europe; 2018

The storage of energy in the motion of free electrons in plasmonic systems is inherently lossy due to the multiple absorption processes and states available for electron transitions above the Fermi level.1 Nevertheless, the associated tens of femtoseconds lifetime of plasmons allow the efficient excitation of vibrational modes in nanostructures with oscillation periods orders of magnitude longer, a process which can be effectively addressed, and probed, by sub-ps laser pulses.2, 3 The resulting acoustic phonons, whose frequency is defined by multiple boundary conditions such as size, composition and environment of the nanoantennas, make these resonators extremely suitable for coherent generation of spectrally narrow hypersound4, photoacoustic imaging5, acoustic-optical modulation6 and mass sensing.7 Although the parameter space of boundary conditions for these structures has been systematically explored, with hypersound tunability given by the adhesion layer8, mechanical constraints4 and even mode-selection through a coupled geometry-delayed pump pulse scheme9, this approach has only recently been extended to the coupling between nanoresonators.10 Here we present the detection of generated surface acoustic hypersound by single gold nanoantennas using resonators placed at distances more than one order of magnitude larger than the characteristic resonator dimensions. Pump-probe laser technique and finite element method calculations are used to demonstrate that the anisotropy in wave emission, dictated by the nanoparticle geometry, generates a non-intuitive delayed detection of acoustic waves for receptors positioned differently relative to the source. This work contributes to the understanding of the relaxation processes in nanostructures and their coupling to the substrate and to the design of localized subwavelength sensors.