Long-lived electron emission reveals localized plasmon modes in disordered nanosponge antennas

We report long-lived, highly spatially localized plasmon states on the surface of nanoporous gold nanoparticles—nanosponges—with high excitation efficiency. It is well known that disorder on the nanometer scale, particularly in two-dimensional systems, can lead to plasmon localization and large field enhancements, which can, in turn, be used to enhance nonlinear optical effects and to study and exploit quantum optical processes. Here, we introduce promising, three-dimensional model systems for light capture and plasmon localization as gold nanosponges that are formed by the dewetting of gold/silver bilayers and dealloying. We study light-induced electron emission from single nanosponges, a nonlinear process with exponents of n≈5...7, using ultrashort laser pulse excitation to achieve femtosecond time resolution. The long-lived electron emission process proves, in combination with optical extinction measurements and finite-difference time-domain calculations, the existence of localized modes with lifetimes of more than 20 fs. These electrons couple efficiently to the dipole antenna mode of each individual nanosponge, which in turn couples to the far-field. Thus, individual gold nanosponges are cheap and robust disordered nanoantennas with strong local resonances, and an ensemble of nanosponges constitutes a meta material with a strong polarization independent, nonlinear response over a wide frequency range. Sponge-like gold nanoparticles riddled with pores can act as plasmonic antennas with strong, long-lived local resonances, find a team in Germany. Christoph Lienau at Carl von Ossietzky Universität in Germany and co-workers fabricated such ‘nanosponges’ by dewetting and dealloying gold–silver bilayers. They then investigated the optical properties of the nanosponges by exciting them with femtosecond laser pulses and performing ultrafast analysis of light-induced electron emission. These electron emission measurements indicated that the nanosponges funneled incoming light into a few highly localized plasmon modes with lifetimes of the order of 20 fs on the surface of the nanosponges. Furthermore, ensembles of the nanosponges exhibited a strong nonlinear, polarization-independent response over a wide frequency range. Such nanosponges could be useful for applications such as random lasing, plasmonic sensing and surface-enhanced Raman scattering.