Pump-Selective Spectral Shaping of the Ultrafast Response in Plasmonic Nanostars

Plasmonic nanostructures are, to date, well-known to offer unique possibilities for the tailoring of light–matter interactions at the nanoscale. Most recently, a new route to ultrafast all-optical modulation has been disclosed by combining the resonant features of plasmonic nanostructures with the giant third-order optical nonlinearity of noble metals regulated by highly energetic (hot) carriers. In this framework, a variety of nanostructures have been designed, with special attention to shapes featuring tips, where extreme and highly sensitive field enhancements (hot spots) can be attained. Here, we report on a broadband pump–probe spectroscopy analysis of an ensemble of spiky star-shaped nanoparticles, exploring both the perturbative and nonperturbative regimes of photoexcitation. The experiments are corroborated by semiclassical numerical simulations of the ultrafast optical response of the sample. We found that the peculiar hot spots supported by the star tips allow one to easily control the spectral shape of the transient optical signal, upon tuning of the pump wavelength. Our results elucidate the ultrafast response of hot electrons in star-shaped nanostructures and contribute to the understanding of the tip-mediated enhanced nonlinearities. This work paves the way to the development of ultrafast all-optical plasmonic modulators for pump-selective spectral shaping

Resonant Fully dielectric metasurfaces for ultrafast Terahertz pulse generation

Metasurfaces represent a new frontier in materials science paving for unprecedented methods of controlling electromagnetic waves, with a range of applications spanning from sensing to imaging and communications. For pulsed terahertz generation, metasurfaces offer a gateway to tuneable thin emitters that can be utilised for large-area imaging, microscopy and spectroscopy. In literature THz-emitting metasurfaces generally exhibit high absorption, being based either on metals or on semiconductors excited in highly resonant regimes. Here we propose the use of a fully dielectric semiconductor exploiting morphology-mediated resonances and inherent quadratic nonlinear response. Our system exhibits a remarkable 40-fold efficiency enhancement compared to the unpatterned at the peak of the optimised wavelength range, demonstrating its potential as scalable emitter design