Abstract
Summary form only given. Plasmonic metasurfaces provide a compact platform to engineer the wave-front of optical beams by tuning the material and its morphology, hence enabling advanced functionalities in ultra-thin photonic systems. In standard metasurfaces, however, the optical response is usually static and fixed by design. An appealing possibility to achieve ultrafast dynamical tuning is given by optically-induced plasmonic systems, where the metallic response of narrow-bandgap semiconductors is driven by high-fluence illumination. Under these conditions, the surface of the semiconductor can be overflown with photo-carriers inducing a transient metallic state. An intriguing question is whether the transient metallization could be employed to dynamically engineer the optical response and to control light-matter interactions on the surface. At THz frequency-scales an optically-induced metallisation can have an initial transient much faster than the THz-period. The generation of THz fields from semiconductor surfaces is in fact, the result of a complex interplay between photo-carrier dynamics (e.g. photo-Dember) and nonlinear frequency conversion (e.g. optical rectification). A detailed characterization of the THz emission, therefore, can provide critical insights into the dynamical evolution and physical properties of the photo-induced plasmonic state. In this work, we will discuss our investigation of the THz emission from nanostructured silicon surfaces which are characterised by a near-unity absorption across the visible spectral range and are commonly denoted as Black Silicon (BSi). In BSi, the surface nanopatterning is known to induce a strong enhancement of the THz generation under ultrafast illumination, in sharp contrast with planar silicon where optical-to-THz conversion is mostly negligible. The generation of THz pulses from BSi however, is indeed driven by the presence of several competing mechanisms, including transient metallization and charge localization. We assessed the role played by each of these mechanisms through an embodiment of a dual pump approach. In our experimental setup, an ultrashort probe pulse generates THz within the sample to be detected, while a pump pulse photo-excites the nanopillars to induce the transient metallic state. The use of two optical pulses ensures significant overlap between the generated carriers and the THz generation region. For BSi, we observe a sharp transition between several regimes as a function of pump delay, with the emergence of an additional THz generation process upon photoexcitation, compatible with a plasmonic coupling and thus resulting in the observed phase shift. These results hold significant implications for a tuneable plasmonic response controlled via external pumping parameters with applications in nonlinear imaging.