RESUMEN
Metamaterials and plasmonic structures made from aluminum (Al) have attracted significant interest due to their low cost, long-term stability, and the relative abundance of aluminum compared to the rare metals. Also, aluminum displays distinct dielectric properties allowing for the excitation of surface plasmons in the ultraviolet region with minimal non-radiative losses. Despite these clear advantages, most of the research has been focused on either gold or silver, probably due to difficulties in forming smooth thin films of aluminum. In the present work, we detect and characterize second harmonic generation (SHG) in the optical regime, emanating from triangular hole arrays milled into thin aluminum films in reflection mode, at normal incidence. We report intense nonlinear responses, year-long stability, and overall superior performances with respect to gold. The robustness of the Al structures and high reproducibility of the measured SHG responses allowed us to investigate changes in the directional emission upon tiny modifications in the structure's symmetry. We also demonstrate large-field instantaneous SHG imaging over areas containing several hole arrays using a recent, non-linear single-spinning disk microscope. Such high spatio-temporal resolution imaging has important applications, e.g., when studying chemical transformations occurring at electrode surfaces during charging and discharging cycles, as well as ageing.
RESUMEN
Second harmonic generation (SHG) is forbidden from most bulk metals because metals are characterized by centrosymmetric symmetry. Adsorption or desorption of molecules at the metal interface can break the symmetry and lead to SHG responses. Yet, the response is relatively low, and minute changes occurring at the interface, especially at solid/liquid interfaces, like in battery electrodes are difficult to assess. Herein, we use a plasmonic structure milled in a gold electrode to increase the overall SHG signal from the interface and gain information about small changes occurring at the interface. Using a specific homebuilt cell, we monitor changes at the liquid/electrode interface. Specifically, traces of water in dimethoxyethane (DME) have been detected following changes in the SHG responses from the plasmonic structures. We propose that by plasmonic structures this technique can be used for assessing minute changes occurring at solid/liquid interfaces such as battery electrodes.
RESUMEN
Second harmonic generation (SHG) is forbidden for materials with inversion symmetry, such as bulk metals. Symmetry can be broken by morphological or dielectric discontinuities, yet SHG from a smooth continuous metallic surface is negligible. Using non-linear microscopy, we experimentally demonstrate enhanced SHG within an area of smooth silver film surrounded by nanocavities. Nanocavity-assisted SHG is locally enhanced by more than one order of magnitude compared to a neighboring silver surface area. Linear optical measurements and cathodoluminescence (CL) imaging substantiate these observations. We suggest that plasmonic modes launched from the edges of the nanocavities propagate onto the smooth silver film and annihilate, locally generating SHG. In addition, we show that these hotspots can be dynamically controlled in intensity and location by altering the polarization of the incoming field. Our results show that switchable nonlinear hotspots can be generated on smooth metallic films, with important applications in photocatalysis, single-molecule spectroscopy and non-linear surface imaging.
