Switching a polymer Electrically switchable metasurfaces and plasmonic materials will enable the development of active nanophotonic technology. Karst et al . show that a metallic polymer can be used for electrical switching of plasmonic nanoantenna resonances. The plasmonic resonance can be completely switched ON and OFF with switching speeds up to 30 hertz (video rate), low switching voltages of ±1 volt (complementary metal-oxide semiconductor compatible), and a switching contrast of 100%. The results could have applications in nanophotonic devices such as those used in augmented and virtual reality imaging applications. —ISO

Plasmonic nanostructures are used today for a variety of applications. Choosing the best suited plasmonic material for a specific application depends on several criteria, such as chemical and thermal stability, bulk plasma frequency, nonlinear response, and fabrication constraints. To provide a comprehensive summary, we compare these properties for eight different plasmonic materials, namely, Ag, Al, Au, Cu, Mg, Ni, Pd, and Pt. All these materials can be fabricated with electron beam lithography and subsequent evaporation of the desired material. First, we heated rod-antenna-type nanostructures made from these materials up to 1100 °C in air and investigated their linear optical response. Most structures lose their plasmonic properties at temperatures far below the melting point of the respective material. Gold, silver, and platinum structurally deform, whereas the other materials appear to chemically degrade. Second, to improve the thermal stability, structures with a 4 nm thin Al2O3 capping layer are fabricated. The thermal stability is significantly increased with the capping layer for all materials except for copper and magnesium. Lastly, the laser damage threshold is investigated for silver, aluminum, gold, and copper, which exhibit high nonlinear optical susceptibilities and are therefore particularly interesting for nonlinear optical applications.

Switchable metasurfaces can actively control the functionality of integrated metadevices with high efficiency and on ultra-small length scales. Such metadevices include active lenses, dynamic diffractive optical elements, or switchable holograms. Especially, for applications in emerging technologies such as AR (augmented reality) and VR (virtual reality) devices, sophisticated metaoptics with unique functionalities are crucially important. In particular, metaoptics which can be switched electrically on or off will allow to change the routing, focusing, or functionality in general of miniaturized optical components on demand. Here, we demonstrate metalenses-on-demand made from metallic polymer plasmonic nanoantennas which are electrically switchable at CMOS (complementary metal-oxide-semiconductor) compatible voltages of ±1 V. The nanoantennas exhibit plasmonic resonances which can be reversibly switched ON and OFF via the applied voltage, utilizing the optical metal-to-insulator transition of the metallic polymer. Ultimately, we realize an electro-active non-volatile multi-functional metaobjective composed of two metalenses, whose unique optical states can be set on demand. Overall, our work opens up the possibility for a new level of electro-optical elements for ultra-compact photonic integration.

Arrays of metallic nanoparticles can be used for plasmonic color printing. However, the development of dynamic plasmonic color displays capable of controllable and reversible switching of individual pixels is still in its infancy. Here, an active plasmonic device that operates at the border between the visible and the near-infrared spectral region using inverse copper nanoantennas is introduced. This is a suitable choice, as copper can easily and reversibly be oxidized and reduced with the method of cyclic voltammetry. The inverse sample setup allows for the easy application of an electrical contact. With this configuration, a centroid wavelength shift of up to 210 nm (Δλ/λ ≈ 25%) within less than 3 min can be achieved, which is extraordinarily large. The resonance shift can be reversed, and the response increases with every voltage cycle, which is attributed to structural changes on the copper surface, leading to an increased surface area.

Infrared vibrational spectroscopy is a powerful tool for the identification of changes of the secondary protein structure, which are associated with many human diseases, such as Alzheimer's or Parkinson's disease. In order to obtain deeper insight into the mechanisms that lead to such changes, it is important to investigate the protein conformation on minimum sample concentrations and volumes, ideally on the single-protein level, and at short measurement times. For this purpose, surface-enhanced infrared absorption in combination with Fourier-transform infrared (FTIR) micro-spectroscopy is a highly suitable technique, but is usually limited to large numbers of proteins and long integration times due to the low brilliance of the standard thermal infrared light sources, such as Globar light sources. Here, we push this inherent limit down by using a highly brilliant, broadband mid-IR laser in combination with a standard FTIR microscope. Thus, detecting the secondary protein structure in a sample volume that is ∼100 times smaller than previously demonstrated becomes feasible with our tabletop FTIR system. We utilize polypeptides as a model system, which we functionalize on a single resonant gold nanoantenna, and demonstrate the capabilities of our ultrasensitive system to detect the protein conformation in a living environment within 10 min, without the necessity of frequency tuning the mid-IR laser or any data postprocessing. For comparison, measurements conducted with a synchrotron light source were also performed, which support the results obtained with the laser source. We believe that with further advances it will be possible to scale the process to the ultimate limit of a few or even single proteins and observe the conformational behavior of a few or individual entities in an aqueous environment.