As a multifunctional device for sensing experiments and fundamental research, tailor-made plasmonic nanostructures with continuously tunable resonances are created by preparing bow tie-shaped nanostructures on a flexible substrate. The bow ties are fabricated by electron beam lithography on a chromium sacrificial layer and transferred to a polydimethylsiloxane (PDMS) substrate. The structures on PDMS are analyzed by reflection dark-field spectroscopy and scanning electron microscopy. Dark-field spectra of individual nano-antennas are obtained while the substrate is relaxed, and while strain is applied and the substrate is elastically stretched. Depending on the alignment of the bow ties relative to the direction of the strain, the deformation of the substrates leads to an increase or decrease of the nanostructure gaps, and therefore to a fully reversible decrease or increase of the antenna coupling, respectively. The continuous change in coupling is visible as a blue-shift in the resonance of the coupling mode for increasing gap widths, and a red-shift for decreasing gap widths. This configuration offers interesting perspectives for molecular transport and sensing investigations under variable coupling conditions as well as for tunable SERS substrates and optical strain sensor applications. In particular, very narrow gaps are within reach in the transversal configuration.

Conventional opaque electrodes in microelectrode array (MEA) technology obstruct the view of cells in their immediate vicinity (e.g., ≈50 µm) from which the strongest extracellular action potentials are recorded. This limitation has been overcome by transparent graphene electrodes which allow for optical access essential for novel applications such as optogenetics and calcium imaging. Downscaling, necessary for high resolution single‐unit electrophysiological recordings, has been a significant challenge due to inferior electrochemical impedance and correspondingly lower signal‐to‐noise ratio. Here, the combination of graphene with the conductive polymer poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as a transparent microelectrode material for in vitro MEAs is presented and their application with optical imaging and electrophysiology is demonstrated. Optimal graphene/PEDOT:PSS microelectrodes display transparencies of 84% over the visible spectrum and impedance magnitude of (166 ± 13) kΩ at 1 kHz. The balance of transparency and 1 kHz impedance can be tuned from ≈90% and 700 kΩ to 50% and 42 kΩ.

SummaryPlasmonic resonances in metallic nano-triangles have been investigated by irradiating these structures with short laser pulses and imaging the resulting ablation and melting patterns. The triangular gold structures were prepared on Si substrates and had a thickness of 40 nm and a side length of ca. 500 nm. Irradiation was carried out with single femtosecond and picosecond laser pulses at a wavelength of 800 nm, which excited higher order plasmon modes in these triangles. The ablation distribution as well as the local melting of small parts of the nanostructures reflect the regions of large near-field enhancement. The observed patterns are reproduced in great detail by FDTD simulations with a 3-dimensional model, provided that the calculations are not based on idealized, but on realistic structures. In this realistic model, details like the exact shape of the triangle edges and the dielectric environment of the structures are taken into account. The experimental numbers found for the field enhancement are typically somewhat smaller than the calculated ones. The results demonstrate the caveats for FDTD simulations and the potential and the limitations of “near field photography” by local ablation and melting for the mapping of complex plasmon fields and their applications.

We systematically investigate the metallic photoluminescence (MPL) emitted from plasmonic nanoparticles (NPs) upon excitation with ultrafast laser pulses using a scanning confocal optical microscope (SCOM). By comparing the emission spectra of Au NPs of varying dimensions with the corresponding dark-field scattering spectra, indications are found that MPL encompasses two emission channels: the particle plasmons (PPs) and the electron-hole (e-h) pair recombination. The plasmons can be interpreted to play a twofold role: in the excitation process they provide the local field enhancement, and in the emission process they offer extra radiation channels.