Fig. 1

Optical field enhancement and femtosecond photoemission in resonant nanostars. a, b TEM images of nanostars on Si₃N₄ membranes (500 nm scale bars). Insets show close-ups of single stars (50 nm scale bars). c Measured optical extinction spectra (solid lines) for ensembles of the two nanostar batches (see color-coded frames in a, b), compared to simulated spectra of individual nanostars from the respective batch (color-coded, dashed). d Simulated intensity enhancement (magnitude square of surface-normal electric field component) for a single nanostar with dimensions extracted from a. An arrow indicates the incident polarization (20 nm scale bar). e Schematic of the experimental setup: light is focused onto the sample (nanostars deposited on the glass substrate side facing toward the electron detector); electrons are detected using a microchannel plate (MCP) phosphor-screen assembly. f Photoemission map from a nanostar sample (850 nm resonance wavelength, surface density of 0.1 µm⁻²). The inset shows a fine scan over a single star, with the dashed circle indicating the FWHM intensity spot size of the laser focus (scale bar: 2 µm). g Scanning electron micrograph of the same region scanned in f (scale bar: 5 µm; scale bar in g also applies to f). The insets show close-ups of individual nanostars (50 nm scale bars). The circles in f and g indicate the positions of nanostars leading to considerable photoemission