Continuous-Wave Multiphoton Photoemission from Plasmonic Nanostars

Highly nonlinear optical processes, such as multiphoton photoemission, require high intensities, typically achieved with ultrashort laser pulses and, hence, were first observed with the advent of picosecond laser technology. An alternative approach for reaching the required field intensities is offered by localized optical resonances such as plasmons. Here, we demonstrate localized multiphoton photoemission from plasmonic nanostructures under continuous-wave illumination. We use synthesized plasmonic gold nanostars, which exhibit sharp tips with structural features smaller than 5 nm, leading to near-field-intensity enhancements exceeding 1000. This large enhancement facilitates 3-photon photoemission driven by a simple continuous-wave laser diode. We characterize the intensity and polarization dependencies of the photoemission yield from both individual nanostars and ensembles. Numerical simulations of the plasmonic enhancement, the near-field distributions, and the photoemission intensities are in good agreement with experiment. Our results open a new avenue for the design of nanoscale electron sources.

In general, nonlinear optical signals can be enhanced by confining a given incident average power in time and/or space. While temporal confinement is ubiquitous in the use of ultrashort laser pulses, additional spatial confinement is realized in optical nanostructures, defining the field of ultrafast nano-optics 32,33 . In particular, extensive theoretical and experimental work has shown a growing level of control over the near-field localization associated with resonant modes in optimized nanostructure geometries 34,35 . Exceedingly large field-enhancements in plasmonic nanostructures suggest the observation of highly nonlinear processes even under continuouswave (CW) illumination conditions.
Here, we demonstrate nonlinear photoelectron emission from individual resonant gold nanostars under CW excitation at incident intensities below 1 MWcm -2 , using a 630-nm lowpower laser diode. We characterize the CW multiphoton photoemission yield as a function of incident intensity and polarization, and further provide spatial scans to identify emission from individual nanostars. These findings are compared with photoemission measurements using 10 fs laser pulses at 800 nm central wavelength. Additionally, we present simulations of the electromagnetic near-field distributions and the resulting photoelectron yield that further support the nanoscale plasmonic origin of CW nonlinear photoemission at the single-particle level. Our results illustrate the potential of plasmonic field confinement in tailored resonant nanostructures to widely proliferate nonlinear nano-optics beyond ultrafast science.
The nanostars used in our experiments are grown by a seed-mediated approach 36,37 (see Methods for details) and exhibit multiple protuberances terminating in sharp tips, with radii as small as 4 nm (insets to Figure 1a,b). Despite the particle-to-particle variability in the detailed nanostar morphology, the controlled growth conditions used in the synthesis allow us to tune their plasmonic response close to the laser operation wavelengths of either 660-nm (CW) or 800 nm (fs-pulses with 190 nm full-width-at-half-maximum (FWHM) spectral bandwidth). Figure 1c shows the measured ensemble optical extinction spectra for both sets of nanostars deposited on glass slides (solid curves). Electromagnetic simulations of individual nanosstars (Fig. 1d) from each sample batch, with structural features sizes extracted from the TEM images in Figs. 1a and b, yield spectra (dashed curves) agreeing well with the central wavelength of measured response function. The simulated spectra are essentially dominated by one of the protruding tips of the particle, and therefore, notably narrowed compared to the experimental ensemble spectra. The calculated intensity enhancement for single 3D nanostars, as presented in Figure 1d, exceeds 10 3 at tip regions a few nanometers in diameter. Figure 1d plots the magnitude squared of the optical field component which is locally perpendicular to the surface, as the surface-parallel component does not contribute significantly to photoemission due to low quantum efficiency 38 .
In the photoemission experiments, nanostars (cf. Figures 1a and 1b) dispensed on a fused silica substrate with conductive indium-tin-oxide (ITO) coating are illuminated with focused CW or fspulsed laser radiation (see Methods for details), as depicted in Figure 1e. The focal-spot diameters (FWHM of intensity) are 3.5 µm  1.1 µm (major  minor axis) and 5 µm for the CW and femtosecond-pulsed illumination, respectively, enabling the excitation of single nanostars for samples with a surface coverage of 0.1 particles/µm². Polarization and intensity control is realized with a broadband half-wave plate and a thin-film polarizer. The photoemission measurements are conducted in a high-vacuum chamber at background pressures of 10 -7 mbar.
Emitted photoelectrons are detected using a phosphor-screen microchannel-plate (MCP), imaged by a charge-coupled device camera for a moderate bias voltage (-10 to -30 V) applied to the sample, drawing emitted electrons towards the grounded detector front plate. Spatial photoemission maps are obtained by scanning the samples relative to the laser focus using a precision 3D translation stage.  In order to better understand the nonlinear photoemission process, we carry out perturbative simulations of the photoemission yield from a single nanostar under CW excitation (see solid red curve in Fig. 2a), based upon a description of conduction electrons as independent particles subject to a rectangular step potential to describe the surface barrier (see Methods for details).
The results are in good quantitative agreement with the experimentally observed electron yield, justifying the employed perturbative treatment.
In both experiment and simulation, the far-field coupling to the resonant modes of individual nanostars strongly depends on the incident laser polarization. Figure 3   In conclusion, we demonstrated 3-photon photoemission from individual gold nanoparticles using low-power CW laser radiation at a wavelength of 660 nm (1.8 eV photon energy). This type of nonlinear processes requires large light intensities typically realized by employing ultrafast laser pulses. Instead, by harnessing a 1000-fold optical CW near-field intensity enhancement via localized plasmons at the tips of gold nanostars, we achieve a >10 9 fold total enhancement of the 3-photon electron yield, which agrees with calculations from a perturbative model. The findings suggest the use of very sharp tips (> 4 nm radii) as coherent electron sources in future nanoscale free-electron devices with potential applications in microscopy, spectroscopy, sensing, and signal processing.

Photoemission experiment
Two sources are used to illuminate the samples with (i) few-femtosecond, nano-joule laser pulses having a central wavelength of 800 nm at 80 MHz repetition rate, and (ii) continuous-wave radiation at a wavelength of 660 nm from a low-budget (sub-100 €) laser diode. Au nanostars deposition on SiN TEM grids. Au Nanostars were deposited on a TEM SiN grid via spin coating (5 μL; 1 st ramp at 500 rpm for 10 s; 2 nd ramp at 3000 rpm for 30 s with an acceleration rate for both ramps of 500 rpm/s) from two different Au concentrations (8×10 -5 M, 4×10 -4 M) to achieve particle densities of 0.18 and 0.6 particles/μm 2 .

Au nanostars deposition on glass slides for solid UV-VIS characterization. Solutions of both
types of Au Nanostars with concentrations of 5×10 -4 M were prepared and spin coated (50 μL, 500 rpm, 60 s) on microscope cover-slip glass slides to achieve a low particle density sufficient to avoid interparticle coupling while enabling UV-vis spectra to be recorded.

Theoretical methods
Electromagnetic simulations. Extinction spectra and near-field distributions are calculated using a finite-difference method (COMSOL) to solve Maxwell's equations under external plane-wave illumination for characteristic nanostar morphologies (see Figure 1d). The dielectric function of gold is taken from optical data 51 .
Multiphoton photoemission. An estimate of the photoemission yield is obtained by considering a flat surface exposed to a normal electric field with an amplitude given by the maximum intensity of the locally normal near-field resulting from the electromagnetic calculations for the nanostars.
An effective hotspot area of 5×5 nm 2 is assumed (i.e., we multiply the electron emission current density by this area). The flat-surface approximation is justified by the small electron wavelength (~1 nm at the Fermi level of gold) compared with the nanostar tip rounding radius (~4 nm). We describe the gold flat-surface through a square-step potential (depth where ′ is the initial electron wave vector inside gold, ′ = {√ ℏ − ℏ }, and ′ = √ /ℏ.