Plasmonic nanostar photocathodes for optically-controlled directional currents

Femtosecond optical control over nanoscale currents is crucial for various applications, including ultrafast electron diffraction and microscopy, x-ray free-electron lasers, and terahertz optoelectronic circuits. Plasmonic metal nanostructures show promise as photocathodes, femtosecond photodiodes, and photodetectors for integration into nanoscale devices. The particle geometry and resulting field enhancements primarily govern the mapping from optical field parameters to near-field electron dynamics. Particle geometry defines hot-spot regions where electrons are excited and escape as photoemission or photovoltaic currents. Geometry and patterning also control the frequency response, allowing for tailoring of broadband photodetection, photocurrent polarity, and selective multi-mode lasing. Asymmetric particles, such as gold nanostars, support multiple resonances, providing opportunities for spatiotemporal photocurrent control through frequency and polarization-selective excitation of spatially separated, differently oriented plasmonic hot spots. While electric near-field hot spots have been investigated with photoemission electron microscopy (PEEM), direct observation of photoelectron momentum-space distributions has been challenging. This research aims to address this gap by demonstrating optically-controlled directional multi-photon photoemission (MPPE) from single gold nanostars using 2D photoelectron velocity mapping and 3D reconstruction, providing detailed characterization of angular and energy distributions. The study focuses on the weak-field (multiphoton) regime to minimize nanostar heating and avoid space-charge effects, ensuring high temporal coherence of the emitted electrons.

Previous studies have explored the correlation between photoemission and plasmonic hot spots using PEEM, achieving spatial resolution of ~20 nm. These studies, combined with optical pulse shaping, have demonstrated coherent control over spatial photoemission distributions on femtosecond timescales. However, direct observation of photoelectron momentum-space distributions from single, resonantly excited nanoparticles has been limited. Recent advancements have enabled angle-resolved photoelectron mapping from individual gold nanorods and bow-tie nanoantennas, clarifying nanoplasmonic angular photoemission distributions. Angular photoelectron mapping and steering have also been demonstrated for gold and tungsten nanotips, primarily in the field emission regime. Despite these advances, many aspects of the nanoplasmonic photoelectron emission mechanism and opportunities for angular control, particularly in the multiphoton regime, remain to be fully elucidated.

Gold nanostars with sharp tips (3.4 ± 0.4 nm radii) were synthesized using a seedless growth method and sorted by size to select for simple geometries. Electron micrographs showed nanostars with an average of three tips in the surface plane. Plasmonic properties were characterized using normal-incidence laser light (700–1000 nm, 50 fs, 75 MHz). A scanning sample stage enabled diffraction-limited photoemission mapping for locating single nanostars on an ITO-coated glass substrate. Photoelectron velocity distributions were collected as a function of laser frequency, polarization, and intensity using a velocity map imaging (VMI) electrostatic lens configuration. Multiphoton process orders were determined by measuring photoemission rates as a function of laser pulse intensity. Finite element simulations, using SEM-measured nanostar geometries (accounting for a 3 nm Pt coating), were performed to determine plasmonic field enhancements. The simulations included the ITO-coated substrate and HEPES ligand layer. Calculations based on coherent surface MPPE theory were carried out to determine photoemission rates and velocity distributions, using the simulated plasmonic fields and considering both direct and surface-rescattered quantum amplitudes. The Fermi-Dirac electron distribution at a pulse-averaged temperature (calculated via the two-temperature model) was considered. The effect of the ponderomotive energy was also included in the calculations. The study then involved selective excitation of multiple nanostar hot spots by independently tuning frequency and polarization. The resulting velocity distributions at different frequencies and polarizations were measured and compared to theoretical predictions. The calculations took into account various factors including the geometry of the nanostars, the dielectric environment, and the multiphoton process orders.

The study demonstrated that individual nanostar tips behave as locally bright, point-like electron sources with a high degree of spatial coherence. The n-photon photoemission rate varied as I_PPE ~ cos²(θ - θ_tip) as the polarization angle θ was rotated away from the resonant tip. Multiphoton process orders (3PPE to 4PPE) were observed depending on the laser wavelength. The measured 2D and reconstructed 3D photoelectron velocity map images showed clear directionality along the resonant tip axis, consistent with surface-mediated MPPE. The theoretical calculations, based on coherent surface MPPE theory, showed good agreement with the experimental results, confirming the tip-aligned directionality. Selective excitation of different tips on a single nanostar was achieved by independently tuning laser frequency and polarization. Frequency control allowed for entirely frequency-controlled tip selectivity, while polarization control enabled 90° rotation of photoemission directionality with only 20° polarization rotation. The results demonstrated versatile photoemission switching/steering by independently tuning frequency or linear polarization, leaving the other degree of freedom available for modifying control characteristics. The photoemission switching timescale was estimated to be limited by plasmon dephasing time and optical cycle, suggesting the possibility of attosecond spatiotemporal control.

The findings demonstrate the potential of gold nanostars as all-optical photocurrent control elements. The ability to independently control photoemission directionality using frequency and polarization provides versatile manipulation of nanoscale currents. This opens opportunities for applications such as femtosecond electron imaging and diffraction, polarization-sensitive photodetection, and terahertz nanoelectronics. The strong agreement between experimental observations and theoretical calculations based on surface-mediated MPPE strongly supports the dominant role of this mechanism. The high degree of spatial coherence observed suggests possibilities for ultrafast point projection and diffractive imaging of nearby nanoscale objects. The work establishes a simple mechanism for femtosecond spatiotemporal current control in designer nanosystems.

This research demonstrated versatile, optically-controlled directional photoemission from plasmonic gold nanostars. Independent tuning of laser frequency and polarization allowed for selective excitation of different nanostar tips, resulting in precise control over the directionality of emitted photocurrents. The findings are well-supported by theoretical calculations and highlight the potential of using designer plasmonic nanoparticles as all-optical photocurrent control elements in various applications. Future research could explore the strong-field regime and investigate the integration of these nanostars into functional nanoelectronic devices.

The study focused primarily on the weak-field multiphoton photoemission regime. While the theoretical framework extends to the strong-field regime, experimental validation in this regime would strengthen the conclusions. The model used simplifies the complex interactions at the nanostar-substrate interface; a more detailed model of these interactions could potentially improve the accuracy of the theoretical predictions. The sample preparation involved a platinum coating for improved SEM imaging. The effects of this coating on the plasmonic properties of the nanostars warrant further investigation. The simulations simplified the complex tip-tip interactions, which may be relevant in some cases.