28, 29 on individual gold nanorods and bow-tie nanoantennas using time-of-flight momentum PEEM 28, 29, which serves to clarify nanoplasmonic angular photoemission distributions and phenomena such as the transition into the optical field emission regime.
#4150 VELOCITY STACK FULL#
Full photoelectron momentum and energy characterization has been achieved by Lehr et al. Such capabilities have only appeared recently 25, 27, 28, 29 as a comprehensive understanding of photocurrent distributions is becoming crucial for the design and implementation of nanocathodes in nascent ultrafast nanoelectronics and electron imaging applications. However, direct observation of the corresponding photoelectron momentum-space distributions has remained a challenge, requiring angle-resolved photoelectron mapping from single, resonantly excited nanoparticles. Furthermore, these techniques have been combined with optical pulse shaping 26 to achieve coherent control over spatial photoemission distributions on femtosecond timescales 18. Electric near-field hot spots have been extensively investigated in nanoplasmonic systems, with photoemission electron microscopy (PEEM) studies establishing the correlation between photoemission and plasmonic hot spots with ~20 nm spatial resolution 16, 17. With nanostars and other multi-resonant particles, important opportunities for spatiotemporal photocurrent control emerge via frequency- and polarization-selective excitation of different plasmonic hot spots, which are often spatially separated and oriented in different directions 16, 17, 22, 24, 25. Spectral characteristics can even be controlled at the single-particle level for asymmetric particles that support multiple resonances, such as gold nanostars 16, 22, 23, 24. Geometry and particle array patterning also govern the frequency response of plasmonic systems, which have been tailored for broadband photodetection 19, photocurrent polarity control 20, and selective multi-mode lasing with narrow spectral linewidths 21. Particle geometry defines the surface field-enhanced hot-spot regions where conduction electrons build up during collective oscillations, become excited, and escape as photoemission or photovoltaic currents. In plasmonic systems, the mapping from optical field parameters onto near-field electron dynamics is primarily governed by the particle geometry and corresponding field enhancements, which can be crafted with high precision by synthetic or lithographic methods 11, 13, 16, 17, 18. Among recent advances, plasmonic metal nanostructures have shown considerable versatility and promise as bright photocathodes 8, 9, 13, femtosecond photodiodes 10, 11, and carrier-envelope-phase-sensitive photodetectors 14, 15 that can be integrated into nanoscale, chip-based devices. We thus establish a simple mechanism for femtosecond spatiotemporal current control in designer nanosystems.įemtosecond optical control over nanoscale currents is essential for ultrafast electron diffraction and microscopy 1, 2, 3, 4, 5, 6, x-ray free-electron lasers 7, 8, 9, and terahertz optoelectronic circuits 10, 12, 12. Classical plasmonic field simulations combined with quantum photoemission theory elucidate the role of surface-mediated nonlinear excitation for plasmonic field enhancements highly concentrated at the sharp tips ( R tip = 3.4 nm). Versatile angular control is achieved by selectively exciting different tips on single nanostars via laser frequency or linear polarization, thereby rotating the tip-aligned directional photoemission as observed with angle-resolved 2D velocity mapping and 3D reconstruction. Here, we provide a direct momentum-space characterization of multiphoton photoemission from plasmonic gold nanostars and demonstrate all-optical control over these currents. However, angular photocurrent distributions in nanoplasmonic systems remain poorly understood and are therefore difficult to anticipate and control. Plasmonic nanocathodes offer unique opportunities for optically driving, switching, and steering femtosecond photocurrents in nanoelectronic devices and pulsed electron sources.
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