Fig. 4: Frequency-controlled directional photoemission. | Nature Communications

Fig. 4: Frequency-controlled directional photoemission.

From: Plasmonic nanostar photocathodes for optically-controlled directional currents

Fig. 4

a Correlated nanostar scanning electron micrograph with the shorter and longer resonant tips at 50° and 315°, respectively (50 nm scale bar). b Experimental polarization dependence for frequency at the short-tip resonance (725 nm, blue), between resonances (775 nm, orange), and at the long-tip resonance (825 nm, red) with nonlinear cosine fits for the 3-photon process. c Calculated multiphoton surface current distribution at different frequencies, showing the transition from one tip hot spot to the other with circular polarization. d Experimental and e theoretical vz-projected velocity maps on and between the two resonances using circular polarization. Vectors indicate the peak directions determined by Gaussian fits around the peaks of the angular distributions. Peak photoemission angles range from 55° (50°) at 725 nm to 325° (315°) at 825 nm for the experimental (theoretical) velocity distributions, i.e. a 90° photoemission rotation. The vector magnitude represents the speed (vF) of photoelectrons excited from the Fermi level, which decreases with decreasing photon energy by energy conservation, \(\frac{1}{2}m_ev_{\mathrm{F}}^2 = n\hbar \omega - \phi\), with tip work function ϕ = 4.5 eV and n = 3 in this excitation energy range (\(\hbar \omega =\) 1.5–1.7 eV). The Fermi velocity does not coincide with the apparent edge of the 825 nm (red) distribution due to the velocity dependence of the photoemission amplitude, which leads to deviations from a simple Fermi–Dirac distribution (particularly near-zero velocity) and shifts the effective edge outward relative to vF for near-threshold photon energies.

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