Fig. 1. Optical resonance enhanced Ag nano-structured photocathode: (a) Spicer’s three-step model of photoemission; (b) illustration of the Ag nano-structured photocathode; (c) illustration of the Ag film photocathode; (d) cross-section of the FDTD setup used for simulating the optical properties of the Ag nanoparticles on ITO substrate.

Fig. 2. Fabrication and activation process of the Ag nanosphere photocathode: (a) Schematics of the fabrication process for Ag nanosphere photocathode; (b) SEM image of the Ag film; (c) SEM image of the Ag nanosphere; (d) surface Cs activation process of the Ag nanosphere.

Fig. 3. Mie resonance characteristics of Ag nanospheres: (a) Light scattering, absorption and extinction efficiency (Q_{sca, abs, ext}) spectra of the Ag nanosphere (D = 120 nm) in air and on substrates; (b) and (c) are the normalized magnetic field intensity (|H|², color) and lines (white lines) in vertical crosscuts through the center of the Ag nanosphere in air and on Ag/ITO substrate with the electric dipole resonance wavelength of 439 and 505 nm, respectively; (d) the normalized magnetic field intensity (|H|², color) and lines (white lines) in vertical crosscuts through the center of the Ag nanosphere with the electric quadrupole resonance wavelength of 350 nm.

Fig. 4. η_a spectra of the square arranged Ag nanosphere array on Ag/ITO substrate: (a) Dependence of η_a spectra upon D when P = 400 nm; (b) dependence of η_a spectra upon P when D = 120 nm.

Fig. 5. Light absorption spectrums of the Ag nanosphere array and Ag film: (a) Measured results. (b) simulated results.

Fig. 6. (a) Measured photoemission quantum efficiency of the Ag nanosphere array and film photocathode; (b) calculated light absorption spectra of the Ag nanosphere array photocathode.