Plasmon-Enhanced Photocathode for High Brightness and High Repetition Rate X-Ray Sources

In this Letter, we report on the efficient generation of electrons from metals using multiphoton photoemission by use of nanostructured plasmonic surfaces to trap, localize, and enhance optical fields. The plasmonic surface increases absorption over normal metals by more than an order of magnitude, and due to the localization of fields, this results in over 6 orders of magnitude increase in effective nonlinear quantum yield. We demonstrate that the achieved quantum yield is high enough for use in rf photoinjectors operating as electron sources for MHz repetition rate x-ray free electron lasers.

Figure 1

A set of nanocavities (NCs) were fabricated from a template produced by electron beam lithography (a). FDTD simulations show that the NCs cause an order of magnitude electric field enhancement at the mouth of the NC (b) and in magnetic field at the base of the NC (c). Light is normal to the surface with the electric vector perpendicular to the cavity axis ($p$ polarization).

Figure 2

Optical response of the plasmon-enhanced cathode. Experimentally determined spectral bandwidth, in good agreement with the FDTD simulation, shows broadband response with an on-resonance (720 nm) reflectivity less than 1% (a). The inset shows an optical image of the patterned area demonstrating the large absorption increase from $s$-to $p$-polarized normal incidence illumination. Unlike a classical grating, the coupling to the resonant plasmon modes inside the nanocavities leads a very wide angular bandwidth (b).

Figure 3

Plasmonic cathode multiphoton photoemission enhancement imaged in the PEEM. The NC array is prepared on a flat metal substrate which photoemits under UV radiation (a). When illuminated only by an 800 nm laser in $p$ polarization at 75 deg angle of incidence (b) light couples to the plasmonic NCs producing resonant absorption and field enhancement, which leads to a dramatic increase in the multiphoton photoemission producing a bright image only in the patterned area. $s$-polarized light, however, does not couple to the plasmonic modes and yields the same photocurrent as the flat surface (c). The contrast between the light and the dark area normalized intensity with subtracted background in (b) represents the photoemission enhancement.

Figure 4

Experimentally determined photoemission current as a function of laser intensity from the NC array plotted on a log-log scale. Linear fit to the data yields a fourth order photoemission scaling and an enhancement over a flat surface (Schweikhard et al. [21]) of more than 6 orders of magnitude. At the pulse charges required, the multiphoton process is shown to be more efficient than the one-photon process (UV). The inset shows the polarization dependence of the photocurrent where 0 deg corresponds to $E$ vector oriented in $p$ polarization (along the $x$ axis): the max-to-min ratio corresponds to the plasmonic photoemission enhancement.