Phys. Rev. Lett. 110, 076802 (2013)

Scientists in the USA have designed a plasmon-enhanced photocathode for bright, high-repetition-rate X-ray sources. The idea of Aleksandr Polyakov and co-workers from Lawrence Berkeley National Laboratory and Pacific Northwest National Laboratory was to use nanostructured plasmonic surfaces to trap light and greatly enhance the photoemission of electrons. Their chosen structure was aluminium with grooves 15 nm in width, 60 nm in depth and 100 nm in period and a plasmonic resonance at a wavelength of about 720 nm. By experimentally measuring the photoemission current from the sample using a 20-fs laser with a central wavelength of 800 nm, the researchers confirmed that the process was caused by four-photon photoemission. The generated photocurrent was 2.45 pA for a beam intensity of 0.065 GW cm−2. The fourth-order photoemission was enhanced by over six orders of magnitude relative to that from a flat gold surface.