Influence of a Thin Dielectric Layer on the Electron Emission Properties of a Noble Metal Surface, the CuO/Cu Case

Abstract Electron emission by a material surface under intense ultrashort pulsed laser light is actively studied for its application in various fields, from biochemistry to aerospace. Different strategies are proposed to push back against the electronic breakdown happening at high laser irradiance. Here, we investigate the effect of adding a thin dielectric top layer of copper oxide (CuO) on a noble plasmonic metal, copper (Cu), on the electron emission processes. Using a low energy and photoemission electron microscope (LEEM - PEEM), we measured, at a subwavelength spatial scale, electron emission spatial distribution and total electron energy distribution (TEED) of both pristine and thermally oxidized copper. We report that (i) the preferred electron emission sites on the surface correspond to step edge bunches and surface defects of the Cu lattice, (ii) the growth of a thin CuO layer improves the electron emission properties of Cu surface. The thin dielectric CuO coating is a source of near-field plasmon-polariton waveguiding and confinement in the immediate vicinity of the metal surface. The electron emission regime changes from a photoelectric weak field regime to a strong field tunnelling regime beyond a light irradiance threshold of 0.7 GW/cm² (0.073 V/nm). This irradiance threshold value is one order of magnitude lower than that required before thermal oxidation. Under equivalent illumination conditions (irradiance, exposure time) above a threshold irradiance, the total charge extracted from Cu with a CuO surface layer is 100 times greater than that of a clean metal.