Strong-Field Above-Threshold Photoemission from Sharp Metal Tips

Abstract

We present energy-resolved measurements of electron emission from sharp metal tips driven with low energy pulses from a few-cycle laser oscillator. We observe above-threshold photoemission with a photon order of up to 9. At a laser intensity of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ the suppression of the lowest order peak occurs, indicating the onset of strong-field effects. We also observe peak shifting linearly with intensity, with a slope of around $\ensuremath{-}1.0\text{ }\text{ }\mathrm{eV}/({10}^{12}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2})$. We attribute the magnitude of the laser field effects to field enhancement taking place at the tip's surface.