Strong-Field Above-Threshold Photoemission from Sharp Metal Tips

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 similar to 2 x 10(11) W/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 -1.0 eV= (10(12) W/cm(2)). We attribute the magnitude of the laser field effects to field enhancement taking place at the tip's surface.