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 $\sim 2\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$ 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 $-1.0\mathrm{eV}/({10}^{12}\mathrm{W}/{\mathrm{cm}}^2)$. We attribute the magnitude of the laser field effects to field enhancement taking place at the tip’s surface.

Figure 1

(a) Sketch of the experimental setup (not to scale). The retarding field spectrometer can be moved away from the tip (out of plane) to make room for the imaging MCP detector. The laser output viewport is used for alignment. OAP: Off-axis parabolic focusing mirror. (b) Energy diagram of the metal-vacuum interface with respect to the Fermi energy with an applied dc field of $0.8\mathrm{GV}/\mathrm{m}$.

Figure 2

Electron count rate as a function of the electron energy. From bottom to top the curves are taken at laser intensities of $\{0.9,1.2,1.4,1.6,1.9,2.1,2.3\}\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$. Upper inset: Positions of the peaks of the curve at $1.2\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$. The peak positions are extracted from a fit to the data with multiple Gaussians. Lower inset: Total tip current vs intensity in a double-logarithmic plot. Details in the text.

Figure 3

Positions of the $(K+S)=4$ and 5 maxima (squares and diamonds) and of the next lower minima (balls and triangles) as extracted from Fig. 2 as a function of laser intensity. The slopes are in the range of $-2.3\dots -0.4\times {10}^{-12}\mathrm{eV}/(\mathrm{W}/{\mathrm{cm}}^2)$.