Figure 1
Conceptual diagrams of the laser-induced electron emission, electron spectra and experimental setup.
(a) Illustration of different emission mechanisms, referred to as Models A to C, discussed in this work. (b) Schematic representation of the experimental setup. A tungsten tip oriented towards the [011] crystal direction is mounted in a vacuum chamber. 7 fs laser pulses (centre wavelength: 830 nm; repetition rate: 80 MHz) are focused onto the apex of the tip. The beam waist at the tip apex is estimated to be approximately 3.5 μm in diameter. A pinhole plate covered with a phosphor is installed in front of the tip to observe the field emission pattern and to define the emission sites for spectroscopic measurements. A hemispherical electrostatic analyzer was used to measure the energy spectra. (See Methods Section for details). Different emission processes under strong fields are indicated by orange and green arrows. (c) Evolution of electron emission spectra with average laser intensities (values are 2 × 1012 W/cm2, 4 × 1012 W/cm2, 6 × 1012 W/cm2, 8 × 1012 W/cm2 and 10 × 1012 W/cm2, which correspond to laser powers of 18 mW, 36 mW, 54 mW, 72 mW and 90 mW, respectively), with electron numbers per pulse indicated for each curve. Intensities are given in a logarithmic scale. The electron numbers per pulse were plotted as a function of laser peak intensities in the inset, where the solid line is fitting curves with power functions; exponents are 2.6.
