Terahertz-Induced Electron Emission from a Gold Surface

Strong-field photoemission of electrons has been extensively studied [1,2]. Enabled by the development of high-intensity THz sources, THz field driven emission from a nanotip [3] up to fC bunch charge [4], and with pC charge from an antenna structure [5] have been demonstrated. Here, we present an experimental study of electron emission from a plane gold surface irradiated by intense single-cycle THz pulses with 0.35 THz mean frequency, focused to a peak field strength up to 300 kV/cm (corresponding to a Keldysh parameter of γ ∼ 0.4). Fig. 1(a) shows the measured electron current, induced by the p-polarized THz pulses at 45° angle of incidence and amplified by an electron multiplier, as function of the THz peak electric field. Electron emission was observed even at moderate peak field strengths below 50 kV/cm. Flipping the polarity of the THz pulses resulted in a significantly different current signal. The direction of the field polarity has been determined by applying a dc voltage across a LiNbO3 slab and measuring the electro-optic signal. The electron signal, measured as function of the THz polarization angle (ϕ = 0: in the plane of incidence), showed double maxima (Fig. 1b). In order to compare the photoemission measurement results with theoretical model, we employed the Fowler-Nordheim (FN) equation. The enhanced photocurrent with polarity flip (while using the symmetric waveform of THz field) indicates towards rescattering process. In order to investigate the effect of rescattering we numerically solved the time dependent Schrodinger equation to find the emission current probability quantum mechanically.