Coherent Field Emission Upon Ultrafast Laser Irradiation of the Tip Plasmon

Abstract

Irradiation of sharp silver tips with femtosecond laser pulses leads to photoassisted coherent field emission without a static field. We reconstruct the time profile of the emission, and show that the process is entirely governed by the collective response of the tip plasmon and its field emission. Weak-field optical excitation leads to multiphoton absorption and field emission from the tip apex due to the enhanced local field. The attendant sharp field gradient ensures ponderomotive acceleration of emitted electrons and non-local light-matter interaction. The crossover regime in which simultaneous multiphoton absorption and optical field emission take place is evidenced by the time profile of electron emission correlation, laser power dependence, and polarization angle dependence of each harmonic current.

Figures (4)

• Figure 1: (a) Schematic of the experiment: Electron emission measured under irradiation of the STM junction by cross-polarized pulse trains. (b) Typical spectrum of the current shows a progression of four harmonics of the double-beat frequency ($\sim$1 kHz). (c) Left panel: Cross-correlation current as a function of delay between pulse trains, recorded at harmonics of the double-beat. Right panel: Fourier transform of the time correlations showing harmonics of the optical frequency ($\lambda$ = 800 nm). The reconstruction of the emission time-profile (in black) is obtained by forward simulation using Eq. 1. (d) Snapshot of charge density difference of harmonics.
• Figure 2: Semilog plot of the four harmonics of the field emission as a function of inverse applied electric field. Each curve is fitted using Eq. \ref{['current']}
• Figure 3: (a) Computed enhancement of $E_z$ on a silver tip (cone diameter = 20 nm). $\beta=E_z/E_0$ is $\sim$80 at the apex. (b) Bias dependence of the normalized current and the extracted electron energy spectrum at $I=2\times 10^{13}$ W/m$^2$. The inset shows the linear shift of the spectral peak as a function of applied laser intensity, as predicted for ponderomotive acceleration.
• Figure 4: Angular distribution of photocurrent. (a) Geometry of cross-polarized illumination: the tip is along $\hat{z}$; the propagation is along $\hat{x}$+$\hat{z}$; the polarization angle $\phi$ is zero when $\hat{e_{1}}$ and $\hat{e_{2}}$ are at $\pm 45 ^\circ$. (b) In parallel polarization, the normalized angular distributions of the harmonics are superimposable. (c-f) Angular distributions of 1-4th harmonics in cross-polarized excitation. (g) Decomposition of 2nd harmonic in (d) fitted with $a_1:a_2 =1:23$. (h) Decomposition of 3rd harmonic in (e) with $a_2:a_3 = 1:5.4$. (i) Decompostion of 4th harmonic in (f) with $a_1:a_4:b_4 = 1:-2.6:0.21$. In both polarization configurations, the tip is slightly tilted with respect to the laboratory $z$-axis.