Ultrafast Electron Pulses from a Tungsten Tip Triggered by Low-Power Femtosecond Laser Pulses

We present an experimental and numerical study of electron emission from a sharp tungsten tip triggered by sub-8-fs low-power laser pulses. This process is nonlinear in the laser electric field, and the nonlinearity can be tuned via the dc voltage applied to the tip. Numerical simulations of this system show that electron emission takes place within less than one optical period of the exciting laser pulse, so that an 8 fs 800 nm laser pulse is capable of producing a single electron pulse of less than 1 fs duration. Furthermore, we find that the carrier-envelope phase dependence of the emission process is smaller than 0.1% for an 8 fs pulse but is steeply increasing with decreasing laser pulse duration.

Figure 1

Sketch of the experimental setup. The laser beam traverses a dispersion balanced interferometer, whose output is focused onto the field emission tip.

Figure 2

Autocorrelation traces with tunable nonlinearity. (a) Autocorrelation traces for three different dc voltages but identical laser parameters. (b) Peak-to-baseline ratio vs dc tip voltage (blue points: data). The solid blue line is the simulation result with no adjustable parameter; the dashed red line is a fit of the simple optical field emission model to the data. The numbers in circles correspond to the traces given in (a). Inset: peak current and baseline current vs dc tip voltage.

Figure 3

Model and electron pulse. Model potential used in the calculations, shown here with an applied electric field of $F=4.4\mathrm{GV}/\mathrm{m}$ (solid black line). $E=0$ is the vacuum energy. Initial potential and initial wave function are shown in light blue. The inset shows the electric field generated by a three cycle laser pulse ($F_{\mathrm{laser}}\approx 2.7\mathrm{GV}/\mathrm{m}$, $F_{\mathrm{dc}}=0.2\mathrm{GV}/\mathrm{m}$, CE phase $\varphi =\pi$, bottom), and the resulting probability flux (top). A single electron pulse with a width of 660 as (FWHM) is emitted. The measurement position was at $z=2\mathrm{nm}$ for this plot.

Figure 4

Contour plot (log scale) of the modulation depth as a function of dc field and fluence for a 2 cycle laser pulse ($\tau =5.3\mathrm{fs}$). Line spacing is 8 contours per decade.

Figure 5

Modulation depth as a function of dc field and fluence for different pulse durations.