Field Emission Tip as a Nanometer Source of Free Electron Femtosecond Pulses

We report a source of free electron pulses based on a field emission tip irradiated by a low-power femtosecond laser. The electron pulses are shorter than 70 fs and originate from a tip with an emission area diameter down to 2 nm. Depending on the operating regime we observe either photofield emission or optical field emission with up to 200 electrons per pulse at a repetition rate of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator, could serve as an efficient source for time-resolved electron interferometry, for time-resolved nanometric imaging and for synchrotrons.

Figure 1

Photofield emission (a) and optical field emission (b) energy diagrams. In photofield emission an electron is excited by a laser photon to an intermediate state and then tunnels through the barrier, which is generated by a dc voltage applied to the tip. In optical field emission, the laser field instantaneously wiggles the barrier. If the barrier is sufficiently thin, electrons tunnel from the Fermi level. This process is dominant for high fields. Dashed line: no field applied on the tip.

Figure 2

Experimental configuration, see text for details.

Figure 3

Emission for low laser power. (a) Fowler-Nordheim plots of the dc current and the additional photo current (squares and circles). A fit to the dc data (dashed line) yields a tip radius $r$ of $(134\pm 3)\mathrm{nm}$. The solid line is a fit to photofield emission current with $r=134\mathrm{nm}$ and an effective work function $\Phi ={\Phi }_W-h\nu =3\mathrm{eV}$ (laser power $P=260\mathrm{mW}$, ${\Phi }_W=4.5\mathrm{eV}$). The presence of the laser field $F_{\mathrm{laser}}$ at the tip further reduces the barrier in addition to the field due to the dc voltage applied to the tip. Therefore, we leave $F_{\mathrm{laser}}$ as a free parameter. The solid red line is drawn with the best fit value of $F_{\mathrm{laser}}=1.1\mathrm{GV}/\mathrm{m}$, and the dotted lines represent a deviation of $\pm 25\%$. With the tip’s radius of curvature much smaller than the laser wavelength we expect the laser field to be enhanced at the tip apex by a factor of $\sim 5$ [15]. By comparing the fitted $F_{\mathrm{laser}}$ to the maximum field calculated in the focal spot in the absence of any material, we infer an enhancement factor of 4.1, in good agreement with the expected factor. (b) Polarization dependence of the photocurrent with $U_{\mathrm{tip}}=-1500\mathrm{V}$ and $P=260\mathrm{mW}$ for $r=134\mathrm{nm}$. The data are well fit with a ${cos}^2\theta$ on a flat background (red line). The inset in (a) shows the definition of the angle $\theta$ (in plane perpendicular to laser propagation direction). (c) 1 GHz repetition rate signal measured in the electron current. The measured linewidth is about 1 Hz at $-3\mathrm{dBc}$, which corresponds to the resolution limit of the spectrum analyzer. (d) The photo current increases linearly with laser power ($r\approx 40\mathrm{nm}$, $U_{\mathrm{tip}}=-720\mathrm{V}$).

Figure 4

Few atom electron source. (a) Field ion microscope image of a 30 nm radius tip. The zoom in reveals the central $(111)$ emission area encircled by a ring of 7 atoms, spanning an area of $\lesssim 2\mathrm{nm}$ diameter. (b) dc-field emission image of a 30 nm radius tip; only emission from the central $(111)$ plane is visible, i.e., from the seven atom ring structure in (a). (c) Same tip with laser on, but much smaller ($100\times$ reduced) MCP gain than in (b). (d) Scanning electron microscope image (SEM) of a typical tip, scale bar $1\mu \mathrm{m}$.

Figure 5

Polarization dependence for high laser power. Photocurrent as a function of $\theta$ for a stub as shown in the inset (SEM image, scale bar 500 nm). The tip ends as a fairly flat surface facing the MCP. Because of the sharpness of parts of the edges (radius of curvature $\lesssim 100\mathrm{nm}$) field enhancement takes place as for sharp tips ($P=530\mathrm{mW}$, $U_{\mathrm{tip}}=-100\mathrm{V}$). The line is a fit of the data to optical field emission (see text).