Optical Control of Field-Emission Sites by Femtosecond Laser Pulses

We have investigated field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses. Strongly asymmetric modulations of the field-emission intensity distributions are observed depending on the polarization of the light and the laser incidence direction relative to the azimuthal orientation of tip apex. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity. Simulations of local fields on the tip apex and of electron emission patterns based on photoexcited nonequilibrium electron distributions explain our observations quantitatively.

Figure 1

Schematic diagram of the experimental setup. A tungsten tip is mounted inside a vacuum chamber. Laser pulses are generated outside the vacuum chamber. An aspherical lens is located just next to the tip to focus the laser onto the tip apex. Emitted electrons are detected by a position-sensitive detector in front of the tip. The polarization angle ${\theta }_P$ is defined in the inset, where the laser beam propagates towards the reader’s eye (see text for further description).

Figure 2

Electron emission patterns without laser (a), and with laser irradiation (b). $V_{\mathrm{tip}}$ indicates the dc potential applied to the tip and $P_L$ indicates the laser power measured outside the vacuum chamber. The inset of (a) shows the front view of the atomic structure of a tip apex based on a ball model. The inset of (b) shows a schematic side view of the laser-induced field-emission geometry. A dashed white line denotes a mirror symmetry line of the atomic structure in each picture.

Figure 3

Comparison of measured and simulated laser-induced FEM (LFEM) images for different light polarization angles ${\theta }_P$ and for different azimuthal orientations $\phi$ of the tip. The leftmost column gives the FEM images without laser irradiation for four different azimuthal angles ($V_{\mathrm{tip}}=-2250\mathrm{V}$). For the same azimuthal angles, observed LFEM images are shown as a function of polarization angle ${\theta }_P$ in 30° steps ($V_{\mathrm{tip}}\approx -1500\mathrm{V}$ and $P_L=20\mathrm{mW}$). The simulated LFEM images from the photo field-emission model, in which $V_{\mathrm{tip}}$ and $P_L$ were set as in the corresponding experiments, are shown on the right-hand side of the observed LFEM images. The color scale and laser propagation direction are the same as in Fig. 2.

Figure 4

Schematic illustration of the excitation and interference of surface EM waves (a). The calculated time-averaged field distribution around the model tip is shown in a linear color scale in (b) for ${\theta }_P=0°$. Highest field values are represented in yellow (brightest color). In (c), the time-averaged field distributions are given in a front view of the model tip for different polarization directions ${\theta }_P$. The laser propagation direction is indicated by dashed red arrows, it is the same as in the experiment. (d) shows a schematic diagram of photo field-emission from a nonequilibrium electron distribution.

Figure 5

Line profiles extracted from the observed FEM images (red line with squares), LFEM images (blue line with circles), and from LFEM images simulated by the photo field-emission model (green solid line) and the optical field-emission model (black dashed line) at [$\phi =0°$, ${\theta }_P=0°$]. The whole scanned line corresponds to the unfolded rectangle indicated by the dashed blue line in the FEM figures above, and the corresponding sides are indicated by white arrows. Each line profile has been normalized by the maximum value.