Subpicosecond Ultracold Electron Source

We present the first observation of subpicosecond electron bunches from an ultracold electron source. This source is based on near-threshold, two-step, femtosecond photoionization of laser-cooled rubidium gas in a grating magneto-optical trap. Bunch lengths as short as $735\pm 7\mathrm{fs}$ (rms) have been measured in the self-compression point of the source by means of ponderomotive scattering of the electrons by a 25 fs, 800 nm laser pulse. The observed temporal structure of the electron bunch depends on the central wavelength of the ionization laser pulse, in agreement with detailed simulations of the atomic photoionization process. This shows that the bunch length limit imposed by the atomic photoionization process has been reached.

Figure 1

Schematic representation of the UCES setup.

Figure 2

(a)–(e) The measured number $N_e$ of electrons scattered per bunch as function of the delay between the interaction laser pulse and the electron bunch in the self-compression point, with the ionization laser polarization parallel (red) and perpendicular (blue) to the accelerating field, and with central ionization wavelengths of (a) 474.4 nm, (b) 488.7 nm, and (c) 498.1 nm. Insets (d) and (e) show detector images recorded at a delay of 0.6 ps without and with interaction laser, respectively. $N_e$ is obtained by summing the counts inside the red outlined areas in the insets (d) and (e). (f)–(h) gpt simulations of the time profiles of the atomic photoionization process of stark shifted Rb atoms in an electric field of $1.2\mathrm{MV}/\mathrm{m}$. In the simulations the measured laser spectra are used. (i)–(k) gpt simulations of the measured temporal distributions of the bunches in the self-compression point based on the time profiles of (f)–(h) and a realistic beamline model.

Figure 3

gpt simulations of trajectories of electrons escaping a singly ionized rubidium atom at $(x,y)=(0,0)$. Indicated are the (color-coded) potential energy and equipotential lines (thin black curves) due to the ionic Coulomb field in an external uniform electric field of $1.2\mathrm{MV}/\mathrm{m}$. Five simulated trajectories are shown (thick colored curves), corresponding to five different initial angles and ionization wavelengths of 474.4 nm (red, purple, and blue trajectories) and 492.0 nm (white trajectory). The relative arrival times at $z=1\mathrm{mm}$ are indicated. Note that $z=0$ in this figure corresponds to $z=-d_{\mathrm{MOT}}$ in Fig. 2.