Extraction dynamics of electrons from magneto-optically trapped atoms

Abstract

Pulsed photoionization of laser-cooled atoms in a magneto-optical trap (MOT) has the potential to create cold electron beams of few meV bandwidths and few ps pulse lengths. Such a source would be highly attractive for the study of fast low-energy processes like coherent phonon excitation. To study the suitability of MOT-based sources for the production of simultaneously cold and fast electrons, we study the photoionization dynamics of trapped Cs atoms. A momentum-microscope-like setup with a delay-line detector allows for the simultaneous measurement of spatial and temporal electron distributions. The measured patterns are complex, due to the Lorentz force inducing spiral trajectories. Ray-tracing simulations reproduce the main features. We find that the production of electron bunches with bandwidths of a few meV is straightforward; however, pulses in the ps-range are more demanding and require beam blanking or partial blocking.

Narrow-band pulsed electron source based on near-threshold photoionization of Cs in a magneto-optical trap

“…The spatio-temporal and spectroscopic analysis of the electron beam is performed with a high-time-resolution detector (~200 ps) and the Time-of-Flight (ToF) momentum microscopy technique. This is the follow-up of our previous work [8] that was conducted in the presence of a static magnetic field. Under those conditions the magnetic field broadened the ToF and induced a complicated mode structure due to spiral trajectories [8].…”

“…This is the follow-up of our previous work [8] that was conducted in the presence of a static magnetic field. Under those conditions the magnetic field broadened the ToF and induced a complicated mode structure due to spiral trajectories [8]. In the current paper an improved experimental setup with a rapidly switched magnetic field is presented.…”

“…2(b)) produces to the optical axis and second ̶ the ones emitted backwards (close to  = 180°). The electrons emitted away from the DLD spend more time (because of their longer path) in the MOT region before they are turned around by the extraction electric field [8,19], that is why they arrive later, forming the peak labelled with B in Fig. 3(c), compared to the one ejected directly toward the detector, forming the peak labelled with A in Fig.…”

“…The experimental setup is similar to the one used in our previous work [8] but now with the magnetic field switched off during the pulse of the ionizing laser, see Fig. 1.…”

“…Furthermore, such sources can employ large active areas (hundreds of micrometres) that allows extracting a large number of electrons to record the diffraction patterns [5,6]. Typically ∼10 8 atoms could be trapped in such a cold cloud with rms radius of ∼1 mm, at a source temperature of T ~ 150 µK [7,8]. The loading time of the magneto-optical trap is around 100 ms and depends on the experimental conditions [9].…”