Compression of single-electron pulses with a microwave cavity

Abstract

Few-femtosecond to attosecond electron pulses are required for advancing ultrafast diffraction and microscopy to the regime of electrons in motion. Here, we report the combination of a single-electron source with a microwave cavity for pulse compression. In such an arrangement, the electron pulses can become significantly shorter than the laser pulses used for electron generation. This comes at the expense of an increase in energy spread. We report the use of an energy analyzer for characterizing microwave-compressed singleelectron pulses. Phase effects, linearity, focal distances, incoming pulse durations and laser-microwave jitter are measured for three different synchronization approaches. The results demonstrate the applicability of a microwave cavity in the single-electron regime and identify jitter as the current limitation on the way to few-femtosecond, eventually attosecond pulses of single electrons.

Passive optical enhancement of laser-microwave synchronization

“…First, repetition rate drifts of the laser translate into timing drifts for a passive optical filter because of its phase slope (7.45 fs/kHz). Our free-running laser drifts by less than 1 Hz over 1 h, 17 which corresponds to about 1.2 kHz at the 1216th harmonic or about 9 fs of timing drift. Likewise, the distance between the cavity mirrors must be quite stable, since a mechanical displacement of 10 nm already introduces about 20 fs of timing drift.…”

“…For our studies of structural dynamics with ultrafast electron diffraction in the single-electron regime, 16 we aim for generating a microwave at 6.237 GHz, used to compress single-electron pulses to few-femtosecond duration by using time-dependent acceleration fields. 17 The laser system is a mode-locked long-cavity Ti:Sapphire oscillator, 18 providing 0.5 lJ of pulse energy at 800 nm central wavelength at a repetition rate of 5.129 MHz (Femtosource XL, Femtolasers Produktions GmbH). The microwave frequency is the 1216th harmonic of the laser's repetition rate.…”

“…Additional technical noise is introduced whenever high microwave power and hence amplification of the photodiode's signal is needed. 17,22 Here the mode filter reduces the required amplifier gain by providing a higher microwave power level directly from the photodiode.…”

“…28 Alternatively, dense electron packets can be compressed to femtosecond duration. 21,29,30 The singleelectron source, 16 the microwave compressor, 17 an isochronic magnetic lens system, 31 tilted excitation pulses 32 and a coherent diffraction methodology 33 are mostly ready; just the quality of synchronization is not at the desired fewfemtosecond level yet. With the here described approach, probably in combination with an optical mixer 2 for compensating long-term drifts, we may reach into the fewfemtosecond, ultimately attosecond regime of structural dynamics.…”

Imaging population transfer in atoms with ultrafast electron pulses

“…Recently, single-electron pulses with a full-width at half-maximum (fwhm) duration of 28 fs have been demonstrated [15]. Various schemes for further compression of these pulses to attosecond durations [16][17][18][19][20] and for reaching attosecond resolution by optical gating [21] have been proposed. Simulations of various electron scattering processes employing attosecond duration incident electron pulses, whether treated simply as potential scattering processes [22] or more rigorously as coherent scattering processes [23][24][25], have demonstrated the ability of such ultrashort electron pulses to image electronic motions in target atoms and molecules.…”