Passive optical enhancement of laser-microwave synchronization

Abstract

Thermal noise is a fundamental limitation for synchronizing microwaves to high-power lasers of low repetition rate. Here, we describe an optical enhancement scheme that concentrates the output power of a fast photodiode into a narrow range of harmonics around a microwave frequency. The scheme is entirely passive and requires no feedback or lock. Using a 5-MHz laser and a microwave at 6.2 GHz, we demonstrate an enhancement of optical-to-microwave conversion by a factor of 4000. The uncorrelated noise on time scales up to 8 min amounts to less than 4 fs, with laser pulses intense enough for pump-probe experiments of structural dynamics.

Electron microscopy of electromagnetic waveforms

“…In [59,63], it is shown how timing jitter and long-term drifts between the electron pulse arrival at the compression stage and into at streaking stage, as well as amplitude fluctuations of the compression and streaking can dramatically worsen not only the resolution but also the advantage of using compression altogether. With the all-optical concept, where the compression and streaking fields are naturally synchronized to the electron pulse generation via the same laser pulse, the timing jitter can be improved passively, reaching values of <5 fs (root mean square) [36], meaning that measurements of electromagnetic waveforms by electron microscopy in the THz frequency range will not suffer from timing jitter and drifts.…”

“…The longitudinal momentum gain p x can be used for compression to create a train of ultrashort pulses at high optical frequencies [62], or an individual ultrashort pulse with microwave or THz fields [36,63]. In the latter concept, the initial electron pulse duration should be shorter than one half-period of the wave oscillation.…”

Femtosecond single-electron pulses generated by two-photon photoemission close to the work function

“…Assuming that the longitudinal velocity spread is similar to the transverse one [21], i.e. a halfspherical initial shape of the initial phase space, sub-100 fs pulses are achievable with 10 kV mm −1 electrostatic acceleration [21] without requiring microwave compression [22] and advanced synchronization [28]. More importantly, if single-electron pulse compression is applied with time-dependent microwave fields [22], any smaller phase space volume in the time/energy domain before compression either implies an improved monochromaticity or shorter pulses at target [22].…”

“…All subsequent beam manipulations, for example with magnetic lenses or temporal compression, cannot practically reduce the initial phase space volume. The physics of photoemission hence determines the best achievable compromises at target, for example between pulse duration and monochromaticity [28], between coherence and beam size [29] and between divergence and temporal distortions [30], among others [31]. A fundamental study and optimization of photoemission in the femtosecond regime is therefore essential for advancing ultrafast imaging with electrons towards novel resolution regimes.…”

Shaped cathodes for the production of ultra-short multi-electron pulses

“…Shorter multi-electron pulses are expected from this approach for which synchronization noise has limited the instrument response to about 80–150 fs (rms) 60–62 . However, a recent phase-lock scheme based on passive optical enhancement has reduced this timing jitter to only $\cong$5 fs (rms) 65–67 . Therefore, RF pill-lens electron pulse rebunching still holds great promise in providing sub-20 fs (rms) temporal resolution with bright multi-electron bunches 56 .…”