Laser-microwave synchronisation for ultrafast electron diffraction

Abstract

Ultrafast electron diffraction is a pump--probe technique that allows the visualisation of molecular dynamics with atomic scale resolution. However, the fastest electronic and atomic dynamics in light-driven matter transformations are, as yet, unmeasureable with this technique. This is because the temporal resolution in ultrafast electron diffraction is limited by difficulties in producing the shortest electron pulses, caused by the electron charge, via Coulomb repulsion (space charge), and rest mass, via vacuum dispersion of the electron wavefunction. Space charge effects and a finite energy bandwidth both lead to temporal broadening of electron pulses. Methods to compress such pulses in microwave fields have been developed, but these are fundamentally limited by the achievable temporal synchronisation of the employed microwave with the excitation laser pulses. This work is aimed at breaking this limitation and thereby advancing ultrafast electron diffraction towards the ultimate temporal resolution of any realistic light--matter interaction. Firstly, a high-resolution optical-microwave phase detector based on optical interferometry is designed for operation around the 800-nm wavelength of Ti:sapphire lasers best suited for sample excitation. The phase detector provides a resolution of 3 fs and the capability of functioning as an integral component in a phase-locked loop for synchronising a low-noise dielectric resonator oscillator with the Ti:sapphire laser. Furthermore, we demonstrate a separate, novel, passive synchronisation technique through direct microwave extraction of a harmonic of the laser repetition rate by photodetection. A record-low residual phase noise over nine frequency decades (mHz--MHz) is achieved through implementation of an optical-mode filter which circumvents thermal noise problems at low pulses energies to simultaneously reduce detrimental amplitude-to-phase noise conversion in the photodetection process. An amplification chain is designed to achieve a microwave power suitable for electron compression while preserving this excellent phase noise. Rigorous out-of-loop characterisation of the synchronisation with the optical-microwave phase detector shows a root-mean-square (rms) timing stability of 4.8 fs. This superior synchronisation has allowed the generation of 12 fs (rms) electron pulses, the shortest to our knowledge. Lastly, stability of the laser--electron synchronisation over many hours is also demonstrated on a sub-five-femtosecond scale through in-situ measurement and subsequent compensation for the entire range of possible long-term drifts. This shows that incorporating these techniques can allow ultrafast electron diffraction experiments to observe the fastest reversible atomic-scale light--matter interaction dynamics