Laser-microwave synchronisation for ultrafast electron diffraction

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.