All-optical tabletop strong-field terahertz pump—Ultrafast x-ray probe platform

We report the design and implementation of the first operational compact platform for strong-field terahertz (THz) pump-ultrafast x-ray probe measurements, which were previously accessible only at large-scale accelerator-based facilities, such as SLAC. Femtosecond laser-driven THz and x-ray sources are optimized by appropriately tuning the chirp of the driving laser pulses to maximize their generation efficiencies. The intense THz pump pulse generated via laser optical rectification in organic crystals delivers peak fields of up to 3 MV/cm and covers the spectral range of 0.1-3 THz. The ultrafast x-ray probe pulse is produced via ultraintense femtosecond laser interactions with tape-like copper foils, yielding a flux of around 105 copper Kα photons/pulse impinging on the sample at a repetition rate of 100 Hz. The spatial overlap and temporal synchronization of THz and x-ray pulses are achieved with the aid of an auxiliary near-infrared laser pulse. THz pump-x-ray probe measurements on the ultrafast structural phase transition of vanadium dioxide are performed as an example to demonstrate the capability of the platform. Such an all-optical tabletop platform enables the selective excitation and control and in situ high-temporal-resolution detection of ultrafast lattice structural dynamics, offering a unique tool for the study of ultrafast materials science and non-equilibrium physics.