Generation and Characterization of Broadband Spatiotemporal Light Springs at Relativistic Intensities

Spatiotemporal shaping of relativistic laser light is a powerful tool for particle acceleration, radiation sources, and extreme field science. A striking example is the light spring, where pulse intensity helically modulates around the propagation axis. This structure creates spatiotemporal ponderomotive forces and enables efficient coupling to helical plasma waves.

We recently demonstrated such light springs in broadband (~30 nm) laser pulses at relativistic intensities—exceeding past experiments by orders of magnitude. Our method spectrally splits the pulse, imprints distinct orbital angular momentum states onto each component, and coherently recombines them into a beam whose transverse velocity, rotation, and mode evolve in time.

We review this experimental platform and introduce a bichromatic Laguerre-Gaussian model that yields analytical expressions for angular momentum density, ponderomotive force, and plasma density perturbations. We compare our model to particle-in-cell (PIC) simulations, offering a framework to predict how high-intensity broadband light springs couple to plasmas.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344 and funded by the Laboratory Directed Research and Development Program at LLNL under project tracking code 24-ERD-031. This material is based upon work supported by the National Science Foundation under Grant No. PHY-1753165 and DMR-1548924.