Bound electron nonlinearity beyond the ionization threshold

Although high field laser-induced ionization is a fundamental process underlying many applications, there have been no absolute measurements of the nonlinear polarizability of atoms and molecules in the presence of ionization. Such information is crucial, for example, for understanding the propagation of high intensity ultrashort pulses in matter. Here, we present absolute space- and time-resolved measurements of the ultrafast laser-driven nonlinear polarizability in argon, krypton, xenon, nitrogen, and oxygen up to an ionization fraction of a few percent. These measurements enable determination of the non-perturbative bound electron nonlinearity well beyond the ionization threshold, where it is found to be approximately linear in intensity

Spatiotemporal torquing of light

We demonstrate the controlled spatiotemporal transfer of transverse orbital angular momentum (OAM) to electromagnetic waves: the spatiotemporal torquing of light. This is a radically different situation than OAM transfer to longitudinal, spatially-defined OAM light by stationary or slowly varying refractive index structures such as phase plates or air turbulence. We show that transverse OAM can be imparted to a short light pulse only for (1) sufficiently fast transient phase perturbations overlapped with the pulse in spacetime, or (2) energy removal from a pulse that already has transverse OAM. Our OAM theory for spatiotemporal optical vortex (STOV) pulses [Phys. Rev. Lett. 127, 193901 (2021)] correctly quantifies the light-matter interaction of this experiment, and provides a torque-based explanation for the first measurement of STOVs [Phys. Rev. X 6, 031037 (2016)]