Ultrafast spatiotemporal control of photoionization and THz generation*

.A laser pulse composed of a fundamental and properly phased second harmonic exhibits an asymmetric electric field that can drive a time-dependent current of photoionized electrons. The current produces an ultrashort burst of terahertz (THz) radiation. When driven by a conventional laser pulse, the THz radiation is emitted into a cone with an angle determined by the dispersion of the medium. Here we demonstrate that the programmable velocity intensity peak of a spatiotemporally structured laser pulse can be used to control the emission angle, focal spot, and spectrum of the THz radiation. Specifically, a subluminal ultrafast flying focus can drive a highly focusable, single-cycle THz pulse ideal for probing picosecond dynamics or pumping new states of matter. The recently demonstrated ultrafast flying focus employs an axiparabola to focus different radial locations in the near field to different axial locations in the far field and an echelon to adjust the relative timing of the foci. While effective for THz generation, the optical configuration required for the ultrafast flying focus constrains properties of the intensity peak, such as the transverse profile and orbital angular momentum (OAM). Recently proposed nonlinear flying focus techniques offer additional opportunities for spatiotemporal control that can overcome these constraints. The self-flying focus combines temporal pulse shaping with the inherent nonlinearity of a medium, avoiding the need for custom optics. The flying focus X applies cross-phase modulation in a Kerr lens to create an ultrashort flying focus with a customized radial profile, with or without OAM. The resulting pulses can drive arbitrary trajectory ionization fronts that mitigate ionization refraction and allow for the formation of long, contiguous plasma channels—a critical component of advanced particle and photon accelerators.