Flying Focus: Spatiotemporal control of intensity for laser applications*

A chromatic focusing system combined with chirped laser pulses was used to create a “flying focus” [1]. This advanced focusing scheme provides unprecedented spatiotemporal control over the laser focal volume by enabling a small-diameter laser focus to propagate over 100x its Rayleigh length. Furthermore, the flying focus decouples the speed at which the peak intensity propagates from the group velocity of the laser pulse, allowing the laser focus to co- or counter-propagate along its axis at any velocity. Experiments have demonstrated a nearly constant intensity over 4.5 mm, while the velocity of the focus ranged from subluminal (0.01c) to superluminal (15c). When increasing the laser intensity above the ionization threshold of the background gas, an ionization wave was measured to track the ionization threshold intensity isosurface as it propagates and ionization waves of arbitrary velocity were demonstrated [2],[3]. All backward and all superluminal cases mitigated the issue of ionization-induced refraction that typically challenges the formation of long, contiguous plasma channels. These properties provide the opportunity to overcome current fundamental limitations in laser-plasma amplifiers [4], laser-wakefield accelerators, photon accelerators, ion accelerators, THz generation, and high-order frequency conversion.

[1] D. H. Froula et al., Nat. Photonics 12, 262 (2018).

[2] D. Turnbull et al., “Ionization Waves of Arbitrary Velocity,” 120, 225001 (2018).

[3] J. P. Palastro et al., Phys. Rev. A 97, 033835 (2018).

[4] D. Turnbull et al., Phys. Rev. Lett. 120, 024801 (2018).