Strong-field physics meets quantum electronics

In this work, we have examined the connection between ultrashort pulse strong-field physics and quantum electronic effects in noble gases. In particular, interferometric measurements of plasma densities created by strong-field ionization of argon (Ar), krypton (Kr), and xenon (Xe) by ultrashort linearly and circularly polarized laser pulses are compared to strong-field ionization theories and numerical simulations. Measured ionization rates match theory and simulation for long wavelengths (800 and 400 nm), but at shorter wavelengths (around ∼260nm), significant discrepancies are observed due to resonant multiphoton processes involving specific atomic excited states. The largest discrepancy is observed for argon and explained using theory and simulations that take into account individual multi-photon resonances. These resonances give rise to several quantum electronic effects such as cascaded superfluorescent lasing and quantum interference from the excited populations induced by ultrashort UV pulses. Using strong-field 260 nm light, new applications such as in situ injection of spin-polarized electron beams in plasma accelerators by ionization of Xe atoms by CP photons and cavity-free backward lasing for remote sensing using LP photons to excite Ar, are likely from this new understanding.