Across the threshold: From ultracold plasma to dense Rydberg gases*

Ultrashort laser pulses allow for local and controlled ionization of a quantum gas on femtosecond timescales. We report on the transition from ultracold plasma to dense Rydberg gases when tuning the central wavelength of a single laser pulse of 166 fs duration across the two-photon ionization threshold of ultracold ⁸⁷Rb.

Above the threshold, strong-field ionization triggers the formation of an ultracold plasma. While the initial thermal energy of the ions can be neglected, the electrons, carrying the excess energy, experience rapid cooling on picosecond time scales.

Below the ionization threshold, we observe the formation of dense Rydberg gases since the Rydberg blockade is circumvented by the large bandwidth of the femtosecond pulse.

Our setup allows direct detection of the energy distribution of ions and electrons as well as atoms in Rydberg states. We have performed molecular dynamics simulations that provide insight into the underlying dynamics governed by long-range Coulomb interactions.