Laser Stabilised Ionising Transitions

We investigate a ionising electronic transition under resonant pumping. We demonstrate that, above a critical value of the pump intensity, a novel metastable electronic bound state is created, which can decay into the free electron continuum by two-photon ionization. We calculate the system's resonant fluorescence spectrum, finding results qualitatively different from the Mollow triplet expected in a bound-to-bound transition. The fluorescent emission can be used to measure the time-resolved population of the novel metastable state. Contrary to Kramers-Hennenberger atoms, stabilised by non-perturbative, non-resonant laser pulses, the physics we observe is inherently resonant and relies on perturbative level repulsion. In analogy to how the AC-Stark shift is a semiclassical version of the single-photon Rabi splitting observed in photonic cavity, the phenomenon we describe is better understood as a semiclassical version of recently observed excitons bound by a single cavity photon. Our results demonstrate a novel way to stabilise electronic states with intense laser fields, increasing our capability to design and engineer non-classical states of matter.