Amplification of intense light fields by ‘bound states of free electrons’

Summary form only given. Light is used to modify or control properties in many quantum systems, leading to phenomena such as electromagnetically induced transparency, the generation of slow light or bright coherent XUV radiation. Using light fields, with strengths comparable to the Coulomb field which binds valence electrons in atoms, particularly unusual quantum states can be created which describe a nearly free electron oscillating in the laser field yet still loosely bound to the core
^{1, 2}
. We demonstrate that such states can arise not only in isolated atoms
³
, but also in gases at a few atmospheric pressures, guiding intense laser pulses, where they can act as a gain medium. This gain is created within just a few cycles of the guided field. Using shaped pulses
⁴
with 5-10 fs risetimes, we create pulse conditions, within a laser filament, where these states can persist, and we observe in the corresponding emission, the signatures of these states. This work demonstrates a general, new, ultrafast gain mechanism during filamentation, in which any driven atom can achieve inversion intra-pulse, using pulse shaping techniques.