Amplification of intense light fields by nearly free electrons

TL;DR

This paper demonstrates that nearly free electrons in rare gases, created by intense laser fields, can act as a gain medium during laser filamentation, enabling ultrafast light amplification through laser-dressed states.

Contribution

It reveals that Kramers-Henneberger states in neutral atoms can be exploited for ultrafast gain during laser filamentation, a novel application of laser-dressed states.

Findings

Gain achieved within a few laser cycles

Lasing emission indicates population inversion

States are stable against ionization

Abstract

Light can be used to modify and control properties of media, as in the case of electromagnetically induced transparency or, more recently, for the generation of slow light or bright coherent XUV and X-ray radiation. Particularly unusual states of matter can be created by light fields with strengths comparable to the Coulomb field that binds valence electrons in atoms, leading to nearly-free electrons oscillating in the laser field and yet still loosely bound to the core [1,2]. These are known as Kramers-Henneberger states [3], a specific example of laser-dressed states [2]. Here, we demonstrate that these states arise not only in isolated atoms [4,5], but also in rare gases, at and above atmospheric pressure, where they can act as a gain medium during laser filamentation. Using shaped laser pulses, gain in these states is achieved within just a few cycles of the guided field. The corresponding lasing emission is a signature of population inversion in these states and of their stability against ionization. Our work demonstrates that these unusual states of neutral atoms can be exploited to create a general ultrafast gain mechanism during laser filamentation.