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

Amplification of intense light fields by nearly free electrons