Abstract
In standard lasers, light amplification requires population inversion between an upper and a lower state to break the reciprocity between absorption and stimulated emission. However, in a medium prepared in a specific superposition state, quantum interference may fully suppress absorption while leaving stimulated emission intact, opening the possibility of lasing without inversion. Here we show that lasing without inversion arises naturally during propagation of intense femtosecond laser pulses in air. It is triggered by the combination of molecular ionization and molecular alignment, both unavoidable in intense light fields. The effect could enable inversionless amplification of broadband radiation in many molecular gases, opening unusual opportunities for remote sensing.
Highlights
A resonant light propagating through a medium will be absorbed by the medium in a lower state and will stimulate emission if the medium is excited
The basic idea behind lasing without inversion (LWI) [9,10,11] is similar to conventional lasers—to prepare a medium where emission is favored over absorption so that resonant light is amplified as it propagates in the medium
We present a scheme that does not follow this tradition. It uses only the natural dynamics of a multilevel quantum system and requires no coherence between the excited and the lower electronic states; effectively, lasing without inversion comes “for free.”. We show that this mechanism is active in the highly efficient generation of 391 nm radiation during propagation of intense femtosecond laser pulses in air [12,13,14,15,16], under standard conditions, where the process known as laser filamentation [17,18,19] leads to self-guiding of light
Summary
A resonant light propagating through a medium will be absorbed by the medium in a lower state and will stimulate emission if the medium is excited. We present a scheme that does not follow this tradition It uses only the natural dynamics of a multilevel quantum system and requires no coherence between the excited and the lower electronic states; effectively, lasing without inversion comes “for free.”. [35], a different lasing without an inversion mechanism has been proposed to cause the gain at the 391 nm line based on the existence of long-lived coherent polarizations coupling simultaneously the states X , B, and the intermediate first excited electronic state A in N+2. The present amplification scheme relies on rotational dynamics and does not require coherences between the electronic states of the ion, even though such coherence is inevitably created by the pump pulse driving the molecular ion after ionization of the neutral. Compared to the model in Ref. [35], we account for the light-induced vibrational and rotational dynamics of the molecule, which allows us to capture the effect of rotational quantum beat lasing without inversion
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