Two-level lasers are thermodynamically ruled out, because population inversion cannot be achieved at equilibrium.
In a recent paper ArXiv:1102.1207, Viola Folli and Claudio Conti show that, by employing the interaction between a Self-Induced Transparency Soliton and Surface Anderson Localization of Light, it is possible to achieve a two-level laser-like emission in a dynamical regime, when the optical pump pulse has the same carrier frequency of the resonant system, at variance with standard lasers where additional non-radiative (and energetically inefficient) levels are commonly employed.
This shows that the interaction between two apparently un-related forms of light localization, i.e., solitons and Anderson localizations, may provide novel fascinating regimes for light-matter interaction.
The picture above (after parallel FDTD Maxwell-Bloch simulations) shows the comparison between the case when the SIT soliton propagates almost undistorted in the random system (low-index contrast) and the case when it transfers energy to Anderson localizations located in proximity of the surface (high-index contrast).
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