Degenerate mirrorless lasing in thermal alkali vapors

For a degenerate two-level alkali atom system driven by a linearly polarized continuous-wave pump field, the steady-state gain for an orthogonally polarized probe field was theoretically predicted (Ramaswamy et al., Mirrorless Lasing: A Theoretical Perspective. Opt. Memory and Neural Networks 32, S443-S466 (2023)). Using linear response theory, we calculated the probe absorption spectrum when the pump was detuned from resonance. A sub-natural linewidth dispersive structure was observed near the pump resonance, exhibiting both gain and absorption. Additionally, a distinct pure gain peak appeared at a sideband corresponding to a dressed-state transition. These effects typically do not persist outside the ultracold regime due to inhomogeneous broadening caused by Doppler broadening, which washes out the fine spectral details. We demonstrate that the sideband gain peak is sustained in the warm vapor regime when both the pump Rabi frequency and detuning exceed the Doppler width. Our results can enable degenerate mirrorless lasing in thermal alkali atom vapors, offering a significant enhancement in the signal-to-noise ratio for fluoroscopic remote magnetic sensing applications.