Degenerate mirrorless lasing in thermal alkali vapors is achieved through Doppler compensation using one-photon detuning

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 for non-zero pump detuning. A sub-natural linewidth dispersive profile was observed near the pump resonance, exhibiting gain on one side and absorption on the other. 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 the atomic velocity distribution, which washes out the fine spectral details. We demonstrate that the sideband gain peak is sustained beyond the Doppler broadening limit when both the pump Rabi frequency and detuning exceed the Doppler width, Ω_P≫Δ_P≫Δ_dop. 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.