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

In a degenerate two-level alkali atom system driven by a linearly polarized continuous-wave pump field, theoretical predictions (Ramaswamy et al., Mirrorless Lasing: A Theoretical Perspective, Opt. Memory and Neural Networks 32, S443-S466 (2023)) indicated steady-state gain for an orthogonally polarized probe field. Using linear response theory, we computed the probe absorption spectrum with non-zero pump detuning. A dispersive profile with sub-natural linewidth appeared near the pump resonance, featuring gain on one side and absorption on the other. Furthermore, a clear gain peak was observed at a sideband associated with a dressed-state transition. These spectral features are typically diminished in non-ultracold environments due to Doppler broadening from atomic motion. Nevertheless, we show that the sideband gain peak remains intact when the pump Rabi frequency and detuning are both greater than the Doppler width. Our results enable degenerate mirrorless lasing in thermal alkali atom vapors, significantly improving the signal-to-noise ratio for fluoroscopic remote magnetic sensing applications.