Degenerate mirrorless lasing in thermal vapors

Abstract

Theoretical predictions were made for the steady-state gain of an orthogonally polarized probe field in a degenerate two-level alkali atom system driven by a linearly polarized continuous-wave pump field in [Opt. Mem. Neural Networks 32 (Suppl 3), S443-S446 (2023)]. Employing linear response theory, we computed the probe absorption spectrum under conditions where the pump was detuned from resonance. The results revealed a sub-natural linewidth dispersive feature near the pump resonance, characterized by both gain and absorption. Furthermore, a distinct pure gain peak emerged at a sideband associated with a dressed-state transition. These phenomena are generally absent outside the ultracold regime due to inhomogeneous broadening, primarily from Doppler effects, which obscure the fine spectral structure. In this paper, it is demonstrated that the sideband gain peak is sustained in the warm vapor regime when both the pump Rabi frequency and detuning exceed the Doppler width, $Ω_P > Δ_P \gg Δ_{Dop}$. 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. The theoretical model studied in this paper is also a complete description of atomic vapors with isolated $J = 2 \to J' = 3$ transitions, such as atomic samarium.

Figures (6)

• Figure 1: Vapor cell configuration studied in this work. The cell has absorbing boundary conditions for light and is filled with Rb-85 atoms. A CW strong pump with field strength $E_P$ is used to provide the conditions for degenerate mirrorless lasing for both forward and backward generated emissions.
• Figure 2: Level scheme studied in this work. We consider the Rb-85 D2 transition $(F=2\rightarrow F^{\prime}=3)$. The pump field $E_P$ is $z$-polarized and drives transitions with the same $m$ while the output field $E_O$ is $x$-polarized.
• Figure 3: Diagram showing the transformation of the pump and probe frequencies from the lab frame to the rest frame. The asymmetry of the relative Doppler shifts between the pump and emitted light for the co- and counter-propagating cases is a critical reason for the difference in the observed velocity-averaged spectral line-shapes. The atoms emit a continuum of frequencies but we pick a specific frequency $\omega^{\prime}_{em}$ to show how a single emission frequency is Doppler shifted in the lab frame.
• Figure 4: Non Doppler-broadened spectral line-shapes for absorption (top) and emission (bottom) for strong driving ($\Omega=4\Gamma$) for different values of the pump detuning $\Delta_P$. The EIA peak feature is observed at resonant driving (middle) and gain sidebands and the AT spikes are observed for $\Delta_P=\pm\Gamma$ (left and right).
• Figure 5: Doppler broadened spectral line-shapes for absorption (top) and emission (bottom) for the co-propagating (left) and counter-propagating (right) cases. The parameters are $\Omega_P=4\Gamma$, $\Delta_P=\Gamma$ and $\Delta_{Dop}=8.5\Gamma$. For the calculation, 21 velocity classes were used for the Doppler averaging to demonstrate the effect of the relative Doppler shift in displacing the peaks from different velocity groups for the counter-propagating case.