Mechanisms for Lasing with Cold Atoms as the Gain Medium

We realize a laser with a cloud of cold rubidium atoms as gain medium, placed in a low-finesse cavity. Three different regimes of laser emission are observed corresponding, respectively, to Mollow, Raman, and four-wave mixing mechanisms. We measure an output power of up to $300\mu \mathrm{W}$ and present the main properties of these different lasers in each regime.

Figure 1

Transverse modes of cold-atom lasers. (a) Gaussian ${\mathrm{TEM}}_{00}$ mode, obtained by inserting a small diaphragm in the cavity. Typical modes of (b) the Mollow laser, (c) the Raman laser, and (d) the four-wave mixing laser.

Figure 2

Laser power (squares) and Mollow gain (open circles) versus pump power, with $b_0=11$ and $\Delta =+\Gamma$. Lasing threshold (vertical dashed line) is expected to appear with a gain of about 21% (horizontal dashed line), in good agreement with the experimental data. Inset: Typical weak-probe transmission spectrum.

Figure 3

Laser power (squares) and Raman gain (open circles) versus pump power, with $b_0=10$ and $\Delta =-7\Gamma$. Lasing threshold (vertical dashed line) is expected to appear with a gain of about 21% (horizontal dashed line), in good agreement with the experimental data. Inset: Typical weak-probe transmission spectrum.

Figure 4

Laser power (squares) and phase-conjugate reflectivity due to four-wave mixing (open circles) versus pump power, with $b_0=10$ and $\Delta =-8\Gamma$. Lasing threshold (vertical dashed line) is expected for a reflectivity around 1% (horizontal dashed line), in good agreement with the experimental data. Inset: Example of a weak-probe reflectivity spectrum.