Observation of Collective-Emission-Induced Cooling of Atoms in an Optical Cavity

We report the observation of collective-emission-induced, velocity-dependent light forces. One-third of a falling sample containing $3\times {10}^6$ cesium atoms illuminated by a horizontal standing wave is stopped by cooperatively emitting light into a vertically oriented, confocal resonator. We observe decelerations up to $1500\mathrm{m}/{\mathrm{s}}^2$ and cooling to temperatures as low as $7\mu \mathrm{K}$, well below the free-space Doppler limit. The measured forces substantially exceed those predicted for a single two-level atom.

Figure 1

(a) Experimental setup for observing emission-induced forces. Cs atoms, illuminated by two horizontal laser beams and falling along the resonator, experience velocity-dependent forces in the $xz$ plane. (b) Cs hyperfine structure.

Figure 2

Fall time $t_f$ of atoms with $v_0=15\mathrm{c}\mathrm{m}/\mathrm{s}$ versus light exposure time $t_e$ for ${\Delta }_c/2\pi =-150\mathrm{M}\mathrm{H}\mathrm{z}$, $I/I_s=16$, and ${\delta }_a/2\pi =-63\mathrm{M}\mathrm{H}\mathrm{z}$. The inset shows the TOF signal of the remnant MOT cloud and the delayed atoms for $t_e=2\mathrm{m}\mathrm{s}$.

Figure 3

Scattering ratio $\eta ={\Gamma }_c/{\Gamma }_{\mathrm{f}\mathrm{s}}$ versus atomic saturation parameter $p$ for ${10}^6$ atoms, ${\delta }_a/2\pi =-78\mathrm{M}\mathrm{H}\mathrm{z}$, and ${\Delta }_c/2\pi =-150\mathrm{M}\mathrm{H}\mathrm{z}$. The inset shows the scattering rate into free space ${\Gamma }_{\mathrm{f}\mathrm{s}}$ (open circles) and into the cavity ${\Gamma }_c$ (solid circles) per atom. The solid line is the calculated ${\Gamma }_{\mathrm{f}\mathrm{s}}$.

Figure 4

Evolution of the delayed atom sample’s vertical temperature $T_z$ for ${\Delta }_c/2\pi =-150\mathrm{M}\mathrm{H}\mathrm{z}$, $I/I_s=20$, ${\delta }_a/2\pi =-78\mathrm{M}\mathrm{H}\mathrm{z}$. The inset shows $T_z$ versus initial MOT temperature $T_{\mathrm{M}\mathrm{O}\mathrm{T}}$ in $\mu \mathrm{K}$ for $I/I_s=13$ and ${\delta }_a/2\pi =-58\mathrm{M}\mathrm{H}\mathrm{z}$ with all other parameters as given in Fig. 5.

Figure 5

Vertical temperature $T_z$ versus ${\delta }_a$ for $I/I_s=22$ (open) and $I/I_s=11$ (solid) along with typical error bars. All data were taken with ${10}^6$ atoms for ${\Delta }_c/2\pi =-200\mathrm{M}\mathrm{H}\mathrm{z}$, 5 ms exposure time, and slow light extinction within 0.4 ms.