Realization of coherent optically dense media via buffer-gas cooling

We demonstrate that buffer-gas cooling combined with laser ablation can be used to create coherent optical media with high optical depth and low Doppler broadening that offers metastable states with low collisional and motional decoherence. Demonstration of this generic technique opens pathways to coherent optics with a large variety of atoms and molecules. We use helium buffer gas to cool ${}^{87}\mathrm{Rb}$ atoms to below $7\mathrm{K}$ and slow atom diffusion to the walls. Electromagnetically induced transparency in this medium allows for 50% transmission in a medium with initial optical depth $D>70$ and for slow pulse propagation with large delay-bandwidth products. In the high-$D$ regime, we observe high-contrast spectrum oscillations due to efficient four-wave mixing.

Figure 1

(Color online) Experimental setup (PBS, polarizing beam splitter; QWP, quarter-wave plate; $L$, lens; $D$, photodiode detector). The inset shows the ${}^{87}\mathrm{Rb}$ level structure.

Figure 2

(Color online) $D$ estimates in the presence of EIT. (a) Solid blue line is the negative logarithm of probe transmission as a function of time, as the probe frequency is scanned back and forth through two-photon resonance resulting in the sharp EIT dips. Red dashed line is an exponential fit to the background optical depth. The inset zooms in on the plot around $t=66\mathrm{ms}$. (b) Probe transmission (solid lines) at two different $D$’s exhibiting an EIT peak on the background of a Doppler-broadened single-photon absorption dip. Dashed curves are theoretical fits to the single-photon absorption dip.

Figure 3

(Color online) Interference fringes in the EIT spectrum caused by four-wave mixing. Control field powers are (i) $0.58\mathrm{mW}$, (ii) $0.21\mathrm{mW}$, and (iii) $0.077\mathrm{mW}$, with beam diameter of $0.4\mathrm{mm}$ (the latter deduced from the Rabi frequency fit). (a) Experiment. Since the absolute value of $\delta$ was not calibrated experimentally, $\delta =0$ is chosen to approximately match the theory. (b) Theory.

Figure 4

(Color online) Pulse propagation through EIT medium at three different optical depths increasing from (i) to (iii). Intensity of 1 corresponds to the maximum intensity of the incident pulse, a down-scaled version of which (gray) is centered at $\text{time}=0$. The three experimental data sets (solid lines) and the simulations (dashed lines) are shifted vertically relative to each other for easier viewing. The circled piece of data set (iii) and the corresponding theory are magnified by a factor of 10 for easier viewing. Control field of $400\mu \mathrm{W}$ and $3.2\mathrm{mm}$ diameter (the latter deduced from the Rabi frequency fit) is used. The absolute delays are (i) $6\mu \mathrm{s}$, (ii) $16\mu \mathrm{s}$, and (iii) $59\mu \mathrm{s}$, yielding delay-bandwidth products (where bandwidth is given by the output pulse) of (i) 0.7, (ii) 1.2, and (iii) 2.9. From the theoretical fit, the fraction of energy transmitted is: (i) 18%, (ii) 8%, and (iii) 0.4%.