Resolving multiple peaks using a sub-transit-linewidth cross-correlation resonance

In a recent paper [L. Feng et al., Phys. Rev. Lett. 109, 233006 (2012)], we demonstrated a sub-transit-linewidth resonance based on cross correlation between optical fields in an electromagnetically induced transparency (EIT) system. Here, we investigate the ability of such resonance to resolve multiple resonance peaks. We first report the implementation of the cross-correlation resonance in a hyperfine EIT system, with a linewidth of about $1/16$ of the transit EIT width. Then, a magnetic field is applied to create multipeak EIT resonance with tunable spacing. Cross-correlation resonance with multipeaks is observed and the line shape agrees qualitatively with our numerical simulations. We find that the multipeak correlation resonance has better contrast than the corresponding EIT resonance, but with a lower signal to noise. Furthermore, this correlation resonance cannot resolve peaks with spacing close to or less than the intrinsic width, like any other resonance techniques. We analyze the origin of this limitation and make connections to the Ramsey-type subnatural spectroscopy technique. This work shows potential applications of the correlation resonance in atomic frequency standard and laser spectroscopy.

Figure 1

Energy levels for the multipeak hyperfine EIT using two orthogonal linearly polarized laser fields.

Figure 2

The experiment schematics. See text for details.

Figure 3

Measured single-peak $g^{\left(2\right)}\left(0\right)$ resonance spectrum. The FWHM is about 11.6 kHz, which is $1/16$ of the transit EIT width of 190 kHz. The total laser power at the cell input is 26 $\mu$W.

Figure 4

Measured EIT and $g^{\left(2\right)}\left(0\right)$ spectra for peak spacing equal to 3.5 times the transit EIT FWHM. (a) EIT spectrum. (b) $g^{\left(2\right)}\left(0\right)$ spectrum. The blue curve is to guide the eye. Total laser power was 26 $\mu$W for both (a) and (b).

Figure 5

Measured EIT and $g^{\left(2\right)}\left(0\right)$ spectra for peak spacing equal to 1.5 times the transit EIT FWHM. (a) EIT spectrum. (b) $g^{\left(2\right)}\left(0\right)$ spectrum. The blue curve is to guide the eye. Total laser power was 26 $\mu$W for both (a) and (b).

Figure 6

Simplified energy levels for the multipeak EIT used in our simulation.

Figure 7

Simulation results of the three $\Lambda$ energy system. In (a), the level spacing was 3.5 times the transit EIT FWHM. Both EIT and $g^{\left(2\right)}\left(0\right)$ resonance can resolve the energy levels. In (b), the level spacing was 1.5 times the transit EIT FWHM. The EIT peaks begin to overlap, and the $g^{\left(2\right)}\left(0\right)$ shows individual peaks well. See text for details.

Figure 8

Experiment results from the time-delayed subnatural Ramsey resonance. Traditional absorption spectra for a (a) single peak and (b) three-peak case. (c),(d) Corresponding Ramsey spectra of (a) and (b). Adapted from figures in [7].