Frequency Matching in Light-Storage Spectroscopy of Atomic Raman Transitions

We investigate the storage of light in an atomic sample with a $\Lambda$-type coupling scheme driven by optical fields at variable two-photon detuning. In the presence of electromagnetically induced transparency (EIT), light is stored and retrieved from the sample by dynamically varying the group velocity. It is found that for any two-photon detuning of the input light pulse within the EIT transparency window, the carrier frequency of the retrieved light pulse matches the two-photon resonance frequency with the atomic ground state transition and the control field. This effect which is not based on spectral filtering is investigated both theoretically and experimentally. It can be used for high-speed precision measurements of the two-photon resonance as employed, e.g., in optical magnetometry.

Figure 1

Three-level scheme used for storage and retrieval of light. The levels $|g_{+}⟩$ and $|e⟩$ and $|g_{-}⟩$ and $|e⟩$ are coupled by a ${\sigma }^{-}$-polarized control field and a ${\sigma }^{+}$-polarized signal field with optical frequencies ${\omega }_c$ and ${\omega }_s$, respectively. The two-photon detuning is given by $\delta ={\omega }_s-{\omega }_c-2g_F{\mu }_{B}B$, where ${\mu }_B$ is the Bohr magneton, $g_F$ the hyperfine $g$ factor, and $B$ the magnetic field.

Figure 2

Scheme of experimental setup.

Figure 3

(a) Typical stored light signal for a storage period of $20\mu \mathrm{s}$. At time $t=0$, the control field and the signal field were switched off adiabatically, and at time $t\approx 20\mu \mathrm{s}$, only the control field was switched on, restoring the signal field. The inset shows a portion of the retrieved pulse fitted with a sinusoidally modulated function [22]. (b) Measured beat frequency of the released signal light as a function of the applied magnetic bias field. All data points were recorded with the same value of the two-photon detuning of the initial signal field. The size of the error bars of the data points is smaller than the drawing size of the dots.

Figure 4

Beat frequencies for different values of the two-photon detuning for the pulses before storage (open squares) and after the retrieval procedure (circles). For all shown data points, the atomic sample was exposed to the same magnetic bias field. The slopes of the fitted linear curves are (within uncertainties calculated by the fitting routine) one for the input pulses (dashed line) and zero for the pulses detected after storage (solid line). The size of the error bars is within the drawing size of the data points.