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arXiv:1408.2316 · New Journal of Physics

Electromagnetically induced transparency and four-wave mixing in a cold atomic ensemble with large optical depth

Jiao Geng, Geoff Campbell, J. Bernu, Daniel B. Higginbottom, Ben M. Sparkes, Syed M. Assad, Weiping Zhang, Nicholas P. Robins, Ping Koy Lam, B. C. Buchler

Published Nov 24, 2014• 38 citations

Abstract

We report on the delay of optical pulses using electromagnetically induced transparency (EIT) in an ensemble of cold atoms with an optical depth exceeding 500. To identify the regimes in which four-wave mixing (4WM) impacts on EIT behaviour, we conduct the experiment in both 85 Rb and 87 Rb. Comparison with theory shows excellent agreement in both isotopes. In 87 Rb negligible 4WM was observed and we obtained one pulse-width of delay with 50% efficiency. In 85 Rb 4WM contributes to the output. In this regime we achieve a delay-bandwidth product of 3.7 at 50% efficiency, allowing temporally multimode delay, which we demonstrate by compressing two pulses into the memory medium.

DOI: 10.1088/1367-2630/16/11/113053

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We report on the delay of optical pulses using electromagnetically induced transparency (EIT) in an ensemble of cold atoms with an optical depth exceeding 500. To identify the regimes in which four-wave mixing (4WM) impacts on EIT behaviour, we conduct the experiment in both 85 Rb and 87 Rb. Comparison with theory shows excellent agreement in both isotopes. In 87 Rb negligible 4WM was observed and we obtained one pulse-width of delay with 50% efficiency. In 85 Rb 4WM contributes to the output. In this regime we achieve a delay-bandwidth product of 3.7 at 50% efficiency, allowing temporally multimode delay, which we demonstrate by compressing two pulses into the memory medium.

We observed electromagnetically induced transparency-based four-wave mixing (FWM) in the pulsed regime at low light levels. The FWM conversion efficiency of

3.8(9)%

was observed in a four-level system of cold

{}^{87}\mathrm{Rb}

atoms using a driving laser pulse with a peak intensity of $\ensuremath{\approx}80$ $\ensuremath{\mu}\mathrm{W}/{\mathrm{cm}}^{2}$, corresponding to an energy of $\ensuremath{\approx}60$ photons per atomic cross section. Comparison between the experimental data and the theoretical predictions proposed by Harris and Hau [Phys. Rev. Lett. 82, 4611 (1999)] showed good agreement. Additionally, a high conversion efficiency of

46(2)%

was demonstrated when applying this scheme using a driving laser intensity of $\ensuremath{\approx}1.8$

\mathrm{mW}/{\mathrm{cm}}^{2}

. According to our theoretical predictions, this FWM scheme can achieve a conversion efficiency of nearly

100%

when using a dense medium with an optical depth of 500.

We study the modification of a traditional electromagnetically induced transparency (EIT) stored-light technique that includes both EIT and four-wave mixing (FWM) in an ensemble of hot Rb atoms. The standard treatment of light storage involves the coherent and reversible mapping of one photonic mode onto a collective spin coherence. It has been shown that unwanted, competing processes such as four-wave mixing are enhanced by EIT and can significantly modify the signal optical pulse propagation. We present theoretical and experimental evidence to indicate that, while a Stokes field is indeed detected upon retrieval of the signal field, any information originally encoded in a seeded Stokes field is not independently preserved during the storage process. We present a simple model that describes the propagation dynamics of the fields and the impact of FWM on the spin wave.

Electromagnetically induced transparency (EIT) in a four-level atomic system suppresses linear absorption and leads to resonantly enhanced slow-light four-wave mixing at low light intensities. We analyse the backward four-wave mixing in the four-level system under the EIT conditions, derive the four-wave-mixing efficiency and discuss slow-light propagation conditions. We present experimental measurements of resonant four-wave mixing with the backward scheme in cold Rb atoms and compare the experimental results with the theoretical calculations.

We report an experimental observation of slow light propagation in cold Rb atoms exhibiting cavity electromagnetically induced transparency (EIT). The steep slope of the atomic dispersion manifested by EIT reduces the light group velocity. The cavity filtering and feedback further contribute to the slowdown and delay of the light pulse propagation. A combination of the cavity and the EIT atomic system significantly improves the performance of the slow light propagation. A propagation time delay of approximately 200 ns was observed in the cavity and Rb EIT system, which is approximately 70 times greater than the time delay calculated for the light pulse propagation through the same Rb EIT system without the cavity.

We report the observation of Sommerfeld-Brillouin optical precursors generated from a long square-modulated laser pulse propagating through a cold atomic ensemble with electromagnetically induced transparency. The optical depth (alpha(0)L) of the medium can be varied from 0 up to 50. We demonstrated that the step-on rising and step-off falling edges propagate with the speed of light in vacuum without a slow light effect. At high alpha(0)L, the precursor is separated from the delayed main pulse at the rising edge, while at the falling edge, we observe damped oscillatory structures resulting from the interference between the precursor and main field.