Dark and bright coherent states

Depending upon the parameters of the optical transitions, a coherent superposition state may be either ‘dark’ or ‘bright’, producing electromagnetically induced transparency (EIT) or absorption (EIA) [Akulshin et al, PRA 57, 2996 (1998), Lezama et al, PRA 59, 4732 (1999)]. These intriguing states not only provide the possibility for practical applications but they are also important for better understanding fundamental physics of atom-light interaction. Some details still remain not well understood. As an example, we have observed, characterized and explained a strong and unexpected resonance of enhanced absorption (pseudo EIA) under experimental conditions where no narrow structure was expected. In addition, we have demonstrated in experiments with a ⁶Li atomic beam that both coherent population oscillations and long-lived Zeeman coherences may be responsible for very narrow absorption resonances.

Papers:
J. Fuchs et al, J. Phys. B: At. Mol. Opt. Phys. 40, 1117 (2007)
J. Fuchs et al, J. Phys. B: At. Mol. Opt. Phys. 39, 3479-3489 (2006)

Delay lines in coherent media

Slow light (SL) has become an essential part of optics. SL is the passage of light pulses through a medium with group velocity reduced by several orders of magnitude. EIT is able to produce the necessary steep dispersion. In a laser cooled Na cloud under conditions of EIT the famous 17m/s group velocity was obtained by the team led by L.Hau [Nature 397, 594 (1999)].
We have shown that in a warm Rb cell containing 1 Torr of Kr it is possible to make a longer absolute delay (~ 19 μs) with higher transparency (~50%), though the group velocity is reduced by a more modest five orders of magnitude.
In a ‘bright’ state the dispersion is also steep but negative [Phys. Rev. Lett. 83, 4277 (1999)], leading to the intriguing situation where the peak of a pulse actually leaves the medium before it enters it. But Einstein can rest easy as this does not mean that information is transmitted faster than c.
We are not dealing with slow or fast moving photons! Slow and fast light is a result of interference within the wave packet.

Papers:
A.M. Akulshin et al, Laser Physics, 15, 1252-6 (2005)
A.M. Akulshin et al, Phys. Rev. A, 67 , 011801(R) (2003)
A.M. Akulshin et al, J. Opt. B: Quantum Semiclass. Opt., 5, S479 (2003)

Fast and slow light in linear atomic media

The small spectral width of EIT or EIA resonances imposes a severe limitation on pulse duration. The pulse must be long enough that all its spectral components are within the region of steep dispersion. Shorter pulses require a wider region of constant dispersion. Broadband slow-light media, which are characterized by very large relative delay and high transmission of ns pulses, have been realized in a Rb vapour cell. This simple delay line is much more robust than an EIT-based one and suitable for direct 2D image processing
Fast light in linear atomic media can be achieved by exploiting the steep anomalous dispersion associated with laser-cooled atoms in a MOT. We have observed a group velocity of -c/360 in this way.

Papers:
W.G.A. Brown et al, J. Opt. Soc. Am. B 25 C82-C86 (2008) arXiv 0805.2993
A. Akulshin et al, Optics Express 16, 15463-8 (2008)
M.R. Vanner et al, J. Phys. B: At. Mol. Opt. Phys. 41, 051004 (2008) arXiv 0711.4172
Presentations:
Link to ICO-21 talk (Sydney, Jul 2008) (pdf, 650 kB)
Link to CAOUS talk (Feb 2008) (pdf, 1.1 MB)
Link to Poster (Quantum Atom Optics Downunder, Wollongong, Dec 2007) (pdf, 1.0 MB)

Storage of Light in Fast- and Slow-Light Media

In fact, ‘stopped light’ or ‘storage of light’ is the storage and retrieval of information carried by light. The scheme of our light storage experiment is similar to the first demonstration [Phillips et al, PRL. 86, 783(2001)]; however, we use a fast-light atomic medium instead, where EIT does not exist and the concept of the dark state polariton [Fleischhauer and Lukin, PRL. 84, 5094 (2000)] does not apply. We have demonstrated the controlled release of light pulses in both slow- and fast-light coherent media. The remarkable similarity of the atomic responses after a dark interval, despite dramatically different optical properties, strongly indicates that there is common physics of ‘storage of light’ in the two cases.

Papers:
A. Lezama et al, Phys. Rev. A 73, 033806 (2006)
A.M. Akulshin et al, J. Phys. B: At. Mol. Opt. Phys. 38, L365 (2005)

Enhanced Kerr Nonlinearity

The refractive index of coherent media at a Raman resonance depends on the intensity of the driving field. This means that a large Kerr nonlinearity of an atomic sample can be obtained at very weak light intensity. We have recently observed and measured the giant Kerr coefficient n₂ of the refractive index of atomic media prepared in dark and bright coherent states. Higher order nonlinearity is also large in both cases. This is an entirely unique situation at such low light intensity.

Paper: A.M. Akulshin et al, J. Opt. B: Quantum Semiclass. Opt. 5, S479 (2003)

Wave mixing and generation of new optical fields

The extraordinarily large Kerr nonlinearity induced by very weak optical fields can result in efficient wave mixing and generation of new optical fields. We have shown that spatial nonlinear effects such as diffraction and self-focussing in a dense atomic sample play an important role even at a very low light intensity.

Paper: A.M. Akulshin et al, J. Opt. B: Quantum Semiclass. Opt. 6, 491 (2004)

Frequency up-conversion

Frequency up-conversion in a vapour cell can take place when Rb atoms are excited by resonant radiation into the 5D state. In addition to a striking blue fluorescence, highly collimated and coherent blue radiation is generated.
We show that stimulated four wave mixing is responsible for the collimated blue light. The intensity of the blue light depends on frequency detuning of the both IR laser fields. Other parameters such as intensity, polarization and atomic density are also very important.