Brief Introduction to EIT

Graham Purves, Charles Adams, Ifan Hughes

Those who are already familiar with the effects of two radiation fields interacting with three atomic levels should move straight on to EIT at Durham. Those unfamiliar with the field may be interested in the following, very brief introduction.

General EIT Scheme

Electromagnetically Induced Transparency (EIT), is an effect which can occur in atoms, when we consider two radiation fields interacting, close to resonance with three levels in the atom. In the regime of EIT one of the radiation fields, the probe beam, is of a lower intensity than the other, strong pump beam.

Transmission / Absorption

When measuring the transmission of the weak probe beam through the medium exhibiting EIT, we will see a narrow transmission (the induced transparency) windowin the broad absorption due to the single photon hyperfine transition that we are close to resonance with. Depending upon the exact experimental parameters, these features can be substantially narrower than the natural linewidth of either of the two electric dipole transitions that when considered together, give rise to the EIT. EIT widths five orders of magnitude narrower than the natural linewidth have been reported.

Dispersion

Associated with the narrow transparency there is a concomitant steep dispersion. Steep dispersion features are of interest for a range of different reasons. Their shape provides an ideal signal for locking a laser to a specific point in a spectral profile (numerous different types of laser locking methods take advantage of such shaped spectral features) - similarly such a signal can be used to detect very small changes of frequency caused by some other physical mechanism, such as magnetic fields. Measuring the dispersion of a medium directly is something we have investigated over the last couple of years.

Here at Durham we are interested in using this aspect of EIT, with the aim of constructing an interferometer, that would be capable of making very sensitive measurements of both magnetic fields and rotations. For more information on our research, please follow this link - EIT at Durham.

Slow Light

One other fascinating application of steep dispersion signals is to the field of slow light. Changes in the refractive index of a medium lead to modifications of the group velocity of light. The steep dispersion features associated with EIT, are such that they significantly reduce the group velocity of light.

If instead of using a continuous wave (CW) probe field, we instead use a probe pulse, then we would find that the time taken for the pulse to propagate through the medium, would be longer with the application of the pump field, than without it. It is even possible to turn off the pump field whilst the probe pulse is in the medium. This then stores the probe pulse in the medium. Turning the pump field on some time later, it is possible to regenerate the probe pulse.

Content © G. T. Purves, Durham University 2005