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Journal of Physics B Atomic Molecular and Optical Physics

Controlling the speed of light pulses

Peter W. Milonni

Published Mar 13, 2002• 124 citations
Publisher

Abstract

Recent experiments have demonstrated that light pulses can be made to propagate `superluminally' or, at the opposite extreme, to come to a complete stop. Some of the basic physics underlying these studies and their antecedents are reviewed.
DOI
10.1088/0953-4075/35/6/201

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The propagation of light in dispersive media is a subject of fundamental as well as practical importance. In recent years attention has focused in particular on how refractive index can vary with frequency in such a way that the group velocities of optical pulses can be much greater or much smaller than the speed of light in vacuum, or in which the refractive index can be negative. Treating these topics at an introductory to intermediate level, Fast Light, Slow Light and Left-Handed Light focuses on the basic theory and describes the significant experimental progress made during the past decade. The book pays considerable attention to the fact that superluminal group velocities are not in conflict with special relativity and to the role of quantum effects in preventing superluminal communication and violations of Einstein causality. It also explores some of the basic physics at the opposite extreme of very slow group velocities as well as stopped and regenerated light, including the concepts of electromagnetically induced transparency and dark-state polaritons. Another very active aspect of the subject discussed concerns the possibility of designing metamaterials in which the refractive index can be negative and propagating light is left-handed in the sense that the phase and group velocities are in opposite directions. The last two chapters are an introduction to some of the basic theory and consequences of negative refractive index, with emphasis on the seminal work carried out since 2000. The possibility that "perfect" lenses can be made from negative-index metamaterials-which has been perhaps the most controversial aspect of the field-is introduced and discussed in some detail.
I n 1998, laser pulses were slowed [1] in a Bose-Einstein condensate (BEe) [2] ofsodium to only 17 m/s, more than seven orders of magnitude lower than the speed of light in vacuum. Associated with the dramatic reduction factor for the light speed was a spatial compression of the pulses by the same large factor. A light pulse, which was more than 1 km long in vacuum, was compressed to a size of -50 flm, and at that point was completely contained within the condensate [1]. This allowed the light-slowing experiments to be brought to their ultimate extreme [3]: in the summer of 2000, light pulses were completely stopped, stored, and subsequently revived in an atomic medium, with millisecond storage times [4]. The initial ultra-slow light experiments spurred a flurry of slow light investigations, and slow or partially stopped light has now been observed in limited geometries in warm rubidium vapours [5-7], liquid-nitrogen cooled crystals [8], and recently in room temperature crystals [9]. Here, we begin with a discussion ofultra-slow and stopped light. We describe how cold atoms and Bose-Einstein condensates have been manipulated to generate media with extreme optical properties. While the initial experiments concentrated on the light propagation, we have recently begun a number of investigations of the effects that slow light has on the medium in which it propagates. Effects are profound because both the velocity and length scales associated with propagating light pulses have been brought down to match the characteristic velocity and length scales of the medium. With the most recent extension, the light roadblock [10], we have compressed light pulses to a length ofonly 2 f!ill. Here, we describe the use of ultra-compressed light pulses to probe superfluidity and the creation of quantized vortices in BEes through formation of'superfluid shock. waves'. We also present the observation of an ultra-slow-light-based, pulsed atom laser. Furthermore, we demonstrate the use of slow and stopped light for manipulation of optical information, in particular in Bose-Einstein condensates that allow for phase coherent processing of three-dimensional, compressed patterns of stored optical information.