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arXiv:quant-ph/0505191 · Physical Review A

Transverse localization and slow propagation of light

Jing Cheng, Shensheng Han, YiJing Yan

Published Aug 15, 2005• 16 citations
arXivHTMLPDF

Abstract

The effect of a finite control beam on the transverse spatial profile of the slow light propagation in an electromagnetically induced transparency medium is studied. We arrive at a general criterion in terms of eigenequation, and demonstrate the existence of a set of localized, stationary transverse modes for the negative detuning of the probe signal field. Each of these diffraction-free transverse modes has its own characteristic group velocity, smaller than the conventional theoretical result without considering the transverse spatial effect.

DOI
10.1103/physreva.72.021801

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The effect of a finite control beam on the transverse spatial profile of the slow light propagation in an electromagnetically induced transparency medium is studied. We arrive at a general criterion in terms of eigenequation, and demonstrate the existence of a set of localized, stationary transverse modes for the negative detuning of the probe signal field. Each of these diffraction-free transverse modes has its own characteristic group velocity, smaller than the conventional theoretical result without considering the transverse spatial effect.
The slow light propagation effect in electromagnetically induced transparency with a spatial partially coherent coupling field has been studied. We find that the group velocity of the probe beam can be significantly reduced when the spatial coherence of the coupling field decreases. It shows that the group delay of the probe beam can achieve about 10 times that of the fully coherent case. This work shows the effect of spatial coherence of the coupling field in a coherent atomic medium and provides a slow-light mechanism in optical coherence systems.
We describe the temporal and spatial dynamics of propagating electromagnetically induced transparency pulses in an optically thick medium. Results include pulse velocities as slow as c/165c/165c/165 with 55% transmission, strong-probe-field effects, and the observation of near diffraction-limited transmitted beam quality.
Due to the finite transverse distribution of the control field and cold atoms, there can exist a set of guiding modes for ultraslow signal field propagating in an atomic medium with electromagnetically induced transparency. For cold four-level atoms, these guiding modes can be manipulated by varying the laser fields and atomic distribution. By carefully designing the distribution of the induced refractive index, we show that it is possible to realize single mode operation for slow light propagation which may have practical applications.
The speed of light is nominally given by c/n, where n is the refractive index of the medium in which the light is traveling. Since the refractive index of free space is 1, it is natural to expect that light travels at c in free space. We show that this is not the case if one considers real beams that have finite transverse extent. The modification of the wavevectors comprising the beam result in a change in the phase and group velocities. We study the group velocity of single photons, whose arrival times are unambiguously established by a click in a single-photon detector.We generate time-correlated photon pairs via spontaneous parametric down-conversion. One photon is sent through a fiber delay of a fixed length; the arrival time of this photon is the reference time to which we compare the arrival time of the other photon, which we can choose to be either unstructured or structured. The structuring is done by spatial light modulators that can be switched on and off, hence allowing photons to travel through exactly the same optical path, much like in a race. We consider photons in a Bessel mode and a focused Gaussian mode, and show that in both cases, the reduction in group velocity results in a delay of several micrometers over a propagation distance of 1 m or ~30 femtoseconds in terms of arrival time. The effect is well-understood from a geometric argument and from a rigorous calculation of the harmonic average of the group velocity, and hence, is not anomalous at all. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.
Magnetic-free nonreciprocal optical devices that prevent backscattering of signals are essential for optical information processing. The achieved nonreciprocal behaviors mostly rely on various optical dispersive effects, leading to dispersive modulations of the spatial beam profiles, such as broadening and discretization, of incident signals. Such deformation inevitably reduces the matching with subsequent functional components. Here, we experimentally demonstrate the nonreciprocal transverse localization of light in a moiré photonic lattice in atomic vapors. When the probe field co- or counter-propagates with the forward coupling field with a honeycomb moiré profile, the output pattern can exhibit localized or dispersive behavior. The nonreciprocal behaviors (in both beam size and transmitted intensity) are from the atomic thermal motion. A backward Gaussian coupling field is also introduced to modulate the size and intensity of the backward probe, which experiences dispersive propagation. The current work provides an approach to control the transverse beam profile in nonreciprocal transmission. The authors demonstrate nonreciprocal transverse localisation of light in a reconfigurable moiré photonic lattice within a rubidium atomic vapor under electromagnetic-induced transparency conditions. This shows selective control of localised or dispersive optical features.
A light beam normally incident upon an uniformly moving dielectric medium is, in general, subject to bendings due to a transverse Fresnel-Fizeau light drag effect. In most familiar dielectrics, the magnitude of this bending effect is very small and hard to detect. Yet, the effect can be dramatically enhanced in strongly dispersive media where slow group velocities in the m/s range have been recently observed taking advantage of the electromagnetically induced transparency effect. In addition to the usual downstream drag that takes place for positive group velocities, we discuss a significant anomalous upstream drag which is expected to occur for negative group velocities. Furthermore, for sufficiently fast speeds of the medium, higher-order dispersion terms are found to play an important role and to be responsible for light propagation along curved paths or the restoration of the time and space coherence of an incident noisy beam. The physics underlying this class of slow-light effects is thoroughly discussed.