Wave propagation and group velocity

The ubiquitous role of optical fibres in modern photonic systems has stimulated research to realize slow and fast light devices directly in this close-to-perfect transmission line. Recent progress in developing optically controlled delays in optical fibres, operating under normal environmental conditions and at telecommunication wavelengths, has paved the way towards real applications for slow and fast light. This review presents the state-of-the-art research in this fascinating field and possible outcomes in the near future.

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Slow and fast light in optical fibres

It is now possible to exercise a high degree of control over the velocity at which light pulses pass through material media. This velocity, known as the group velocity, can be made to be very different from the speed of light in a vacuum c. Specifically, the group velocity of light can be made much smaller than c, greater than c, or even negative. We present a survey of methods for establishing extreme values of the group velocity, concentrating especially on methods that work in room-temperature solids. We also describe some applications of slow light.

https://science.sciencemag.org/content/sci/326/5956/1074.full.pdf

Controlling the Velocity of Light Pulses

Controlling the group velocity of an optical pulse typically requires traversing a material or structure whose dispersion is judiciously crafted. Alternatively, the group velocity can be modified in free space by spatially structuring the beam profile, but the realizable deviation from the speed of light in vacuum is small. Here we demonstrate precise and versatile control over the group velocity of a propagation-invariant optical wave packet in free space through sculpting its spatio-temporal spectrum. By jointly modulating the spatial and temporal degrees of freedom, arbitrary group velocities are unambiguously observed in free space above or below the speed of light in vacuum, whether in the forward direction propagating away from the source or even traveling backwards towards it.

/pdf/optical-space-time-wave-packets-having-arbitrary-group-1f5hrambln.pdf

Optical space-time wave packets having arbitrary group velocities in free space.

We review progress in the development of methods for controlling the group velocity of light. These methods allow one to create situations in which the group velocity of light is much smaller than the velocity of light in vacuum c, in which the group velocity is greater than c, or in which the group velocity is negative. We present a survey of methods for establishing extreme values of the group velocity, concentrating especially on methods that can work in room temperature solids. We also describe some of the applications of slow and fast light that are currently under development.

Slow and fast light: fundamentals and applications

The nature of pulse propagation through a material with a negative value of the group velocity has been mysterious, as simple models seem to predict that pulses will propagate "backward" through such a material. Using an erbium-doped optical fiber and measuring the time evolution of the pulse intensity at many points within the fiber, we demonstrate that the peak of the pulse does propagate backward inside the fiber, even though the energy flow is always in the forward direction.

https://science.sciencemag.org/content/sci/312/5775/895.full.pdf

Observation of Backward Pulse Propagation Through a Medium with a Negative Group Velocity

We exploit the versatility of erbium doped fiber to study pulse propagation in situations of positive and negative group velocities. We also study effects resulting from the use of shaped pulses containing broad spectral components.

Christopher Barsi

Papers

Observation of Backward Pulse Propagation Through a Medium with a Negative Group Velocity

Observation of exotic pulse propagation in erbium-doped optical fibers