Slow light

  Slow light is the literal slowing of the speed of light. It is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. The term is usually only applied when the velocity is at least hundreds of times slower than the speed of light in a vacuum.

Researchers at UC Berkeley slowed the speed of light traveling through a semiconductor to 6 miles per second in 2004. This was in an effort to develop computers that will use only a fraction of the energy of today's machines.^[1]

In 2005, IBM created a microchip that can slow down light, claiming that its light-slowing device is the first to be fashioned out of fairly standard materials, potentially paving the way toward commercial adoption.^[2]

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Background

While the speed of light in a vacuum is a well-known physical constant, when light is subject to physical conditions other than a vacuum, the effective velocity of the electromagnetic waves that make up the light can change. The most well-known instance of this is when light is transmitted through a refractive material that has an electric permittivity and a magnetic permeability different from a vacuum. When this occurs the index of refraction can be determined for a material as follows:

n = c / v

where n is the index of refraction, c is the speed of light in a vacuum, and v is the apparent speed of light through the material. As the index of refraction increases, the observed speed of light in the medium decreases. Theoretically, there is no limit to the index of refraction, and it is possible to obtain values for n that approach infinity. When this happens, the speed of light in the material becomes effectively equal to zero, and light has "stopped moving".

Microphysically, in the quantum mechanical view, photons of light are interacting through a lattice arrangement of atoms in a material either through repeating processes of absorption and re-emission, or by many different scattering processes. In the case of absorption and re-emission, there is a finite amount of time for a material to absorb and re-emit a photon and this lag time will cause an effective "slowing" of the observed photon speed. Between absorptions and re-emissions, however, the photon is traveling at c. For scattering processes, the photon takes a path that is a longer distance than the metric displacement along its trajectory. As the photon is scattered, it follows a circuitous path which is constantly changing direction but on average is propagating in a straight-line direction. This also has the effect of appearing to slow down the speed of light since the photons are traveling the speed of light along a longer distance.

Procedure

There are many mechanisms which can generate slow light, all of which create narrow spectral regions with high dispersion, i.e. peaks in the dispersion relation. Schemes are generally grouped into two categories: material dispersion and waveguide dispersion. Material dispersion mechanisms such as Electromagnetically Induced Transparency (EIT), Coherent Population Oscillation (CPO), and various Four Wave Mixing (FWM) schemes produce a rapid change in refractive index as a function of optical frequency, i.e. they modify the temporal component of a propagating wave. This is done by using a nonlinear effect to modify the dipole response of a medium to a signal or "probe" field. Waveguide dispersion mechanisms such as photonic crystals, Coupled Resonator Optical Waveguides (CROW), and other micro-resonator structures modify the spatial component (k-vector) of a propagating wave.

A predominant figure of merit of slow light schemes is the Delay-Bandwidth Product (DBP). Most slow light schemes can actually offer an arbitrarily long delay for a given device length (length/delay = signal velocity) at the expense of bandwidth. The product of the two is roughly constant. A related figure of merit is the fractional delay, the time a pulse is delayed divided by the total time of the pulse.

Potential use

Slow light could be used to greatly reduce noise, which could allow all types of information to be transmitted more efficiently. Also, optical switches controlled by slow light could cut power requirements a million-fold compared to switches now operating everything from telephone equipment to supercomputers. ^[1] Slowing light could lead to a more orderly traffic flow in networks.

Slow light in fiction

Slow glass is a fictional material in Bob Shaw's short story "Light of other days" (Analog, 1966), and several subsequent stories. The glass, which delays the passage of light by years or decades, is used to construct windows, called scenedows, that enable city dwellers, submariners and prisoners to watch "live" countryside scenes. In the original story, Shaw implied that "slow glass" simply was a material with a enormously high index of refraction. In a later story, though, he replaced this with a convoluted explanation where the delay light takes in passing through the glass is attributed to photons passing "...through a spiral tunnel coiled outside the radius of capture of each atom in the glass.". The simple "high refractive index" explanation wouldn't work for a simple reason; with a refractive index somewhere in the quadrillions, Fresnel reflection would result in a surface that would be almost perfectly reflective.

Bob Shaw later reworked the stories into the novel Other Days, Other Eyes (1972).

The slow light experiments are mentioned in Dave Eggers' novel You Shall Know Our Velocity!. In the novel, the speed of light is described as a "sunday crawl".

On Discworld, where Terry Prattchet's novel series takes place, light travels only a few hundred miles per hour due to Discworld's high magic field.

References

• Lene Vestergaard Hau, S.E. Harris, Zachary Dutton, Cyrus H. Behroozi, Nature v.397, p.594 (1999).
• "IBM's new photonic wave-guide". Nature, November 2004.
• ^ Harvard.edu
• Slow and Fast Light Topical Meeting