In optics, slow light is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. Slow light occurs when a propagating pulse is substantially slowed by the interaction with the medium in which the propagation takes place. Group velocities below the speed of light in vacuum c were known to be possible as far back as 1880, but could not be realized in a useful manner until 1991, when Stephen Harris and collaborators demonstrated electromagnetically induced transparency in trapped strontium atoms. Reduction of the speed of light by a factor of 165 was reported in 1995. In 1998, Danish physicist Lene Vestergaard Hau led a combined team from Harvard University and the Rowland Institute for Science which realized much lower group velocities of light. They succeeded in slowing a beam of light to about 17 meters per second. In 2004, researchers at UC Berkeley first demonstrated slow light in a semiconductor, with a group velocity 9.6 kilometers per second. Hau and her colleagues later succeeded in stopping light completely, and developed methods by which it can be stopped and later restarted. In 2005, IBM created a microchip that can slow light, fashioned out of fairly standard materials, potentially paving the way toward commercial adoption.
Background
When light propagates through a material, it travels slower than the vacuum speed, c. This is a change in the phase velocity of the light and is manifested in physical effects such as refraction. This reduction in speed is quantified by the ratio between c and the phase velocity. This ratio is called the refractive index of the material. Slow light is a dramatic reduction in the group velocity of light, not the phase velocity. Slow light effects are not due to abnormally large refractive indices, as will be explained below. The simplest picture of light given by classical physics is of a wave or disturbance in the electromagnetic field. In a vacuum, Maxwell's equations predict that these disturbances will travel at a specific speed, denoted by the symbol c. This well-known physical constant is commonly referred to as the speed of light. The postulate of the constancy of the speed of light in all inertial reference frames lies at the heart of special relativity and has given rise to a popular notion that the "speed of light is always the same". However, in many situations light is more than a disturbance in the electromagnetic field. Light traveling within a medium is not merely a disturbance solely of the electromagnetic field, but rather a disturbance of the field and the positions and velocities of the charged particles (electrons) within the material. The motion of the electrons is determined by the field (due to the Lorentz force) but the field is determined by the positions and velocities of the electrons (due to Gauss' law and Ampère's law). The behavior of a disturbance of this combined electromagnetic-charge density field (i.e. light) is still determined by Maxwell's equations, but the solutions are complicated because of the intimate link between the medium and the field. Understanding the behavior of light in a material is simplified by limiting the types of disturbances studied to sinusoidal functions of time. For these types of disturbances Maxwell's equations transform into algebraic equations and are easily solved. These special disturbances propagate through a material at a speed slower than c called the phase velocity. The ratio between c and the phase velocity is called the refractive index or index of refraction of the material (n). The index of refraction is not a constant for a given material, but depends on temperature, pressure, and upon the frequency of the (sinusoidal) light wave. The latter leads to dispersion. A human eye perceives the intensity of the sinusoidal disturbance as the brightness of the light and the frequency as the color. If a light is turned on or off at a specific time or otherwise modulated, then the amplitude of the sinusoidal disturbance is also time-dependent. The time-varying amplitude does not propagate at the phase velocity but rather at the group velocity. The group velocity depends not only on the refractive index of the material, but also on the way in which the refractive index changes with frequency (i.e. the derivative of refractive index with respect to frequency). Slow light refers to a very low group velocity of light. If the dispersion relation of the refractive index is such that the index changes rapidly over a small range of frequencies, then the group velocity might be very low, thousands or millions of times less than c, even though the index of refraction is still a typical value (between 1.5 and 3.5 for glasses and semiconductors).
Preparation 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 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. Dispersion mechanisms such as photonic crystals at red and blue edges, coupled resonator optical waveguides (CROW), and other micro-resonator structures modify the spatial component (k-vector) of a propagating wave.
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