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 down by the interaction with the medium in which the propagation takes place.
In 1998, Danish physicist Lene Vestergaard Hau led a combined team from Harvard University and the Rowland Institute for Science which succeeded in slowing a beam of light to about 17 meters per second, and researchers at UC Berkeley slowed the speed of light traveling through a semiconductor to 9.7 kilometers per second in 2004. Hau later succeeded in stopping light completely, and developed methods by which it can be stopped and later restarted. This was in an effort to develop computers that will use only a fraction of the energy of today's machines.
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 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.
In addition to propagating through a vacuum, light may also propagate through many types of matter, denoted as the medium. Light traveling within a medium is no longer 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 Ampere'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. 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. This leads to an effect called dispersion.
A human perceives the amplitude 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 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 large reduction in the 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).
Different ways to achieve slow light
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. Slowlight can also be achieved exploiting the dispersion properties of planar waveguides realized with single negative metamaterials (SNM)  or double negative metamaterials (DNM).
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. Plasmon induced transparency – an analog of EIT - provides another approach based on the destructive interference between different resonance modes. Recent work has now demonstrated this effect over a broad transparency window across a frequency range greater than 0.40 THz
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. Slowing light could lead to a more orderly traffic flow in networks. Meanwhile, slow light can be used to build interferometers that are far more sensitive to frequency shift as compared to conventional interferometers. This property can be used to build better, smaller frequency sensor and compact high resolution spectrometers.
Slow light in fiction
These window panes are of a composition through which light is slowed down in the same way as when it passes through water. You know well, Péronne, how one can hear more quickly a sound through, for example, a metal conduit or some other solid than through simple space. Well, Péronne, all this is of the same family of phenomena!
- Here is the solution. These panes of glass slow down the light at an incredible rate since there need be only a relatively thin sheet to slow it down a hundred years. It takes one hundred years for a ray of light to pass through this slice of matter! It would take one year for it to pass through one hundredth of this depth.
Subsequent fictional works that address slow light are noted below.
- The slow light experiments are mentioned in Dave Eggers' novel You Shall Know Our Velocity (2002), in which the speed of light is described as a "Sunday crawl".
- On Discworld, where Terry Pratchett's novel series takes place, light travels only a few hundred miles per hour due to Discworld's "embarrassingly strong" magic field. [(Pratchett' first Discworld novel is The Colour of Magic (1983).]
- 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. "Slow glass" is a material 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."
- Shaw later reworked the stories into the novel Other Days, Other Eyes (1972).
- Cromie, William J. (1999-02-18). "Physicists Slow Speed of Light". The Harvard University Gazette. Retrieved 2008-01-26.
- "Light Changed to Matter, Then Stopped and Moved". Photonics.com. Retrieved 10 June 2013.
- Kanellos, Michael (2004-09-28). "Slowing the speed of light to improve networking". ZDNet News. Archived from the original on 2008-02-28. Retrieved 2008-01-26.
- Kanellos, Michael (2005-11-02). "IBM slows light, readies it for networking". ZDNet News. Archived from the original on 2007-12-19. Retrieved 2008-01-26.
- Lee, Myungjun; et al (2010). "Systematic design study of an all-optical delay line based on Brillouin scattering enhanced cascade coupled ring resonators". Journal of Optics A 12 (10).
- Wentao T. Lu, Savatore Savo; B. Didier F. Casse; Srinivas Sridhar (2009). "Slow microwave waveguide made of negative permeability metamaterials". Microwave and Optical Technology Letters 51 (11): 2705–2709. doi:10.1002/mop.24727.
- Savatore Savo, Wentao T. Lu; B. Didier F. Casse; Srinivas Sridhar (2011). "Observation of slow-light in a metamaterials waveguide at microwave frequencies". Applied Physics Letters 98 (17): 1719079. Bibcode:2011ApPhL..98q1907S. doi:10.1063/1.3583521.
- K.L. Tsakmakidis, O. Hess; A.D. Boardman (2007). "Trapped rainbow storage of light in metamaterials". Nature 450 (7168): 397–401. Bibcode:2007Natur.450..397T. doi:10.1038/nature06285. PMID 18004380.
- Zhu, Zhihua; et al (2013). "Broadband plasmon induced transparency in terahertz metamaterials". Nanotechnology 24 (21). Bibcode:2013Nanot..24u4003Z. doi:10.1088/0957-4484/24/21/214003.
- Pollitt, Michael (2008-02-07). "Light touch could boost fibre optic networks". The Guardian. Retrieved 2008-04-04.
- Renard, Maurice (1933). The Master of Light.
- Evans, Arthur B. "The Fantastic Science Fiction of Maurice Renard". Science Fiction Studies, #64, Vol.21, Part 3, November 1994. Retrieved 23 February 2011.
- Shaw, Bob. Othe rDays, Other Eyesdate=1972. ISBN 9780330238939.
- 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.
- Slow and Fast Light Topical Meeting
- J. Scheuer, G. T. Paloczi, J. K. S. Poon and A. Yariv, “Coupled Resonator Optical Waveguides: Towards Slowing and Storing of Light”, Opt. Photon. News, Vol. 16 (2005) 36.