Slow light
Encyclopedia
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 take place.
Researchers at the Rowland Institute for Science
slowed the group velocity of light to 38 miles per hour in 1999, and researchers at UC Berkeley
slowed the speed of light traveling through a semiconductor
to 9.7 km/s (34900 kilometers per hour or 21700 miles per hour) in 2004. This was in an effort to develop computers that will use only a fraction of the energy of today's machines.
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.
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 (electron
s) 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 due to 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 the phase velocity and c 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).
, 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.
, 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.
'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).
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 Pratchett
's novel series takes place, light travels only a few hundred miles per hour due to Discworld's high magic field.
In Maurice Renard
's novel Le maître de la lumière (The Master of Light, 1933), the description of “luminite” might be one of the earliest mentions of slow glass.
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
. Slow light occurs when a propagating pulse is substantially slowed down by the interaction with the medium in which the propagation take place.
Researchers at the Rowland Institute for Science
Rowland Institute for Science
The Rowland Institute for Science was founded by Edwin H. Land, founder of Polaroid Corporation, as a nonprofit basic research organization in 1980. The Rowland, as it is commonly referred to, is dedicated to experimental science across a wide range of disciplines...
slowed the group velocity of light to 38 miles per hour in 1999, and researchers at UC Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
slowed the speed of light traveling through a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
to 9.7 km/s (34900 kilometers per hour or 21700 miles per hour) in 2004. This was in an effort to develop computers that will use only a fraction of the energy of today's machines.
In 2005, IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
created a microchip
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
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.
Background
When light propagates through a material, it travels slower than the vacuum speed. This is a change in the phase velocityPhase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
of the light and is manifested in physical effects such as refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
. This reduction in speed is quantified by the ratio between c and the phase velocity. This ratio is called the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of the material. Slow light is a dramatic reduction in the group velocity
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
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
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
given by classical physics is of a wave or disturbance in the electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
. In a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, Maxwell's equations
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. The postulate of the constancy of the speed of light in all inertial reference frames lies at the heart of special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
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
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
, 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 (electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s) within the material. The motion of the electrons is determined by the field (due to the Lorentz force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
) but the field is determined by the positions and velocities of the electrons (due to Gauss' law and Ampere's law
Ampère's force law
In magnetostatics, the force of attraction or repulsion between two current-carrying wires is often called Ampère's force 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 due to 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
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
. The ratio between the phase velocity and c is called the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
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
Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
.
A human perceives the amplitude of the sinusoidal disturbance as the brightness
Brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...
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
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
. 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
Derivative
In calculus, a branch of mathematics, the derivative is a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much one quantity is changing in response to changes in some other quantity; for example, the derivative of the position of a...
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 dispersionDispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
, i.e. peaks in the dispersion relation
Dispersion relation
In physics and electrical engineering, dispersion most often refers to frequency-dependent effects in wave propagation. Note, however, that there are several other uses of the word "dispersion" in the physical sciences....
. Schemes are generally grouped into two categories: material dispersion and waveguide dispersion. Material dispersion mechanisms such as Electromagnetically Induced Transparency
Electromagnetically induced transparency
Electromagnetically induced transparency is a coherent optical nonlinearity which renders a medium transparent over a narrow spectral range within an absorption line...
(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.
Potential use
Slow light could be used to greatly reduce noiseNoise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
, which could allow all types of information to be transmitted
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
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
Slow glass is a fictional material in Bob ShawBob Shaw
Bob Shaw, born Robert Shaw, was a science fiction author and fan from Northern Ireland. He was noted for his originality and wit. He won the Hugo Award for Best Fan Writer in 1979 and 1980...
's short story "Light of other days" (Analog
Analog Science Fiction and Fact
Analog Science Fiction and Fact is an American science fiction magazine. As of 2011, it is the longest running continuously published magazine of that genre...
, 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).
The slow light experiments are mentioned in Dave Eggers
Dave Eggers
Dave Eggers is an American writer, editor, and publisher. He is known for the best-selling memoir A Heartbreaking Work of Staggering Genius and for his more recent work as a screenwriter. He is also the co-founder of the literacy project 826 Valencia.-Life:Eggers was born in Boston, Massachusetts,...
' novel You Shall Know Our Velocity
You Shall Know Our Velocity
You Shall Know Our Velocity! is a 2002 novel by Dave Eggers. It was Eggers's debut novel, following the success of his memoir A Heartbreaking Work of Staggering Genius ....
. In the novel, the speed of light is described as a "Sunday crawl".
On Discworld
Discworld
Discworld is a comic fantasy book series by English author Sir Terry Pratchett, set on the Discworld, a flat world balanced on the backs of four elephants which, in turn, stand on the back of a giant turtle, Great A'Tuin. The books frequently parody, or at least take inspiration from, J. R. R....
, where Terry Pratchett
Terry Pratchett
Sir Terence David John "Terry" Pratchett, OBE is an English novelist, known for his frequently comical work in the fantasy genre. He is best known for his popular and long-running Discworld series of comic fantasy novels...
's novel series takes place, light travels only a few hundred miles per hour due to Discworld's high magic field.
In Maurice Renard
Maurice Renard
-Overview:Maurice Renard was born in Châlons-en-Champagne.He was the author of the archetypal mad scientist novel Le Docteur Lerne - Sous-Dieu [Dr. Lerne - Undergod] , which he dedicated to H. G. Wells...
's novel Le maître de la lumière (The Master of Light, 1933), the description of “luminite” might be one of the earliest mentions of slow glass.
- 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.