Red (660 & 635 nm), green (532 & 520 nm) and blue-violet (445 & 405 nm) lasers
Laser light is a type of stimulated emission of radiation.
A laser beam used for welding
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A helium–neon laser demonstration. The glow running through the center of the tube is an electric discharge. This glowing plasma is the gain medium for the laser. The laser produces a tiny, intense spot on the screen to the right. The center of the spot appears white because the image is overexposed there.
Spectrum of a helium–neon laser. The actual bandwidth is much narrower than shown; the spectrum is limited by the measuring apparatus.
Lidar measurements of lunar topography made by Clementine mission.
Laserlink point to point optical wireless network
Mercury Laser Altimeter (MLA) of the MESSENGER spacecraft
Aleksandr Prokhorov
Charles H. Townes
LASER notebook: First page of the notebook wherein Gordon Gould coined the acronym LASER, and described the elements required to construct one. Manuscript text: "Some rough calculations on the feasibility / of a LASER: Light Amplification by Stimulated / Emission of Radiation. /
Conceive a tube terminated by optically flat / [Sketch of a tube] / partially reflecting parallel mirrors..."
Graph showing the history of maximum laser pulse intensity throughout the past 40 years.
Wavelengths of commercially available lasers. Laser types with distinct laser lines are shown above the wavelength bar, while below are shown lasers that can emit in a wavelength range. The color codifies the type of laser material (see the figure description for more details).
A 50 W FASOR, based on a Nd:YAG laser, used at the Starfire Optical Range
A 5.6 mm 'closed can' commercial laser diode, such as those used in a CD or DVD player
Close-up of a table-top dye laser based on Rhodamine 6G
The free-electron laser FELIX at the FOM Institute for Plasma Physics Rijnhuizen, Nieuwegein
Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments.
The US–Israeli Tactical High Energy weapon has been used to shoot down rockets and artillery shells.
Laser application in astronomical adaptive optics imaging

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation.

- Laser

Such a gain medium, along with an optical resonator, is at the heart of a laser or maser.

- Stimulated emission
Red (660 & 635 nm), green (532 & 520 nm) and blue-violet (445 & 405 nm) lasers

10 related topics

Alpha

Einstein in 1921, by Ferdinand Schmutzer

Albert Einstein

German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time.

German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time.

Einstein in 1921, by Ferdinand Schmutzer
Einstein at the age of three in 1882
Albert Einstein in 1893 (age 14)
Einstein's Matura certificate, 1896
Albert Einstein and Mileva Marić Einstein, 1912
Einstein in 1904 (age 25)
Olympia Academy founders: Conrad Habicht, Maurice Solovine and Albert Einstein
The New York Times reported confirmation of "the Einstein theory" (specifically, the bending of light by gravitation) based on 29 May 1919 eclipse observations in Principe (Africa) and Sobral (Brazil), after the findings were presented on 6 November 1919 to a joint meeting in London of the Royal Society and the Royal Astronomical Society. (Full text)
Einstein with his second wife, Elsa, in 1921
Einstein's official portrait after receiving the 1921 Nobel Prize in Physics
Albert Einstein at a session of the International Committee on Intellectual Cooperation (League of Nations) of which he was a member from 1922 to 1932.
Albert Einstein (left) and Charlie Chaplin at the Hollywood premiere of City Lights, January 1931
Cartoon of Einstein after shedding his "pacifism" wings (Charles R. Macauley, c. 1933)
Albert Einstein's landing card (26 May 1933), when he landed in Dover (United Kingdom) from Ostend (Belgium) to visit Oxford.
Portrait of Einstein taken in 1935 at Princeton
Einstein accepting US citizenship certificate from judge Phillip Forman
Einstein in 1947
Albert Einstein (right) with writer, musician and Nobel laureate Rabindranath Tagore, 1930
Albert Einstein with his wife Elsa Einstein and Zionist leaders, including future President of Israel Chaim Weizmann, his wife Vera Weizmann, Menahem Ussishkin, and Ben-Zion Mossinson on arrival in New York City in 1921
Eddington's photograph of a solar eclipse
Einstein with Millikan and Georges Lemaître at the California Institute of Technology in January 1933.
Einstein at his office, University of Berlin, 1920
The photoelectric effect. Incoming photons on the left strike a metal plate (bottom), and eject electrons, depicted as flying off to the right.
Einstein during his visit to the United States
Newspaper headline on 4 May 1935
Einstein and Niels Bohr, 1925
The 1927 Solvay Conference in Brussels, a gathering of the world's top physicists. Einstein is in the center.
Einstein (second from left) at a picnic in Oslo during the visit to Denmark and Norway in 1920. Heinrich Goldschmidt (left), Ole Colbjørnsen (seated in center) and Jørgen Vogt behind Ilse Einstein.

In 1917, at the height of his work on relativity, Einstein published an article in Physikalische Zeitschrift that proposed the possibility of stimulated emission, the physical process that makes possible the maser and the laser.

Atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy vol. 1 (1808)

Atom

Smallest unit of ordinary matter that forms a chemical element.

Smallest unit of ordinary matter that forms a chemical element.

Atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy vol. 1 (1808)
The Geiger–Marsden experiment:
Left: Expected results: alpha particles passing through the plum pudding model of the atom with negligible deflection.
Right: Observed results: a small portion of the particles were deflected by the concentrated positive charge of the nucleus.
The Bohr model of the atom, with an electron making instantaneous "quantum leaps" from one orbit to another with gain or loss of energy. This model of electrons in orbits is obsolete.
The binding energy needed for a nucleon to escape the nucleus, for various isotopes
A potential well, showing, according to classical mechanics, the minimum energy V(x) needed to reach each position x. Classically, a particle with energy E is constrained to a range of positions between x1 and x2.
3D views of some hydrogen-like atomic orbitals showing probability density and phase (g orbitals and higher are not shown)
This diagram shows the half-life (T½) of various isotopes with Z protons and N neutrons.
These electron's energy levels (not to scale) are sufficient for ground states of atoms up to cadmium (5s2 4d10) inclusively. Do not forget that even the top of the diagram is lower than an unbound electron state.
An example of absorption lines in a spectrum
Graphic illustrating the formation of a Bose–Einstein condensate
Scanning tunneling microscope image showing the individual atoms making up this gold (100) surface. The surface atoms deviate from the bulk crystal structure and arrange in columns several atoms wide with pits between them (See surface reconstruction).
Periodic table showing the origin of each element. Elements from carbon up to sulfur may be made in small stars by the alpha process. Elements beyond iron are made in large stars with slow neutron capture (s-process). Elements heavier than iron may be made in neutron star mergers or supernovae after the r-process.

If a bound electron is in an excited state, an interacting photon with the proper energy can cause stimulated emission of a photon with a matching energy level.

This physical property is used to make lasers, which can emit a coherent beam of light energy in a narrow frequency band.

Photons are emitted by a cyan laser beam outside, orange laser beam inside calcite and its fluorescence

Photon

Elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force.

Elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force.

Photons are emitted by a cyan laser beam outside, orange laser beam inside calcite and its fluorescence
Photoelectric effect: the emission of electrons from a metal plate caused by light quanta – photons.
The cone shows possible values of wave 4-vector of a photon. The "time" axis gives the angular frequency (rad⋅s−1) and the "space" axis represents the angular wavenumber (rad⋅m−1). Green and indigo represent left and right polarization
Thomas Young's double-slit experiment in 1801 showed that light can act as a wave, helping to invalidate early particle theories of light.
In 1900, Maxwell's theoretical model of light as oscillating electric and magnetic fields seemed complete. However, several observations could not be explained by any wave model of electromagnetic radiation, leading to the idea that light-energy was packaged into quanta described by . Later experiments showed that these light-quanta also carry momentum and, thus, can be considered particles: The photon concept was born, leading to a deeper understanding of the electric and magnetic fields themselves.
Up to 1923, most physicists were reluctant to accept that light itself was quantized. Instead, they tried to explain photon behaviour by quantizing only matter, as in the Bohr model of the hydrogen atom (shown here). Even though these semiclassical models were only a first approximation, they were accurate for simple systems and they led to quantum mechanics.
Photons in a Mach–Zehnder interferometer exhibit wave-like interference and particle-like detection at single-photon detectors.
Stimulated emission (in which photons "clone" themselves) was predicted by Einstein in his kinetic analysis, and led to the development of the laser. Einstein's derivation inspired further developments in the quantum treatment of light, which led to the statistical interpretation of quantum mechanics.
Different electromagnetic modes (such as those depicted here) can be treated as independent simple harmonic oscillators. A photon corresponds to a unit of energy E = hν in its electromagnetic mode.

The photon concept has led to momentous advances in experimental and theoretical physics, including lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics.

The laser is an extremely important application and is discussed above under stimulated emission.

A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated.

Light

Electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye.

Electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye.

A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated.
The electromagnetic spectrum, with the visible portion highlighted
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Beam of sun light inside the cavity of Rocca ill'Abissu at Fondachelli-Fantina, Sicily
Due to refraction, the straw dipped in water appears bent and the ruler scale compressed when viewed from a shallow angle.
Hong Kong illuminated by colourful artificial lighting.
Pierre Gassendi.
Christiaan Huygens.
Thomas Young's sketch of a double-slit experiment showing diffraction. Young's experiments supported the theory that light consists of waves.
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Emission can also be stimulated, as in a laser or a microwave maser.

First prototype ammonia maser and inventor Charles H. Townes. The ammonia nozzle is at left in the box, the four brass rods at center are the quadrupole state selector, and the resonant cavity is at right. The 24 GHz microwaves exit through the vertical waveguide Townes is adjusting. At bottom are the vacuum pumps.

Maser

First prototype ammonia maser and inventor Charles H. Townes. The ammonia nozzle is at left in the box, the four brass rods at center are the quadrupole state selector, and the resonant cavity is at right. The 24 GHz microwaves exit through the vertical waveguide Townes is adjusting. At bottom are the vacuum pumps.
A hydrogen radio frequency discharge, the first element inside a hydrogen maser (see description below)
A hydrogen maser.

A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission.

The laser works by the same principle as the maser but produces higher frequency coherent radiation at visible wavelengths.

Wave functions of the electron in a hydrogen atom at different energy levels. Quantum mechanics cannot predict the exact location of a particle in space, only the probability of finding it at different locations. The brighter areas represent a higher probability of finding the electron.

Spontaneous emission

Process in which a quantum mechanical system transits from an excited energy state to a lower energy state (e.g., its ground state) and emits a quantized amount of energy in the form of a photon.

Process in which a quantum mechanical system transits from an excited energy state to a lower energy state (e.g., its ground state) and emits a quantized amount of energy in the form of a photon.

Wave functions of the electron in a hydrogen atom at different energy levels. Quantum mechanics cannot predict the exact location of a particle in space, only the probability of finding it at different locations. The brighter areas represent a higher probability of finding the electron.

Lasers start via spontaneous emission, then during continuous operation work by stimulated emission.

Optical amplifiers are used to create laser guide stars which provide feedback to the adaptive optics control systems which dynamically adjust the shape of the mirrors in the largest astronomical telescopes.

Optical amplifier

Device that amplifies an optical signal directly, without the need to first convert it to an electrical signal.

Device that amplifies an optical signal directly, without the need to first convert it to an electrical signal.

Optical amplifiers are used to create laser guide stars which provide feedback to the adaptive optics control systems which dynamically adjust the shape of the mirrors in the largest astronomical telescopes.
Schematic diagram of a simple Doped Fiber Amplifier

An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed.

In doped fiber amplifiers and bulk lasers, stimulated emission in the amplifier's gain medium causes amplification of incoming light.

A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated.

Amplified spontaneous emission

A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated.

Amplified spontaneous emission (ASE) or superluminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium.

ASE is produced when a laser gain medium is pumped to produce a population inversion.

Emission lines and absorption lines compared to a continuous spectrum

Einstein coefficients

Atom or molecule.

Atom or molecule.

Emission lines and absorption lines compared to a continuous spectrum
Schematic diagram of atomic spontaneous emission
Schematic diagram of atomic stimulated emission
Schematic diagram of atomic absorption

The Einstein A coefficients are related to the rate of spontaneous emission of light, and the Einstein B coefficients are related to the absorption and stimulated emission of light.

In the cases of thermodynamic equilibrium and of local thermodynamic equilibrium, the number densities of the atoms, both excited and unexcited, may be calculated from the Maxwell–Boltzmann distribution, but for other cases, (e.g. lasers) the calculation is more complicated.

Fig.1. Simplified scheme of levels a gain medium

Active laser medium

Fig.1. Simplified scheme of levels a gain medium
Laser rods (from left to right): Ruby, Alexandrite, Er:YAG, Nd:YAP

The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser.

The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source.