Optical pumping is a process in which
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
is used to raise (or "pump")
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s from a lower
energy levelA quantum mechanical system or particle that is bound, confined spatially, can only take on certain discrete values of energy, as opposed to classical particles, which can have any energy. These values are called energy levels. The term is most commonly used for the energy levels of electrons in...
in an
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
or
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
to a higher one. It is commonly used in
laser constructionA laser is constructed from three principal parts:*An energy source ,*A gain medium or laser medium, and*Two or more mirrors that form an optical resonator.-Pump source:...
, to
pumpLaser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
the
active laser mediumThe active laser medium or gain medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...
so as to achieve
population inversionIn physics, specifically statistical mechanics, a population inversion occurs when a system exists in state with more members in an excited state than in lower energy states...
. The technique was developed by 1966
Nobel PrizeThe Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in chemistry, Nobel Prize in literature, Nobel Peace Prize, and...
winner
Alfred KastlerAlfred Kastler was a German-born French physicist, and Nobel Prize laureate.Kastler was born in Guebwiller and later attended the Lycée Bartholdi in Colmar, Alsace, and École Normale Supérieure in Paris in 1921...
in the early 1950s.
Optical pumping is also used to cyclically pump electrons bound within an atom or molecule to a well-defined
quantum stateIn quantum physics, a quantum state is a mathematical object that fully describes a quantum system. One typically imagines some experimental apparatus and procedure which "prepares" this quantum state; the mathematical object then reflects the setup of the apparatus. Quantum states can be...
.
Optical pumping is a process in which
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
is used to raise (or "pump")
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s from a lower
energy levelA quantum mechanical system or particle that is bound, confined spatially, can only take on certain discrete values of energy, as opposed to classical particles, which can have any energy. These values are called energy levels. The term is most commonly used for the energy levels of electrons in...
in an
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
or
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
to a higher one. It is commonly used in
laser constructionA laser is constructed from three principal parts:*An energy source ,*A gain medium or laser medium, and*Two or more mirrors that form an optical resonator.-Pump source:...
, to
pumpLaser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
the
active laser mediumThe active laser medium or gain medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...
so as to achieve
population inversionIn physics, specifically statistical mechanics, a population inversion occurs when a system exists in state with more members in an excited state than in lower energy states...
. The technique was developed by 1966
Nobel PrizeThe Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in chemistry, Nobel Prize in literature, Nobel Peace Prize, and...
winner
Alfred KastlerAlfred Kastler was a German-born French physicist, and Nobel Prize laureate.Kastler was born in Guebwiller and later attended the Lycée Bartholdi in Colmar, Alsace, and École Normale Supérieure in Paris in 1921...
in the early 1950s.
Optical pumping is also used to cyclically pump electrons bound within an atom or molecule to a well-defined
quantum stateIn quantum physics, a quantum state is a mathematical object that fully describes a quantum system. One typically imagines some experimental apparatus and procedure which "prepares" this quantum state; the mathematical object then reflects the setup of the apparatus. Quantum states can be...
. For the simplest case of
coherentIn physics, coherence is a property of waves, that enables stationary interference. More generally, coherence describes all properties of the electronic correlation between physical quantities of a wave....
two-level optical pumping of an atomic species containing a single
outer-shellAn electron shell may be thought of as an orbit followed by electrons around an atom nucleus. Because each shell can contain only a fixed number of electrons, each shell is associated with a particular range of electron energy, and thus...
electron, this means that the electron is coherently pumped to a single
hyperfine sublevelThe term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with the...
(labeled ), which is defined by the
polarizationPolarization is a property of waves that describes the orientation of their oscillations. This article primarily covers the polarization of electromagnetic waves such as light, although other types of wave also exhibit polarization....
of the pump
laserA laser is a device that emits light through a process called stimulated emission. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses...
along with the quantum selection rules. Upon optical pumping, the atom is said to be
oriented in a particular sublevel, however due to the cyclic nature of optical pumping the bound electron will actually be undergoing repeated
excitation and decayExcitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
between upper and lower state sublevels. The
frequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
and polarization of the pump laser determines which sublevel the atom is oriented in.
In practice, completely coherent optical pumping may not occur due to power-broadening of the linewidth of a transition and undesirable effects such as hyperfine structure trapping and
radiation trappingRadiation trapping, imprisonment of resonance radiation, radiative transfer of spectral lines, line transfer or radiation diffusion is a phenomenon in physics whereby radiation may be "trapped" in a system as it is emitted by one atom and absorbed by another....
. Therefore the orientation of the atom depends more generally on the frequency, intensity, polarization, spectral bandwidth of the laser as well as the linewidth and transition probability of the absorbing transition.
An optical pumping experiment is commonly found in physics undergraduate laboratories, using
rubidiumRubidium is a chemical element with the symbol Rb and atomic number 37. Rb is a soft, silvery-white metallic element of the alkali metal group....
gas isotopes and displaying the ability of radiofrequency (MHz)
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
to effectively pump and unpump these
isotopeIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
s.