Optical pumping is a process in which
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
is used to raise (or "pump")
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s from a lower
energy level
A quantum mechanics, quantum mechanical system or particle that is bound state, bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical mechanics, classical pa ...
in an
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
or
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
to a higher one. It is commonly used in
laser construction to
pump
A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy.
Mechanical pumps serve in a wide range of application ...
the
active laser medium so as to achieve
population inversion. The technique was developed by the 1966
Nobel Prize
The Nobel Prizes ( ; ; ) are awards administered by the Nobel Foundation and granted in accordance with the principle of "for the greatest benefit to humankind". The prizes were first awarded in 1901, marking the fifth anniversary of Alfred N ...
winner
Alfred Kastler in the early 1950s.
[ Page 56.]
Optical pumping is also used to cyclically pump electrons bound within an atom or molecule to a well-defined
quantum state
In quantum physics, a quantum state is a mathematical entity that embodies the knowledge of a quantum system. Quantum mechanics specifies the construction, evolution, and measurement of a quantum state. The result is a prediction for the system ...
. For the simplest case of
coherent two-level optical pumping of an atomic species containing a single
outer-shell electron, this means that the electron is coherently pumped to a single
hyperfine sublevel (labeled
), which is defined by the
polarization of the pump
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
along with the quantum
selection rules. Upon optical pumping, the atom is said to be ''oriented'' in a specific
sublevel, however, due to the cyclic nature of optical pumping, the bound electron will actually be undergoing repeated
excitation and decay between the upper and lower state sublevels. The
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
and polarization of the pump laser determine the
sublevel in which the atom is oriented.
In practice, completely coherent optical pumping may not occur due to power-broadening of the
linewidth
A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...
of a transition and undesirable effects such as hyperfine structure trapping and
radiation trapping. Therefore the orientation of the atom depends more generally on the frequency, intensity, polarization, and 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
rubidium
Rubidium is a chemical element; it has Symbol (chemistry), symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have ...
gas isotopes and displaying the ability of
radiofrequency (MHz)
electromagnetic radiation
In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
to effectively pump and unpump these
isotope
Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
s.
See also
*
Atomic coherence
*
Laser pumping
*
Optical cavity
An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors or other optical elements that confines light waves similarly to how a cavity resonator confines microwaves. Optical cavities are a major component of lasers, ...
*
Rabi cycle
References
{{Authority control
Quantum optics