Optical pumping is a process in which
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
is used to raise (or "pump")
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
s from a lower
energy level in an
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, a ...
or
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
to a higher one. It is commonly used in
laser construction to
pump the
active laser medium so as to achieve
population inversion
In science, specifically statistical mechanics, a population inversion occurs while a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energy ...
. The technique was developed by the 1966
Nobel Prize
The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...
winner
Alfred Kastler in the early 1950s.
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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 provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution i ...
. For the simplest case of
coherent
Coherence, coherency, or coherent may refer to the following:
Physics
* Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference
* Coherence (units of measurement), a deriv ...
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" is an acronym for "light amplification by stimulated emission of radiation". The ...
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. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
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 dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...
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 gas isotopes and displaying the ability of
radiofrequency (MHz)
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
to effectively pump and unpump these
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
s.
See also
*
Atomic coherence
*
Laser pumping
*
Optical cavity
*
Rabi cycle
References
{{Authority control
Quantum optics