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 te ...
is used to raise (or "pump")
electron
The electron (, or in nuclear reactions) 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 partic ...
s from a lower
energy level
A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The ...
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 ...
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 bio ...
to a higher one. It is commonly used in
laser construction
A laser is constructed from three principal parts:
*An energy source (usually referred to as the ''pump'' or ''pump source''),
*A ''gain medium'' or ''laser medium'', and
*Two or more mirrors that form an ''optical resonator''.
Pump source
The ...
to
pump
A pump is a device that moves fluids ( liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method the ...
the
active laser medium so as to achieve
population inversion. 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." Alfre ...
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 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 in ...
. 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
Polarization or polarisation may refer to:
Mathematics
*Polarization of an Abelian variety, in the mathematics of complex manifolds
*Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
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 firs ...
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 ...
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
Rubidium is the chemical element with the 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 a density hig ...
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, (visible ...
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 number ...
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 forms a cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and pro ...
*
Rabi cycle
References
{{Authority control
Quantum optics