Optical pumping

(′äp·tə·kəl ′pəmp·iŋ)

(optics) The process of causing strong deviations from thermal equilibrium populations of selected quantized states of different energy in atomic or molecular systems by the use of electromagnetic radiation in or near the visible region.

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Sci-Tech Encyclopedia: Optical pumping

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The process of causing strong deviations from thermal equilibrium populations of selected quantized states of different energy in atomic or molecular systems by the use of optical radiation (that is, light of wavelengths in or near the visible spectrum), called the pumping radiation.

Optical pumping is vital for light amplification by stimulated emission in an important class of lasers. For example, the action of the ruby laser involves the fluorescent emission of red light by a transition from an excited level E₂ to the ground level E₁. In this case E₂ is relatively high above E_l and the equilibrium population of E₂ is practically zero. Amplification of the red light by laser action requires that number of atoms N₂ exceed N₁ (population inversion). The inversion is accomplished by intense green and violet light from an external source which excites the chromium ion in the ruby to a band of levels, E₃ above E₂. From E₃ the ion rapidly drops without radiation to E₂, in which its lifetime is relatively long for an excited state. Sufficiently intense pumping forces more luminescent ions into E₂ by way of the E₃ levels than remain in the ground state E₁, and amplification of the red emission of the ruby by stimulated emission can then occur. See also Laser.

Wikipedia: Optical pumping

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Optical pumping of a laser rod (bottom) with an arc lamp (top). Red: hot. Blue: cold. Green: light. Non-green arrows: water flow. Solid colors: metal. Light colors: fused quartz. Refs: [1], [2],[3]

Optical pumping is a process in which light is used to raise (or "pump") electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction, to pump the active laser medium so as to achieve population inversion. The technique was developed by 1966 Nobel Prize winner Alfred Kastler in the early 1950s.^[1]

Optical pumping is also used to cyclically pump electrons bound within an atom or molecule to a well-defined quantum state. 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 $m_F\!$ ), which is defined by the polarization of the pump laser along with the quantum selection rules. Upon optical pumping, the atom is said to be oriented in a particular $m_F\!$ sublevel, however due to the cyclic nature of optical pumping the bound electron will actually be undergoing repeated excitation and decay between upper and lower state sublevels. The frequency and polarization of the pump laser determines which $m_F\!$ 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 trapping. 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.^[2]

An optical pumping experiment is commonly found in physics undergraduate laboratories, using rubidium gas isotopes and displaying the ability of radiofrequency (MHz) electromagnetic radiation to effectively pump and unpump these isotopes.

References

^ Taylor, Nick (2000). LASER: The inventor, the Nobel laureate, and the thirty-year patent war. New York: Simon & Schuster. ISBN 0-684-83515-0. Page 56.
^ Demtroder, W. (1998). Laser Spectroscopy: Basic Concepts and Instrumentation. Berlin: Springer.

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