- Energy transfer upconversion
Energy Transfer Upconversion or ETU is a physical principle (most commonly encountered in
solid-state laserphysics) that involves the excitation of a laser-active ionto a level above that which would be achieved by simple absorption of a pump photon, the required additional energy being transferred from another laser-active ion undergoing nonradiative deexcitation.
ETU involves two fundamental ideas: "energy transfer" and "upconversion". The analysis below will discuss ETU in the context of an optically pumped [see
optical pumping] solid-state laser.
A solid-state laser [see also
laser] has laser-active ions embedded in a host medium. Energy may be transferred between these by dipole-dipole interaction(over short distances) or by fluorescenceand reabsorption (over longer distances). In the case of ETU it is primarily dipole-dipole energy transfer that is of interest.
If a laser-active ion is in an excited state, it can decay to a lower state either radiatively (i.e. energy is conserved by the emission of a photon, as required for laser operation) or nonradiatively. Nonradiative emission may be via
Auger decayor via energy transfer to another laser-active ion. If this occurs, the ion receiving the energy will be excited to a higher energy state than that already achieved by absorption of a pump photon. This process of further exciting an already excited laser-active ion is known as "upconversion".
ETU is normally an unwanted effect when building lasers. Nonradiative decay is itself an inefficiency (in a perfect laser every downward transition would be a
stimulated emissionevent), whilst the excitation of the energy-receiving ion can result in heating of the gain medium. When ETU occurs due to a clustering of ions within the host medium, it is sometimes termed "concentration quenching".
* [http://www.rp-photonics.com/encyclopedia.html Encyclopedia of Laser Physics and Technology]
* CLARKSON W A  , "J. Phys. D: Appl. Phys." 34, pp. 2381-95
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