Energy Transfer Upconversion or ETU is a physical principle (most commonly encountered in

solid-state laser A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from ...

physics) that involves the excitation of a laser-active ion to 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. Clarkson, W.A.

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''Journal of Physics D: Applied Physics'' 34(16): p. 2381-95 ''heat effects and their mitigation in end-pumped solid-state lasers.'' ETU involves two fundamental ideas: ''energy transfer'' and ''upconversion''. The analysis below will discuss ETU in the context of an optically pumped ee_optical_pumping.html" ;"title="optical_pumping.html" ;"title="ee optical pumping">ee optical pumping">optical_pumping.html" ;"title="ee optical pumping">ee optical pumpingsolid-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 fluorescence and 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 decay The Auger effect or Auger−Meitner effect is a physical phenomenon in which the filling of an inner-shell vacancy of an atom is accompanied by the emission of an electron from the same atom. When a core electron is removed, leaving a vacancy, a ...

or 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

photon upconversion A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always m ...

. 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 emission Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to th ...

event), whilst the excitation of the energy-receiving ion can result in heating of the

gain medium The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a ...

. When ETU occurs due to a clustering of ions within the host medium, it is sometimes termed ''concentration quenching''.

References

{{Solid-state laser Solid-state lasers