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 active laser medium, 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 ...
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. 001 ''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
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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 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
Photon upconversion (UC) is a process in which the sequential Absorption (electromagnetic radiation), absorption of two or more photons leads to the Emission spectrum, emission of light at shorter wavelength than the excitation wavelength. It is ...
.
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 ...
event), whilst the excitation of the energy-receiving ion can result in heating of the
gain medium
The active laser medium (also called a 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 ...
. 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