Energy transfer upconversion


Energy Transfer Upconversion or ETU is a physical principle 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.
ETU involves two fundamental ideas: energy transfer and upconversion. The analysis below will discuss ETU in the context of an optically pumped solid-state laser.
A solid-state laser has laser-active ions embedded in a host medium. Energy may be transferred between these by dipole-dipole interaction or by fluorescence and reabsorption. 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 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.
ETU is normally an unwanted effect when building lasers. Nonradiative decay is itself an inefficiency, 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.