Energy transfer upconversion
Encyclopedia
Energy Transfer Upconversion or ETU is a physical principle (most commonly encountered in solid-state laser
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.
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 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
.
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
event), 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.
Solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such 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 solid-state lasers .-Solid-state...
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.
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
Optical pumping
Optical pumping is a process in which light is used to raise 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...
] solid-state laser.
A solid-state laser [see also laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
] 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
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
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
Photon upconversion is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength. It is a anti-Stokes type emission. An example is the conversion of infrared light to visible light. Materials by which...
.
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
In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
event), 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.