Photodarkening
Encyclopedia
Photodarkening is optical effect observed in interaction of laser radiation with
amorphous media (glasses) in optical fibers.
Until now, such creation of color centers was reported only in glass
fibers
. Photodarkening limits the density of excitations in fiber laser
s.
According to the Encyclopedia of Laser Physics and Technology, photodarkening is the effect that the optical losses in a medium can grow when the medium is irradiated with light at certain wavelengths. We may also define photodarkening as reversible creation of absorption centers in optical media at the illumination with
light.
s. At the high concentration of excitations, the absorption
jumps up, causing the avalanche
of the broadband luminescence
.
Increase of absorption can be caused by formation of color centers by electron
s in the conduction band
, created by several neighboring excited ions. (The energy of one or two excitations is not sufficient to pop an electron
into the conduction band
). This explains, why the rate of darkening is strong function of the intensity of the exciting beam (as in the case with optical fibers discussed above). In the experiments, the
thermal effects are important; therefore only the initial stage of the avalanche
can be interpreted as photodarkening, and such interpretation is not yet confirmed. Recent work pointed out the role of thulium
contamination. Through laser
pump and signal absorption
, and energy transfer from ytterbium
; thulium
is able to emit UV light, known to create color centers in silica glass.
amorphous media (glasses) in optical fibers.
Until now, such creation of color centers was reported only in glass
fibers
. Photodarkening limits the density of excitations in fiber laser
Fiber laser
A fiber laser or fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium. They are related to doped fiber amplifiers, which provide light amplification without lasing...
s.
Definition
One could expect the term photodarkening to refer to any process when any object becomes non-transparent (dark) due to illumination with light. Formally, the darkening of the photo-emulsion also could be considered as photodarkening. However, recent papers use this term meaning reversible creation of absorbing color centers in optical fibers. One may expect that the effect is not specific for fibers; therefore, the definition should cover wide class of phenomena, excluding, perhaps, non-reversible darkening of photographic emulsions.According to the Encyclopedia of Laser Physics and Technology, photodarkening is the effect that the optical losses in a medium can grow when the medium is irradiated with light at certain wavelengths. We may also define photodarkening as reversible creation of absorption centers in optical media at the illumination with
light.
Photodarkening rate
The inverse of the timescale at which photodarkening occurs can be interpreted as photodarkening rateColor centers
Usually, photodarkening is attributed to creation of color centers due to resonant interaction of electromagnetic field with an active mediumPossible mechanisms of photodarkening
The phenomenon, similar to photodarkening in fibers, was recently observed in chunks of Yb-doped ceramics and crystalCrystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s. At the high concentration of excitations, the absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
jumps up, causing the avalanche
Avalanche
An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the...
of the broadband luminescence
Luminescence
Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light emitted by a...
.
Increase of absorption can be caused by formation of color centers by electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s in the conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
, created by several neighboring excited ions. (The energy of one or two excitations is not sufficient to pop an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
into the conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
). This explains, why the rate of darkening is strong function of the intensity of the exciting beam (as in the case with optical fibers discussed above). In the experiments, the
thermal effects are important; therefore only the initial stage of the avalanche
Avalanche
An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the...
can be interpreted as photodarkening, and such interpretation is not yet confirmed. Recent work pointed out the role of thulium
Thulium
Thulium is a chemical element that has the symbol Tm and atomic number 69. Thulium is the second least abundant of the lanthanides . It is an easily workable metal with a bright silvery-gray luster...
contamination. Through 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...
pump and signal absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
, and energy transfer from ytterbium
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...
; thulium
Thulium
Thulium is a chemical element that has the symbol Tm and atomic number 69. Thulium is the second least abundant of the lanthanides . It is an easily workable metal with a bright silvery-gray luster...
is able to emit UV light, known to create color centers in silica glass.