Random laser
Encyclopedia
A random laser is a laser that uses a highly disordered gain medium. A random laser uses no optical cavity but the remaining principles of operation remain the same as for a conventional laser
. Random laser action has been observed in many different media, including semiconductor
powder, nanostructured and non-nanostructured thin films, laser dyes
, ceramics and many more.
Developments in nanoparticles have demonstrated that large amounts of optical scattering
can occur when photons are incident. In this way, light can be diffused around a medium in much the same way as it is on white paint
and in clouds.
(UV
emission - bandgap 3.3 eV), light from a pump source ( e.g. frequency-tripled Nd:YAG laser) will induce spontaneous emission
of light at around 350 nm within the gain medium. These spontaneously emitted photons will then stimulate other radiative transitions in the gain medium to take place, unleashing yet more photons. This is, in many ways analogous to the chain reaction that occurs in the fission
of neutrons in a nuclear reactor and has been referred to by R.H. Dicke as an "optical bomb".
is a well-known phenomenon which occurs when electrons become trapped in a disordered metallic structure and this metal goes through a phase transition from conductor
to insulator
. These electrons are said to be Anderson Localized. The conditions for this localization are that there is a high enough density of scatters in the metal (other electrons, spins, etc.) to cause free electrons to follow a single looped path.
In analogy to this, we can imagine photons diffusing through a medium scattering off nanoparticles of diameter 10 - 100 nm. If the Ioffe-Regel criterion, describing the ratio of photon wave-vector k to mean free-path length (of a photon not colliding with anything) l, is met:
, then there is a probability that photons will become trapped in much the same way as electrons are observed to be trapped under Anderson localization. In this way, while the photon is trapped, the scatters may act as an optical cavity. The gain medium in which the scatterers lie will allow for stimulated emission to occur. As in an ordinary laser, if the gain is greater than the losses incurred, the lasing threshold will be broken and lasing can occur.
Photons traveling in this loop will also interfere with each other. The well defined cavity length (1- 10 um) will ensure that the interference
is constructive and will allow certain modes to oscillate. The competition for gain permits one mode to oscillate once the lasing threshold has been reached.
Recent studies show that these weak localization processes are not the governing phenomena for the onset of Random Lasing. Random Lasing also occurs for
! This is in agreement with experimental findings. Even though travelling of light on exactly 'closed loops' would explain the occurrence of confined lasing spots intuitively, the question is still open if, e.g. the stimulated emission processes are correlated with those processes.
The theory of 'preformed cavities' is however not confirmed.
Typical amounts of gain medium required to exceed the lasing threshold depend heavily on the scatterer density.
production has been observed to be around 500°C for powders. This is in contrast to producing an ordinary laser crystal at temperatures exceeding 700°C.
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...
. Random laser action has been observed in many different media, including semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
powder, nanostructured and non-nanostructured thin films, laser dyes
Dye laser
A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths. The wide bandwidth makes them particularly suitable for tunable lasers and...
, ceramics and many more.
Developments in nanoparticles have demonstrated that large amounts of optical scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
can occur when photons are incident. In this way, light can be diffused around a medium in much the same way as it is on white paint
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
and in clouds.
Incoherent regime
If nano particles are embedded in an optical gain medium, for example, zinc oxideZinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
(UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
emission - bandgap 3.3 eV), light from a pump source ( e.g. frequency-tripled Nd:YAG laser) will induce spontaneous emission
Spontaneous emission
Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy, e.g., the ground state and emits a photon...
of light at around 350 nm within the gain medium. These spontaneously emitted photons will then stimulate other radiative transitions in the gain medium to take place, unleashing yet more photons. This is, in many ways analogous to the chain reaction that occurs in the fission
Fission
Fission is a splitting of something into two parts.Fission may refer to:*In physics, nuclear fission is a process where a large atomic nucleus is split into two smaller particles....
of neutrons in a nuclear reactor and has been referred to by R.H. Dicke as an "optical bomb".
Anderson localization
Anderson localizationAnderson localization
In condensed matter physics, Anderson localization, also known as strong localization, is the absence of diffusion of waves in a disordered medium. This phenomenon is named after the American physicist P. W...
is a well-known phenomenon which occurs when electrons become trapped in a disordered metallic structure and this metal goes through a phase transition from conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
to insulator
Insulator
Insulator may refer to:* Insulator , a substance that resists the flow of electric current* Insulator , an element in the genetic code* Thermal insulation, a material used to resist the flow of heat...
. These electrons are said to be Anderson Localized. The conditions for this localization are that there is a high enough density of scatters in the metal (other electrons, spins, etc.) to cause free electrons to follow a single looped path.
In analogy to this, we can imagine photons diffusing through a medium scattering off nanoparticles of diameter 10 - 100 nm. If the Ioffe-Regel criterion, describing the ratio of photon wave-vector k to mean free-path length (of a photon not colliding with anything) l, is met:
, then there is a probability that photons will become trapped in much the same way as electrons are observed to be trapped under Anderson localization. In this way, while the photon is trapped, the scatters may act as an optical cavity. The gain medium in which the scatterers lie will allow for stimulated emission to occur. As in an ordinary laser, if the gain is greater than the losses incurred, the lasing threshold will be broken and lasing can occur.Photons traveling in this loop will also interfere with each other. The well defined cavity length (1- 10 um) will ensure that the interference
Interference
In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have...
is constructive and will allow certain modes to oscillate. The competition for gain permits one mode to oscillate once the lasing threshold has been reached.
Random Laser Theory
Theory however shows that for multiple scattering in amplifying random media "Anderson" localization of light does not occur at all - even though the calculation of interferences are essential to prove that fact. In contrary so called weak localizations processes can be proven, but it is vividly discussed, wheter those mechanisms play the key role in the mode stistics or not.Recent studies show that these weak localization processes are not the governing phenomena for the onset of Random Lasing. Random Lasing also occurs for
! This is in agreement with experimental findings. Even though travelling of light on exactly 'closed loops' would explain the occurrence of confined lasing spots intuitively, the question is still open if, e.g. the stimulated emission processes are correlated with those processes.The theory of 'preformed cavities' is however not confirmed.
Typical amounts of gain medium required to exceed the lasing threshold depend heavily on the scatterer density.
Applications
This field is relatively young and as such does not have many realized applications. However, random lasers based on ZnO are promising candidates for electrically pumped UV lasers, biosensors and optical information processing. This is due to the low production cost and that the optimum temperature for substrateSubstrate (semiconductor)
Substrate is a solid substance onto which a layer of another substance is applied, and to which that second substance adheres. In solid-state electronics, this term refers to a thin slice of material such as silicon, silicon dioxide, aluminum oxide, sapphire, germanium, gallium arsenide , an alloy...
production has been observed to be around 500°C for powders. This is in contrast to producing an ordinary laser crystal at temperatures exceeding 700°C.
External links
- Journal of Optics. Special issue: nano and random lasers. February 2010 http://iopscience.iop.org/2040-8986/12/2