Second harmonic generation
Encyclopedia
An optical frequency multiplier is a nonlinear optical
device, in which photon
s interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency (and shorter wavelength
). Two types of devices are currently common, frequency doublers often based on lithium niobate
(LN), lithium tantalate
(LT), potassium titanyl phosphate
(KTP) or lithium triborate
(LBO), and frequency triplers typically made of potassium dihydrogen phosphate or commonly known as KDP. Both are widely used in optical experiments that use laser
s as a light source.
There are two processes that are commonly used to achieve the conversion, second harmonic generation
(SHG, also called frequency doubling), or sum frequency generation
which sums two non-similar frequencies. Direct third harmonic generation (THG, also called frequency tripling) also exists, and can be used to detect interface between material of different excitability. For example, it has been used to extract the outline of cells in embryos, where the cells are separated by water..
Optical frequency multipliers are very common in high-power lasers, notably those used for inertial confinement fusion
(ICF) experiments. ICF attempts to use a laser to heat and compress a target containing fusion fuel, and it was found in experiments with the Shiva laser
that the infrared
frequencies generated by the laser lost most of its energy in the hot electron
s being generated early in the heating process. In order to avoid this problem much shorter wavelengths needed to be used, and experiments on the OMEGA laser and NOVETTE laser
validated the use of frequency tripling KDP
crystals to convert the laser light into the ultraviolet
, a process that has been used on almost every laser-driven ICF experiment since then, including the National Ignition Facility
.
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
device, in which photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency (and shorter wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
). Two types of devices are currently common, frequency doublers often based on lithium niobate
Lithium niobate
Lithium niobate is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.-Properties:...
(LN), lithium tantalate
Lithium tantalate
Lithium tantalate , is a crystalline solid which possesses unique optical, piezoelectric and pyroelectric properties which make it valuable for nonlinear optics, passive infrared sensors such as motion detectors, terahertz generation and detection, surface acoustic wave applications, cell phones...
(LT), potassium titanyl phosphate
Potassium titanyl phosphate
Potassium titanyl phosphate or KTP is a nonlinear optical material which is commonly used for frequency doubling diode pumped solid-state lasers such as Nd:YAG and other neodymium-doped lasers. The material has a relatively high optical damage threshold , a great optical nonlinearity and excellent...
(KTP) or lithium triborate
Lithium triborate
Lithium triborate LBO is a non-linear optics crystal. It has a wide transparency range, moderately high nonlinear coupling, high damage threshold and desirable chemical and mechanical properties. This crystal is often used for second harmonic generation of Nd:YAG lasers...
(LBO), and frequency triplers typically made of potassium dihydrogen phosphate or commonly known as KDP. Both are widely used in optical experiments that use 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...
s as a light source.
There are two processes that are commonly used to achieve the conversion, second harmonic generation
Second harmonic generation
An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...
(SHG, also called frequency doubling), or sum frequency generation
Sum frequency generation
Sum-frequency generation is a non-linear optical process. This phenomenon is based on the annihilation of two input photons at angular frequencies \omega_1 and \omega_2 while, simultaneously, one photon at frequency \omega_3 is generated...
which sums two non-similar frequencies. Direct third harmonic generation (THG, also called frequency tripling) also exists, and can be used to detect interface between material of different excitability. For example, it has been used to extract the outline of cells in embryos, where the cells are separated by water..
Optical frequency multipliers are very common in high-power lasers, notably those used for inertial confinement fusion
Inertial confinement fusion
Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....
(ICF) experiments. ICF attempts to use a laser to heat and compress a target containing fusion fuel, and it was found in experiments with the Shiva laser
Shiva laser
The Shiva laser was a powerful 20-beam infrared neodymium glass laser built at Lawrence Livermore National Laboratory in 1977 for the study of inertial confinement fusion and long-scale-length laser-plasma interactions. The device was named after the multi-armed form of the Hindu god Shiva, due...
that the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
frequencies generated by the laser lost most of its energy in the hot 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 being generated early in the heating process. In order to avoid this problem much shorter wavelengths needed to be used, and experiments on the OMEGA laser and NOVETTE laser
Novette laser
Novette was a two beam neodymium glass testbed laser built at Lawrence Livermore National Laboratory in about 15 months throughout 1981 and 1982 and was completed in January 1983. Novette was made using recycled parts from the dismantled Shiva and Argus lasers and borrowed parts from the future...
validated the use of frequency tripling KDP
Monopotassium phosphate
Monopotassium phosphate -- 24 -- is a soluble salt which is used as a fertilizer, a food additive and a fungicide. It is a source of phosphorus and potassium. It is also a buffering agent...
crystals to convert the laser light into the ultraviolet
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...
, a process that has been used on almost every laser-driven ICF experiment since then, including the National Ignition Facility
National Ignition Facility
The National Ignition Facility, or NIF is a large, laser-based inertial confinement fusion research device located at the Lawrence Livermore National Laboratory in Livermore, California. NIF uses powerful lasers to heat and compress a small amount of hydrogen fuel to the point where nuclear fusion...
.