An optical frequency multiplier is a

nonlinear optical Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typica ...

device in which

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...

s interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency (and shorter

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

). Two types of devices are currently common: ''frequency doublers,'' often based on

lithium niobate Lithium niobate () is a non-naturally-occurring salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other line ...

(LN), lithium tantalate (LT), potassium titanyl phosphate (KTP) or lithium triborate (LBO), and ''frequency triplers'' typically made of

potassium dihydrogen phosphate Monopotassium phosphate (MKP) (also, potassium dihydrogenphosphate, KDP, or monobasic potassium phosphate) is the inorganic compound with the formula KH2PO4. Together with dipotassium phosphate (K2HPO4.(H2O)x) it is often used as a fertilizer, ...

(KDP). Both are widely used in optical experiments that use

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

s as a light source.

Harmonic generation

There are two processes that are commonly used to achieve the conversion:

second-harmonic generation Second-harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of ...

(''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 an interface between materials of different excitability. For example, it has been used to extract the outline of cells in embryos, where the cells are separated by water.Nonlinear microscopy and tissue morphogenesis
Laboratoire d'Optique et Biosciences, École Polytechnique, France

Lasers

Optical frequency multipliers are common in high-power lasers, notably those used for

inertial confinement fusion Inertial confinement fusion (ICF) is a fusion energy process that initiates nuclear fusion reactions by compressing and heating targets filled with thermonuclear fuel. In modern machines, the targets are small spherical pellets about the size o ...

(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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...

frequencies generated by the laser lost most of its energy in the hot

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...

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 Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...

, a process that has been used on almost every laser-driven ICF experiment since then, including the National Ignition Facility.

References

{{DEFAULTSORT:Optical Frequency Multiplier Second-harmonic generation Frequency multiplier Nonlinear optics Laser science