Non linear optics, a branch of modern optics, studies the nonlinear phenomena and applications of media under the action of strong coherent light.The study of nonlinear optics is of great significance to the development of laser technology, spectroscopy and material structure analysis.Common second-orderNonlinear optical crystalyesPotassium dihydrogen phosphate（KDP）、Ammonium dihydrogen phosphate（ADP）、Potassium deuterium phosphate(KD * P), barium sodium niobate, etc.In addition, many third-orderNonlinear optical material。Before the advent of laser, it was basically to study the propagation of weak beams in the medium and determine the optical properties of the mediumRefractive indexorPolarizabilityIs a constant independent of light intensity, and the polarization intensity of the medium is proportional tolight waveElectric field strength ofE, the light wave superposition follows the linear superposition principle (seeIndependent propagation principle of light）。The study of optical problems under the above conditions is called linear optics.For very strong lasers, such aselectric field intensityAccessibleatomInternalCoulomb fieldThe interaction between light and medium will producenonlinear effect , the physical quantities reflecting the properties of the medium (such as polarization intensity) are not only related to the field strengthEIs also determined byEThis leads to many new phenomena that are not obvious in linear optics.[1]
The nonlinear effect isEItems to the power of, and items to a higher power than
The result of working together.[1]
development
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Pockels effect
The early work of nonlinear optics can be traced back to 1906Pockels effectThe discovery ofKerr effectDiscovery of.However, nonlinear optics has become such an important discipline today, which should be said to start after the emergence of laser.The appearance of laser provides people with beams with high intensity and good coherence.Such beams are necessary to discover various nonlinear optical effects (generally, the power density is greater than 10tenW/cmtwo, but there are great differences for different media and different effects).
Since 1961, P. A. Franken et al. first discovered the optical secondaryharmonicSince then, the development of nonlinear optics has roughly gone through three different periods.
The first period was from 1961 to 1965.This period is characterized by the rapid emergence of new nonlinear optical effects.Such as optical harmonics, optical sum frequency and difference frequency, optical parametric amplification and oscillationMultiphoton absorption, beamAutofocusAnd laser scattering were discovered in this period.
The second period was from 1965 to 1969.On the one hand, some new nonlinear optical effects continue to be found in this period, such as nonlinear spectral effects, various transient coherence effects, light induced breakdown, etc;On the other hand, it is mainly devoted to the further understanding of the discovered effects and the development of various nonlinear optical devices.
The third period is from the 1970s to the present.This is a period when nonlinear optics is maturing.Its characteristics are as follows: the research on nonlinear effects of solids is expanded to include gasesatomResearch on nonlinear effects of vapor, liquid, solid and even liquid crystal;The research on second-order nonlinear effects has developed to third-order, fifth order and higher order effects;From general nonlinear effect to resonance nonlinear effect;As far as the time category is concerned, the nanosecond enters the picosecond field.These characteristics are closely related to the development of laser tuning technology and ultrashort pulse laser technology.[1]
Optical phenomenon
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①Optical rectification。EtwoThe existence of the term will cause the constant polarization term of the medium, resulting in constant polarizationchargeAnd correspondingPotential difference, the potential difference is proportional to the light intensity andfrequencyIrrelevant, similar to the DC voltage obtained after the AC is rectified by the rectifier tube.
② GenerationHigh order harmonics。The frequency of weak light remains unchanged after entering the medium.After the strong light enters the medium, due to the nonlinear effect of the mediumωIn addition, 2 will appearω、3ω,..., etc.PA. Franken and his colleagues observed the second harmonic generation experimentally for the first time.They put rubiesLaser3kW red (6943 ∨) emittedLaser pulseFocusing on the quartz chip, the ultraviolet second harmonic with a wavelength of 3471.5 ∨ was observed.If a piece of sodium barium niobate crystal is placed in the laser cavity with a wavelength of 1W and 1.06 μ m, a continuous 1W second harmonic laser with a wavelength of 5323 ∨ (1 ∨=10-10m）。This kind of frequency doubling effect of nonlinear media has important applications in laser technology.
③optical mixing 。When the frequency of two beams isωoneandωtwo（ωone>ωtwo）If only the polarization intensity is consideredPThe quadratic term of will produce a frequency of（ωone+ωtwo）The sum frequency term and frequency of are（ωone-ωtwo）The difference frequency term of.It can be fabricated by optical mixing effectOptical parametric oscillator, which can be tuned in a wide rangeLaserIt can emit coherent radiation from infrared to ultraviolet.
④ StimulatedRaman scattering。The Raman scattering generated by ordinary light source is spontaneous Raman scattering, and the scattered light is incoherent.Whenincident lightWhen a very strong laser is usedLaser radiationThe strong interaction with material molecules makes the scattering processStimulated radiationIs calledStimulated Raman scattering。The generated Raman scattering light has high coherence, and its intensity is much stronger than the spontaneous Raman scattering light.Stimulated Raman scattering can be used to obtain a variety of new wavelengths of coherent radiation, and provide a means for in-depth study of the interaction between strong light and matter.
⑤Autofocus。The refractive index of the medium will increase with the increase of light intensity under the action of strong light.The intensity of the laser beam hasGaussian distributionThe light intensity is the largest at the central axis and decreases outwards, so there is a large refractive index near the axis of the laser beam. Like a convex lens, the beam will automatically converge to the axis until the beam reaches a filamentlimit(Diameter approx. 5 × 10-6m），And can generate total reflection within the filament range, as if the lightOptical fibreInternal transmission is the same.
⑥ Light induced transparency.Absorption of medium under weak lightcoefficient(See absorption of light) is independent of light intensity, but for very strong lasers, the absorption coefficient of the medium is dependent on light intensity, and the absorption coefficient of some originally opaque media will become zero under the action of strong light.
Study the effect of nonlinear optics on laser technologyspectroscopyIt is of great significance to analyze the material structure and the development of.The study of nonlinear optics is one of the common laws of nonlinear phenomena in various systemsCross science。Research hotspots in nonlinear optics include: research and search for newNonlinear optical materialFor example, organic polymers or organic crystals.And discuss whether these materials can be used as two wave mixing, four wave mixing and spontaneousoscillationAnd phase reversal optical amplifiers, and even spatial optical solids.Common second-orderNonlinear optical crystalyesPotassium dihydrogen phosphate（KDP）、Ammonium dihydrogen phosphate(ADP), potassium deuterium phosphate (KD * P)Niobic acidBarium sodium, etc.In addition, many third-order nonlinear optical materials have been found.[2]
application
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From the technical field to the research field, the application of nonlinear optics is very extensive.For example, ① various nonlinear crystals are used to make electro-optic switches and realize laser modulation. ②Using the generation of second and third harmonics, the second and third order optical sum frequency andDifferential frequencyRealize the conversion of laser frequency to obtain various lasers ranging from short ultraviolet, vacuum ultraviolet to far infrared;At the same time, difficulties in infrared reception can be overcome by up conversion of infrared frequency. ③Optical parametric oscillation is used to tune the laser frequency.Combined with frequency doubling and mixing technology, it can realize tuning in a wide range from mid infrared to vacuum ultraviolet. ④The phase conjugation characteristics of the output beam in some nonlinear optical effects are used to process optical information, improve the imaging quality and beam quality. ⑤Nonlinear etalon and various bistable devices are made by using the properties of refractive index changing with light intensity. ⑥Utilize various nonlinear optical effects, especially resonance nonlinear optical effects and variousTransient coherent optical effect, study the high excited state and high resolution spectrum of matter, the transfer process of energy and excitation in matter and othersRelaxation processEtc.[2]
Optical crystal
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Optical crystal is used asOptical medium materialsOfCrystalline material。It is mainly used to make ultraviolet and infrared area windows, lenses and prisms.Presscrystal structureIt is divided into single crystal and polycrystalline.becauseSingle crystal With high crystal integrity, light transmittance and low input loss, the commonly used optical crystals are mainly single crystals.
Optical communication andIntegrated optics usuallyuseNonlinear optical crystal, includingQuasi phase matching(QPM) multi domain structure crystal materials and components;Laser televisionRed, green, blueTricolorNonlinear optical crystal used for light source;Semiconductor for the Next Generation Optical Disk Blue Light SourceFrequency doubling crystal(e.g. KN and some possible QPM products);R&D and production of new infrared, ultraviolet and deep ultraviolet nonlinear crystals.[2]
