Nonlinear optical materials are a class of optoelectronic functional materials that have broad application prospects in the fields of photoelectric conversion, optical switching, and optical information processing. In the current era of rapid development of information technology, the optoelectronic industry is developing rapidly, and the demand for optoelectronic functional materials is also increasing. In the optoelectronic industry, such as optical switches, optical communications, optical information processing, optical computers, and laser technology, nonlinear optical materials are required as basic materials. Therefore, nonlinear optical materials have attracted extensive attention in recent decades. Its research is also developing at a rapid pace.

In the current era of rapid development of information technology, the optoelectronic industry is developing rapidly, and the demand for optoelectronic functional materials is also increasing. In the optoelectronic industry, such as optical switches, optical communications, optical information processing, optical computers, and laser technology, nonlinear optical materials are required as basic materials. Therefore, nonlinear optical materials have attracted extensive attention in recent decades. Its research is also developing at a rapid pace. Nonlinear optical materials refer to a class of materials that change in frequency, phase, and amplitude under the action of external light fields, electric fields, and strain fields, resulting in changes in refractive index, light absorption, and light scattering. When a laser is used as a light source, the nonlinear optical phenomenon produced by the interaction between the laser and the medium will lead to frequency doubling, frequency combination, frequency difference, parametric oscillation, and parametric amplification of light, resulting in harmonics. Using the frequency conversion and photorefractive functions of nonlinear optical materials, especially the frequency doubling and triple frequency capabilities, they can be widely used in signal converters and optical switches, optical modulators, frequency multipliers, Limiters, amplifiers, rectifier lenses and transducers. Under the action of the electromagnetic field, the positive and negative charge centers of the atoms will migrate, that is, polarization will occur, resulting in an induced dipole moment p. When the light intensity is not very high, the induced dipole moment p of the molecule is linearly proportional to the electric field intensity E of the light. However, when light intensity is large enough like a laser, new electromagnetic fields are created that vary in frequency, phase, polarization and other transmission properties of non-classical optics. The molecular-induced dipole moment p becomes a nonlinear function of the electric field intensity E, which is expressed as follows: p=αE+βE2+γE3+…where α is the microscopic linear polarizability of the molecule; β is the first-order molecular hyperpolarizability (second-order effect), γ is the second-order molecular hyperpolarizability (third-order effect). That is, the electric polarization effect induced by the n-th power of the electric field intensity E is called the n-order nonlinear optical effect. A good nonlinear optical material should be easily polarized, with asymmetric charge distribution, with a large π-electron conjugated system, and a non-centrosymmetric molecular material. In addition, it can achieve phase matching at the working wavelength, has a high power breakdown threshold, wide transmittance, good optical integrity, uniformity, hardness and chemical stability of the material, and is easy to perform various mechanical and optical processing. required. Ease of production and low price are also factors that should be considered. At present, the second-order and third-order nonlinear optical effects are mostly studied.

Since its birth in the 1960s, the research of nonlinear optical materials has made great progress, and many of them have entered the practical stage [1-3]. Nonlinear optical materials can be roughly classified into inorganic nonlinear optical materials, organic nonlinear optical materials, inorganic-organic hybrid materials, and the like according to the composition.

In the application of quadratic nonlinear optical materials, inorganic materials have been in the main position for a long time and have made great progress, and have been applied in many devices so far [4-6]. Inorganic materials are generally more stable than organic materials, many of them allow anisotropic ion exchange, making them useful for wave guide materials, and they all have crystalline forms that are more pure than organic materials. These include KTP (KTiO-PO4) type materials, KDP (KH2PO4) type materials, perovskite type (LiNbO3, KNbO3, etc.) materials, semiconductor materials (Te, Ag3AsS3, CdSe, etc.), borate series materials (including KB5, BBO, LBO and KBBF), etc., as well as zeolite molecular sieve-based materials, glass-type and complex-type materials.

It mainly includes some isomorphs of KH2PO4 and tetragonal crystal system and their deuterated crystals. Such crystals are simple to grow, easy to obtain high-quality single crystals, can obtain 90° phase matching, and are suitable for high-power frequency doubling. Although their nonlinear coefficients are small, they do not prevent high conversion efficiencies at high powers.

It mainly includes KTiOPO4 and isomorphs of orthorhombic system. KTP crystal has the advantages of large nonlinear coefficient, low absorption coefficient, not easy to deliquescence, difficult to embrittlement, good chemical stability, easy processing and high frequency doubling conversion efficiency. It is an excellent nonlinear optical crystal. The difference limits its application in the ultraviolet region.

Such as barium metaborate (BBO), lithium triborate (LBO), etc. The common feature of these crystals is a particularly wide range of UV transmission. Among them, the advantages of BBO and LBO are large nonlinear coefficient, high conversion efficiency, wide light transmission range, high light damage threshold, good chemical stability and easy machining.

Such as Te, Ag3AsS3, CdSe, GaP, GaAs, α-SiC and β-SiC, etc., by adjusting the energy gap of the material, the transition probability of electrons can be effectively changed, thereby controlling the nonlinear optical response of the material. Most of these materials have high nonlinear optical coefficients, but the disadvantage is that the crystal quality is not high and the light damage threshold is too low.

It mainly includes LiNbO3, LiTaO3 and LixNbyO3 ferroelectric crystals with different Li/Nb atomic ratios. They all have good nonlinear optical effects and have been widely used. Lithium niobate single crystal is a ferroelectric material with excellent linear and nonlinear optical properties, large electro-optic coefficient, wide light transmission range, and excellent thermal and chemical stability. It is widely used in manufacturing. Electro-optic modulators, electro-optic deflectors, electro-optic switches and inorganic crystal materials that are ideal for the manufacture of integrated optical devices. At the same time, the piezoelectric properties of lithium niobate make it a key material for the manufacture of ultrasonic transducers, sound meter devices, and can be used for video and microwave signal processing. At present, most of lithium niobate is used for telecommunication. However, lithium niobate is prone to light damage, which limits its application in strong laser

Nanoclusters of nonlinear optical materials can be obtained by molecular assembly based on zeolite molecular sieves. Because a certain molecular sieve can only allow molecules of a certain size to enter, its pore structure plays an extremely important role in the assembly process. At present, more researches are on the assembly of organic nonlinear optical effect substances in zeolite. For example, the polymer grown by polymerization in the pores of the molecular sieve has good microscopic order, which avoids the defect that the molecular order of the polymer is easily destroyed. At the same time, the molecular sieve as a matrix can protect the organic molecules of the guest and enhance the photothermal stability of the guest. In addition, the dielectric constant of the molecular sieve framework can be adjusted by adjusting the electrochemical composition, and the influence between the host and the guest can be adjusted, thereby enhancing the nonlinear optical effect. For another example, nonlinear optical effects are generated for some organic molecules with symmetry centers after they are assembled in some molecular sieves. For example, after assembling p-nitroaniline in A1PO4-5 molecular sieve by gas-phase loading method, it is found that the generated inclusion compound shows a certain frequency doubling effect, which may be caused by the structure of A1PO4-5 without symmetrical center, while the zeolite with symmetrical center does not produce this effect.

The nonlinear optical effect of glass is mostly due to the resonance effect caused by the nonlinear polarization of the atoms or ions of the material under the irradiation of a strong electric field. Although glass is isotropic, its structure can be changed when subjected to effects such as electrical polarization, thermal polarization, laser-induced polarization, electron beam radiation polarization, etc. , so as to break the inversion symmetry of the glass and make it have a second-order nonlinear optical effect. It can be used to prepare double frequency doublers, hybrid bistables, ultraviolet lasers, infrared lasers, electro-optic modulators, etc. The third-order nonlinear optical effect of glass can be used to prepare ultra-high-speed optical switches, optical memories, optical computing elements, and new optical fibers. For example, the tellurium niobium zinc system glass is a kind of third-order nonlinear optical glass material with excellent performance. Rare earth ions are introduced into the tellurium niobium zinc system glass, and the transition of its 4f electrons is used to improve the possibility of harmonic photon excitation, thereby improving the third-order optical nonlinearity of the glass. Due to the diverse composition of glass, superior performance, good light transmittance, good chemical stability and thermal stability, easy fabrication and processing and easy doping and a series of advantages, it has attracted more and more people’s attention, and it is also a kind of good application. Prospects of nonlinear optical materials.

In the early stage of nonlinear optical materials research, a series of organic compounds such as urea, picric acid, and dinitroaniline were found to have nonlinear optical effects. Since organic molecules with large delocalized π-conjugated electron systems have strong optoelectronic coupling characteristics, high response values and relatively large optical coefficients can be obtained. After the 1980s, organic nonlinear optical materials developed rapidly. Compared with inorganic materials, organic materials have the advantages of high nonlinear optical coefficient, fast response, easy modification, high optical damage threshold, easy processing and strong molecular variability. Many organic nonlinear optical materials have been discovered or synthesized, including various organic low molecular nonlinear optical materials, high polymer nonlinear optical materials, and metal-organic complex nonlinear optical materials.

It mainly includes a series of chromophore-containing π-conjugated compounds such as urea and its derivatives, Schiff base compounds, azo compounds, stilbene compounds, fused heterocyclic compounds, phthalocyanine compounds, and organic salts. Chain of near-ultraviolet absorbing small-molecule compound materials. Organic molecules have large delocalized π-electron conjugated structures, are easily polarized, have large nonlinear optical coefficients, are easy to design and combine, are easy to process and form, and are easy to device. In addition, they have relatively low cost, low dielectric constant, fast optical response, and non-resonant optical polarizability comparable to or far exceeding that of ferroelectric inorganic crystals. Therefore, new structural materials can be developed by changing the structure through molecular design and synthesis.

Polymer nonlinear optical materials not only have the advantages of large nonlinear optical coefficient, fast response speed, low DC dielectric constant, etc., but also because the molecular chains are connected by covalent bonds, high mechanical strength, good chemical stability, and excellent processing performance, It has strong structural variability and can be made into various forms such as films, sheets, fibers, etc. It has broad application prospects in many aspects such as optical modulation devices, neural networks for optical computing, spatial light modulators, optical switching devices and all-optical serial processing elements. In the synthesis of polymer nonlinear optical materials, although the polymer itself has a non-centrosymmetric unit, the orientation of its dipole moment is irregular, and its nonlinear optical performance is weak. Therefore, by applying an electric field, the orientation of the molecules can be aligned, thereby enhancing their nonlinear optical properties. The polarization orientation of the polymer chain can only occur above the glass transition temperature, and the orientation freezing must be below the glass transition temperature, which requires the polymer material to have a higher glass transition temperature. The polymer should also be a transparent material to minimize light loss. According to the polymer structure, it can be roughly divided into host-guest type polymers, side-chain and main-chain type polymers, cross-linked polymers, conjugated polymer nonlinear optical materials, and the like.

The guest organic conjugated molecule with high nonlinear optical coefficient and the host polymer are mixed to form the nonlinear optical material of the host-guest system, also known as the doped nonlinear material. Such polymers have good nonlinear optical properties, are easy to prepare and purify, but often have poor host-guest compatibility, and it is difficult to increase the amount of doping. In addition, the addition of low-molecular dopants will also reduce the glass transition temperature of the material and affect its orientation stability.

The chromophore molecules are bound to the polymer backbone or side chains by covalent or ionic bonds. Compared with doped materials, such polymers have more chromophore content, increase orientation stability, and have higher nonlinearity. However, the field-induced non-centrosymmetric arrangement of polymers is prone to relaxation, resulting in poor performance.

The chromophore molecules are cross-linked in the polymer network, and the chromophore orientation is polarized before the cross-linking reaction occurs or during the cross-linking process, and the chromophore orientation stability is significantly improved, thereby obtaining better optical properties.

The higher the degree of molecular delocalization, the better the nonlinear optical properties of the material. Conjugated polymers can be used as good second-order nonlinear optical materials. Such polymer nonlinear optical materials mainly include polydiacetylene (PDA), polyacetylene (PA), polythiophene (PTh), polyphenylene vinylene (PPV), polyaniline (PAn), polybenzothiazole (PBT) , Polybenzimidazole (PBI), polyimide and its derivatives. In addition, there are also inorganic polymers such as polyphosphazene, polysiloxane and polyalkylsilicon, which all exhibit good nonlinear optical properties and have better thermal and chemical stability.

Mainly include metallocene complexes, metal carbonyl complexes, metal olefin organic complexes, metal polyacetylene polymers, metal porphyrin organic complexes, metal phthalocyanine organic complexes and other complex nonlinear optical materials, etc. . In 1986, C.C.Frazier et al first reported the second harmonic effect of metal-organic compounds. Since then, some nonlinear effects of metal-organic compounds have been discovered one after another [7]. Molecular configuration has a direct impact on the nonlinear optical properties and color of metal-organic complexes. Due to the diversity of ligands and metals, metal-organic compounds also have various structures, which have more advantages than nonlinear optical materials composed of simple organic molecules. Since metal atoms have different numbers of d or f electrons, different oxidation states and coordination numbers, different three-dimensional structures can be formed, resulting in unique optoelectronic properties. For example, the redox change of the central metal may lead to a larger molecular hyperpolarizability; the central metal can also become a chiral center, and a non-centrosymmetric crystal can be obtained after splitting; the introduction of metal atoms can combine the magnetic, electrical properties and optical properties. The properties combine to produce magneto-optical and electro-optical effects. In addition, metal-organic complexes have more absorption bands, there are photons transitioning from metal to ligand and from ligand to metal, and there is a larger ground state dipole moment and polarizability, and the energy between the ground state and the excited state. The level difference is small, which is beneficial to improve the photoelectric response speed of the material. By designing and synthesizing novel ligands with certain structural characteristics, it will be beneficial to the further development of complex research. The following rules are summarized: if you want to explore second-order nonlinear optical materials that are completely transparent in the visible light region, you can design tetrahedral, tetrahedral or octahedral molecules; if you want to explore new chromophores or third-order nonlinear optical materials ( Such materials do not require complete transparency in the visible light region), and a planar quadrilateral metal-organic compound should be designed.

Inorganic/organic hybrid nonlinear optical materials integrate the advantages of inorganic materials and organic materials, and incorporate organic functional molecules or polymers into inorganic networks through methods such as salt formation or sol/gel technology, and between inorganic/organic molecules A new class of materials that form chemical bonds. The main advantage of the preparation by sol/gel technology is that the organic/inorganic hybrid materials can be prepared by bonding inorganic glass with organic chromophores at a temperature lower than the decomposition temperature of organic chromophores. Through the rigid amorphous two-dimensional structure and excellent high temperature stability of inorganic glass, the orientation relaxation of chromophores can be suppressed, and the thermal stability of the material can be improved. In addition, it has good film-forming properties and is a class of materials with good application prospects [9,10]. Nano-doped microcrystalline semiconductor glass is the most widely used third-order nonlinear optical material.