How Much Do You Know About Laser Protection Materials? （Part 2）

Laser protection materials must have sufficient performance stability when exposed to laser irradiation. When exposed to laser irradiation, the absorbent can not be significantly discolored or faded to affect the absorption effect, and the base material can not be aged or broken down.

2. Laser protection materials based on the nonlinear optical principle

The theoretical basis of nonlinear protective materials is nonlinear optics, that is, the nonlinear polarization theory of the medium caused by the interaction between the strong electromagnetic field and the medium. The ideal nonlinear laser protection material should have a very short nonlinear response time in the whole protection band and can adapt to the protection of Q-switched and mode-locked lasers. The nonlinear optical coefficient, especially the third-order nonlinear optical coefficient, should be large, the anti-laser damage threshold should be high, and the stable limiting band of physical and chemical properties should be wide.

2.1 Nonlinear absorption

The nonlinear absorbing material is the reverse saturation absorbing material. The absorption coefficient of the anti-saturated absorbing material is approximately linear under the weak laser irradiation, and the absorption coefficient of the medium increases with the increase of the input light intensity under the strong laser irradiation, so as to realize the protection of the laser. At present, there are many reports on the research of reverse saturation absorbent materials. Including toluene, o-xylene, tetrachloroethane as solvent C6o solution, C6o/pst composite material C60/pmma composite material; Indanthrin and its derivatives such as indenturing oxide, indenturing chloride, indenturing oligomer, etc.; Organometallic compounds; Fullerene derivatives 17; Sacrifice compound; Third-order nonlinear polymer materials, such as poly double fast, poly Z fast, polystyrene fast materials.

C6o is a sphere composed of 60 carbon atoms, with a three-dimensional spherical meso-electron covalent structure, and thus has a stronger optical nonlinearity. The laser protection value of Yindanshilin is high, the high energy laser transmittance is very low, and the protection performance is good; The laser absorption threshold of Yindanshilin derivatives is relatively low, and the laser protection performance is poor.

Heavy metal CAI Xianjing compounds have high co-heavy two-dimensional. The electronic system can be combined with more than 60 different metal atoms. Due to the large de-regionalization of the electron system and the charge transfer of the metal-ligand bond, it exhibits a large nonlinear polarization.

2.2 Nonlinear Scattering

A nonlinear optical material is composed of two media with matching linear refractive indexes but the different nonlinear refractive indexes have the characteristics of nonlinear optical material under the action of the laser. The characteristic of nonlinear scattering is that there is no scattering effect when the weak light incident, shows high transmission. When a strong light incident, the scattering effect is greatly enhanced, showing low transmission. They occur only in nonlinear scatterers composed of two different materials. This nonlinear scatterer can be a suspension of liquid and particles, such as a colloidal carbon suspension, or it can be a solid mixed medium composed of two solids.

2.3 Nonlinear refraction

Nonlinear refraction includes photorefractive, self-focusing, and self-defocusing three kinds of photorefractive limiting is the use of photorefractive effect to cause the scattered light amplification, so that the human light to the scattered light transfer energy, so as to limit the intensity of the transmitted light: self-convergent (diffuse) focus refers to the nonlinear material under the action of Gaussian beam, the refractive index of the axisymmetric change n=n0+n21, 7 material refractive index coefficient; Is the light intensity. At n20, the self-focusing method is used to converge the beams. At 0, the self-defocusing method is used to make the light diverge so that the purpose of limiting the output light is strong and small can be achieved. Such materials include inorganic substances such as ferroelectric tungsten-copper SNB60 and BSKNN-60 single crystals and some organic polymers.

2.4 Nonlinear Reflection

The interface composed of linear material and nonlinear material is called a nonlinear interface. If the refractive index of the nonlinear material is positive, the refractive index of the nonlinear material is required to be greater than the refractive index of the nonlinear material. When the light incident from the linear material to the interface, the incident Angle meets the critical Angle condition of total reflection. When the weak light incident, because the condition of total reflection is met, the incident light is completely reflected off. When the strong laser is fired, the nonlinear refractive index changes greatly, the total reflection condition is destroyed, part of the light energy is transmitted, and the reflected light intensity is weakened, so as to achieve the purpose of protection. Of course, the nonlinear interface composed of two nonlinear optical materials with opposite nonlinear symbols can also achieve the effect of laser protection.

3. Thermal phase convenient protective materials

Since the interaction between laser and material is firstly thermal effect, the mechanism of thermal phase transition of some materials can be used to achieve strong laser protection. At present, VO thin film material is the most studied. VO film is a kind of thermotropic phase change material. It has a monoclinal structure near room temperature and a semiconductor state. The band gap is 0.7eV and the corresponding cutoff wavelength is 1800nm. When the temperature rises to 68C, the structure becomes orthogonal and metallic. When the laser beam is irradiated on the VO thin film, the material is heated, and the semiconductor-metal phase transition process will occur, accompanied by this process, its photoelectric characteristics will change greatly, especially the infrared characteristics will change from high transmission to high reflection. The optical properties of VO films can be used to block the attack of infrared light and electromagnetic radiation, thus achieving laser protection.

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