Infrared optical properties of infrared lens materials

1. Basic knowledge of infrared

Infrared rays are electromagnetic waves in nature like visible light. Its wavelength range is very wide (0.7～1000 mm). According to wavelength, it can be divided into three spectral regions: near infrared (0.7～15 mm), mid-infrared (15～50mm), far infrared (50～1000mm). Infrared rays also have wave-particle duality, obeying the law of reflection and refraction of waves, and interference and diffraction effects will also occur under certain conditions. The difference between infrared and visible light is that infrared is invisible to the naked eye, and there are a series of transparent bands with very low absorption in the infrared band in the atmosphere, such as 1 ~ 1.1 mm, 1.6 ~ 1.75 mm, 2.1 ~ 2.4 mm, 3.4 ~ 4.2 For bands such as mm, the transmittance of the atmosphere is above 80%; for 8-12 mm, the transmittance of the atmosphere is 60% to 70%. These characteristics make infrared rays have many applications in military, engineering and biomedicine.

Second, the performance of infrared materials

Infrared materials should have transmittance, refractive index and dispersion to different wavelengths of infrared. Of course, the strength and hardness of the material, corrosion resistance and deliquescence resistance, density, heat rate, thermal expansion coefficient, specific heat capacity, etc. are used in infrared optical devices (such as Lenses, prisms, filters and fairings, etc.) also need to be considered in the preparation and application. The spectral transmittance of a material is related to the structure of the material, especially the chemical bond and atomic weight. Any material can only have a high transmittance in a certain waveband. For pure crystalline materials, if the absorption of impurities is not considered, the short-wave transmission limit s l depends on the absorption of electrons, that is, the light absorption that causes electrons to be excited from the valence band to the conduction band. Therefore, generally speaking, the short-wave cut-off wavelength is roughly equivalent to the optical frequency corresponding to the energy of the band gap of the crystal. The long-wave transmission limit of 1 l mainly depends on phonon absorption, that is, lattice vibration absorption. It can be the first harmonic vibration absorption or the higher harmonic vibration absorption. Phonon absorption is related to the crystal structure, the average molecular weight of the crystal elements and chemical bonds. In the case of the same crystal structure, the larger the average molecular weight, the longer the wavelength of phonon absorption, and the longer the infrared transmission long-wave cut-off wavelength 1 l of the material. For diamond, germanium, silicon and other crystals with a diamond structure, since there is no active first-order harmonic lattice vibration in the infrared region, the higher harmonics are also weak, so it is a type of high transmittance and wide transmission band The excellent infrared optical materials are also commonly used. Refractive index and dispersion are another important characteristic of infrared optical materials. First of all, the refractive index and reflectance loss are closely related. The larger the refractive index, the higher the reflection loss. Secondly, for different purposes, there are different requirements for the refractive index.

For example, for the optical materials used to manufacture windows and fairings, in order to reduce reflection loss, lower refractive index is required; materials used to manufacture prisms, lenses and other optical components in optical systems with high magnification and wide angle of view require The refractive index is higher. For example, sometimes for achromatic or other aberrations, it is not only necessary to use materials with different refractive indexes as compound lenses, but also have certain requirements for dispersion. As the prism of the dispersive element in the spectrophotometer, its performance is directly related to the refractive index and dispersion of the material. In addition to transmittance, refractive index and dispersion, the mechanical properties, corrosion resistance, and deliquescence resistance of materials are also very important for a good optical device. For example, although the green sodium crystal is a good infrared optical material, it is easy to deliquesce and is not suitable for use in the field; germanium is also a good infrared optical material, but when the temperature rises, the transmittance decreases significantly, and it is relatively brittle , The temperature of soft talk is too low, so it is not suitable for fairing.

Similarly, although diamond has excellent various properties, it cannot be made into a large-size device, and the price is too expensive, so few people use it as an actual optical material. In addition, special attention should be paid to the sub-radiation characteristics of the material when it is heated. In order to avoid false signals in the detector, the sub-radiation of the heated material in the working band should be small. This is especially important in search and tracking systems. In infrared optical systems, some commonly used components have different requirements for material properties. For the detector window material, it is required that the window must have a high transmittance in the response band of the detector (so the absorptivity and reflectivity are required to be very low).

In this way, the radiation from the target can be transmitted well, but the self-radiation is very small. For refrigeration detectors, the window must be well sealed with glass or other detector housing materials, so the coefficient of thermal expansion must match, and the transmittance of the window should not change significantly with temperature changes. Generally, the window should be exposed to the air. Therefore, it should not be afraid of moisture, have good chemical stability, and will not be moldy or hairy for a long time. The influence of scattering will reduce the transmittance. In addition, the window material should be easy to process and cut into various shapes. In order to reduce the reflection loss, a material with a low refractive index can be selected as the window material. If a material with a high refractive index is necessary, it is easy to coat an antireflection coating. At the same time, the window is generally thin, and the material should have sufficient strength. The requirement for the rectifying material is that the rectifier must have a high transmittance in the corresponding band of the detector, and the self-radiation should be small to avoid false signals. Some materials have high transmittance at room temperature, but at high temperatures, only the carrier absorption increases and the transmittance characteristics deteriorate significantly (such as germanium), so this material cannot be used as a fairing. The rectifier is installed in the front of the optical system of high-speed flying bodies such as airplanes, missiles, and spacecraft. Due to aerodynamic heating, the temperature of the fairing is very high. Therefore, the melting point and softening temperature of the fairing are required to be high, and the material The thermal stability is better, and it must be able to withstand thermal shock. The fairing should be harder, so that on the one hand it is beneficial to processing, grinding and polishing, on the other hand it will not be scratched by flying dust and sand. Since the fairing is exposed to the air, it has good chemical stability and must be able to prevent corrosion from salt solutions or corrosive gases in the atmosphere, and is not afraid of deliquescent. It should be pointed out that the general window size is small, while the size of the fairing is often larger (a few tens of millimeters to hundreds of millimeters in diameter), and the refractive index must be continuous to avoid scattering.

Therefore, the fairing is often required to be made of a single crystal or a uniform polycrystal with no sudden change in refractive index at the crystal grain boundary. The curvilinear rate of the fairing is often very large, so it must have sufficient strength to facilitate processing, assembly, and withstand vibration and air waves. The requirements for lenses and prisms are that the lens and prism materials should be pure and uniform, and the requirements for refractive index are strict, and other requirements are similar to those of window materials. However, the requirement for the coefficient of thermal expansion is important only in the immersion lens. It is thought that if the detector is cooled, if the expansion coefficient is not well matched, the immersion lens and the detector may become detached, increasing the reflection loss. An outstanding requirement for prism materials is that it has a wide projection band and large dispersion.

The polymer material is cheap, has good acid and alkali resistance and corrosion resistance, is insoluble in water, and has good transmittance in the near infrared and far infrared, which is its advantage. However, the structure of polymer materials is complex. The molecular vibration and rotation absorption band and the lattice vibration absorption band are just in the mid-infrared band. Therefore, the transmittance of the plastic in the mid-infrared band is very low, and the softening temperature of the plastic is low, and the strength is not high. , Can only be used for windows and protective films at lower temperatures. A few plastics can be used as lenses, but not fairings. To

Plastics are mainly used in the far-infrared region, and the mid-near infrared is less used. The commonly used plastic is plexiglass, that is, polymethyl methacrylate, which transmits visible light and near infrared, and is often used as a protective film, antireflection film and window material. Polyethylene is not transparent to visible light, but has a high far-infrared transmittance. It is a far-infrared optical material used at room temperature. High-density polypropylene is harder than polyethylene, and has a certain transmittance in some mid-infrared bands.