Optics & optical coatings

Guide | Technical Referance

Optical Crystals

Some crystals exhibit excellent optical properties that glass materials cannot achieve.
On the other hand, crystals also have properties not desirable for optics such as deliquescent or birefringence properties.
Optics utilize useful crystals after comprehensive evaluation of these properties.

Optical Window Materials

Optical Window Materials Comparison Chart of Refractive Index of Crystal Materials (Reference Data) Comparison Chart of Refractive Index of Crystal Materials
Calcium Fluoride CaF2

The natural mineral is called fluorite and although it is rare, some fluoresce when irradiated with ultraviolet light. Artificial calcium fluoride is used for high-grade camera lenses or beamsplitters of spectroscopes.
It has optical properties such as high transmittance in a wide range from vacuum ultraviolet to infrared, and small dispersion of refractive index and different dispersion curve (abnormal partial dispersion) compared to average optical glass, making it very advantageous when designing achromatic lenses. Its isotropic crystal structure does not cause birefringence.
It is more generally used than lithium fluoride or magnesium fluoride due to its relative chemical stability and low deliquescence.

Sapphire AI2O3

Sapphire is a hard mineral second to diamond, and is a crystal difficult to scratch. It has long been popular for use in watches as crystals, shafts of gears or bearings. Sometimes called sapphire glass, but its structure is not vitreous but crystalline.
Sapphire is chemically very stable making it useful as an alternative to glass. Artificial sapphire is transparent and colorless, and has a wide transmission range from ultraviolet to infrared. Since sapphire is a single crystal, birefringence occurs depending on the orientation of the crystal. While it has good insulation properties, its thermal conductivity is relatively high.
It is used as titanium-sapphire (Ti: sapphire) for an excitable medium of ultrashort pulse lasers or substrates that grow purple LED.

Zinc Selenide ZnSe

It is an amber color crystal, capable of transmitting up to long-wavelength infrared of 20µm. However, it does not transmit blue and green in the visible range.
Zinc selenide is often used as a lens material of CO2 lasers. Red laser light is sometimes used as the guide light of a CO2 laser, and zinc selenide can transmit this guide light together with the CO2 laser light. It is legally classified as a poisonous and deleterious substance, and some products require a document of transfer of poisonous and deleterious substances. In addition, disposal of used zinc selenide products as general waste is prohibited. Zinc selenide products no longer used must be returned to the place of purchase. It is insoluble in water, but reacts with acids to form toxic hydrogen selenide. Transmission loss by surface reflectance is high due to its high refractive index, but it can achieve 99% or higher transmittance with the addition of anti-reflection coating.

Silicon

It is single crystal silicon used in semiconductors, has metallic luster and although it does not transmit light, it transmits infrared light from wavelengths of 2 to 6µm. It can also be used as a filter for infrared detectors.
Its excellent thermal conductivity also makes it useful as substrates of gold coating mirrors used for high-output CO2

Germanium Ge

This material has metallic luster and although it does not transmit light, it transmits a wide infrared range from 2 to 20µm. It is used as a material of thermography camera lenses.
Since it has a high refractive index of 4, its transmittance goes down to lower than 50% due to transmission loss by surface reflectance if used without an anti-reflection coating.

Attention
  • Infrared-transparent materials need to consider the influence of the radiation spectrum due to temperature in addition to transmission and reflection.
  • When observing infrared of wavelength 10μm or higher, or using in an environment where an optical system is 30°C or higher, all substances emit infrared radiant light making it impossible to observe the infrared spectrum of an analyte.

Birefringence Materials

There are isotropic and anisotropic crystals, and birefringence is found in anisotropic crystals.
Especially single crystals among anisotropic crystals are utilized as polarization optics such as waveplates or polarizers.

Quartz Crystal SiO2

This material is a single crystal quartz without impurities (trigonal) and exhibits small birefringence.
Artificial crystals are mass produced as materials for low pass filters of crystal oscillators or CCD imaging devices.
Production of waveplates utilizes small refractive index difference between ordinary beam (no) and extraordinary beam (ne).

Wavelength [nm] Refractive Index
Ordinary Beam Extraordinary Beam
404.7 1.5572 1.5667
546.1 1.5462 1.5553
589.3 1.5443 1.5534
656.3 1.5419 1.5509
Transmitted Wavelength Range [μµm] 0.2 - 2
Density [g/cm3] 2.65
Thermal Conductivity [Wm-1K-1] (70℃) 9.3/5.4
Coefficient of Thermal Expansion [×10-6/℃] (20℃) 6.8/12.2
Excerpt from Chronological Scientific Tables
Calcite CaCO3

Calcite is transparent and colorless, and abundant in nature.
It is a single crystal (trigonal) and exhibits large birefringence.
Its refractive index difference between ordinary beam (no) and extraordinary beam (ne) is utilized in Glan-Thompson polarizers with high extinction ratio performance.
Calcite is a soft crystal and easily scratched.

* Since it is a natural mineral, the transmittance property differs by individual crystal.
Wavelength [nm] Refractive Index
Ordinary Beam Extraordinary Beam
404.7 1.6813 1.4969
546.1 1.6616 1.4879
589.3 1.6584 1.4864
656.3 1.6544 1.4846
Transmitted Wavelength Range [µm] 0.35 - 2.3*
Density [g/cm3] 2.71
Thermal Conductivity [Wm-1K-1] (0°C) 5.39/4.51
Coefficient of Thermal Expansion
[ ×10-6/°C〕(0 - 80°C)
26.3/5.44
Excerpt from Chronological Scientific Tables

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