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
Comparison Chart of Refractive Index of Crystal Materials (Reference Data)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 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.
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.
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
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.
- 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.
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 |
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.
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 |