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Crystal Shines in Optics
Optical crystals were the primary optical materials utilized by humans long before optical glass. Before the advent of optical glass, natural optical crystals were employed for crafting lenses and plane mirrors. Unlike ordinary optical glass, which is confined to visible light and near-ultraviolet regions, optical crystals exhibit excellent transmittance across ultraviolet, visible, near-infrared, and even infrared spectrums. This makes optical crystals the preferred material for designing optical components in the infrared and ultraviolet domains.
Moreover, optical crystals possess anisotropic and nonlinear characteristics absent in optical glasses. As optoelectronic technology advances, the distinctive attributes and significance of optical crystals in this field have become increasingly apparent. Given that natural optical crystals fall short of meeting modern demands, contemporary optical crystals are predominantly synthesized artificially.
What is Crystal
The molecules, atoms or ions in the crystal are arranged in an orderly and highly regular manner to form a lattice structure, which is called a lattice structure. This gives the crystal uniformity, anisotropy and symmetry.
Optical crystals are a special type of crystal with excellent optical properties. They are classified by use and process performance and include UV, IR, laser, polarizing and apochromatic crystals. These crystals find a wide range of applications in different optical systems, meeting a variety of needs. Hard crystals, such as quartz and ruby, are typically used in high-wear environments, while soft crystals, such as fluorite, may be better suited for other uses. Water-soluble crystals require special attention in moisture-sensitive environments.
Optical crystals play a key role in many fields such as optical sensing and laser technology due to their unique structures and properties. Their diversity makes them indispensable materials in optical systems.
Basic Properties of Optical Crystals
Due to its inherent unique properties, crystals have a wide range of applications in the field of optics and are as important as optical glass. Its key features include:
1) Uniformity
The properties and arrangement of different parts inside the crystal are the same, and they have uniform physical and chemical properties. This uniformity is the basic condition for crystals to be used as optical materials.
2) Anisotropy
The light beam propagates in the crystal in different directions, and its optical properties also change accordingly. This anisotropic characteristic is manifested in the form of birefringence, and the degree of birefringence changes regularly with the direction. This characteristic Just what is needed to achieve precise control of light using optical crystals.
3) Symmetry
Crystals have the same properties in different directions or positions, showing a certain symmetry. When manufacturing optical parts, the symmetry of the crystal needs to be maintained to ensure consistent performance.
4) Self-regulation
Crystals have the characteristic of spontaneously forming a closed geometric polyhedral shape, which is caused by the orderly accumulation of its internal regular lattice structure in three-dimensional space. This self-exemplification affects the shape and structure of the crystal.
5) Minimum internal energy
From a thermodynamic point of view, among objects of the same composition, the internal energy of the crystalline state is the smallest. The so-called internal energy refers to the sum of the kinetic energy of the irregular motion of the particles inside the object and the potential energy determined by the mutual positions of the particles. The internal energy of the crystal is minimal, indicating that the crystal is in a relatively stable state.
6) Stability
Because the internal energy is minimal, crystals have higher stability than other states. This allows the crystal’s performance to remain relatively stable over time, which is a major advantage as an optical material.
Physical Properties of Crystals
To harness the optical properties of crystals effectively in the optical field, one must carefully consider their physical properties. Here’s an analysis:
- Cleavage: Crystal cleavage, the ability to split under directional force, varies among different crystals or facets. Although cleavage poses challenges in processing, it’s a feature processors leverage for precision cutting.
- Hardness: Crystals exhibit anisotropic and symmetrical hardness, influencing the selection of processing parameters, abrasives, and polishing agents.
- Solubility: The crystal’s solubility, indicating its dissolving capacity in water at a specific temperature, affects processing and usage conditions. Solubility increases with temperature, distinguishing processing techniques for deliquescent and non-deliquescent crystals.
- High Melting Point and Thermal Stability: Crucial for applications in high-temperature environments, crystals boast a high melting point and excellent thermal stability. This quality ensures reliability in scenarios demanding exposure to elevated temperatures.
Optical Properties of Crystals
These characteristics give the crystal unique optical properties, making it play an irreplaceable role in laser technology, communications, optical imaging and other fields.
1) Wide light transmission band
Generally, optical crystals have a much wider transmittance band range than optical glass, especially in the long-wavelength band, which can reach 60um. Although optical crystals are also divided into ultraviolet optical crystals, visible optical crystals and infrared optical crystals according to different working bands, the optical crystals of each wavelength band still have a wide band range. Generally, light element compound optical crystals have a wider wavelength band in the ultraviolet region, while heavy metal element compound optical crystals have a wider wavelength band in the infrared region.
2) Low absorption rate
Optical crystals not only have a much wider light transmission band than optical glass, but also absorb much less light. The absorption of light by optical crystals is also anisotropic. Depending on the vibration direction of the incident light wave, the degree of absorption is also different. Crystals have strong absorption or transmission characteristics for light of specific wavelengths, making them unique applications in optical filters and lasers.
3) Small dispersion
Another difference with glass is that the refractive index of optical crystals changes less with wavelength, that is to say, the dispersion of optical crystals is smaller. Optical systems made of silicon (Si) or germanium (Ge), like those used in infrared optics, require little correction for chromatic aberration.
4) Birefringence
Except for the equiaxed crystals of the advanced crystal family, which are isotropic and do not produce birefringence, the other six major crystal systems are all anisotropic. When a light beam propagates in any direction other than the optical axis, birefringence occurs and changes regularly. The crystal’s birefringence properties for light in specific directions make it an ideal choice for manufacturing polarization devices and modulation devices, which are widely used in laser technology and communication systems.
5) Optical activity
In some optical crystals, when a plane polarized wave propagates along the optical axis, the polarization plane rotates at a certain angle, which is called optical rotation. There are left-handed and right-handed crystals, and the rotation angle varies with the wavelength. The shorter the wavelength, the larger the rotation angle.
6) Pleochroism
Since optical crystals absorb light of different frequencies anisotropically, except for equiaxed crystals, different colors will appear in different directions of the same crystal, which is called pleochroism of the crystal. Pleochroism is closely related to absorbency. If the absorbency is significant, the pleochroism must also be significant.
Types of Optical Crystals
Optical crystals are usually divided into two categories: single crystal and polycrystalline. Single crystal materials have high crystal integrity, transmittance, and low insertion loss, so optical crystals are mostly single crystals.
According to the chemical composition, optical single crystals can be divided into halide single crystals, oxide single crystals, inorganic salt compound single crystals, sulfide single crystals and semiconductor single crystals. Commonly used optical crystals include oxygen optical crystals, halide optical single crystals, semiconductor optical single crystals and hot-pressed optical polycrystals.
Optical crystals are divided into the following types according to their uses:
- Ultraviolet and infrared crystals: used in windows of spacecraft, artificial satellites, missiles, etc., including quartz, silicon, germanium, etc.
- Apochromatic crystal: used for advanced apochromatic objectives
- Polarizing crystal: making polarizing devices
- Laser crystal: used to manufacture lasers
- Electro-optical crystals, acousto-optical crystals and nonlinear crystals, etc.
Five Common Optical Crystals
There are many types of optical crystals, but the following five crystals are most commonly used in optical applications:
Calcium fluoride (CaF2)
Fluorite, natural or artificial, is known for its high transparency and fluorescent properties and is used in the manufacture of high-performance optical components. Its excellent transmittance and low dispersion in the ultraviolet to infrared spectrum make it an ideal choice for lasers and optical components, especially when designing achromatic lenses. It is more commonly used than lithium fluoride or magnesium fluoride due to its relative chemical stability and low deliquescence.
Sapphire (AI2O3)
Sapphire is a crystal whose hardness is second only to diamond. Its chemical properties are very stable and it has good insulation properties. It can often replace optical glass in some optical applications. Synthetic sapphire is transparent and colorless, with a wide transmission range from ultraviolet to infrared. Its single-crystal structure induces birefringence, but its widespread use in lasers and LEDs demonstrates its unique value in high-performance optics. It is used as titanium sapphire (Ti: sapphire) as an excitable medium for ultrashort pulse lasers or as a substrate for growing violet LEDs.
Zinc Selenide (ZnSe)
Zinc selenide is an amber crystal capable of transmitting long-wavelength infrared light up to 20μm. Especially suitable for CO2 laser lenses. Despite moisture sensitivity and toxicity, its superior transparency and chemical stability make it a critical material in infrared optics. Due to its high refractive index, transmission losses due to surface reflectivity are high, but by adding custom anti-reflective coatings, transmission rates of 99% or higher can be achieved.
Silicon (Si)
Single crystal silicon is excellent in high thermal conductivity and infrared transparency. It can transmit infrared light with a wavelength of 2 to 6 μm and is used for semiconductor and infrared detection. Its versatility makes it ideal for use in optical components, especially in a wide range of applications in high-tech industries.
Germanium (Ge)
This material has a metallic luster and, while opaque, transmits a wide range of infrared rays from 2 to 20 μm. It is used as a material for thermal imaging camera lenses. Due to its high refractive index of 4, it becomes a key material in infrared optics through the use of anti-reflective coatings, although its high refractive index results in transmission losses.
Optical Crystal Processing
Optical crystal processing involves precision cutting, polishing and coating of crystal materials to meet the high requirements for precision and surface quality of optical components. The following are some key technical points of optical crystal processing:
1) Material selection
Select a crystal material suitable for optical applications, considering refractive index, dispersion properties, etc. Carefully inspect and select the quality of crystals, because most crystals are artificial or natural, but they all have defects and defects of varying degrees, such as dislocations, impurities, nodules, clouds, stones, interlayers, double Jing et al.
2) Find the optical axis
Some crystals, such as those belonging to the equiaxed crystal system, are optically uniform; while other crystals produce birefringence. There is only one direction in the crystal in which birefringence does not occur. This direction is called the optical axis direction. This type of crystal is called a uniaxial crystal. Trigonal crystal system, tetragonal crystal system and hexagonal crystal system are all uniaxial crystals. When processing uniaxial crystals, the optical axis must be aligned and a surface perpendicular to the optical axis must be ground.
3) Select processing method
- Cutting process: Use high-precision CNC grinding technology to ensure high precision in the shape and size of the processed crystal components. This may involve different cutting processes such as drilling, milling, turning, etc.
- Surface grinding and polishing: The cut crystals are surface ground and polished to obtain extremely high surface quality. This typically involves using grinding and polishing tools of varying grits, resulting in a flat, clear, scratch-free surface.
- Waterjet and Sawing Technologies: Waterjet and sawing are common optical crystal cutting technologies. Water jets allow precise cutting of various shapes, while sawing is one of the traditional mechanical cutting methods.
- Selection of cutting abrasives: Crystals with different hardnesses require the selection of abrasives with different hardnesses. For crystals that are harder than glass, you can choose quartz powder, agate powder, silicon carbide, white sapphire, natural garnet and diamond powder; for soft crystals, you can use abrasives such as iron oxide, chromium oxide, titanium oxide, tin oxide and magnesium oxide. .
- Selection of polishing fluid: Commonly used polishing fluids include water, absolute alcohol, kerosene, salt and saturated solution.
- Selection of grinding molds and polishing molds: For hard crystals, iron, copper and aluminum molds can be used, hard polishing molds are used for polishing, and polyurethane, phenolic tape and hardwood polishing molds are used for mechanical polishing. For soft crystals, use copper and glass molds, and when polishing, use wax, asphalt beeswax, and asphalt rosin polishing molds.
4) Control temperature and humidity
Consider the thermal conductivity properties of crystals and control temperature and humidity, especially for water-soluble crystals. Some crystals have different heat conduction speeds in different directions, and large temperature differences can easily cause the crystals to explode. Even if the heat transfer rate is the same in all directions, sudden cooling or heating may cause cracking.
5) Control vibration and external forces
Because the crystal is fragile, it needs to be handled carefully to prevent it from exploding due to external force and vibration.
6) Pay attention to labor protection
Since the crystals may contain substances harmful to the human body, labor protection is required, especially when toxic gases are produced. For example, thallium compounds, phosphides and arsenides are harmful to the human body. Germanium tetroxide gas will be produced during germanium polishing, which is highly toxic.
7) Detection and measurement
Use high-precision optical measurement equipment, such as interferometers, laser interferometers, etc., to detect and measure the optical properties of the processed crystal components to ensure that they meet the design specifications.
8) Quality control
Implement a comprehensive quality control system, including testing key parameters in each processing step, as well as comprehensive testing of the final product, to ensure the quality of the final product.
Conclusion
With a decade-long presence in the optical industry, Noni is a trusted supplier that excels in delivering high-precision optical lenses tailored to customers’ specific requirements and designs. Our vast experience in the field enables us to provide optimal solutions based on customers’ samples and drawings. Our expertise includes the processing of Sapphire optics, ZnSe optics, Germanium optics, and custom optics of other crystals.