The Optical Transmission and Properties of Sapphire Windows
Sapphire Windows, often called Sapphire Glass Windows or Sapphire Optical Windows, made of single crystal synthetic sapphire (Al2O3), are becoming more and more indispensable in the exclusive fields of electro-optics due to their combination of superior optical, chemical, and mechanical properties. Sapphire windows, renowned for their unparalleled hardness, robustness against external aggression, and the broad transmission wavelength range of sapphire from 0.17μm in the UV region to 5.5μm in the mid-wave IR region find versatile usages in a wide range of applications under harsh environments. Typical applications of sapphire optical windows include laser processing protective windows, touch screen windows, detector windows, IR optical fronts, etc. This article that Shalom EO presents concentrates on the optical transmission and the optical properties of sapphire windows, including the transmission wavelength range of sapphire, the transmission rate, the birefringence and orientations. We have also analyzed the different growth methods and grades of sapphire crystals.
You might also learn more about sapphire windows in our relevant blogs:
- Sapphire (Al2O3)
- Exploration of Current Sapphire Window Production Techniques
- Advantages and Applications of Sapphire Windows
Introduction to Sapphire Windows
Optical Windows are windows that function to transmit light of the wanted wavelength with minimum absorption or distortion, while also provide protection for sensitive optics from harmful environments. When it comes to manufacturing optical windows, there are miscellaneous choices of materials, including optical glass materials (like the most common N-BK7, UV fused silica, or flint glass for making lenses) and crystal materials (such as Ge, caf2 with excellent ir transmission for manufacturing IR windows). Each optical material is selected based on the considerations and careful engineering of its optical transmission, mechanical strength, scratch resistance, thermal properties, etc.
Sapphire Single Crystals, the chemical composition of Aluminum Oxide (Al2O3), has long been recognized as one of the most durable and versatile materials available for making optical windows as well as protective windows. With the distinguished optical properties of wide optical transmission wavelengths spanning UV, VIS to MWIR, and low absorption, sapphire windows can transmit light signals with high efficiencies. The other remarkable properties of sapphire windows encompassing an unrivaled hardness rated 9 on the Mohs scale, high-temperature endurance, and resistance to chemical corrosion and abrasions make sapphire windows a popular choice for a wide range of optical applications, especially where high strength, optical clarity, and resistance to harsh conditions are paramount. For example, sapphire windows are ideal as a touch screen of smartphones, because the hard sapphire window can protect the interiors of the phone and are not prone to scratches; sapphire windows are also excellent when integrated into detectors, where their hard character shield the cores of detectors while its wide transmission from UV to IR ensuring efficient light transfer.
The Optical Properties of Sapphire
Sapphire’s optical properties are one of the crucial reasons it is favored for use as an optical window. There are multiple factors that will affect the optical properties of sapphire. In this section, we explore some of the critical optical characteristics of sapphire, including its transmission wavelength range, birefringence, absorption, reflection, and refractive index, and the essential elements that will influence sapphire's optical properties.
Understanding Synthetic Sapphire
Crystals are solid materials composed of atoms or molecules arranged in a certain pattern, with specific structures and properties. Among them, the optical properties of crystals refer to the absorption, transmission, reflection, and refraction of light of crystals. The arrangement of atoms or ions within the crystal lattice determines how light interacts with the material. Crystals with different symmetries, such as cubic, tetragonal, or hexagonal, exhibit distinct optical behaviors. Therefore we must understand the structural and fundamental characteristics of sapphire before we can get a full understanding of the optical properties of sapphire.
The optical properties of sapphire crystals, as the same with other crystals are dependent on their lattice structure. The lattice is a spatial grid network of atoms or molecules arranged in a coherent manner inside the crystal. Different lattice structures have an important impact on the propagation and interference of light waves. For example, the refractive index of a crystal is an important parameter in optical properties. It is the ratio of the speed of light propagation in a substance to the speed of light propagation in a vacuum. The refractive index of a crystal is related to the periodicities of its lattice. Generally speaking, the stronger the periodicities of the lattice, the higher the refractive index of the crystal.
At the same time, the lattice structure of a crystal is also related to its properties such as absorption, transmission, and reflection of light. Crystal absorbing light is the process of electrons absorbing energy and transitioning from a low-energy state to a high-energy state. In a crystal, the transition of electrons is restricted by the lattice, and only photons whose energy meets certain conditions can be absorbed by the crystal. Therefore, the absorption spectrum of a crystal is determined by the lattice structure. Similarly, the transmission and reflection properties of a crystal are also affected by the lattice structure. Reflection occurs after the crystal is polished, while the transmission properties of a crystal are related to the scattering and absorption of light by its lattice.
In addition to the lattice structure, the band structure also has an important influence on the optical properties of the crystal. The energy band is a model that describes the energy electronic state in a solid. The electrons in a crystal are distributed in a series of energy bands, of which the valence band and the conduction band are the two most important energy bands. The valence band refers to the energy level where the electron is in the default state, while the conduction band refers to the energy state where the electron can move freely. When light is irradiated into a crystal, its energies can excite the electrons in the crystal to transition from the valence band to the conduction band, a process called photoexcitation.
The band structure of a crystal directly affects its optical properties. For example, the electrons present in the conduction band can absorb the energy of light and be excited, thereby causing the absorption of light. If there are no available electrons in the conduction band, the light cannot be absorbed by the crystal.
As opposed to jewels made of natural sapphires which is a rare and expensive mineral, Optical Sapphire Windows are made of Synthetic Single Ccrystal Sapphire, which offers a more cost-effective and available alternative. The production of synthetic sapphire has become a specialized process, allowing manufacturers to create large, defect-free crystals suitable for various applications, including sapphire windows. Synthetic sapphire is a duplication of natural sapphire but is purer and water-clear. Without the inclusions and contaminants purposefully eliminated to avoid degradation of performance, synthetic sapphires appear colorless and transparent in contrast with the colorful natural sapphires with their colors attributed to the impurities.
Chemical Composition and Crystal Structure of Sapphire
Synthetic Sapphire is composed of aluminum oxide (Al2O3), there are nine types of sapphire, the technical or optical grade sapphire we use is alpha-sapphire, often called alumina or α-alumina. Alpha-Sapphire has a hexagonal structure. The lattice constant is a=b=4.758A, c=12.991A.
Sapphire’s crystal structure also allows it to be grown in a controlled manner, ensuring the production of high-quality material with consistent properties. The crystal’s orientations can have significant impacts on the optical and mechanical properties of the resulting sapphire window.
Growth Methods of Sapphire
Structural defects, dislocations, and impurities within the sapphire crystal can alter the propagation of light. These defects may lead to scattering, absorption, or the introduction of additional absorption bands. Therefore, we will go through a discussion about the growth methods and quality grades of sapphire crystal.
There are various sapphire growth methods, including:
1. Verneuil: The Verneuil process (also known as the flame fusion method) is one of the oldest and most simple methods for growing synthetic sapphire crystals. The process involves feeding aluminum oxide powder (Al2O3) is into a high-temperature flame. The Verneuil is cheap for applications where the sapphire crystal quality and optical transmission of the sapphire windows are not paramount, such as protective sapphire windows, or mechanical components.
2. EFG (Stepanov): The EFG method (also known as the Stepanov method) is a crystal growth technique that allows for the controlled growth of large sapphire crystals with a defined geometries. The EFG method delivers better quality than the Verneuil but is more expensive than the latter.
3. Czochralski: The Czochralski crystal growth method, or the CZ-method) is a method of dipping a seed sapphire crystal into a melt of aluminum oxide (Al₂O₃) in a crucible and pulling out the seed crystal. The Czochralski method excels at growing sapphire of outstanding optical properties and high purities.
4. Kyropoulos: The Kyropoulos method or the KY technique is a modification of the Czochralski method, compared with the CZ sapphire growth method, the crystals are formed inside the crucible and the KY method is charaterized by a temperature control process to enable the gradual cooling of the single crystal, resulting in less thermal stress and mechanical shock. The procedure is explained below:
First, Aluminum oxide (Al2O3) or other appropriate raw materials are heated to their melting point, forming a molten metal bath in the crucible. The seed crystal is pulled upward (at a controlled rate). The molten material solidifies at the solid-liquid interface as the seed crystal is pulled upwards. The solidification rate is controlled through cooling, which allows the sapphire crystal to form from the top down. The result KY growth method a pear-shaped single sapphire crystal with a controlled solidification process, minimizing defects and yielding high-quality material suitable for advanced optical and industrial applications.

5. HEM: The HEM (Heat Exchanger Method) process is optimized to produce large, defect-free crystals with low internal stress. However, the method requires more sophisticated and expensive equipment and is very power-consuming.
6. SAMPAC: The SAMPAC (sapphire growth technique with micro-pulling and shoulder expanding at the cooled center) method is a novel sapphire crystal growth method invented for growing large size sapphire crystals.
At present, only a limited number of methods such as HEM, Temperature Gradient Technique (TGT), and Kyropoulos are capable of growing optical-grade large-size sapphire crystals. However, The sapphire crystals grown using the heat exchange method can indeed reach large sizes and are good in quality but require a large amount of helium as a coolant, which is expensive.
The quality of sapphire crystals grown using the TGT method is comparable to that of products produced by the heat exchange method, but the crystal blanks need to be annealed in high-temperature oxidation and reduction atmospheres, and the subsequent processing of the blanks is complicated.
At Shalom EO, we harness the Kyropoulos-grown (KY-grown) synthetic sapphire (click here to learn more about Kyropoulos-grown sapphire) and SAPMAC-grown sapphire for manufacturing sapphire windows. These cutting-edge sapphire growth techniques help us to deliver sapphire windows of large sizes, optimized optical properties, and excellent mechanical properties suitable for a wide range of applications.
Grades
The quality of sapphire crystals can only be justified by comprehensively assessing the combination of sapphire’s optical and physical properties.
While the professionals failed to have a consensus agreement on the quality grading of sapphires, In broad terms, sapphires can be categorized into three main grades based on their intended application:
- High-Grade Sapphire: This grade of sapphire is grown with less lattice dislocation and imperfections, contributing to minimal or no light scatter or absorption loss. The sapphires are used for applications where optical functionalities are imperative, such as optical windows, lenses, and optical components, etc.
- Ultraviolet (UV) Grade Sapphire: The UV grade sapphire is a type of sapphire that goes through special treatment to reduce the oxygen vacancies or F-centers in sapphire crystals. UV grade sapphires are designed with the absence of UV absorption peak and high resistance to UV darkening, to make them capable of handling applications in the ultraviolet wavelength range.
- Lower-grade Sapphire: The lower-grade sapphire exhibits higher degrees of defects, such as light scatter or lattice distortions. This grade is suitable for mechanical parts and components, where optical distortion is of insignificant concern.
Below is an additional chart that categorizes sapphire into optical and technical quality grades, where grades 1-4 are regarded as qualities competent for optical applications, and grades 5-6 are regarded as qualities competent for mechanical and other technical applications:
Grade | Insertions, block boundaries, and twins | Micro-bubbles, and Scattering Centers | Applications |
Grade 1 Sapphire | No | No | Critical optical applications demand maximized optical clarity. |
Grade 2 Sapphire | No | Micro-bubbles <10 µm, spaced at least 10 mm apart | Demanding optical applications, where minor imperfection is tolerable. |
Grade 3 Sapphire | No | Individual bubbles <20 µm, located at least 10 mm apart | Average optical uses. |
Grade 4 Sapphire | No | Individual bubbles <20 µm, located at least 10 mm apart. Bubble clusters, which might include individual bubbles ~50 µm, are allowed within a total cluster size of <200 µm, not closer than 10 mm to one another. | Can still be applied as optical grade material but for scenarios where optical properties can be compromised. |
Grade 5 Sapphire | No | Bubbles <20 µm in size spaced at least 2 mm apart. Larger bubble clusters (~500 µm in size) spaced at least 5 mm apart. | Mechanical or industrial applications that do not demand optical excellence. |
Grade 6 Sapphire | No | Defective areas with bubble clusters larger than 500 µm | General industrial applications with less stringent standards. |
Optical Transmission Wavelength Range and Absorption of Sapphire
Sapphire windows are transparent across a broad wavelength range, from 170nm in the ultraviolet (UV) spectrum to 5.5µm in the infrared (IR) spectrum. This wide transmission range makes sapphire an ideal material for multispectral optical applications. For the UV spectrum, sapphire windows are immune to UV darkening, which is referred to as a phenomenon where a material absorbs UV radiation and undergoes a change in its optical properties. Windows made of materials susceptible to UV darkening will undergo yellowing or discoloration. UV grade sapphire windows are more resilient to UV exposure, making them ideal for UV windows and viewports in laboratories, and space telescopes.Sapphire is known for its excellent transmission properties, especially in the UV, visible, and IR ranges. This is because the strong covalent bonds in its lattice result in minimal scattering and absorption of light.
Furthermore, because of the inherently high structural strength of sapphire, much thinner thicknesses can be obtained with sapphire windows than in the case of other windows, enabling further enhanced optical transmission.
The absorption loss is another important issue for optical windows, where the absorption of power manifests itself as heat. Sapphire windows handle the heating up problem well with excellent thermal properties. In the visible and near-IR ranges, sapphire exhibits very low absorption, making it an excellent material for optical windows, lenses, and other optical components. The lattice structure also influences the absorption spectrum, particularly at specific wavelengths where electron transitions can occur, such as in the ultraviolet (UV) range.
Transmission of Sapphire Windows without Coating from UV to 1100nm
Transmission of Sapphire Windows without Coating from 2.5μm to 8.0μm
Sapphire (Al₂O₃) is an aluminum oxide crystal with a hexagonal lattice structure (a variant of the corundum crystal system). The periodicity and symmetry of this lattice significantly influence the propagation of light through the crystal.
Birefringence and Orientations
Birefringence (also known as double refraction) is a characteristic of materials where light passing through the material experiences different refractive indices depending on the polarization direction. This causes the light to split into two rays (ordinary ray and extraordinary ray), each traveling at different speeds and along different paths.
The birefringence of sapphire can be controlled by selecting the appropriate cutting orientation, which allows manufacturers to optimize the material for specific optical applications.
Sapphire has a relatively high refractive index, typically around 1.76 in the visible spectrum. This is due to the regular and strong periodicities of its crystal lattice, which ensures a consistent bending of light waves passing through it. The lattice's high symmetry leads to a significant difference in the refractive index in different directions, making sapphire an anisotropic material (its refractive index varies depending on the crystal orientation).
Sapphire is a uniaxial birefringent crystal (although only with a mild birefringence), its optical axis is the c-axis <0001>. For applications like optical windows, birefringence is undesirable, because this effect leads to optical distortion and image degradation in optical systems requiring clear, undistorted transmission. The birefringence and refractive indices of sapphire crystals are shown in the figure below:
Figure 4. Refractive Indices of Sapphire
Crystal orientation refers to the arrangement of the atoms, ions, or molecules within a crystal lattice and how these microstructures align with the external surface of the crystal. When manufacturing sapphire windows, the crystal’s orientation is critical because the orientation affects the birefringence, thermal expansion, and other important behaviors and properties. The orientations of the sapphire windows are set during the cutting/slicing stage.
C-cut sapphire windows, alternatively called c-plane/z-cut sapphire windows/0° plane cut sapphire windows are the most popular cutting orientation chosen to deliver moderate birefringence while optimizing transmission. C-cut refers to a sapphire crystal that is cut along the C-axis of the crystal, which is the [0001] crystallographic axis. This direction is perpendicular to the base plane of the sapphire crystal. One of the critical advantages of C-cut sapphire windows is that it minimizes birefringence along the C-axis. This is because the C-axis is the direction of the lowest anisotropic nature in the crystal structure, which leads to very low or negligible birefringence, this makes C-cut sapphire windows preferred for certain applications where authentic transmission of optical signals the prior requirement (e.g. laser windows).
Other common orientations or planes of sapphire windows include A-plane, M-plane, R-plane, and Random-cut, each orientation exhibiting different optical, mechanical, or thermal properties. The details of the properties of sapphire’s orientations are as below:
A-plane (11-20) sapphire: The A-plane (11-20) sapphire windows exhibit outstanding mechanical hardness and scratch resistance. A-cut sapphire substrates are useful for thin film growth of III-V nitrides, superconductors, and magnetic films with small lattice mismatches and reliable chemical/physical performance. A-plane sapphires have high thermal conduction, which makes them an excellent choice for solar cells and other high-tech applications.
M-plane (10-10) sapphire: The M-plane (10-10) sapphire windows and or substrates are typically used for epitaxial thin film growth of MgxZn1-xO thin films.
R-plane (1-102) sapphire: R-plane sapphire substrates are preferred for the hetero-epitaxial deposition of silicon used in microelectronic IC applications.
Random-Cut sapphire windows: As opposed to specified cutting orientation, random-cut sapphire windows mean sapphire windows without a deliberately chosen cutting orientation. Random-cut sapphire windows are often used where slight birefringence is less of a concern.
The UV Absorption Peak of Sapphire
During the growth process of sapphire, especially in high-purity and high-temperature conditions (such as over 2000°C), the material is often grown in an environment with a reducing atmosphere. This can lead to the loss of oxygen atoms, creating oxygen vacancies or F-centers. F-centers (also known as color centers) are defects where oxygen vacancies are created in the sapphire crystal, and the vacancies are filled by electrons. These defects can absorb light and affect the material's optical transmission.
The introduction of oxygen defects or F-centers can cause specific absorption bands, most notably around 200 nm, which corresponds to the deep UV absorption peak of sapphire. This absorption occurs because the F-centers can absorb photons with energies corresponding to the band gap and defective states in the material.
To address this issue, Hangzhou Shalom EO offers UV-grade sapphire windows, the sapphire is specially fabricated with eliminated oxygen defects to exhibit no absorption peak at the critical 200 nm UV wavelength.
The optical transmission curves of our UV-grade sapphire windows are as shown below:
a. Optical transmission curves of our UV-grade sapphire windows (uncoated)
b. Optical transmission curves of UV-grade sapphire windows D57.15x4mm (uncoated)
c. Optical transmission curves of UV-grade sapphire windows D65.15x4mm (uncoated)
To minimize the introduction of these defects, the growth process is controlled with caution. Using a high-oxygen atmosphere and maintaining a low-temperature gradient during the crystal growth can help reduce the formation of oxygen vacancies and improve the overall quality of the sapphire.
After sapphire is grown, it might undergo various annealing treatments in an oxygen-rich atmosphere to repair defects and restore the material's transparencies, particularly in the UV range. This process can help reduce the effects of F-centers and improve the optical properties.
This makes our UV-grade sapphire windows ideal for applications that require the transmission of UV light without significant absorption, such as UV spectroscopes, laser systems, and scientific research.
AR Coated Sapphire Windows
The optical transmission of sapphire windows can be further enhanced by depositing anti-reflection thin film coatings (AR) coatings on sapphire substrates. Sapphire itself is known for its exceptional hardness, durabilities, transmission in UV, visible, and infrared ranges, and its resistance to scratching and corrosion. When combined with an AR coating, it can be used in various high-performance optical systems, such as cameras, sensors, and other imaging devices, as well as in harsh environments where other materials might not withstand the conditions.
Other Factors That Affect the Optical Properties of Sapphire Windows
Temperature: The optical properties of the sapphire window, such as refractive index and absorption coefficient, can change with temperature due to the thermal expansion of the lattice and changes in electronic properties.
Strain and Stress: Mechanical strain within the crystal lattice can alter its optical properties by changing the refractive index, especially in anisotropic materials. This is important in applications where precise control of optical characteristics is required.
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