Introduction to Common Optical Materials

The first step in all optical manufacturing processes is selecting appropriate optical materials. The optical parameters (refractive index, Abbe number, transmittance, reflectance), physical properties (hardness, deformation resistance, striae/bubble content, Poisson's ratio), and even temperature characteristics (thermal expansion coefficient, refractive index vs. temperature) of optical materials directly impact the performance of optical components and systems. This article provides a brief overview of common optical materials and their characteristic properties.

Optical materials are generally classified into three major categories:

optical glasses, optical crystals, and specialty optical materials.

01 Optical Glasses

Optical glass is an amorphous (vitreous) optical medium capable of transmitting light. It alters light propagation direction, phase, and intensity, commonly used in fabricating optical elements such as prisms, lenses, mirrors, windows, and filters for optical instruments or systems. Optical glass exhibits exceptional transparency, chemical stability, and high uniformity in physical structure and properties, with precisely defined optical constants. Retaining a non-crystalline structure from its molten state, ideal optical glass demonstrates isotropic properties (e.g., refractive index, thermal expansion coefficient, hardness, thermal conductivity, electrical conductivity, elastic modulus) in all directions under optimal conditions.

Leading manufacturers of optical glass include SCHOTT (Germany), Corning (USA), OHARA (Japan), and Chengdu Guangming (CDGM, China). For UV applications, common materials include UV-grade fused silica (UVFS). Notable quartz materials comprise domestic JGS1, JGS2, JGS3, Corning 7980, and OHARA’s high-purity quartz glass series (SK-1300, SK-1310, SK-1320L, SK-1321, etc.). Our optical components typically employ JGS1, which maintains high transmissivity starting at 180 nm. Corning 7980, favored for laser optics due to its homogeneity and low bubble/impurity content, offers enhanced laser damage threshold. In visible and near-infrared ranges, popular choices include SCHOTT N-BK7, float glass B270, and CDGM’s H-K9L. N-BK7 and H-K9L share similar properties and are interchangeable, providing high transmission across 350 nm–2.0 µm. Finely annealed H-K9L optical glass, a hard, scratch-resistant material with strong chemical durability, is widely used in high-quality optical elements. Its minimal bubble content and low impurities make it suitable for precision components like lenses, windows, and prisms.

02 Optical Crystals

Optical crystals refer to crystalline materials used as optical media. Due to their structural properties, they are widely employed in UV and infrared applications, such as windows, lenses, and prisms. Optical crystals are further divided into single-crystal and polycrystalline materials. Single-crystal materials, with higher crystalline integrity, optical transmittance, and lower insertion loss, dominate practical use.

· Common UV/IR crystal materials: Quartz (SiO₂), fluorite (CaF₂), lithium fluoride (LiF), rock salt (NaCl), silicon (Si), germanium (Ge), etc.

· Polarizing crystals: Calcite (CaCO₃), quartz (SiO₂), and sodium nitrate (nitre) are frequently used.

· Apochromatic crystals: Utilizing unique dispersion properties, crystals like fluorite (CaF₂) combined with glass can eliminate spherical aberration and secondary spectrum in apochromatic objectives.

· Laser crystals: Used as solid-state laser media, e.g., ruby, calcium fluoride, and neodymium-doped yttrium aluminum garnet (Nd:YAG).

Natural crystals are rare, while artificial growth is challenging, size-limited, and expensive. Crystals are typically selected when glass materials are insufficient, often for non-visible light applications in semiconductors, lasers, and other industries.

03 Specialty Optical Materials

Glass-ceramics (or microcrystalline glass) represent a special class of optical material between glass and crystalline structures. Unlike ordinary optical glass, they possess a crystalline phase, but finer than traditional ceramics. Characterized by low thermal expansion, high strength, hardness, low density, and exceptional stability, glass-ceramics are widely used in applications such as flat reference plates, standard meter scales, large reflective mirrors, and laser-guided gyroscopes.

The above categories of optical materials, collectively referred to as optical media, alter light propagation direction, phase, and intensity. Beyond these, other optical materials include optical fibers, thin films, liquid crystals, and luminescent materials. The advancement of optical technology relies on progress in optical material science. We anticipate further breakthroughs in China’s optical material technologies.