Optical materials


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Optical materials

All substances used in the construction of devices or instruments whose function is to alter or control electromagnetic radiation in the ultraviolet, visible, or infrared spectral regions. Optical materials are fabricated into optical elements such as lenses, mirrors, windows, prisms, polarizers, detectors, and modulators. These materials serve to refract, reflect, transmit, disperse, polarize, detect, and transform light. The term “light” refers here not only to visible light but also to radiation in the adjoining ultraviolet and infrared spectral regions. At the microscopic level, atoms and their electronic configurations in the material interact with the electromagnetic radiation (photons) to determine the material's macroscopic optical properties such as transmission and refraction. These optical properties are functions of the wavelength of the incident light, the temperature of the material, the applied pressure on the material, and in certain instances the external electric and magnetic fields applied to the material. See Atomic structure and spectra, Dispersion (radiation), Electromagnetic radiation, Electrooptics, Infrared radiation, Lens (optics), Light, Magnetooptics, Mirror optics, Optical modulators, Optical prism, Polarized light, Reflection of electromagnetic radiation, Refraction of waves, Ultraviolet radiation

There is a wide range of substances that are useful as optical materials. Most optical elements are fabricated from glass, crystalline materials, polymers, or plastic materials. In the choice of a material, the most important properties are often the degree of transparency and the refractive index, along with each property's spectral dependency. The uniformity of the material, the strength and hardness, temperature limits, hygroscopicity, chemical resistivity, and availability of suitable coatings may also need to be considered.

Glass technology provided the foundation for classical optical elements, such as lenses, prisms, and filters. Glasses developed for use in the visible region have internal transmittances of over 99% throughout the wavelength range of 380–780 nanometers. However, the silicate structure in glasses limits their transmission to about 2.5 micrometers in the infrared. Chalcogenide glasses, heavy-metal fluoride glasses, and heavy-metal oxide glasses extend this transmission to 8–12 μm. See Color filter

Advances in the process for manufacturing optical fibers led to the present fiber-optic communication systems that operate in the near-infrared region with windows at wavelengths of 850, 1310, 1550, and 1625 nm. An advanced fiber-optic system, LEAF (Large Effective Area Fiber), was designed to minimize nonlinearities by spreading the optical power over large areas. See Optical fibers

The use of photolithography for printing integrated circuits has necessitated the improvement in the transmission of glasses for the ultraviolet region. Fused silica, which transmits to about 180 nm, is well suited for the lithography in the ultraviolet region. However, the crystalline material calcium fluoride, which transmits into the ultraviolet region to about 140 nm, outperforms any glass in printing microchips using fluorine excimer lasers. Deep-ultraviolet applications of fused-silica glasses include high-energy lasers, spacecraft windows, blanks for large astronomical mirrors, optical imaging, and cancer detection using ultraviolet-laser-induced autofluorescence. See Fluorescence, Telescope

The need for an inexpensive, unbreakable lens that could be easily mass-produced precipitated the introduction of plastic optics in the mid-1930s. Although the variety of plastics suitable for precision optics is limited compared to glass or crystalline materials, plastics are often preferred when difficult or unusual shapes, lightweight elements, or economical mass-production techniques are required.

The softness, inhomogeneity, and susceptibility to abrasion intrinsic to plastics often restrict their application. Haze (which is the light scattering due to microscopic defects) and birefringence (resulting from stresses) are inherent to plastics. Plastics also exhibit large variations in the refractive index with changes in temperature. Shrinkage resulting during the processing must be considered. See Birefringence

Organic synthetic polymers are emerging as key materials for information technologies. Polymers often have an advantage over inorganic materials because they can be designed and synthesized into compositions and architectures not possible with crystals, glasses, or plastics. They are manufactured to be durable, optically efficient, reliable, and inexpensive. Many uses of polymers in photonic and optoelectronic devices have emerged, including light-emitting diodes, liquid-crystal–polymer photodetectors, polymer-dispersed liquid-crystal devices (for projection television), optical-fiber amplifiers doped with organic dyes (rhodamine), organic thin-film optics, and electrooptic modulators. See Light-emitting diode

Although most of the early improvements in optical devices were due to advancements in the production of glasses, the crystalline state has taken on increasing importance. Historically, the naturally occurring crystals such as rock salt, quartz, and fluorite plus suitable detectors permitted the first extension of visible optical techniques to harness the invisible ultraviolet and infrared rays. Synthetic crystal-growing techniques have made available single crystals such as lithium fluoride (of special value in the ultraviolet region, since it transmits at wavelengths down to about 120 nm), calcium fluoride, and potassium bromide (useful as a prism at wavelengths up to about 25 μm in the infrared). Many alkali-halide crystals are important because they transmit into the far-infrared. See Crystal growth, Crystal structure, Single crystal

Following the invention of the transistor, germanium and silicon ushered in the use of semiconductors as infrared optical elements or detectors. Polycrystalline forms of these semiconductors could be fabricated into windows, prisms, lenses, and domes by casting, grinding, and polishing. Compound semiconductors such as gallium arsenide (GaAs), ternary compounds such as gallium aluminum arsenide (Ga1-xAlxAs), and quaternary compounds such as indium gallium arsenide phosphide (InGaAsP) now serve as lasers, light-emitting diodes, and photodetectors. See Semiconductor

Single crystals are indispensable for transforming, amplifying, and modulating light. Birefringent crystals serve as retarders, or wave plates, which are used to convert the polarization state of the light. In many cases, it is desirable that the crystals not only be birefringent, but also behave nonlinearly when exposed to very large fields such as those generated by intense laser beams. A few examples of such nonlinear crystals are ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate (KDP), beta barium borate (BBO), lithium borate (LBO), and potassium titanyl phosphate (KTP). See Crystal optics, Nonlinear optics

Other optical materials are the liquid crystals used in displays as light valves, materials used in erasable optical disks for computers and in liquid cells (Kerr cells), laser dyes, dielectric multilayer films, filter materials, and the many metals (aluminum, gold, beryllium, and so forth) and alloys that are important as coating materials. See Kerr effect

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