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Published on May 20th, 2009 | By: pwray@ceramics.org


Ceramics & Optical Materials

Published on May 20th, 2009 | By: pwray@ceramics.org

[Image above] Credit: Paul Hocksenar; Flickr CC BY-NC-SA 2.0


Materials with special light reflecting, transmitting or other optical properties include a wide range of glass compositions, glass ceramics, and selected ceramics. These materials are found in literally hundreds of products in almost every industry: aerospace, telecommunications, electronics, industrial, medical, military, and homeland security.


Glass substrates, ball lenses, aspheric lenses and other items are used for optical communication devices. Other products include telescope mirrors, lasers, photonic devices, imaging systems, cathode ray tubes, flat screens for computer monitors/TVs, infrared windows/domes, glass substrates for liquid crystal displays, and optical benches.


Conventional applications include binoculars and spotting scopes; 35-mm, APS and digital still cameras and video cameras; interchangeable camera lenses; and digital projectors. For more than 35 years, a special type of low-expansion glass ceramic has also been used for mirror substrates of earthbound and orbital telescopes, including the Mars Reconnaissance Observer HiRISE (High Resolution Imaging Science Experiment) telescope, and the Keck Telescopes in Hawaii.


Glass-ceramic mirrors help explore outer space


A special type of low-expansion glass ceramic manufactured by SCHOTT North America will be used for the Magdalena Ridge Observatory (MRO) interferometer’s primary, secondary and tertiary mirrors. This new multi-telescope interferometer is composed of ten 1.4 meter telescopes and synthesizes the ten telescopes light-gathering into a single image, thereby simulating a single telescope of 400 meters. Scheduled for completion in 2008 in New Mexico, the MRO will be primarily used for deep space work, studying objects of interest beyond the Solar System such as galactic formation, supernovae and black holes.


SCHOTT has also recently patented a similar glass ceramic material with improved properties. Made using a proprietary thermal process, this glass ceramic possesses higher heat resistance than the conventional one, and near-zero thermal expansion. The thermal transformation process produces a glass ceramic material that contains over 90% keatite, a special type of silica (silicon dioxide) crystal. This keatite crystal structure enables a higher operating temperature (850°C) for long periods of time without alterations.


Created to answer specifications for NASA’s Constellation X telescope, this patented material has many other potential applications where higher heat resistance is required, including mechanical and optical components within high energy laser systems, mould materials in hot forming processes, ceramic engine components, and calibration standards for optical and mechanical probes.


The Constellation-X Observatory is a combination of several X-ray telescopes working in unison to generate the observing power of one giant telescope. With the Observatory, scientists will investigate black holes, Einstein’s Theory of General Relativity, galaxy formation, the evolution of the Universe on the largest scales, the recycling of matter and energy, and the nature of dark matter and dark energy.


Like all X-ray telescopes, Constellation-X must be positioned in space because X-ray light does not penetrate the Earth’s atmosphere. Yet, in designing Constellation-X, scientists wanted an X-ray telescope similar to the large earth-bound telescopes to collect as much X-ray light as possible. These requirements led to the unique multi-telescope design of Constellation-X. The four telescopes will combine to provide a sensitivity 100 times greater than any past or current X-ray satellite mission. Essentially, scientists will be able to collect more data in an hour than they would have collected in days or weeks with current X-ray telescopes.


Tiny ceramic rods produce ideal anti-reflection coating

A team of researchers from Rensselaer Polytechnic Institute has created the world’s first material that reflects virtually no light. This optical coating is made from a material that enables vastly improved control over the basic properties of light. The new material has almost the same refractive index as air, making it an ideal building block for anti-reflection coatings. It sets a world record by decreasing the reflectivity compared to conventional anti-reflection coatings by an order of magnitude. The material has a refractive index of 1.05, which is extremely close to the refractive index of air and the lowest ever reported.


Using a technique called oblique angle deposition, the researchers deposited silica (silicon dioxide or SiO2) nanorods at an angle of precisely 45 degrees on top of a thin film of aluminum nitride (AlN), which is a ceramic-type semiconducting material used in advanced light-emitting diodes (LEDs). The technique allows the researchers to strongly reduce or even eliminate reflection at all wavelengths and incoming angles of light. Conventional anti-reflection coatings, although widely used, work only at a single wavelength and when the light source is positioned directly perpendicular to the material.


The new optical coating could find use in just about any application where light travels into or out of a material, such as:


More efficient solar cells. The new coating could increase the amount of light reaching the active region of a solar cell by several percent, which could have a major impact on its performance.


Brighter LEDs. LEDs are increasingly being used in traffic signals, automotive lighting, and exit signs, because they draw far less electricity and last much longer than conventional fluorescent and incandescent bulbs. But current LEDs are not yet bright enough to replace the standard light bulb. Eliminating reflection could improve the luminance of LEDs, which could accelerate the replacement of conventional light sources by solid-state sources.


“Smart” lighting with vastly improved control of the basic properties of light, which could allow light sources to adjust to specific environments.


Optical interconnects. For many computing applications, it would be ideal to communicate using photons, as opposed to the electrons that are found in electrical circuits. The new materials could help achieve greater control over light, helping to sustain the burgeoning photonics revolution.


High-reflectance mirrors. The ability to precisely control a material’s refractive index could be used to make extremely high-reflectance mirrors, which are used in many optical components including telescopes, optoelectronic devices, and sensors.


Black body radiation. Researchers could use an ideal black body to shed light on quantum mechanics.




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