A team of researchers from Rensselaer Polytechnic Institute has created the world’s first material that reflects virtually no light. Reporting in the March issue of Nature Photonics, they describe an optical coating made from the material that enables vastly improved control over the basic properties of light. The research could open the door to much brighter LEDs, more efficient solar cells, and a new class of "smart" light sources that adjust to specific environments, among many other potential applications.
Most surfaces reflect some light — from a puddle of water all the way to a mirror. 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.
A fundamental property called the refractive index governs the amount of light a material reflects, as well as other optical properties such as diffraction, refraction, and the speed of light inside the material. "The refractive index is the most fundamental quantity in optics and photonics. It goes all the way back to Isaac Newton, who called it the ‘optical density,’" said E. Fred Schubert, the Wellfleet Senior Constellation Professor of the Future Chips Constellation at Rensselaer and senior author of the paper.
Schubert and his coworkers have created a material with a refractive index of 1.05, which is extremely close to the refractive index of air and the lowest ever reported. Window glass, for comparison, has a refractive index of about 1.45.
Incredible New Things in Optics and Photonics
Scientists have attempted for years to create materials that can eliminate unwanted reflections, which can degrade the performance of various optical components and devices. "We started thinking, there is no viable material available in the refractive index range 1.0-1.4," Schubert said. "If we had such a material, we could do incredible new things in optics and photonics."
So the team created one. Using a technique called oblique angle deposition, the researchers deposited silica nanorods at an angle of precisely 45 degrees on top of a thin film of aluminum nitride, which is a semiconducting material used in advanced light-emitting diodes (LEDs). From the side, the films look much like the cross section of a piece of lawn turf with the blades slightly flattened.
The technique allows the researchers to strongly reduce or even eliminate reflection at all wavelengths and incoming angles of light, Schubert said. 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.
A Broad Spectrum of Applications
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. "Conventional coatings are not appropriate for a broad spectral source like the sun," Schubert said. "The sun emits light in the ultraviolet, infrared, and visible spectral range. To use all the energy provided by the sun, we don’t want any energy reflected by the solar cell surface."
-- 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. Not only could improved LEDs provide significant energy savings, they also offer the potential for totally new functionalities. Schubert’s new technique allows for vastly improved control of the basic properties of light, which could allow "smart" light sources to adjust to specific environments. Smart light sources offer the potential to alter human circadian rhythms to match changing work schedules, or to allow an automobile to imperceptibly communicate with the car behind it, according to Schubert.
-- 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. This is the basis of the burgeoning field of photonics. The new materials could help achieve greater control over light, helping to sustain the burgeoning photonics revolution, Schubert said.
-- High-reflectance mirrors. The idea of anti-reflection coatings also could be turned on its head, according to Schubert. 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. The development could also advance fundamental science. A material that reflects no light is known as an ideal "black body." No such material has been available to scientists, until now. Researchers could use an ideal black body to shed light on quantum mechanics, the much-touted theory from physics that explains the inherent "weirdness" of the atomic realm.
Schubert and his coworkers have only made several samples of the new material to prove it can be done, but the oblique angle evaporation technique is already widely used in industry, and the design can be applied to any type of substrate — not just an expensive semiconductor such as aluminum nitride.
Schubert is featured in an interview about the research in the same issue of Nature Photonics.
Several other Rensselaer researchers also were involved with the project: Professors Shawn-Yu Lin and Jong Kyu Kim; and graduate students J.-Q. Xi, Martin F. Schubert, and Minfeng Chen.
The research is funded primarily by the National Science Foundation, with additional support from the U.S. Department of Energy, the U.S. Army Research Office, the New York State Office of Science, Technology and Academic Research (NYSTAR), Sandia National Laboratories, and the Samsung Advanced Institute of Technology in Korea. The substrates were provided by Crystal IS, a manufacturer of single-crystal aluminum nitride substrates for the production of high-power, high-temperature, and optoelectronic devices such as blue and ultraviolet lasers.
Under Schubert’s leadership, the Future Chips Constellation focuses on innovations in materials and devices, in solid state and smart lighting, and applications such as sensing, communications, and biotechnology. A new concept in academia, Rensselaer constellations are led by outstanding faculty in fields of strategic importance. Each constellation is focused on a specific research area and comprises a multidisciplinary mix of senior and junior faculty and postdoctoral and graduate students.
Jason Gorss | EurekAlert!
New gel-like coating beefs up the performance of lithium-sulfur batteries
22.03.2017 | Yale University
Pulverizing electronic waste is green, clean -- and cold
22.03.2017 | Rice University
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to simulate these confined natural conditions in artificial vesicles for the first time. As reported in the academic journal Small, the results are offering better insight into the development of nanoreactors and artificial organelles.
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
23.03.2017 | Life Sciences
23.03.2017 | Power and Electrical Engineering
23.03.2017 | Earth Sciences