Japanese researchers at Osaka University in Suita and the Institute for Laser Technology in Osaka have demonstrated that atomic layer deposition can be used to manipulate the refractive index of an optical thin film by precisely varying the thickness and ratio of its constituent layers. The technique opens the door to improving numerous applications, such as the manufacture of solar cells and laser diodes.Using a specially designed growth chamber, researchers have used atomic layer deposition to tailor the refractive index of a thin film by controlling the thickness of individual layers of Al2O3 and TiO2 grown on the substrate. Courtesy of Shinichi Zaitsu.In chemical vapor deposition, the chemicals react as vapors and coat a surface. Atomic layer deposition, in contrast, applies single-molecule-thick layers of a chemical onto the surface. This process is self-limiting because chemical bonds hold a single layer on the surface: Once all the binding locations are full, chemical deposition stops regardless of how much vapor remains in the chamber.The process first introduces the vapors of a reactant -- either trimethylaluminum [Al(CH3)3] or tetrachlorotitanium (TiCl4) -- that chemisorb to a silica substrate. The researchers pump out the vapor and add water vapor to form an oxide layer. Then they deposit individual layers of Al2O3 and TiO2 onto the substrate until they achieve the desired thickness and refractive index.Shinichi Zaitsu, a doctoral student at the university, said that the scientists selected Al2O3 and TiO2 be-cause the materials already are in use for high-power laser applications. They also are working with SiO2 because it has a lower refractive index and will expand their range of refractive index control.The work is different from conventional thin-film technology in that the researchers create separate layers of each oxide rather than mixing the two in one film. "Generally, the refractive index of composite films [made up of] a mixture of high and low refractive index materials is determined by the mixture ratio of the two components," Zaitsu said. "Because our optical coating consists of alternating layers of high (TiO2) and low (Al2O3) refractive index materials, the mixture ratio is defined by the relative thickness of TiO2 and Al2O3 within a nanoscale pair. Therefore, the control of the refractive index is achieved by adjusting the thickness of each layer."One application that may benefit from the new process is the manufacture of chirped mirrors for dispersion compensation in femtosecond lasers. It also may ease the manufacture of large optical components with precise optical indices for inertial confinement fusion experiments, he said.
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