Metamaterials
A metamaterial is a material which gains its properties from its structure rather than directly from its composition. These materials have been artificially engineered to have unusual electromagnetic properties. The metamaterials made so far are difficult to produce in large quantities.
Now Yi-Jun Jen of Taipei University of Technology, Taiwan, and his colleagues in collaboration with Akhlesh Lakhtakia, Pennsylvania State University have developed a technique that could lead to the large-scale production of optical “metamaterials”. They have used a technique called “oblique angle deposition” (OAD). This involves depositing vapour at an angle onto a substrate held in a vacuum. Akhlesh Lakhtakia says, “Our technique should couple naturally with much of microelectronics.”
By firing electrons into a block of silver Jen and his colleagues were able to vaporize the silver. Then they directed the silver vapor at two-inch substrates of silica material. Silver deposits on the silica substrate in patches but when deposited at certain angle to the substrate norm — nanorods grow preferentially towards the incoming vapor because of the “self-shadowing effect”. The film thickness was 240 nm and the angle between the normal to the substrate and the tilt of the nanorods was maintained at 66 degrees. Silver nanorods were grown to about 650 nm long and 80 nm wide.
In order to find out more about the optical properties of the deposited film the researchers illuminated the film a number of times with light at wavelengths between 300 and 850 nm. They found that light between the wavelengths 532nm and 690 nm was refracted negatively.
Akhlesh Lakhtakia said, “Of course, such films had been grown by others but no one thought to characterize them as biaxial dielectric-magnetic materials.”
Jaime Gomez Rivas of the Nanowire Photonics group at the AMOLF Institute in Eindhoven said, “Their results are interesting because of the simplicity on the fabrication of the layers of nanorods.”
April 2, 2009