Figure 1. (a) Transmittance vs. wavelength for a four-period Ag/Si3N4 (30-nm/90-nm) stack. Note that all wavelengths above the visible are entirely rejected. (b) Modulus of the electromagnetic field at the first resonance inside the structure.
By stacking metal and dielectric layers, researchers have been able to design structures that are highly transparent at particular wavelengths, reflective at others, and can be engineered to suit various optical filtering tasks.1-3 The team from the U.S. Army Aviation & Missile Command at Redstone Arsenal, and Time Domain Corporation of Huntsville, both in Alabama, sees the new structure as being useful for many applications, but particularly those that make use of its conductivity. These include combination windshield/antennas and transparent front electrodes for liquid crystal displays.
The same material properties that make metals good conductors also make them good reflectors. Metals reflect well for most wavelengths of interest to engineers and tend to absorb what they don't reflect. Therefor, they are often used for radiation shielding applications as well as mirrors. As a material's conductivity decreases, through the classes of semiconductors and insulators, its ability to transmit light tends to increase. Glasses, for example, are both excellent insulators and make great windows for many wavelength ranges. For applications where a transmissive conductor is required, there have been relatively few good options.
Alabama researchers have managed to come up with a new option by using thin metallic films stacked between thicker dielectric layers. Normally, a significant part of any reflection of light will take place not at the surface of a metal layer, but at some distance into it (known as the skin depth). For visible light, this distance is of the order of 10 nm (depending on the metal used), while for microwaves it would be a few microns. It was thought metal films much thicker than this would necessarily reflect a great deal of any incoming radiation. However, Michael Scalora and his colleagues found that this need not be the case. By carefully controlling the geometry of the structure, the skin depth could be exceeded -- both in individual layers and in total metal thickness -- while keeping light transmission high.
Figure 2. There are now several different strategies for designing metallo-dielectric photonic band gap structures. These include structures that are (a) periodic; (b) based on Cantor sets (shown is a three-stage Cantor-like multilayer); (c) based on Fibonacci numbers; and (d) based on chirped sets.
The first devices used a classic Fabry-Perot or Bragg-grating-type structure in that the layers were arranged periodically with an approximately l/4 total thickness. Figure 1 shows a transmission spectrum from such a device and an example structure. However, by further analyzing the way the layers were changing the boundary conditions that in turn affected the optical properties of the metals, the Alabama team determined that other patterns were also possible. In particular, structures based on fractal-like mathematical constructs such as Cantor sets and Fibonacci numbers proved have proved efficient, as have "chirped" designs (Figure 2).
Not unlike their reflective counterparts, the metallo-dielectric photonic band gap structures improve with higher numbers of layers. Thus, the more metal used, the more transparent the structures may become. Scalora says that he and his colleagues have already successfully fabricated filters with at least 150 nm of silver (total layer depth) with a sheet resistance of 0.1 W/square. He and his team are currently working to prove the technology for real. As well as the transparent antenna and display applications, the structure's ability to reject both above and below the design wavelength may make it suitable for safety goggles and other radiation shielding applications.
1. M. Scalora, M. J. Bloemer, A. S. Pethel, J. P. Dowling, C. M. Bowden, and A. S. Manka, Transparent, metallo-dielectric, one-dimensional, photonic band-gap structures, J. App. Phys. 83 (5), pp. 2377-2383, 1 March 1998.
2. Mark J. Bloemer, and Michael Scalora, Transmissive properties of Ag/MgF2 photonic band gaps, App. Phys. Lett. 72 (14), pp. 1676- 1678, 6 April 1998.
3. C. Sibilia, M. Scalora, M. Centini, M. Bertolotti, M. J. Bloemer, and C. M. Bowden, Electromagnetic properties of periodic and quasi-periodic one-dimensional, metallo-dielectric photonic band gap structures, J. Opt. A: Pure Appl. Opt. 1, pp. 490-494, 1999.
Sunny Bains is a scientist and journalist based in the San Francisco Bay area. www.sunnybains.com