To date, few good options have existed for applications requiring a transmissive conductor, since electrical conduction and electromagnetic transmission are two materials properties that tend to be mutually exclusive. Conducting metals reflect well for most wavelengths of interest to engineers and tend to absorb what they don't reflect. Metals are therefore 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 too. Glasses, for instance, are both excellent insulators and make great windows for many wavelength ranges.

Michael Scalora at Time Domain (Huntsville, Ala.) along with other workers at USAMC have proposed another option that uses 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 a short distance into it, known as the skin depth. For visible light, this distance is about 10 nm, while for microwaves it would be a few microns. It was thought, said Scalora, that metal films much thicker than this would simply reflect a great deal of any incoming radiation. However, by carefully controlling the geometry of the structure, he said, it was found that the skin depth could be exceeded both in individual layers and in total metal thickness while keeping light transmission high.

Tuning transmission

Scalora believes that the main advantages of these structures are high conductivity, high transparency, a wide pass band, and the ability to tune the transmission window over a wide range of wavelengths. Fabrication techniques are well established and relatively cheap, and there are many potential applications. "The most promising relate to the replacement of ITO [indium-tin oxide]," Scalora said. "Transparent metal stacks have both the transparency and conductivity needed for applications to all types of displays, where they form the top electrode. Other important areas are sensor protection and electromagnetic shielding.

"We have had some criticism that this work is not new or unique," said Scalora. "The critics point to induced transmission filters as equivalent structures. Induced transmission filters typically contain only one metal layer of thickness around 30 nm. A series of dielectric layers are fabricated on either side to 'induce' the light to propagate through the metal layer. We have approached the problem from a totally different viewpoint," he said. "That is, from the point of view of photonic band gaps." Optical bandgap materials are a recent discovery that behave like the optical analog of semiconductors such as silicon, that have electronic band gaps.

Scalora's work has become controversial because researchers specializing in thin-film technology who have been using similar techniques for years say that there is little or nothing new in the recent findings.

According to Peter Sieck, a senior scientist at glass maker AFG Industries Inc. (Kingsport, Tenn.), "The friction between the photonic-band-gap and thin-film schools has appeared before. We, the thin-film experts, watch as many well known properties of thin-film stacks are rediscovered." In particular, Sieck feels his own work has been slighted by these new claims. "This particular 'discovery' struck a nerve with me because I consider myself quite knowledgeable about silver films in particular, and have designed multi-cavity silver films or visibly transparent RF shielding."

Sieck described applications that are already being done with similar structures. "Currently silver films are used as transparent heat reflectors both for their rejection of near-infrared solar energy. Similar transmissive silver films have been used for PDP [plasma display panel] television front screens. Here the emphasis is on RF isolation, and typically performance is measured in ohms/square centimeter." The screens are able to sustain a good resistance to electrical conduction while transmitting 80 percent of the light striking them, he said.

He also pointed to a book written in the 1980s, "Thin-Film Optical Filters" (Macmillan). "The author starts by saying that "70 nm of silver can have 80 percent transmission," Sieck said, "but goes on to show how multi-period films [as described in Scalora et. al.] have an undesirable double-peaked structure that can be designed out." The book also offers a significant list of references to metal-dielectric filter research, he said.

"Due to the large application base, there is no surprise that many patents exist covering just this type of filter," he said. "Of course the competing groups that I am familiar with do not call their designs 'photonic band-gap structures." However, the mathematics involved are virtually identical and certainly the predictions, when based on exact solutions, are identical stemming as they do from Maxwell's equations," Sieck said.

Scalora has a different view. "We tried to build a periodic lattice of alternating metal and dielectric layers that would exhibit a series of pass bands and stop bands analogous to the band gaps for electrons in a semiconductor. Most people believed that this was impossible and that all the light would be reflected or absorbed by the multiple metal layers," he said. "We were surprised to find that we could build structures containing five metal layers having a total thickness of metal equal to 150 nm and maintain greater than 50 percent transmittance," Scalora said.

"We believe that this point is new and has not been addressed by the community," he concluded. "But more to the point, we recognized that these structures have much more utility than simple optical filters."

Whatever the genesis of the idea, metallo-dielectric stacks seem likely to become more widely used. According to Joseph Haus, director of the electro-optics program at the University of Dayton, (Dayton, Ohio), "The high transmission and conductivity together are an important combination of properties that can be put to good use. I foresee applications that could replace ITO coatings: these include display technology, polymer lasers and polymer electro-optic modulators," he said. "Other applications include UV blocking, IR reflective windows, optical interconnects, and wide band pass filters." The nonlinear optical properties of transparent metals should also prove promising, though the work is still at an early stage.