PORTLAND, Ore. Scientists at the University of Utah have demonstrated the world's first bandpass filter to employ an unused gap in the electromagnetic spectrum.
Operating in the terahertz gap, which is centered on 1 trillion cycles per second, the scientists have harnessed surface plasmon polaritons to realize one of the three essential components required for communications systems. This region of unlicensed spectrum is an active area of research worldwide. It has garnered special attention since 9/11 because terahertz radiation penetrates clothing, but not metals, enabling terahertz scanners to easily spot concealed weapons.
Terahertz bandpass filters are electrical and optical hybrids that could enable scanners with x-ray vision, but without their damaging effects, as well as ultrahigh-speed communications systems. "We've discovered that plasmon polaritons coupled to an electromagnetic wave between a perforated metal surface and air acting as a dielectric, constructively interfere at specific frequencies, enabling a quasicrystal that acts as a very efficient bandpass filter in the terahertz band," said Ajay Nahata, an electrical engineer and associate professor at the University of Utah (Salt Lake City).
"Next we plan on building modulators that transmit the terahertz radiation like an optical device, but which can be switched with electrical signals, thereby enabling very high-speed communications as well as better security scanners."
There are however, many remaining technical obstacles that will have to be cleared away before terahertz radiation can be widely used. In particular, electrical engineers will have to invent ways to switch and modulate these elusive waveforms. Some of these issues explain "why this region of the electromagnetic spectrum has never been harnessed," said Nahata.
"From my point of view the most interesting problems are how to make tunable filters, how to make modulators and how to make switches that are true hybrids-devices that are both optical and electrical, not one or the other."
Surface plasmon polaritons consist of a resonating collection of electrons at the boundary between a conductor and an insulator. They meld electrons with photons to form a new order of object called a quasiparticle. Surface plasmon polaritons have already been shown to enhance transmission in certain optical bands, but the Utah implementation is the first to act as a bandpass filter in the terahertz band.
The deceptively simple device, a metal film perforated in the pattern of a crystal, uses the same principle as windows on microwave ovens. Like the perforated metal plate on a microwave window, Nahata's quasicrystal is also perforated with holes that are a fraction of the millimeter wavelength of terahertz radiation. But the size of the holes has been carefully chosen to generate surface plasmon polaritons that resonate at specific frequencies, enabling those bands to pass.
Nahata and his colleagues fabricated various quasicrystal topologies that tailored the width and center frequency of this terahertz bandpass filter by adjusting the size of the holes from a quarter to a half a millimeter.
In addition to applications in communications, the researchers also focused on detecting concealed metal, chemical and biological weapons. Metal weapons could be detected with a terahertz X-ray-like imaging approach. Biological and chemical weapons could be spotted with vibrational spectroscopy that records terahertz-band resonances, the patterns of which are unique to specific chemical compounds.
The researchers also demonstrated the ability to tune the terahertz bandpass filter by tilting the foil to adjust the center frequency.
Next the researchers plan to build hybrid switches and modulators, thereby realizing all the component parts needed for future terahertz communications and security scanners.
Funding was provided by the U.S. Army Research Office and the University of Utah's Synergy program to promote interdisciplinary research.