Scientists at the University of Utah have demonstrated the world's first bandpass filter to employ an unused gap in the electromagnetic spectrum. Working in the terahertz gap, which is centered on 1 trillion cycles/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 has garnered special attention since the 9/11 terror attacks, 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.
When plasmon polaritons are coupled to an electromagnetic wave between a perforated metal surface, with air acting as a dielectric, they "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." Such a scheme would enable "very high-speed communications as well as better security scanners."
Many technical obstacles will have to be cleared away before terahertz radiation can be widely used. In particular, engineers will have to invent ways to switch and modulate these elusive waveforms. That's one reason "why this region of the electromagnetic spectrum has never been harnessed," said Nahata. "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, which are covered with a perforated metal plate. Since the holes in this plate are smaller than the centimeter wavelength of microwaves, the microwaves cannot pass through. But light waves can, enabling you to see inside.
Nahata's quasicrystal is likewise perforated with holes that are a fraction of the millimeter wavelength of terahertz radiation. But the size has been carefully chosen to generate surface plasmon polar- itons 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.
Nahata performed the work with physics professor Valy Vardeny, postdoctoral researcher Tatsunosuke Matsui and EE doctoral candidate Amit Agrawa.
In addition to applications in communications, the researchers also focused on detecting concealed metal, and chemical and biological weapons. Next they plan to build hybrid switches and modulators, thereby realizing all the component parts needed for terahertz systems.