With all the advances in solid-state communications from long-wavelength, very low-frequency radio waves that talk to submerged submarines, to short-wavelength, very high-frequency light waves that communicate with lasers you would think the whole electromagnetic spectrum was covered. You'd be wrong. There exists a gap, centered at 1 trillion cycles per second, that the solid-state era has only begun to bridge.
The terahertz gap is the last frontier in the EM spectrum. Terahertz frequencies are where microwaves meet infrared light waves. Microwaves are "millimeter wavelengths" whereas infrared is "nanometer wavelength," leaving the terahertz gap nestled in between at "micron wavelengths."
At the low end, silicon speeds today are in the gigahertz band centered on 30-cm wavelengths. But it will take years to ascend to the 300-micron wavelength at 100 GHz, much less the 3-µm wavelength at 1,000 GHz, or 1 THz.
At the high end, laser diodes are dipping down toward the terahertz, but today they cease to function well at infrared wavelengths much longer than a micron. The result is the terahertz gap roughly between the wavelengths of 300 and 1 µm or, in frequency, between 100 GHz and 30 THz.
In this terahertz gap, neither optical nor silicon solutions exist today. But they will soon, because the lure is too attractive. Bridging the terahertz gap will open up applications hitherto impossible.
Terahertz signals emanate from objects naturally, going through clothes, clouds, skin, plastics, cardboard, semiconductors and even some walls. Thus they enable a dizzying array of "X-ray vision" style applications. Terahertz systems can also work like radar, sending out a harmless nonionizing pulse and imaging from the echoed signal. But most amazing of all, using two or more different terahertz-band frequencies enables a simple Fourier transform to provide an absorption spectrum that reveals chemical composition. In other words, terahertz scanners should be able to provide not only a three-dimensional image of an object and its innards, but also a readout of the chemical composition of hidden objects spotted in the image.
For instance, terahertz signals naturally emanate from inside the human body, enabling X-ray-like imaging but without the side effects. What's more, a terahertz scan could simultaneously analyze tissues to pinpoint abnormalities, like skin cancer, in the resulting 3-D model.
Likewise, since terahertz signals can be focused at various depths, quality-control tomography could be revolutionized by looking at the top, inside and bottom of a chip with a single sensor. Terahertz signals could revolutionize security scanning, by seeing through clothes, and inside bags and containers, to provide a 3-D model of the contents and their various chemical compositions. Airplanes using terahertz imaging systems could see through clouds since they, too, are transparent to micron-wavelength signals.
Unfortunately, microwave equipment cannot operate at a high enough frequency (above 100 GHz) and solid-state lasers can not go low enough (below 30 THz). So the entire field has languished, awaiting a technology to call its champion.
The wait is not over, but an amazing array of next-generation attempts to plumb the terahertz mysteries are under way and looking good.
Single-frequency terahertz lasers already exist from Coherent Inc. The Santa Clara, Calif., company offers optically pumped lasers at a fixed terahertz-band frequency, or a tunable millimeter-wave (MMW) source coupled into a high-frequency Schottky diode that produces sideband terahertz radiation that the MMW source can tune.
High-end solid-state optical designs for the terahertz gap exist too, such as the demultiplexer used as a gating device. Based on cross-phase modulation in a semiconductor optical amplifier, this device is called a Toad terahertz optical asymmetric demultiplexer. Solid-state designs for switches and networking backplanes are also reaching the terahertz band.
Space shuttle use
As early as 1999 Picometrix Inc. (Ann Arbor, Mich.) teamed with Lucent Technologies' Bell Labs to introduce the world's first commercial terahertz spectrometer not based on vacuum tubes. Picometrix's ultrafast pulsed-laser technology is capable of broadband terahertz radiation (not just at a single frequency). NASA is using it to inspect space shuttle tiles without having to remove them, as would be required if tabletop systems were used. "Our systems can see through the shuttle's foam, plus they highlight any structural damage by imaging the various layers," said Robin Risser, chief executive officer at Picometrix.
The Picometrix system works by remotely locating the terahertz detector at the end of a 50-meter fiber-optic cable from which the terahertz waves emit, enabling it to service field applications that tabletop systems cannot reach. Thus, for the shuttle application, the tethered transceiver (containing both the source and the detector) can be placed right against the foam to image the tiles beneath.
"Our aim from the beginning with terahertz was to ruggedize the system so that it could get out of the lab and into the field," said Risser. "We are developing advanced semiconductors that emit terahertz, like many academic researchers, but our difference is that we build systems that work in the field right now." The company, he said, is now "working on an airport version to scan through your shoes, so you don't have to remove them at airport security checkpoints."
Other semiconductor research groups are bringing their creations out of the lab as well, among them TeraView Ltd., which was spun out from Toshiba Research Europe Ltd. (both based in Cambridge, England) as a subsidiary that is also partially owned by British companies. Tera-View's technology creates terahertz signals by illuminating special semiconductor crystals with ultrafast laser pulses lasting only about 100 femtoseconds.
In the United States, Los Alamos National Laboratory has licensed its terahertz laser technology to Molecular OptoElectronics Corp. for the further development of emitters and detectors based on an electro-optic crystal mosaic a novel class of terahertz emitters developed by researchers Antoinette Taylor and Timothy Carrig of Los Alamos and Kevin Stewart of MOEC.
Meanwhile, Russian and U.S. academics have teamed up to develop terahertz silicon germanium chips (see March 15, page 8) under the direction of James Kolodzey, professor of electrical and computer engineering at the University of Delaware (Newark), and Miron Kagan, director of the Russian Academy's Institute of Radioengineering and Electronics (Moscow). That institute in turn conducts collaborative research in the field with the Ioffe Physico-Technical Institute (St. Petersburg).
Kolodzey's lab in Delaware has created three types of terahertz-frequency emitters. Two are based on quantum confinement effects in epitaxially grown quantum wells (sometimes called quantum dots). The third dispenses with the wells to emit 8-THz photons directly from electrically pumped, boron-doped p-type SiGe at cryogenic temperatures (though the effect is detectable at temperatures as high as 150 Kelvin).
"We are very hopeful that one or more of our three designs can be used to create the first solid-state terahertz laser," said Kolodzey.