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W ithin the electromagnetic spectrum (betweenDC and gamma rays) the last portion to beexamined lies between millimeter waves andlong-wave infrared. In terms of frequency, this regionextends 3 decades, from roughly 300 GHz to about 300

THz. Other ranges may be referred to as THz, but they vary only in the specific upper and lower boundaries. Thekey point to remember is that this range lies betweenwhat is commonly known as the highest microwave fre-quencies and the longest wavelengths of lightwaves inthe infrared region.

THz Research HistoryThe last gap in the spectrum was bridged in 1923,

when Ernest Nichols and J.D.Tear finally completed theirefforts to observe radiation in the THz range. Their workwas a systematic approach, both upwards in frequencyfrom the microwave region and downward from theinfrared region.

Before this more-or-less official conclusion of thesearch, there were many significant contributions to THz-range understanding. Perhaps the most important waspart of the scientific communitys effort to characterizeblackbody radiation versus temperature. In 1900, inGermany, Rubens and Kurlbaum applied earlier workRubens had done in collaboration with Nichols, resulting in accurate experimental data at longer wavelengthsthan had been previously observed. Max Planck wrote theequation that would become Plancks Radiation Law onthe same day that Rubens showed him those results.

Other experimental work in the THz region alsomade significant contributions. Initially, optical tech-niques of gratings, interference, refraction, etc. were onlypartially successful at THz frequencies, and radiometerswere not very sensitive. As a result, much work was doneto analyze data indirectly at harmonics that fell into theinfrared range where measurements could readily bemade. Intensive mathematical work was done to extractthe fundamental frequency from the observed higher-order products. These manual techniques were used untilthe 1950s, when computers could finally perform aFourier transform on the spectral data. Until then, how-

ever, THz research helped advance the ability to analyzemeasurements made using optical methods, including thefamous Michelson interferometer.

Charactertics of the THz Region

There are several regions of transition in the electro-magnetic spectrum. With increasing frequencies, EMradiation becomes more energetic, and the correspond-ingly shorter wavelengths cross various physical bound-aries. As a result, different behaviors become apparent.

For example, at low frequencies, circuitry behaves very close to the idealized manner of DC. At a higher fre-quency, circuit dimensions become a significant percent-age of a wavelength, and transmission line principlesneed to be applied. As we reach the microwave range, thehigher electron transition energy levels result in moreradiation, requiring a change in design techniques tominimize its effects. At higher frequencies yet, it is near-ly impossible to fabricate circuits small enough, relativeto wavelength, and we need to use electron beams, crystalstructures, and other atomic-level and quantum phenom-ena to create and manipulate the EM waves. As we reach

visible light, all generation and radiation involves quan-tum relationships, which are increasingly energetic andcomplex into the X-ray and gamma ray ranges.

Terrestrial propagation also changes with increasing frequency, from surface waves at the lowest frequencies,through ionospheric reflection and refraction, VHF/UHFline-of sight, microwaves affected by precipitation andfoliage, atomic and molecular absorption at the higher

mm-waves, etc. In this respect, THz radiation fallsbetween microwaves and lightwaves. It experiences high-er absorption by water vapor and dust than mm-waves,but can still pass through many materials that areopaque to lightwaves.

At THz frequencies, wavelengths are in the sameorder of magnitude as molecular-level structures. Thissize/wavelength convergence results in a unique set of behaviors that make THz technology very difficult toimplement, but at the same time, creating opportunitiesfor valuable new applications. With wavelengths in thesame range as molecular structures, the THz spectrum is

TECHNOLOGY REPORT

Terahertz (THz) Technology:An Introduction andResearch UpdateFrom February 2008 High Frequency ElectronicsCopyright 2008 Summit Technical Media, LLC

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filled with absorption lines, fortunately, with some signif-icant gaps between them. In theory, each type of moleculehas a unique signature in the THz region that can beextracted using spectroscopic techniques, although thesignal analysis is computationally intensive.

The large number of spectral absorption lines makes

wide bandwidth communications difficult at THz fre-quencies. In the atmosphere, water vapor is the primarysource of propagation losses, so outdoor communicationslinks will rarely be practical. Indoors, over short dis-tances, it appears practical for THz signals to support thetransmission of bandwidths from 10s to 100s of GHz.

Generating THz signals with a useful power level is asignificant problem to be solved. Conventional transistoroscillator and diode multiplier technology can reach intothe lower end of the range, but not much beyond.Miniature backward-wave oscillators (BWOs) have beendemonstrated, constructed using micromachining tech-

niques. Quantum cascade lasers have generated severalmW of power in the 5 THz range, but they require cooling to liquid nitrogen temperatures or below.

Current THz signal sources for researchincluding optical mixing, pulsed optics and mercury-arc broadbandnoise sourcesdo not deliver the necessary power levelsfor many potential applications. New quantum-level tech-niques will be needed to work with energy levels that arelower than those commonly used for related technologiesin the IR and visible spectrum, such as solid state lasers.New materials structures will be neededand are thesubject of current research.

Detectors are perhaps the biggest problem to besolved. Photonic detectors are used in research, but theyrequire cooling since the equivalent temperature of THzenergy is