910 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 50, NO. 3, MARCH 2002Terahertz TechnologyPeter H. Siegel, Fellow, IEEEInvited PaperAbstract—Terahertz technology applications, sensors, andsources are briefly reviewed. Emphasis is placed on the lessfamiliar components, instruments, or subsystems. Science drivers,some historic background, and future trends are also discussed.Index Terms—Applications, submillimeter, technology, THz.I. INTRODUCTIONTHESE DAYS, it is not possible to do justice to an entirefield or give sufficient credit to all its deserving technicalinnovators in one short paper, even in a relatively narrow area ofdevelopment like terahertz technology. If this were the case, wewouldnothavesuchaplethoraofjournalstosubmitto,norconfer-encestoattend.Onethingiscertain,theIEEEMicrowaveTheoryand Techniques Society (IEEE MTT-S), throughits journalsandsponsored conferences, has played a major role in defining, dis-tributinginformationon,andadvancingthefieldofterahertztech-nology since the society’s inception a half-century ago. Duringthe course of this paper, we look back to the infancy of modernterahertz technology, beginning where Wiltse so ably left off in1984 [1], pass through early childhood, and end up at adoles-cence. The field is perched on adulthood and perhaps, in anotherquarter-century, a more complete history can be written, hope-fully by someone reading this paper today.II. BACKGROUNDThe first occurrence of the term terahertz in thisTRANSACTIONS is attributed to Fleming [2] in 1974, where theterm was used to describe the spectral line frequency coverageof a Michelson interferometer. A year earlier, Kerecman [3] ap-plied terahertz to the frequency coverage of point contact diodedetectors in an IEEE MTT-S conference digest paper. Ashleyand Palka [4] used the designation to refer to the resonantfrequency of a water laser in the same digest. Spectroscopistshad much earlier coined the term for emission frequenciesthat fell below the far infrared (IR).1Today, terahertz isbroadly applied to submillimeter-wave energy that fills theManuscript received August 21, 2001. This work was supported by the Cali-fornia Institute of Technology Jet Propulsion Laboratory under a contract withthe National Aeronautics and Space Administration.The author is with the Submillimeter Wave Advanced Technology Group, JetPropulsion Laboratory, Pasadena, CA 91109-8099 USA.Publisher Item Identifier S 0018-9480(02)01958-0.1The Oxford English Dictionary dates the term “terahertz” back to at least1970 where it was used to describe the frequency range of an HeNe laser. In1947, the International Telecommunications Union designated the highest offi-cial radio frequency bands [extremely high frequency (EHF)] as bands 12–14,300 kMc–300 MMc (1 MMc=1 THz).wavelength range between 1000–100 m (300 GHz–3 THz).Below 300 GHz, we cross into the millimeter-wave bands(best delimited in the author’s opinion by the upper operatingfrequency of WR-3 waveguide—330 GHz). Beyond 3 THz,and out to 30m (10 THz) is more or less unclaimed territory,as few if any components exist. The border between far-IR andsubmillimeter is still rather blurry and the designation is likelyto follow the methodology (bulk or modal—photon or wave),which is dominant in the particular instrument.Despite great scientific interest since at least the 1920s [5],the terahertz frequency range remains one of the least tappedregions of the electromagnetic spectrum. Sandwiched betweentraditional microwave and optical technologies where there isa limited atmospheric propagation path [6] (Fig. 1), little com-mercial emphasis has been placed on terahertz systems. Thishas, perhaps fortunately, preservedsome unique science and ap-plications for tomorrow’s technologists. For over 25 years, thesole niche for terahertz technology has been in the high-resolu-tion spectroscopy and remote sensing areas where heterodyneand Fourier transform techniques have allowed astronomers,chemists, Earth, planetary, and space scientists to measure, cat-alog, and map thermal emission lines for a wide variety of light-weight molecules. As it turns out, nowhere else in the electro-magnetic spectrum do we receive so much information aboutthese chemical species. In fact, the universe is bathed in tera-hertz energy; most of it going unnoticed and undetected.This review will examine terahertz technology today withemphasis on frequencies above 500 GHz and on applicationsthat may not be familiar to every reader. We will also try todo justice to molecular spectroscopy for Earth, planetary, andspace science, the chief drivers of terahertz technology to-date.An excellent overview of lower frequency millimeter and sub-millimeter-wave technology can still be found in the review pa-pers of Coleman [7], [8] and Wiltse [1]. Commercial uses forterahertz sensors and sources are just beginning to emerge asthe technology enables new instrumentation and measurementsystems. So-called T-ray imaging is tantalizing the interests ofthe medical community and promises to open the field up tothe general public for the first time. Other less pervasive appli-cations have been proposed, all of which would benefit frombroader-based interest in the field. We will try to cover some ofthese and anticipate others in the course of this paper.We begin with a survey of terahertz applications in Sec-tion III, and follow this with some selected information onterahertz components in Section IV. More detailed discussionson terahertz components, materials, and techniques cannot becovered. We conclude with some fanciful applications andpredictions in Section V.0018–9480/02$17.00 © 2002 IEEESIEGEL: TERAHERTZ TECHNOLOGY 911(a)(b)Fig. 1. Atmospheric transmission in the terahertz region at various locations and altitudes for given precipitable water vapor pressure (in millimeters).(a) 0–500 GHz. (b) 500–2000 GHz. Data from Erich Grossman using his “Airhead Software” [6].III. TERAHERTZ APPLICATIONSThe wavelength range from 1 mm to 100 m correspondsto an approximate photon energy between 1.2–12.4 meV or toan equivalent black body temperature between 14–140 K, wellbelow the ambient background on Earth. A quick look at thespectral signature of an interstellar dust cloud (Fig. 2), however,explains why astronomers are so interested in terahertz sensortechnology. An excellent science review can be found inPhillips and Keene [9]. Fig. 2 shows the radiated power versuswavelength for