Science and Technology in the Submillimeter Region

Science and Technology in the

Submillimeter Region

Frank C. De Lucia

Advances across a broad range of technologies arerapidly transforming the

submillimeter spectral region—long considered the “gap” in

the electromagnetic spectrum—from a region inhabited by

specialists into one that is fulfilling its long anticipated

potential. After considering thephysics underlying interactions

in the submillimeter region,the author describes its rapidlyexpanding presence in funda-

mental laboratory studies aswell as the major astronomical

and atmospheric science facilities that are being

built to explore it.

August 2003 ■ Optics & Photonics News 45

size scale of the scattering media andwavelength makes the SMM attractiveboth for imaging with useful resolutionover modest distances (~1 km) and forproximate imaging through dust, rain,woven dielectric materials and so forth.

Although as expected molecularattenuation peaks in the SMM, the variation is much greater than casualinspection of Fig. 1 might imply. Forexample, attenuation changes from lessthan 1 dB/km (a transmission of about80%) near 100 GHz, to losses of greaterthan 100,000 dB/km (a transmission of10-10,000) near peaks of water linesaround 1 THz.

While this attenuation can compro-mise applications that require goodatmospheric propagation, the same rota-tional interactions also provide thestrong absorptions and emissions thatmake this spectral region favorable formolecular science. Since the frequenciesof the peak interactions are a function ofmolecular size and the vast majority ofmolecules are larger than water, the peak

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somewhere between a few hundred GHzand a few THz, after which point theydrop exponentially.3

The atmosphere: an overview exampleThese strong interactions also cause significant and complex attenuation inEarth’s atmosphere. Because of the cen-tral role played by atmospheric attenua-tion in many SMM applications and alsobecause propagation can serve as a use-ful illustration of much of the physics ofthis spectral region, let us begin by con-sidering Fig. 1. In Fig. 1 are plotted theabsorption due to atmospheric gases (inthe SMM, primarily oxygen and water)and the classical scattering from waterand fog droplets. The scattering attenua-tion is approximately constant, starting atshort wavelengths, until the wavelengthof the radiation is approximately dropletsize (in this model, essentially constantfor rain and more complex for theassumed varying droplet size distributionof fog). This general relation between the

I t has long been known that the regionbetween about 3 mm (100 GHz) and100�m (3.3 THz) is rich in scientific

and technological opportunities.1, 2 It hasalso long been known that reaping themposes significant technological chal-lenges. Some of the opportunities havebeen realized, with major efforts in labo-ratory spectroscopy, astronomy andatmospheric remote sensing firmly estab-lished. Others, in areas such as imaging,communications and the spectroscopy ofsolids, remain in a period of extendedadolescence. In the context of particularapproaches or scientific communities, theregion in question has been referred to bymany names: millimeter, nearmillimeter,submillimeter, far infrared and terahertz.For the purposes of this article I will usethe term submillimeter (SMM), althoughI will seek to address in general terms theproperties and opportunities this regionencompasses.

The basic physics of the SMMThe interaction of electromagnetic radiation with matter determines theobserved character of each spectralregion. Because of the energy scales ofatoms and molecules, radiation in theoptical region interacts with electroniclevels, infrared radiation interacts withmolecular vibrations, SMM interactswith molecular rotations and the radiointeracts with nuclear multipole phe-nomena. Additionally, especially in liq-uids and solids (which in the SMM donot have energy levels that are as sharplydefined as those in other regions), thereare interactions which are more conve-niently described by classical ideas suchas conductivity, dielectric constant orloss tangent.

Because rotational motion ordinarilyrequires a gas phase system, there is a special relationship between the SMMand rotational gas phase interactions.Additionally, the interaction betweensubmillimeter radiation and molecularrotation is very strong and the Dopplerlimited linewidths are very narrow. Thismakes the SMM very favorable formany applications. In general theserotational interactions start in the con-ventional microwave region but havestrengths that grow as the cube of thefrequency until they reach a maximum

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Figure 1. Atmosphericpropagation across thespectrum. [Note: Since itwas originally drawn by B. D. Guenther in the late1970s, the precursor toFig. 1 has promulgatedthroughout the scientificcommunity, culminating in its being copyrighted by Encyclopedia Britannica.Unfortunately, the originaland all of its offspring con-tain a drafting error whichresults in: a missing spec-tral line in the intervalbetween 100 GHz and 500 GHz; and in the depic-tion of the attenuationscaused by atmosphericabsorption between 0.5 THz and 10 THz to be too small by about a decade on the logarithmic graph.]

absorptions of the heavier molecules areshifted to lower frequency relative tothose of water.

What makes the SMM unique?The characteristics that make the submil-limeter region so special are:

Matter/radiation interactionClassical electromagnetic theory simplyscales with wavelength and frequency.Our everyday experience, however, putsus into contact with regions of the elec-tromagnetic spectrum with which specificproperties are associated: for X-rays, pen-etration and resolution; in the visible,imaging and sight; in the infrared,thermal imaging and vibrational spec-troscopy; in the microwave and radio,communications and radar. This enor-mous variety comes about largely becauseof the multiplicity of ways in which radia-tion interacts with matter and the temper-ature of the world we live in. In thefollowing section I will explore theseinteractions in the SMM, a region ofwhich the public and most members ofthe scientific and technological commu-nities do not possess an intuitive picture.

Separation of the wave functionIt is remarkable that so much of mattercan be understood by separating itsdescriptions into those associated withelectronic, vibrational, rotational andnuclear degrees of freedom. At a quan-tum mechanical level, this descriptionresults from the separation of the energyscale of each from its neighbors by oneor more orders of magnitude. Thismakes possible the separation of theHamiltonian into pieces, which ofteninteract only weakly with each other.There are classical pictures as well. In a molecule, for example, the energeticelectrons travel around the nuclei intheir orbits at high speeds in compari-son to the vibrational motion of thenuclei. Thus, the nuclei only “see” theaverage position of the electrons and arelittle affected by the details of the elec-tronic motion.

Thermal populationsThe properties of the world we live in are not determined only by the energylevels available but also by which ones

are occupied or energetically accessible.For systems at or near thermal equilib-rium, the ratio of the populations of twostates is determined by the Boltzmanndistribution Nu /Nl = exp(-h�/kT ). Welive on a planet with a sun the photons ofwhich are energetic enough to interactwith the electronic degrees of freedom.But on Earth kT(T =300 K) = 200 cm-1 =1/40 eV = 6000 GHz is much greater thanh�rot. As a result, our optical technologyoperates in a world where systems at ornear thermal equilibrium are almostentirely in their electronic ground statesand optical excitation energies far exceedthermal dissipative energy scales. In theSMM at ambient temperature, however,h�~kT. The character of both the tech-nology and science in this region is pro-foundly affected as a result.

InteractionsIntimately connected to this populationdistribution is the nature of how matterinteracts with itself to move from oneenergy level to another. In gases theexchange mechanism is typically theexchange that occurs during a collisionbetween translational energy (which is ofthe order kT) and internal degrees offreedom; in condensed matter the energyexchange is more collective. Irrespectiveof the agent of energy exchange, however,when the levels are separated by <kT andconnected by a transition moment (e.g.,a dipole-dipole interaction in the gasphase), the probability of energy transferis high. Likewise, when the separation is>kT, exchange is slow. This energy rela-tionship has significant impact on theselective excitation and relaxation oflevels in lasers and other quantumdevices and the dividing line occurs at �~kT/h .

Typical SMM spectrumWhile atmospheric transmission is animportant example of gas phase interac-tions, the widely spaced energy levels ofwater and oxygen are atypical, as is thehigh atmospheric pressure. Figure 2shows as a series of blow-ups the spec-trum of oxirane between 298 and 363 GHz obtained with a Fast ScanSubmillimeter Spectroscopy Technique(FASSST) system.4 This system substi-tutes fast scanning and digitization for the phase-lock frequency reference

46 Optics & Photonics News ■ August 2003

THE SUBMILLIMETER REGION

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Figure 3. The J = 2–3 rotational transitions ofexcited vibrational states in the plasma of theHCN discharge laser.

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typical of SMM spectroscopy. In thisexample the scan time for the completeregion was approximately 2 seconds, or arate of approximately 105 resolution ele-ments/ second. The information contentof this figure serves to illustrate thepower of the physics that underlies mat-ter-radiation interactions in this spectralregion. To plot the entire spectrum ofthe upper graph on the scale of the still-compressed lower plot would require a graph approximately 50 meters long.Additionally, in the vertical dimension all of the apparent “noise” peaks on thebaseline are, in fact, weak spectral lines.

Why is the SMM unique technologically? The factors that make the submillimeterregion special from a technological pointof view are:

The physics of the gapThe SMM is often referred to as the “gap”in the electromagnetic spectrum becauseof the historical difficulty associated withdeveloping a technology, especially arobust source technology, capable ofoperating in this region. There are, ofcourse, well known fundamental reasonsfor the difficulty. The gap can beexplained by the fact that size limitationson classical microwave sources occurbelow �~kT/h, where quantum systemsbegin to become practical. Classicalsources typically require a size scale smallenough so that, as the electrons traversean interaction region, the phase of thefield does not reverse and reabsorb theenergy that was just transferred from theelectron to the field. This sets a size scaleof approximately � for the interactionstructures, typically significantly less forconventional e-beam voltages and veloci-ties and much less for solid state devices.While modern fabrication technologiescan easily produce high quality structureson this subwavelength scale, as frequencyis increased, the resistivity of metalsincreases, the cross sectional areas avail-able to the electron currents decrease,heat dissipation becomes more difficultand so forth. In contrast to these classicaldevices, lasers and other quantummechanical devices can operate in largehigh order mode structures that coupletogether the collective contributions of

many atomic or molecular systems.However, radiation sources are easiest tobuild at energies that are large comparedto kT, so that the collisional and otherdissipation processes discussed above donot rapidly drive population inversionstoward equilibrium.

Source demonstrationsThese challenges have produced a multi-tude of approaches to the source prob-lem.1,2 Examples include: broadbandsources that use spatial interference forfrequency selection (Fourier transformwith thermal, discharge or time domainradiation sources); fundamental oscilla-tors (classical electron beam, free electronlasers and solid state); harmonic genera-tion from lower frequency sources; andmixing from optical sources.

The spectral width of sourcesWhile it is not the purpose of this articleto review the characteristics and relativemerits of these approaches, it is perhapsuseful to note that most SMM applica-tions can be separated into those withlinewidths which are very broad or per-haps continua (solids, liquids and highpressure gases) with Q <100, and thosewith gases which are in general very nar-row, with Q>105. Appropriate technolo-gies for these applications can often besimilarly divided, although there aremany cases in which spectrally puresources are desirable even for the investi-gation of phenomena without sharp resonances.

Source brightnessFor many applications, an appropriatefigure of merit is the integral of thesource brightness [W/Hz] over the spec-tral width of the phenomena to be stud-

ied. To provide a numerical referencepoint, 1 mW of power within a 1 MHzDoppler limited line corresponds to abrightness temperature of approximately1014 K. Alternatively, this brightness overa terahertz bandwidth requires a totalaverage power of 1000 W. As anothermeasure, a discharge tube of effectivetemperature 2500 K emits 10-11 W into a 1 MHz bandwidth at 300 GHz.

Coherent and incoherent detectorsBecause of the difficulty of generatingpower in this region, considerable atten-tion has been paid to the integration intoSMM systems of sensitive coherent orincoherent detectors. Coherent detectorsrequire either a local oscillator (frequencydomain) or timing pulse (time domain).The sensor elements for coherent andincoherent detection are usually thesame: semiconductor diodes, photocon-ductors, bolometers and so forth.

Noise considerationsIt is fortuitous that because the quantumlimit to noise is much smaller in theSMM than at shorter wavelengths, in thisspectral region it is possible to build sen-sitive receivers of both classes. It is alsofortuitous that the noise that is radi-ated—even from harsh environmentssuch as discharges and high temperaturesystems—is insignificant in the overallnoise performance of most SMM sys-tems. This is because while the thermalpower emitted in this spectral region isrelatively small, the associated noise iseven smaller, typically by many orders ofmagnitude. Consider a typical 4He inco-herent detector with an NEP of 10-12

W/Hz-1/2 and a cooled filter with a 1 THzcutoff. The noise from a blackbody at 300 K over this region is approximately10-14 W/Hz1/2. Even for somewhat highertemperatures and/or higher frequencycut-offs, this is much lower than thedetector noise. Stated in terms of sourcepower, the environmental noise is about 11 orders of magnitude below that of a 1 mW cw source.

Applications: a few examplesA symbiotic confluence of advances andmajor projects is producing a new era forscience and technology in the SMM andmaking possible exciting visions of the


While the thermal power emitted in

this spectral region is relatively small, the

associated noise is even smaller, typically

by many orders of magnitude.

future. In this section I will discuss across section of examples.

Since it can be argued that the mostimportant and unique characteristic ofthe SMM is its strong interaction withthe rotational degrees of freedom ofmolecules, it is not surprising that mostof the applications and projects are basedon this interaction. Examples includefundamental studies of molecules,remote sensing in the atmosphere andthe exploration of the interstellarmedium and the universe. In addition,there has been a longstanding interest inthe use of a wide range of SMM tech-nologies for applications that do notrequire high spectral resolution. Theseinclude radar and imaging, the study ofsemiconductors and other solids andbiomedical applications.While many ofthese are of interest to the SMM commu-nity and a number of contributions havebeen made,1 I will not discuss them heresince they were recently described in afeature article published in OPN.5

Basic science: gas phase molecules

Spectroscopy The source and detector considerationsdiscussed above have led to an active andsuccessful submillimeter spectroscopybased on coherent, tunable, CW sourcesand broadband thermal incoherentdetectors.4, 6-8 The specificity and sensi-tivity shown in the SMM spectrum ofFig. 2 are the basis for both laboratoryand field investigations. In the laboratorythe fundamental application is spec-troscopy itself: an extensive communitystudies line positions, intensities andlinewidths in a variety of molecularspecies. Some of this work is carried outfor its own intrinsic interest; some isdriven by the need for laboratory data forthe interpretation of the astronomicaland atmospheric applications describedbelow. There has also been considerableuse of these capabilities for the investiga-tion of physical and chemical processesunder non-ambient conditions. Becausethe same physics that makes the genera-tion of radiation in the SMM difficultalso makes even harsh environmentsquiet, this spectral region has been usedadvantageously for the study of equilib-

rium systems over 1-2000 K 9,10 andactive laser plasmas11,12 to name but twoof many examples.2 Figure 3 shows the J = 2–3 rotational transitions of HCN infour different excited vibrational states asobserved in the discharge of a HCN farinfrared laser. This spectral region is soquiet that even broadband helium tem-perature detectors do not observe anyadditional noise in these experiments.

Energy transferAs noted above, h�SMM ~h� rot ~kT in theSMM. This makes it a fertile region forthe study of molecular energy transfer. Ofparticular interest in our laboratory hasbeen the study of gas phase collisions attemperatures of only a few degrees K.This required the development of colli-sional cooling, a method for studying gasphase systems in quasi-equilibrium, attemperatures far below their freezingpoints.13,14 In these collisions, the basicphysics changes and many phenomenathat are “classical” at 300 K reveal theirunderlying quantum nature. Figure 4, forexample, shows the divergence of theelastic and inelastic cross sections at lowtemperature for the H2S-He collision sys-tem. This divergence is caused by colli-sions that change the phase of themolecular wave function without causinga change in state. In the elastic cross sec-tion, a thermally averaged resonance canalso be observed near 3 K. This type ofresonance is a feature of low orbitalangular momentum collisions at lowtemperatures in which resonant excita-tions can cause the formation of quasi-bound states.

Astrophysics: the physical conditionsBy far the largest SMM community hasgrown up around astrophysics. To date,more than 100 different chemical specieshave been identified from their emissionlines. Regions that contain molecules aretypically at temperatures of a few Kelvinto a few hundred Kelvin and which arewell suited to emission in the SMM. Thestrong molecular rotation-radiation coupling adds to the sensitivity of thetechnique. In addition to research in thefield of astrochemistry,15 probes based onmolecular emissions in the SMM (espe-cially of CO, which has been shown tohave a nearly constant abundance ratio



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Figure 5. (a) The Horsehead Nebula, asobserved in the J=3–2 emission from CO at 345 GHz by the Cal Tech SubmillimeterObservatory (CSO) and (b) in an opticalimage.

Figure 4. Divergence of elastic (upper) andinelastic (lower) cross sections at low temper-ature for H2S-He collisions. The points belowzero are the pressure shift cross sections.

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with the astrophysically abundant andsignificant hydrogen) have become powerful probes of a wide range of astro-physical processes. Many of these applica-tions are greatly facilitated by the abilityof longer wavelength SMM radiation topenetrate the dust clouds that hide manyof the most interesting astrophysical processes, especially star and planet formation.

The Horsehead Nebula in Orion is aparticularly interesting example. In thewell known optical image [Fig. 5 (b)] thedust which forms the horse’s head byoptical extinction stands out as a darkfeature against the optical emission in thebackground. In the SMM image, quitethe opposite occurs: here the CO withinthe dust, the attenuation of which pro-duces the optical image, propagates freelythrough the dust and reveals the struc-tures within the dust cloud.

Telescopes and technologyThis field has grown to such an extent asto preclude even a brief description of themajor ground, airborne and satellitefacilities that are operating today aroundthe world. However, descriptions of twoof the most recent, ALMA and Herschel,can give some idea of the work that isbeing carried out. ALMA, for AtacamaLarge Millimeter Array, is an array of64 12-m dishes (see Fig. 6) being built at

16,400 ft in the Atacama dessert in Chileto avoid as much atmospheric attenua-tion caused by water vapor as possible.Scientists working at ALMA will studycosmology, star and planetary formation,stellar evolution, molecular clouds andastrochemistry in the ~30 GHz to ~ 400 GHz region. The interferometerbaselines can be configured from approx-imately 150 m to 10 km, which translatesinto better resolution in the SMM thanthe Hubble Space Telescope provides inthe visible. At its full 10 km extent, itsbaseline is approximately 1,000 timesthat used to produce Fig. 5. This addi-tional resolution will provide far moredetail on the dynamics of, for example,star- and planet-forming regions. It willalso remove the spatial averages overregions of significantly varying physicalproperties and make possible a muchbetter understanding of astrochemistry.

Although Mauna Kea (the location ofCSO) and Atacama (the future home ofALMA) are arguably the best sites in theworld for SMM observations, their per-formance at high frequency is severelycompromised by the atmosphere. In par-allel the European Space Agency, with

significant NASA participation, plans in2007 to launch Herschel, a 3.5-m tele-scope with an instrument temperaturemaintained by superfluid helium.Because it operates above Earth’s atmo-sphere, it can observe from 60 to 670 �mwith three instruments: a heterodynehigh resolution spectrometer (HIFI) withbands of 480-1250 GHz, plus 1410-1910 GHz; a photoconductive arraycamera and spectrometer (PACS) for 210-60 �m; and a spectral and photo-metric imaging receiver (SPIRE) forbands centered on 250 �m, 350 �m and500 �m. The system will be used to focuson how galaxies formed in the early uni-verse, as well as on star and planet forma-tion. With its multitude of phase-lockedoscillators, frequency multiplier chains,cooled mixers, bolometers and photode-tectors, (all of which have to survivelaunch with high reliability) the systemtestifies to the maturity of this technol-ogy in the SMM.

Atmospheric scienceAnother important application is theremote sensing of chemical species thatcontribute to the ozone chemistry of theupper atmosphere.16 Observations havebeen made from the ground, from bal-loons and from spacecraft. Spacecraftoffer the advantage of global coverage ona 24-hour basis. In addition, because they


Figure 6. Artist’s rendition of part of theALMA array of 64 12-m antennas.

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work above the atmosphere, attenuationand noise associated with the loweratmosphere are avoided.

A recent example is the EarthObserving System-Microwave LimbSounder (EOS-MLS), which is scheduledto be launched as a part of the Aura mis-sion in 2004. Figure 7 shows the portionof the simulated spectrum around 650 GHz for EOS-MLS. This is a limbscanning instrument that samples theradiation along the line which extendsfrom the spacecraft through the tangentpoint closest to Earth and then to thebackground of space. Since all altitudesabove the tangent point are sampled,somewhat triangular lineshapes result.However, this shape (as well as multiplespectra taken for different tangentheights) can be used to deconvolute thecontributions from the various altitudes.It can be seen in Fig. 7 for the strong linesthat the antenna temperature saturatesnear the temperature of the atmospherefor all tangent heights, whereas forweaker transitions, because the sample isoptically thin, the antenna temperature islower. Another interesting feature of thisspectrum is that the lines narrow as thetangent height is increased, reflecting thereduced pressure broadening along thelimb sounding path.

ImagingWhile there have been demonstrations ofterrestrial imaging based on a wide vari-ety of SMM technologies, these astro-nomical and atmospheric instrumentsare powerful imaging devices that inmany ways serve as guideposts for thedevelopment of practical terrestrial imag-ing systems. Some, like the CSO 12 m,produce images by mechanical scanningof the antenna. Others—like PACS andSPIRE of Herschel—have focal planearrays, while ALMA reconstructs itsimages interferometrically from detailed

frequency and phase information. MLS is able to construct a three dimensionalimage with pointing information from its antenna and altitude informationobtained from deconvolution of the pres-sure broadened lineshapes it observesalong its limb scanning line of sight.

The futureEven with these successes, there have beenorders of magnitude fewer resources andeffort devoted to exploiting the SMMthan to any of the other regions of theelectromagnetic spectrum. The regionhas gone through several cycles of grow-ing expectations, entry of new investiga-tors and disappointing results. But thiswas all in the past. The steady progress ofSMM technology and laboratory applica-tions has culminated in major astrophysi-cal and atmospheric projects that havepushed the technology to a new level ofcapability and robustness.

Nanofabrication and the mass marketsof wireless communications, collisionavoidance radar and so forth are reachingever higher frequencies and approachingthe SMM. These forces, as well as contri-butions from other technical approaches,promise to provide the critical mass nec-essary to move many applications out oftheir extended adolescence and into amaturity and usefulness comparable tothose of other regions of the electromag-netic spectrum.

Finally, as technology in the SMMbecomes a tool rather than a specialty, itis highly probable that the exposure of

this spectral region to a much broadercommunity will result in new and impor-tant applications that are completelyunseen by the present community ofexperts. Let’s not forget that the firstapplication for telescope time by Buhl and Snyder in the 1970s to usemicrowaves to look for molecules in theinterstellar medium was rejected becausethe conventional wisdom of the astro-nomical experts was that polyatomicmolecules did not exist in space.17

AcknowledgmentsI would like to thank ARO, DARPA,NASA and the NSF for their support ofthis work and E. R. Brown and B. D.Guenther for help in updating Fig. 1.

Frank C. De Lucia ([email protected]) is withthe Department of Physics, Ohio State University,Columbus, Ohio.

References

1. Proc. of the IEEE 1966, 447-717.

2. De Lucia, F. C. In Sensing with Terahertz Radiation;Mittleman, D., Ed.; Springer: Berlin, 2003, 39-154.

3. Gordy, W.; Cook, R. L. Microwave Molecular Spectra, Thirded.; John Wiley & Sons: New York, 1984.

4. Petkie, D. T.; Goyette, T. M.; Bettens, R. P. A.; Belov, S. P.;Albert, S.; Helminger, P.; De Lucia, F. C. Rev. Scient.Instrum 1997, 68, 1675-83.

5. van der Weide, D. Optics and Photonics News 2003, 14,48-53.

6. Golant, M. B.; Alekseenko, Z. T.; Korotkova, Z. S.;Lunkina, L. A.; Negirev, A. A.; Petrova, O. P.; Rebrova, T.B.; Savel'ev, V. S. Pribory i Tekhnika Éksperimenta 1969, 3,231-7.

7. Helminger, P.; De Lucia, F. C.; Gordy, W. Phys. Rev. Lett.1970, 25, 1397-9.

8. Winnewisser, G.; Krupnov, A. F.; Tretyakov, M. Y.; Liedtke,M.; Lewen, F.; Saleck, A. H.; Schieder, R.; Shkaev, A. P.;Volokhov, S. V. J. Mol. Spectrosc. 1994, 165, 294-300.

9. Manson Jr., E. L.; Clark, W. W.; De Lucia, F. C.; Gordy, W.Phys. Rev. A15 1977, 15, 223-6.

10. Pearson, J. C.; Anderson, T.; Herbst, E.; De Lucia, F. C.;Helminger, P. Ap. J. Suppl. Ser. 1991, 379, L41-3.

11. De Lucia, F. C. Appl. Phys. Lett. 1977, 31, 606-8.

12. Skatrud, D. D.; De Lucia, F. C. Appl. Phys. 1984, 35, 179-93.

13. Messer, J. K.; De Lucia, F. C. Phys. Rev. Lett. 1984, 53,2555-8.

14. Ball, C. D.; De Lucia, F. C. Phys. Rev. Lett. 1998, 81, 305-8.

15. Herbst, E. Ann. Rev. Phys. Chem. 1995, 46, 27.

16. Waters, J. W. Atmospheric Remote Sensing by MicrowaveRadiometry: Wiley, New York, 1993.

17. Townes, C. H. How the Laser Happened: Adventures of aScientist; Oxford: New York, 1999.



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Science and Technology in the Submillimeter Region