V.  APPLICATIONS

B.  Astronomical Spectroscopy:  Astronomical spectroscopy has attracted perhaps the largest body of scientists and engineers working in the Submillimeter /Terahertz spectral region.   That the line between scientists and the engineers is often blurred in this field is perhaps one of the chief reasons it has been so successful.  The challenging technical problems of astronomy have not only driven and motivated the development of technology, but also have provided generally accepted and verifiable benchmarks. 

        The genesis of this astronomical activity can be traced back to the early days of microwave spectroscopy when techniques developed for millimeter spectroscopy were adapted in 1955 to detect radiation from the sun in the atmospheric window around 94 GHz.16  The first observation of a polyatomic species in the interstellar medium, ammonia (NH3), was in 1968 by Townes and his coworkers Welsh, Rank, Thornton, and Cheung at the Hat Creek observatory.17 This was followed closely by their discovery of water (H2O).18 Interestingly, two young post docs, Lew Snyder and David Buhl, at the National Radio Astronomical Observatory had previously applied for telescope time on their West Virginia facility, but their application to search for molecules in the interstellar medium had been turned down as a foolish waste of telescope time.19 Shortly thereafter they were awarded telescope time and discovered formaldehyde (H2 CO).20  While it is fortuitous that these early searches in the centimeter spectral region were for molecules which turned out to be relatively abundant, none are as wide spread or as abundant as CO, whose 115 GHz J = 1 → 0 transition was the first to be discovered in the millimeter spectral region.21  We will see below that the abundance of CO, along with its high correlation with the cosmically much more abundant but difficult to detect H2 and He, have made it an important beacon for astrophysical study of the structure and dynamics of both our galaxy and active star forming regions.  An excellent history and overview of this field has been written by Winnewisser, Herbst, and Ungerechts.22

        At a basic level astronomical spectroscopy and atmospheric spectroscopy are very similar.  Both detect the emission from molecules and, to first order, both use similar technology to do so:  heterodyne receivers being extended from lower frequencies and optical interferometric techniques being extended from shorter wave-lengths. 

        It is instructive to start by considering the differences in the science (as well as in the resulting technology) for atmospheric and astronomical applications.  In general the interstellar medium is colder, with temperatures typically not too many times that of the microwave background (2.7 K), but with hotter regions as protostellar cores are approached.  Additionally, the astronomical collision times are much longer in all circumstances.  This long collision time, combined with fluxes of energetic particles, produces molecular systems which can be far from equilibrium in rotational state populations, partial pressures of gases (which for almost all species would approach zero under conditions dictated by vapor pressure), and abundances of ions, free radicals, and other reactive species.  A useful measure of this non-equilibrium is that the lifetime of gaseous species in the interstellar medium is ~ 105 – 106 years before they freeze out on dust grains.

      From the point of view of laboratory astrophysics, this leads to a rather different set of problems than those motivated by atmospheric science.  Table V.B.1-1 shows a list of the molecular species that have been detected in the interstellar medium.  A comparison of it with the species found in Figs.V.A-1 and V.A-2 is instructive.  An important first order

effect in this comparison is simply one of atomic abundance.  The atmospheric species are derivative of the major atmospheric components (N2, O2, and H2O) and man-made injections into the atmosphere. The interstellar species are driven more by cosmic abundances (H, C, O, N, . . .).  Additionally, the interstellar list has many prominent ions and free radicals whose lifetimes under terrestrial conditions are very short.  While spectral line frequencies are independent of the molecular environment after correction to rest velocity, their shapes and widths are not.  Moreover, the inelastic rotational energy transfer rates which are closely related to pressure broadening (which is absent in the interstellar medium but of great significance in the recovery of atmospheric parameters from remote sensing data) are similarly necessary for the recovery of astronomical information from the non-equilibrium interstellar medium.

        Thus much of the emphasis in laboratory astrophysics has been on the development of laboratory environments both for the production of reactive species and ones in which low temperature collisional studies can be carried out.  These have been considered in more detail above in Section III.

1. Some telescope facilities:  Table V.B.2-1 lists a number of major millimeter, submillimeter, and far infrared telescope facilities.  These include ground, aircraft, and space based telescopes.  Especially noteworthy for the future of the field are the large new systems in the active development stage:  The Atacama Large Millimeter Array (ALMA) (http://www.alma.nrao.edu/) to be placed at Llano de Chajnantor in Chile and Herschel (FIRST) (http://herschel.esa.esa.int).  By the historical standards of the spectral region, these represent an enormous investment and are a testimony both to the importance of the science that drives these projects and the technological advances that have made them possible.

2.  Two representative THz telescopes:  In this section we will provide an overview of two recent satellite instruments, the Submillimeter-Wave Astronomy Satellite (SWAS) and the Far Infra-Red and Submillimeter Telescope (FIRST), which has been renamed the Herschel Space Observatory.  These two telescopes differ by about a decade in their launch dates and technological development.  For each, we will focus on the relation between the Submillimeter /Terahertz technologies and the astronomical problems that motivated their design. 

        a.  SWAS:  The Submillimeter-Wave Astronomy Satellite (SWAS) was launched in December of 1998 as a part of NASA's Small Explorer Project (SMEX) and can serve as an example for the characteristics of astronomical systems in the Submillimeter Terahertz spectral region.23  SWAS was designed to study the Holy Grail of interstellar astrophysics, star formation from dense clouds.  It does this by specific observation of water, molecular oxygen, atomic carbon, and isotopic carbon monoxide, species that are central to the cooling processes in these clouds that are necessary for gravitational collapse.  To accomplish its goals, SWAS operates in the 487 - 556 GHz spectral region, a region in which observations cannot be made from the ground because of atmospheric attenuation, primarily due to the strong 110  - 101 transition of water itself.

        A block diagram is shown in Fig. V.B.3.a-1. After the photons are collected by a 68 x 53 cm aluminum primary mirror and provision is made for comparison with a calibration load, the power is split with a wire grid polarizer into two channels and sent to separate heterodyne detectors.  Because the background against which the molecular emissions are viewed is in general very cold (often that of the cosmic microwave background at 2.7K), it is advantageous to use the gain in sensitivity that can be obtained with cooled mixers.  However, in order to reduce size and cost and increase the mission lifetime, in SWAS they are only passively cooled to  ~ 170 K rather than to cryogenic temperatures. 

    Thus, the receiver consists of two (one for each polarization) second harmonic mixer diodes, each pumped by its own frequency tripled InP Gunn oscillator.  The first of these systems is centered near 490 GHz to observe O2 and atomic carbon and the second near 555 GHz for the study of 13CO and H2O.  To further conserve precious photons, multiplex operation is achieved by the use of an acousto-optical spectrometer which allows the simultaneous observation of 700 1 MHz channels between 1.4 and 2.1 GHz.  Figure V.B.3.a-2 shows the result of a SWAS study of the prototypical giant molecular cloud core M17SW.24  From these spectra the distribution in space, abundance (calculated from integrated line intensity), and velocity profiles (calculated from Doppler shifts) of both species are obtained.  Each location in the figure represents typically 3 - 7 hours of integration time to observe these weak signals that are only a few tenths of a degree above the 2.7 K microwave background.  Another interesting feature of these results is that the local oscillator and IF frequencies are selected so that the H2O and 13CO can be observed simultaneously by use of the response in each sideband of the heterodyne receiver.  Finally, it should be noted how much stronger the isotopic 13CO line is th an that of H2O.  In fact, CO is so abundant in the interstellar medium that its less abundant isotopic species are often used for astrophysical studies to avoid the problems associated with recovering remote sensing data from optically thick data.   The 13CO spectra are divided by 10 in all cases except at positions (0.0, 0.0) and (-3.2, 0.0), where the spectra are divided by 20. The 16 spectra make up a 4 x 4 map obtained on a regular grid separated by 3.'2. "

        b. Herschel is a cornerstone observatory mission of the European Space Agency (ESA) and is projected for launch in 2009.25   It is designed to study the formation and evolution of galaxies outside of our own galaxy and the energy sources of particularly luminous galaxies in the early universe.  Because of the absorption and re-radiation of ultraviolet radiation by dust grains, galaxies emit large fractions (~30 - 100%) of their energy in the far infrared.  Additionally, distant galaxies have large red-shifts that further shift this radiation to long wavelength.  Within our own galaxy, Herschel will search for proto-stars, study the formation of stars from the interstellar medium, and explore the evolution of planetary systems.  This is a much wider range of scientific goals than those of SWAS and as a result Herschel carries a suite of complementary instruments.

        Herschel is a much larger (7 m high, 4.3 m wide, with a 3.5 m telescope primary and 3.25 ton weight) instrument than SWAS and will represent about an additional 10 years of THz technology development.  Herschel will have three instruments, employing photoconductor (PACS - Photoconductor Array Camera and Spectrometer),26 bolometer (SPIRE -Spectral and Photometric Imaging REceiver),27 and superconducting mixer detectors (HIFI - Heterodyne Instrument for FIRST).28  In order to optimally detect the molecular emissions against the cold background, these detectors and parts of their optics and electronics will be cooled to cryogenic temperatures by a superfluid liquid helium cryostat with a 3 year minimum lifetime.  As an important by-product of this cooling, much smaller local oscillator powers can be used in the heterodyne instruments and operation to much higher frequencies will result.

        The three complementary instruments on Herschel typify systems in the Submillimeter /Terahertz and comparison can illustrate the trade-offs associated with each.   The heterodyne system will provide very high spectral resolution from 610 µ (490 GHz) down to at least 160 µ (1.9 THz).  However, local oscillator and single mode matching requirements restrict both its high frequency limit and broad bandedness.  In comparison, the two 'optical' systems will provide medium resolution spectroscopy and photometry between 60 µ (5 THz) and 600 µ (500 GHz).  Because these systems use 'optical' rather than heterodyne detection, they do not face increasing difficulties in the fabrication of single mode mixers and in the provision of local oscillator power with increasing frequency.  Indeed, PACS employs a 16 x 25 stressed Ge:Ga array for imaging photometry and spectroscopy that has a long wavelength cutoff near 210 µ.

        HIFI:  It is useful to compare the Herschel heterodyne instrument HIFI with that of SWAS.   HIFI uses helium temperature Superconductor-Insulator-Superconductor (SIS) and Hot Electron Bolometer (HEB) mixers.  The much lower local oscillator power requirements of these mixers aids in the design of a relatively broad banded system , with continuous frequency coverage of the 480-1250 GHz band, and also of the 1410-1910 bands.  Figure V.B.3.b-1 shows a functional block diagram.  Inspection of this figure shows that Herschel is similar to SWAS and other heterodyne THz astronomical receivers.  However, it is much more complex, with seven pairs (one for each polarization) of mixers spread over its much wider spectral range.  It’s IF section also uses Acousto-Optic Modulators (AOMs) for multiplex operation, but supplements them with auto-correlator spectrometers to provide a variety of combinations of bandwidth and resolution. 

       In many respects the greatest challenge for a heterodyne system operating at THz frequencies is the production of adequate local oscillator power.  HIFI plans to use 14 separate local oscillator sub-bands.  These synthesized local oscillators will consist of K- to W-band multipliers, high power MMIC amplifiers operating in 5 bands between 71 and 113 GHz and high frequency planar diode multipliers.29   The high spectral resolution of this heterodyne instrument, combined with the narrow spectral features of low temperature astrophysical molecules, makes Herschel an ideal instrument to study a variety of astrophysical processes.  Its wide spectral coverage can provide massive redundancy for the determination of cosmic abundances and interstellar chemistry.  Its high resolution is essential to avoid spectral confusion and line blending in studies of dynamically evolving regions.

        Photoconductor Array Camera and Spectrometer:  The PACS instrument26 is based on a photoconductive detector that uses two 25x16 Ge:Ga  unstressed/stressed arrays30 to cover two bands,  60-130µ and 130-210µ.  A key element of this instrument is the very high sensitivity (NEP ~ 5 x 10-18 W/Hz- 1/2) obtainable with Ge:Ga photoconductive detectors at ~ 2 K.31,32 (32)  However, these detectors have photoconductive thresholds at 130µ (unstressed) and 210µ (stressed), respectively, and for observations at longer wavelength, Herschel uses either heterodyne (HIFI) or bolometer (PACS) detectors.  Figure V.B.3.b-2 shows the relative response of these detectors as a function of wavelength.  In its photometry mode PACS will perform photometry (λ/Δλ~2) simultaneously in the two bands, with a mesh filter selected choice (60 - 90 µ/ 90 - 130µ) available on the unstressed shorter wavelength array.  This two-color system is designed for the study of broad emission features of external galaxies.  In its spectrometer mode PACS has a resolution of ~1500.  This is accomplished with a diffraction grating in a Littrow configuration and a dichroic beam splitter to separate diffraction orders.  In this configuration, multiplex spatial imaging is retained by the use of a 5 x 5 pixel detector array.

PACS complements HIFI in two important aspects and is a good illustration of the technology trade-offs that are still necessary in the THz.  First, because it does not require local oscillator power for mixers, the difficulties of producing power at high frequency are eliminated.  Secondly, it is much easier to build arrays of photo-detectors than it is to build focal plane arrays of heterodyne mixers.  Thus, for astrophysical projects which require large scale photometric surveys, the arrays of PACS provide spatial multiplexing and a large gain in the overall photometric efficiency for the system.

        Spectral and Photometric Imaging Receiver:  The SPIRE instrument is a bolometer detector array instrument with an imaging photometer and an imaging Fourier Transform Spectrometer (FTS).   Rather than the Ge:Ga 2 K photoconductive arrays used in PACS, this instrument is based on arrays of 'spider-web' Ge bolometers.  This eliminates the long wavelength cutoffs of the photoconductive detectors , but to obtain the required sensitivity (~3 x 10-17W/Hz- 1/2) the detectors are operated at 0.3 K via a closed cycle 3He sorption refrigerator.

        SPIRE's photometer mode (Δλ/λ~3) provides broadband response simultaneously in bands centered on 250µ, 350 µ, and 500 µ, with arrays of 149, 88, and 43 elements, respectively.  Since the bolometers are broadband detectors, the pass bands are determined by a series of filters and dichroic beam splitters.  Figure V.B.3.b-3 shows a typical response for the 350 µ band.  Again, as with PACS, this low resolution response is designed for the study of the broad emission features of distant galaxies.

       In its spectrometer mode, SPIRE uses an FTS to obtain resolution which can be adjusted between 0.04 - 2 cm-1 (λ/Δλ from 20 - 1000 at 250 µ).  It has two separate detector arrays of 37 and 19 elements to cover the 200 - 200 µ and 300 - 670 µ bands, respectively.

        The main scientific goals of SPIRE are deep extragalactic and galactic imaging surveys.  Because the bolometer arrays make possible imaging surveys to longer wavelengths than PACS, SPIRE provides an important compliment in the spectral region where distant galaxies, with large red shifts, are expected to radiate much of their energy.

        HERSCHEL summary:  The suite of instruments on Herschel is a particularly interesting example of the current technological state-of-the-art in the THz region.  While taken as a whole, Herschel admirably addresses the scientific issues at hand, there is clearly still a gap between the extension of microwave-like technology up from long wavelength and optical-like technology down from short wavelength. 

        It is probably fair to say that the single technological advance which would most impact similar systems in the future would be the development of widely tunable Submillimeter /Terahertz sources with enough power to use as local oscillators for arrays of mixer elements throughout this spectral region.  This would make possible the development of widely tunable heterodyne receivers (in contrast to receivers in carefully selected spectral regions) whose mixer arrays would have the spatial multiplex advantage of the 'optical' systems in PACS and SPIRE.  This ‘ideal’ system of the future would then be able to use both spatial and frequency multiplex principles to fully utilize the precious and expensive photons collected by the telescope.

        3.  Examples of other results:  In this section we will briefly discuss a selection of astrophysical results based on Submillimeter /Terahertz observations to illustrate both the current technological state of the art as well as some of the science.

        a.  Mapping the galaxy with CO:  Figure V.B.4.a-1 shows a comparison between an optical image of the galaxy (top) and an emission map of the J = 1 →0 of CO at 115 GHz (bottom).  The astrophysical significance of this comparison is the close correlation between the CO emission and the optical obscuration, showing that regions where the gas and dust densities are high enough to allow molecules to form also produce optical 'dark clouds’.33  Figure V.B.4.a-2 shows a more detailed map of the CO emission from giant molecular clouds (GMCs) located in the

Perseus arm of our galaxy.34 These GMCs contain a significant fraction of the mass of the interstellar matter in the Milky Way, each with a mass of the order 106 that of our sun.  These maps are based on data obtained with the 1.2 m telescope listed in Table V.B.2-1 and currently located at the Center for Astrophysics.  This telescope has been dedicated to CO surveys for about 25 years and has produced a significant body of results.  Briefly, it is a multiplex heterodyne system , with an SIS mixer, Gunn local oscillator and a 1.4 GHz IF frequency, and a 256 channel spectrometer.  These maps are also a good example of more general Submillimeter /Terahertz capabilities: the ability to penetrate particulate matter and to accurately measure small frequency shifts.  Although the dust makes up only a relatively small fraction of the interstellar matter, it scatters light so as to make much of the most interesting galactic regions unobservable.  These maps of CO are possible and astrophysically significant for three reasons.  First, CO is relatively abundant and its presence is highly correlated with the numerically more abundant (but difficult to observe H2 and He).  Second, its radiation at 115 GHz readily penetrates the dust content of the clouds.  Finally, its sharp spectral resonance makes possible not only its unambiguous identification, but also the measurement of Doppler shifts which results both from local galactic turbulence as well as the over-all rotation of the galaxy.

 b. Line surveys:  In addition to using a single molecule like CO to study the astrophysical dynamics of galactic properties such as star formation, astronomers are also keenly interested in the chemistry of the interstellar clouds themselves.  The molecular cloud in Orion is particularly well suited for study both because of its size and proximity (~500 pc; 2000 LY).  There have been many line surveys of this source over the years, but a good example is that of Schilke, Groesbeck, Blake, and Phillips.35 For this study the CSO instrument atop Mauna Kea listed in Table V.B.2-

1 was used to survey the 325 – 360 GHz region.  About 2200 lines were observed; an overview is shown in Fig. V.C.4.b-1 and an expanded region around 338.5 GHz is shown in Fig. V.B.4.b-2. This latter figure shows that the richness and denseness of the spectrum is such that spectral confusion rather than instrument sensitivity is becoming the limiting factor in these strong sources.  More recently, a survey in the 607 – 725 GHz region has also been completed.36

        Figure V.B.4.b-1, especially below 330 GHz, shows another important feature of the THz region: atmospheric absorption.  Although Mauna Kea is one of the best submillimeter observing sites in the world, as telescopes push to ever higher frequencies, atmospheric absorption becomes an increasing limitation.  Figure V.B.4.b-3 shows zenith observations of atmospheric transmission at Mauna Kea taken with a Fourier Transform Spectrometer (FTS) with ~200 MHz resolution.37  The notable feature of this comparison is that the data of 1998 were taken under unusually dry conditions (ground level relative humidity of ~2%) and that transmission windows as high as 1035 GHz provided as much as 35% transmission.

Figure V.B.b-3. Zenith atmospheric transmission from Mauna Kea.

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