Fast analysis of gases in the submillimeterâterahertz with âabsoluteâ specificity

Fast analysis of gases in the submillimeterterahertz with “absolute” specificityIvan R. Medvedev, Markus Behnke, and Frank C. De Lucia Citation: Applied Physics Letters 86, 154105 (2005); doi: 10.1063/1.1897442 View online: http://dx.doi.org/10.1063/1.1897442 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/86/15?ver=pdfcov Published by the AIP Publishing

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Fast analysis of gases in the submillimeter/terahertz with “absolute”specificity

Ivan R. Medvedev, Markus Behnke,a! and Frank C. De Luciab!

Department of Physics, Ohio State University, Columbus, Ohio 43210

sReceived 6 October 2004; accepted 16 February 2005; published online 5 April 2005d

A submillimeter/terahertz point detector for gas monitoring and quantification is described. It isbased upon the fasts,15 GHz/sd sweeping of high spectral puritys,1/107d, high brightnesss,1014 Kd microwave sources and a scanning electronic reference for frequency measurement. Thisapproach can quantify the complex rotational spectrum of gases at a rate of,105 spectral resolutionelements/second at high signal to noise. This resolution and the uniqueness of Doppler limitedrotational spectra provide “absolute” specificity and “zero” false alarm rates even in complexmixtures. Moreover, the small size, low power consumption, and the potential of very low cost makethis approach attractive for a number of important applications. ©2005 American Institute ofPhysics. fDOI: 10.1063/1.1897442g

There has been considerable recent interest in the devel-opment of spectroscopic gas sensors. Approaches whichmake use of the rotational signature in the submillimeter/terahertzsSMM/THzd region have been widely described,especially those based on terahertz-time domain spectros-copy sTHz–TDSd and related techniques.1–15

The purpose of this letter is to describe a particularlysimple and powerful alternative that builds upon what is of-ten referred to as “SMM spectroscopy.” The approach de-scribed here is based upon an all solid state, room tempera-ture implementation of a system architecture, the fast scansubmillimeter spectroscopic techniquesFASSSTd.16 Addi-tionally, FASSST is very simple both in comparison to tradi-tional microwave spectrometers and systems based on lasersources.17–19 The keys to this approach ares1d the spectralbrightnesssW/Hzd of a cw source that can provide a milli-watt of power within a Doppler limited linewidthsDn,1 MHzd in this spectral region corresponds to source tem-perature of ,1014 K, and s2d contrary to widely heldopinion,1,2,4,9–11,20the background noise in this spectral re-gion is very low, making possible the use of very sensitivecw detectors.21

It has long been recognized that the SMM/THz is a par-ticularly advantageous region for the study of polar mol-ecules because of the strong maxima of their interactionstrengths in this spectral region.22 Laboratory investigationshave included the spectroscopy of small fundamental spe-cies, ions, and reactive radicals; studies of high temperaturesystems, excited states, and active laser plasmas; and inves-tigation of collisional energy transfer, pressure broadening,and low temperature systems.23–27 Additionally, there havebeen proposed a number of more applied applications includ-ing analytical chemistry, industrial safety, portal and buildingmonitoring, and chemical intelligence. A more detailed dis-cussion has recently been published.28

The potential use of microwave spectroscopy to probethe molecular rotational fingerprint for analytical purposeswas discussed 50 years ago by Townes and Schawlow,29 and

more recently by Hrubesh.30 Townes and Schawlow devotedan entire chapter of their classic book to the subject andexpressed some surprise even then at the slowness of thisapplication to emerge. About 25 years later Hewlett Packardintroduced a commercial instrument, but during its limitedproduction it gained considerably more favor with spectros-copists interested in molecular structure than from the ana-lytical community. In retrospect its sizesit filled a smallroomd, cost sseveral hundred thousand dollarsd, and limitedcapability sit operated below 50 GHzd were significant im-pediments to its wide spread adoption. The approach de-scribed here overcomes all of these limitations.

In general the absorption coefficients of rotational spec-tra increase as the cube of the frequency, reach a maximumsomewhere in the SMM/THz, and then fall exponentially.22

While the water spectrum peaks in the 1–3 THz region,most species of analytical interest are heavier and havemaxima at significantly lower frequency. Specifically, mosthave strong spectra in the region around 300 GHz and asignificant number of lines in any chosen 15 GHz region,31

and it is this region that we have chosen.One of the principal attributes for point detection sys-

tems based in the SMM/THz is that the complex and redun-dant fingerprint associated with the rotational degrees offreedom leads to very high specificity. In order to take fulladvantage of this attribute, Doppler limited spectral resolu-tion ,1 MHz s0.000 03 cm−1d is required. As a result, thehighly specific information content of a Doppler limitedFASSST spectrum is such that the sophisticated algorithmsthat have been considered for the optimal use of low resolu-tion spectral data in the SMM/THzsRef. 4d are not required.

Additionally, optimum spectroscopic sensitivity occurswhen the pressure broadening of the sample is equal to theDoppler/instrument resolution.22,29 Under these conditions,in a 1 m long fundamental waveguide cell, the optimumsamples,10 mTorrd is less than 10−9 mol, and samples sev-eral orders of magnitude smaller can be studied with micro-second integration times.31

While in principle THz–TDS systems could obtain thisresolution with,100 m long delay lines, their low spectralbrightness has resulted in these systems typically being de-signed for ,3000 MHz resolution11,32–34 and, for optimal

adPresent address: Deutsche Forschungsgemeinschaft, Kennedyallee 40,53175 Bonn, Germany.

bdElectronic mail: fcd@mps.ohio-state.edu

APPLIED PHYSICS LETTERS86, 154105s2005d

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sensitivity, correspondingly increased sample pressures.These comparisons are shown in Table I. In this table therange of values for THz–TDS systems reflects the,10 nW–10mW average power reported in theliterature35,36 and assumes that this power is uniformlyspread over a 1 THz bandwidth. Reality is much more com-plex because of the rapid roll-off with increasing frequencyof these sources.

A compact, room temperature, solid-state system basedon this approach is shown in Fig. 1. In it a voltage tunableoscillator sVTOd that can be electronically tuned between10.25 and 10.83 GHz is multiplied by an active transistormultiplier to produce a few milliwatts at W-bands75–110 GHzd. This power is then amplified to,20 mW ina W-band transistor power amplifier and then tripled in adiode multiplier to produce,0.5 mW between 246.8 and261.2 GHz. After passing through the cell, the power is de-tected with a Schottky diode. The sweep is fully electronicand requires no mechanical tuning. The speed of the fre-quency scans,15 GHz/sd freezes any drift associated withthe VTO.

A key element of the development of this compact sys-tem was the replacement of the larges,40 md Fabry–Pérotcavity previously used for frequency calibration in an earlierbackward wave oscillator tube implementation.16 This wasaccomplished with a compact, scanning electronic system.Fixed frequency reference points are provided by first downconverting the VTO frequency to,500 MHz, with a subse-quent comparison to a 10 MHz comb. Because the small

physical size of the VTO precludes rapid variations in itsfrequency-voltage function, it is possible to interpolate be-tween reference points which are spaced by 240 MHz at themeasurement frequency to obtain the measurement accuracytypical of Doppler limited spectroscopy in this spectral re-gion,,1/10 of a linewidth or,50 kHz. Comparisons with acalibration gassSO2d produced a 0.026 MHz rms deviation,within the expected uncertainty of the calibration catalog,and an uncertainty of,1/107.

This system architecture is very simple because it re-places the complexity and slow speed of the synthesizedphase locked multiplier chains typical of microwave spec-trometers with the fast scan of the VTO and fast, paralleldigitization of the signal and reference channels. While thehardware aboveX band is still specialized and made to ordersthe 38 multiplier, W-band amplifier, and33 multipliermounted in waveguide blocks cost $5000–$10 000 eachd, itis fundamentally very simple—only a few diodes and tran-sistors. Moreover, the mass market of wireless communica-tions has recently made available components at the chiplevel at nominals,$100d cost that would, at present, leaveonly the two highest frequency diode multipliers and a diodedetector beyond the reach of this market.

Figure 2 shows a spectrum to illustrate the informationcontent, specificity, and speed of the FASSST system in thecontext of a mixture of four gases. At the nominal scan rateof 15 GHz/s provided by the chirped VTO, the entire spec-tral interval swhich is not shown in the figure because eachresolution element in the graph would occupy only about2 mmd required a sweep time of,1 s. The lower subscan in

TABLE I. Comparison of cw and THz–TDS techniques.

Parameter

Technique

cw SMM THz–TDS

Resolution 0.3 MHz 3000 MHzOptimum pressure 0.01 Torr 100 Torr

Source brightness 10−3 W/MHza 10−14–10−11 W/MHz saveragedb

10−10–10−7 W/MHz speakdb

Source temperaturesDoppler linewidthd 1014 Ka 103–106 K saveragedb

107–1010 K speakdb

Source temperaturesatmospheric linewidthd 231010 Kc 103–106 K saveragedb

107–1010 K speakdb

a1 mW of power averaged over a 1 MHz Doppler linewidth, the spectralpurity and temperature of the source are much higher.bVaries with frequency according to roll-off with frequency of source.c1 mW of power averaged over a 5 GHz atmospheric linewidth, the spectralpurity and temperature of the source are much higher.

FIG. 1. The solid state FASSST analysis system.

FIG. 2. Individual spectra of four gasessat partial pressures of,2 mTorrdand a mixture of the four. The full spectrumsnot shownd was recorded in,1 s, the upper trace in,0.1 s, and the lower trace,0.01 s.

154105-2 Medvedev, Behnke, and De Lucia Appl. Phys. Lett. 86, 154105 ~2005!

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the figure required,10 ms. Mixtures of gases are particu-larly challenging for the traditional analytical tools such asgas chromatographysGCd, mass spectroscopysMSd, infraredspectroscopy, ion mobility spectroscopy, etc., because oftheir combination of many fewer resolvable resolution ele-ments and less unique fingerprints. In practice, hyphenatedtechniques such as GC–MS are used, whereas the specificityillustrated in Fig. 2 comes from FASSST alone. Even in thegraphically compressed upper trace, the identification of thefour individual gases in this mixture is visually straightfor-ward and “absolute.” Numerical measurement of the fre-quencies to,1/107 and the known relative amplitudes of thefingerprint elements add significantly to the information con-tent. The empty analytical space in Fig. 2, combined withthese numerical opportunities make possible the analysis ofconsiderably more complex spectral environments.

While the figures of merit in Table I are useful, in systemdesign and optimization there are many trade-offs amongresolution, sensitivity, integration time, and cell configura-tion. As a result comparisons, especially between frequencyand time domain approaches, can vary considerably with de-sign optimization. Thus, it is useful to make a direct com-parison with a THz–TDS gas detection system optimized forspeed and sensitivity.37 In order to improve sensitivity, thisTHz–TDS system makes use of a multipass white cell ofvolume ,50 000 cm3. With this system it was possible todetect CH3Cl at a pressure of 7.5 mTorr with a signal-to-noise ratiosS/Nd of ,5 with ,5 min of data acquisitiontime. In comparison, Fig. 2 shows a detection of CH3F saspectroscopically similar moleculed at a pressure of 2 mTorrwith a S/N of,500 with a data acquisition time devoted toa single line of,10−3 s. If normalizations for cell volume,sample pressure, S/N, and integration time are used, the cwsystem of Fig. 1 is capable of detecting about 107 lesssample. If minimum detectable partial pressure rather thanminimum detectable sample is the figure of merit, the factorrelated to the additional cell volume of the white cell incomparison to our simple absorption cell should not be in-cluded in the normalization, and this factor is reduced to 105.Even with the impressive performance, relatively simplemodifications to the cw system can extend its sensitivity by anumber of orders of magnitude.

Additionally, for studies of time varying concentrationsthe speed of the swept cw system is set only by the transformlimit of the linewidth; about 1ms. This is because the bright-ness of the cw sources and the sensitivity of the detectorsmake very small integration times common. This speed ismany orders of magnitude faster than recently reported for avariation of the THz–TDS technique optimized for speed.7

In summary, in order to take advantage of the favorablephysics that underlies spectroscopy in the SMM/THz forpoint detection purposes, it is important to have brightsources that can provide enough power within a Dopplerbroadened linewidth to approach molecular saturation. It isthen possible to take advantage of the complex fingerprintsassociated with rotational spectra to establish “absolute”specificity while requiring only the very small sample quan-tities associated with the low pressure Doppler limit in theSMM/THz. A small, low power consumption, all solid statesystem with these attributes has been described. Moreover,the system architecture is fundamentally simple. Finally,

with the growth of technology in this spectral region to sup-port wireless communication, collision avoidance radar, andother applications, the system described here also has thepotential to become very low cost in the near future.

The authors would like to thank the Army Research Of-fice and the Defense Advanced Research Projects Agency fortheir support. They would also like to thank the referee forsuggesting the more specific comparisons of Table I and inthe text.

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154105-3 Medvedev, Behnke, and De Lucia Appl. Phys. Lett. 86, 154105 ~2005!

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