We report a prototype CO2 gas sensor based on a simple blackbody infrared source and a spectrally narrow quantum cascade detector (QCD). The detector absorption spectrum is centered at 2260cm−1 (4.4μm) and has a full width at half maximum of 200cm−1 (25meV). It covers strong absorption bands of two spectrally overlapping CO2 isotopomers, namely the P-branch of 12CO2 and the R-branch of 13CO2. Acquisition of the spectral information and data treatment were performed in a Fourier transform infrared (FTIR) spectrometer. By flushing its sample compartment either with nitrogen, dry fresh air, ambient air, or human breath, we were able to determine CO2 concentrations corresponding to the different gas mixtures. Adetection limit of 500ppb was obtained in these experiment

Beck, M.

Di Francesco, J.

Herzig, H.-P

Hofstetter, D.

Hvozdara, L.

RERO DOC Digital Library

Appl Phys B (2011) 103:967–970DOI 10.1007/s00340-011-4532-1CO2 isotope sensor using a broadband infrared source,a spectrally narrow 4.4 μm quantum cascade detector,and a Fourier spectrometerD. Hofstetter · J. Di Francesco · L. Hvozdara ·H.-P. Herzig · M. BeckReceived: 10 December 2010 / Revised version: 24 March 2011 / Published online: 13 May 2011© Springer-Verlag 2011Abstract We report a prototype CO2 gas sensor based ona simple blackbody infrared source and a spectrally nar-row quantum cascade detector (QCD). The detector absorp-tion spectrum is centered at 2260 cm−1 (4.4 μm) and has afull width at half maximum of 200 cm−1 (25 meV). It cov-ers strong absorption bands of two spectrally overlappingCO2 isotopomers, namely the P-branch of 12CO2 and theR-branch of 13CO2. Acquisition of the spectral informationand data treatment were performed in a Fourier transforminfrared (FTIR) spectrometer. By flushing its sample com-partment either with nitrogen, dry fresh air, ambient air, orhuman breath, we were able to determine CO2 concentra-tions corresponding to the different gas mixtures. A detec-tion limit of 500 ppb was obtained in these experiments.1 IntroductionSemiconductor-based compact gas sensors using optical ab-sorption can be realized with different technologies. Thanksto the recent development of room temperature operatedD. Hofstetter () · J. Di FrancescoInstitute of Physics, University of Neuchâtel,51 Avenue de Bellevaux, 2009 Neuchâtel, Switzerlande-mail: Daniel.hofstetter@unine.chFax: +41-32-7182901L. Hvozdara · H.-P. HerzigOptics and Photonics Technology Laboratory,Institute of Microtechnology, EPFL, A.-L. Breguet 2,2000 Neuchâtel, SwitzerlandM. BeckQuantum Optoelectronics Group, Institute of QuantumElectronics, ETHZ, Wolfgang-Pauli-Strasse 16, 8093 Zurich,Switzerlandcontinuous wave mid-infrared lasers such as the quan-tum cascade laser (QCL), substances being relevant foratmospheric pollution, process control, and environmentalsensing can now be detected at high resolution [1, 2]. Be-sides a good signal to noise ratio and thus a low detectionlimit, the excellent wavelength selectivity of QCLs allowsa straight-forward discrimination of different isotopomers.As a prominent example of the medical area, non-invasive13C-breath tests are performed, where human enzyme ac-tivities, organ functions, and transport processes can be as-sessed [3]. Such tests detect a possible Helicobacter pyloriinfection of the stomach or the duodenum; in addition, theycan be easily performed and have a high patient acceptance.Unfortunately, the elevated cost of QCLs is still a majorissue, which so far prevented them from providing a realbreakthrough in medical sensing. Alternatively, breath testsusing different types of spectroscopy involving several N2Oand CO isotopes can be carried out. Such techniques havebeen developed extensively by M. Murtz et al. [4, 5]. There-fore, the use of a highly sensitive and spectrally narrow de-tector along with a broadband infrared light source couldbe an attractive alternative to laser-based spectroscopy. Aperfectly suitable device for such applications is the photo-voltaic quantum cascade detector (QCD) [6]. For a selectedwavelength range determined by the detector’s responsecurve, this method results in both a good spectral resolu-tion and a high sensitivity. Ideally, the QCD is constructedas a linear pixel array, and will be combined with a miniatur-ized grating spectrometer. It can be fabricated using standardIII–V-semiconductor technology and custom-designed withup to 1024 elements. For the prototype measurements pre-sented in this paper, where only a single pixel detector wasavailable, the spectral information was obtained thanks toa Fourier transform infrared spectrometer (FTIR). In a later968 D. Hofstetter et al.Fig. 1 Schematic representation of one active region period of a typi-cal QCD showing relevant wavefunctions and saw-tooth potential. Thedevice presented in this article has a transition energy of 286 meV(4.4 μm)version, we envisage the use of a linear array of QCDs alongwith a much simpler and well-adapted grating spectrometer.2 FabricationThe QCD used in this experiment had an active surfaceof 200 × 200 μm2. Epitaxial growth of this device wasperformed via molecular beam epitaxy of multiple In-GaAs/InAlAs layers on an InP substrate. One active re-gion period is depicted in Fig. 1. It comprises a Si-doped(1.5 × 1018 cm−3) main quantum well with a thickness of5.2 nm, and an extraction region consisting of 7 additionalquantum wells with increasing thickness. Under absorptionof a photon with E12 = 286 meV, electrons undergo firstan optical transition from the ground state A1 into the firstexcited state A2 of the main well. From there, they performa resonant tunneling transition into the uppermost quantizedstate of the extraction region (B1) and then scatter down tothe ground state of the following period A′1. A QCD typi-cally counts 20 to 30 such active region periods which aresandwiched between two highly-doped contact layers. Thefabrication procedure was based on chemical wet etching ofmesa structures in order to access the lower contact layer ofthe detector. Ti/Ge/Au/Ag/Au (1.5/12/27/50/300 nm) con-tacts are then evaporated on both bottom and top contactlayers. Finally, contact annealing, polishing of a 45◦ wedgeor etching of a diffraction grating for efficient optical cou-pling, and device mounting on a copper holder completedthe processing. Details on the fabrication can be found inRef. [6].3 Experimental setup and results3.1 Experimental setup and QCD characteristicsFor the measurement of both responsivity and CO2 absorp-tion curves, an experimental setup as shown in Fig. 2 wasFig. 2 Schematic representation of the FTIR-based setup for the sen-sor characterization described in this article. The light source is in thiscase a blackbody source at 1460 Kused. On the emitter side, it consisted of an FTIR spectrome-ter (Bruker IFS-66) and its internal glow-bar infrared source.The latter can be regarded as a nearly ideal blackbody emit-ter at 1460 K with a size of 3 mm in diameter. Its outputwas focused onto the 45◦ facet of the QCD using a singleparabolic mirror with a focal distance of 4 inches. The QCDwas mounted in a liquid nitrogen flow cryostat and stabilizedat temperatures ranging from 150 K to 300 K. The purelyphotovoltaic QCD signal was amplified via a low-noise cur-rent amplifier (SR570) with a gain of 0.2 μA/V, bandpass-filtered between 300 Hz and 10 kHz, and fed back to theexternal detector port of the FTIR. Since the characteristicsof the FTIR’s glow-bar light source were calibrated previ-ously, one could exactly determine the responsivity of theQCD. A series of response curves measured between 150 Kand 300 K is presented in Fig. 3. Each measurement shownis the result of 200 averaged rapid scans. Typical responsiv-ity values for such a QCD are on the order of 5 mA/W at300 K and 9 mA/W at 175 K. Since there is no dark cur-rent noise, the dominant noise mechanism of the device isJohnson noise. At an operating temperature of 175 K, whichresulted in maximum performance, we observed a device re-sistance of 175 k and therefore a detectivity of 5 × 108jones. It is important to note that temperatures around 200 Kcan be maintained by thermo-electric coolers, and that theydo not require the use of liquid nitrogen cooling systems. Forthe purpose of comparison, we give here the detectivity val-ues for commercial detectors at similar temperatures. LaserComponents, for instance, sells a broad band photodetectivePbSe sensor at 2300 cm−1 and 243 K with a detectivity of3 × 1010 jones (SCD-15 series). Experience has shown thata spectrally narrow detector could still be somewhat better,on the order of 5 × 1010 jones. At the same time, Hama-matsu markets a two-stage Peltier photoconductive broadband InSb detector (P6606 series) at 243 K with a detectiv-ity of 3 × 109 jones. This is to be compared with our valueof 5×108 jones for a photovoltaic QCD at 175 K. However,CO2 isotope sensor using a broadband infrared source, a spectrally narrow 4.4 μm quantum cascade 969Fig. 3 Responsivity curves of a 4.4 μm QCD as a function of tem-perature. The little bump around 2330 cm−1 is due to CO2 absorptionbandsit must be noted that neither PbSe nor InSb based detectorslend themselves easily for fabrication as linear arrays. For aQCD, this is straight-forward and will certainly be the nat-ural next step to go.3.2 Absorption of CO2 isotopomersSince CO2 is at its present planetary concentration of340 ppm an important green-house gas in earth’s atmosphere,it can be easily measured using the prototype sensor de-scribed in this article. The main absorption bands are dueto the asymmetrical stretch vibration of the CO2 molecule.More particularly, the band centered around 2360 cm−1 iscalled the R-branch, while the one around 2330 cm−1 isthe P-branch. In the following experiments, absorption mea-surements have been carried out under different experimen-tal conditions. To illustrate this, we show in Fig. 4 a com-parison between measurements with a pure nitrogen purge, adry fresh air purge, an ambient air purge, and a sample of hu-man breath. In all experiments, the 30 cm long optical pathbetween the FTIR and the QCD served as absorption cell.In order to maintain a constant gas concentration along thispath, a tubular protection was mounted around the beam. AsFig. 4 shows, there is a substantial signal change betweenthe different measurements. For clarity, three out of fourcurves have been vertically displaced by 1.5 a.u. (ambientair), 3.0 a.u. (dry fresh air), and 4.5 a.u. (nitrogen). Purgingthe FTIR for 10 minutes with pure N2 led to a small CO2 ab-sorption of only about 10% of the peak value (green trace).A simulated spectrum based on the HITRAN database in-dicated that the concentration was in this case 10 ppm [7].Given the observed signal to noise ratio of the absorptioncurve, a detection limit of roughly 500 ppb was estimated forthis experimental configuration. Under dry fresh air purge(blue trace), we observed a moderate absorption on the orderof 50%, whereas the curve without purge resulted in strongabsorption lines going close to 90% of the maximum detec-tor signal (violet trace). The corresponding CO2 concentra-Fig. 4 Comparison between different purge conditions. The green,blue, and violet curves correspond to CO2-concentrations of 10 ppm,100 ppm, and 340 ppm, respectively. The red curve is for a muchhigher concentration, namely 5000 ppm. The resolution of these mea-surements was 0.25 cm−1tions were 100 and 340 ppm, respectively. For the ambientair measurement, we noticed additional features at 2240–2285 cm−1, which cannot be explained with the absorptionlines of 12C16O2. A comparison with published spectra in-dicated that they are due to the P-branch absorption of the13C16O2 isotope of carbondioxide. Its natural abundance of1.11% is excellently confirmed by our experiments [8].In order to make the absorption of the less abundantisotopomer better visible, human breath was exhaled intothe purge tube of the FTIR. It is known from the litera-ture that human breath can contain up to 4% or 40,000 ppmof CO2 [5]. The lowest (red) trace of Fig. 4 shows the ab-sorption spectrum of such a breath sample. There is nowa large contribution of the 13C16O2 molecule absorptionwhich interferes also strongly with the 12C16O2 band around2300 cm−1. Based on the previous experiments, we con-clude that the 12C16O2 concentration was on the order of5000 ppm, whereas 60 ppm was found for the 13C16O2; con-firming again the natural abundance cited previously.3.3 On-off measurementWe finally performed a quick practical test of this sen-sor system, during which the QCD was hooked up to atwo-stage operational amplifier circuit instead of the low-noise Stanford amplifier. In addition, no spectral informa-tion was this time collected by the FTIR; thus revealingthe integrated absorption signal only. As shown in Fig. 5,the purge was switched between dry and ambient air every3 minutes, resulting in an amplified signal change on the or-der of 350 mV. Since this change corresponds to a concen-tration drop or increase of roughly 250 ppm, we can con-servatively assign a sensitivity of 1.4 mV/ppm and a CO2detection limit of 20 ppm to the system. Although this per-formance might be useful for simple CO2 level monitoring,it is evident that such integrated absorption measurementshave limited wavelength selectivity only. However, a rough970 D. Hofstetter et al.Fig. 5 On-off experiment using the glow-bar source, a QCD, and atwo-stage operational amplifier circuitestimation has shown that a compact diffraction spectrome-ter with 4 μm grating period, 100 mm working distance, and100 μm slit width provides a resolution of 2 cm−1, whichwould be by far sufficient to discriminate between the P-and R-branches of the two relevant CO2 isotopomers.4 ConclusionIn conclusion, we have shown a prototype CO2 sensor whichwas operated with a blackbody source, a QCD, and an FTIRspectrometer. In a simplified experimental arrangement, am-plification of the signal was accomplished with a two-stageoperational amplifier circuit. We observed a detection limitof 500 ppb in the somewhat more sophisticated FTIR con-figuration, whereas the direct absorption measurement with-out spectral information resulted in roughly 20 ppm. To-gether with a suitable grating spectrometer, linear arraysof such QCDs will become very promising candidates formid-infrared spectroscopic measurements in medical appli-cations.Acknowledgements The authors would like to acknowledge theGebert-Ruef foundation for initiating this research and the nanoterainitiative (project IRSENS) of the Swiss National Science Foundationfor their generous support.References1. M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems,E. Gini, H. Melchior, Science 295, 301 (2002)2. N. Bandyopadyay, Y. Bai, B. Gokden, A. Myzaferi, S. Tsao,S. Slivken, M. Razeghi, Appl. Phys. Lett. 97, 131117 (2010)3. E. Israeli, Y. Ilan, S.B. Meir, C. Buenavida, E. Goldin, J. Clin. Gas-troenterol. 37, 139 (2003)4. H. Sabana, T. Fritsch, M.B. Onana, O. Bouba, P. Hering, M. Murtz,Appl. Phys. B, Lasers Opt. 96, 535 (2009)5. M. Sowa, M. Murtz, P. Hering, J Breath Res 4, 047101 (2010)6. F.R. Giorgetta, E. Baumann, M. Graf, Q. Yang, C. Manz, K. Köhler,H.E. Beere, D.A. Ritchie, E. Linfield, A.G. Daveis, Y. Fedoryshyn,H. Jäckel, M. Fischer, J. Faist, D. Hofstetter, IEEE J. Quantum Elec-tron. 45, 1039 (2009)7. See for instance http://www.spectralcalc.com/calc/spectralcalc.php8. A. Kindness, I.L. Marr, Appl. Spectrosc. 51, 17 (1997)

CO2 isotope sensor using a broadband infrared source, a spectrally narrow 4.4 μm quantum cascade detector, and a Fourier spectrometer

http://doc.rero.ch/record/310550/files/340_2011_Article_4532.pdf

CO2 isotope sensor using a broadband infrared source, a spectrally narrow 4.4 μm quantum cascade detector, and a Fourier spectrometer

Abstract

Similar works

Full text

Available Versions

RERO DOC Digital Library