We report on a set of high-sensitivity terahertz spectroscopy experiments making use of QCLs to detect rotational molecular transitions in the far-infrared. We demonstrate that using a compact and transportable cryogen-free setup, based on a quantum cascade laser in a closed-cycle Stirling cryostat, and pyroelectric detectors, a considerable improvement in sensitivity can be obtained by implementing a wavelength modulation spectroscopy technique. Indeed, we show that the sensitivity of methanol vapour detection can be improved by a factor ≈ 4 with respect to standard direct absorption approaches, offering perspectives for high sensitivity detection of a number of chemical compounds across the far-infrared spectral range.
References
[1]
Gallagher, T.F.; Kachru, R.; Gounand, F.; Bjorklund, G.C.; Lenth, W. Frequency-modulation spectroscopy with a pulsed dye laser. Opt. Lett. 1982, 7, 28–30.
[2]
Gagliardi, G.; Tamassia, F.; De Natale, P.; Gmachl, C.; Capasso, F.; Sivco, D.L.; Baillargeon, J.N.; Hutchinson, A.L.; Cho, A.Y. Sensitive detection of methane and nitrous oxide isotopomers using a continuous wave quantum cascade laser. Eur. Phys. J. D 2002, 19, 327–331.
[3]
Borri, S.; Bartalini, S.; De Natale, P.; Inguscio, M.; Gmach, C.; Capasso, F.; Sivco, D.L.; Cho, A.Y. Frequency modulation spectroscopy by means of quantum-cascade lasers. Appl. Phys. B 2006, 85, 223–229.
[4]
Wiltse, J.C. History of millimeter and submillimeter waves. IEEE Trans. Microw. Theory Tech. 1984, 32, 1118–1127.
[5]
De Lucia, F.C. Sensing with Terahertz Radiation; Mittleman, D., Ed.; Springer: Berlin, Germany, 2003; pp. 39–115.
[6]
Leisawitz, D.T.; Danchi, W.C.; DiPirro, M.J.; Feinberg, L.D.; Gezari, D.Y.; Hagopian, M.; Langer, W.D.; Mather, J.C.; Moseley, S.H., Jr.; Shao, M.; et al. UV, optical, and IR space telescopes and instruments. Proc. SPIE 2000, 4013, doi:10.1117/12.393957.
[7]
Testi, L.; Zwaan, M. ALMA status and science verification data. The Messenger 2012, 145, 17–18.
[8]
De Natale, P.; Lorini, L.; Inguscio, M.; Nolt, I.G.; Park, J.H.; Di Lonardo, G.; Fusina, L.; Ade, P.A.; Murray, A.G. Accurate frequency measurements for H2O and 16O3 in the 119-cm？1OH atmospheric window. Appl. Opt. 1997, 36, 8526–8532.
[9]
Bellini, M.; De Natale, P.; di Lonardo, G.; Fusina, L.; Inguscio, M.; Prevedelli, M. Tunable far infrared-spectroscopy of 16O3 ozone. J. Mol. Spectrosc. 1992, 152, 256–259.
Evenson, K.M.; Jennings, D.A.; Petersen, F.R. Tunable far infrared-spectroscopy. Appl. Phys. Lett. 1984, 44, 576–578.
[12]
Zink, L.R.; De Natale, P.; Pavone, F.S.; Prevedelli, M.; Evenson, K.M.; Inguscio, M. Rotational far infrared spectrum of 13CO. J. Mol. Spectrosc. 1990, 143, 304–310.
[13]
Nolt, I.G.; Radostitz, J.V.; di Lonardo, G.; Evenson, K.M.; Jennings, D.A.; Leopold, K.R.; Vanek, M.D.; Zink, L.R.; Hinz, A.; Chance, K.V. Accurate rotational constants of CO, HCl, and HF: Spectral standards for the 0.3- to 6-THz (10- to 200-cm？1) region. J. Mol. Spectrosc. 1987, 125, 274–287.
[14]
Odashima, H.; Zink, L.R.; Evenson, K.M. Tunable far-infrared spectroscopy extended to 9.1 THz. Opt. Lett. 1999, 24, 406–407.
[15]
Piau, G.; Brown, F.X.; Dangoisse, D.; Glorieux, P. Heterodyne detection of tunable FIR sidebands. IEEE J. Quant. Electron. 1987, 23, 1388–1391.
[16]
Bellini, M.; de Natale, P.; Inguscio, M.; Fink, E.; Galli, D.; Palla, F. Laboratory measurements of rotational transitions of lithium hydride in the far-infrared. Astrophys. J. 1994, 424, 507–509.
[17]
Fusina, L.; di Lonardo, G.; de Natale, P. The ground state spectroscopic parameters and molecular geometry of SbH3. J. Chem. Phys. 1998, 109, 997–1003.
[18]
Modugno, G.; de Natale, P.; Bellini, M.; Inguscio, M.; Di Lonardo, G.; Fusina, L.; . Vander Auwera, J. Precise measurement of molecular dipole moments with a tunable far-infrared Stark spectrometer: Application to HOCl. J. Opt. Soc. Am. B Opt. Phys. 1996, 13, 1645–1649.
[19]
De Natale, P.; Bellini, M.; Goetz, W.; Prevedelli, M.; Inguscio, M. Hyperfine-structure and isotope shift in the far-infrared ground-state transitions of atomic oxygen. Phys. Rev. A 1993, 48, 3757–3760.
Vitiello, M.S.; Tredicucci, A. Tunable emission in Terahertz quantum cascade lasers. IEEE Trans. THz Sci. Technol. 2011, 1, 76–84.
[23]
Vitiello, M.S.; Consolino, L.; Bartalini, S.; Taschin, A.; Tredicucci, A.; Inguscio, M.; De Natale, P. Quantum-limited frequency fluctuations in a terahertz laser. Nat. Photon. 2012, 6, 525–528.
[24]
Ren, Y.; Hayton, D.J.; Hovenier, J.N.; Cui, M.; Gao, J.R.; Klapwijk, T.M.; Shi, S.C.; Kao, T.Y.; Hu, Q.; Reno, J.L. Frequency and amplitude stabilized terahertz quantum cascade laser as local oscillator. Appl. Phys. Lett. 2012, 101, doi:10.1063/1.4751247.
[25]
Hübers, H.W.; Pavlov, S.G.; Richter, H.; Semenov, A.D.; Mahler, L.; Tredicucci, A.; Beere, H.E.; Ritchie, D.A. High-resolution gas phase spectroscopy with a distributed feedback terahertz quantum cascade laser. Appl. Phys. Lett. 2006, 89, doi:10.1063/1.2335803.
[26]
Hübers, H.W.; Kimmitt, M.F.; Hiromoto, N.; Brundermann, E. Terahertz spectroscopy: System and sensitivity considerations. IEEE Trans. Terahertz Sci. Technol. 2011, 1, 321–331.
[27]
Eichholz, R.; Richter, H.; Pavlov, S.G.; Wienold, M.; Schrottke, L.; Hey, R.; Grahn, H.T.; Hübers, H.W. Multi-channel terahertz grating spectrometer with quantum-cascade laser and microbolometer array. Appl. Phys. Lett. 2011, 99, doi:10.1063/1.3645635.
[28]
Ren, Y.; Hovenier, J.N.; Higgins, R.; Gao, J.R.; Klapwijk, T.M.; Shi, S.C.; Klein, B.; Kao, T.-Y.; Hu, Q.; Reno, J.L. High-resolution heterodyne spectroscopy using a tunable quantum cascade laser around 3.5 THz. Appl. Phys. Lett. 2011, 98, doi:10.1063/1.3599518.
Consolino, L.; Taschin, A.; Bartolini, P.; Bartalini, S.; Cancio, P.; Tredicucci, A.; Beere, H.E.; Ritchie, D.A.; Torre, R.; Vitiello, M.S.; et al. Phase-locking to a free-space terahertz comb for metrological-grade terahertz lasers. Nat. Commun. 2012, 3, doi:10.1038/ncomms2048.
[31]
Pearson, J.C.; Drouin, B.J.; Maestrini, A.; Mehdi, I.; Ward, J.; Lin, R.H.; Yu, S.; Gill, J.J.; Thomas, B.; Lee, C.; et al. Demonstration of a room temperature 2.48–2.75 THz coherent spectroscopy source. Rev. Sci. Instrum. 2011, 82, doi:10.1063/1.3617420.
[32]
Toxicological Profile for Hydrogen Sulfide; U.S. Department of Health and Human Services, Public Health Service Agency for Toxic Substances and Disease Registry: Atlanta, Georgia, USA, 2006; p. 154. Available online: http://www.atsdr.cdc.gov/toxprofiles/tp114-p.pdf (accessed on 11 March 2013).
