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University of Helsinki Faculty of Science

Molecular spectroscopy and theoretical chemistry


Contact Information

Laboratory of Physical Chemistry
Department of Chemistry
A.I. Virtasen aukio 1
(P.O. BOX 55)
FI-00014 University of Helsinki

Group leader
Prof. Lauri Halonen, D.Sc.
Phone: +358(0)2941 50280
email: lauri.halonen


Experimental work in laser spectroscopy

Development of mid-infrared spectroscopy methods

Fig. 1. (a) The absolute frequency of a mid-infrared beam produced by cw difference-frequency generation (DFG) or OPO can be determined by measuring the frequencies of the pump and signal beams with a near-infrared optical frequency comb (OFC), see for example [4] (b) Our new method reported here is based on second harmonic generation (SHG) of the mid-infrared beam. This significantly simplifies the scanning of the mid-infrared frequency while it is locked to the OFC, see [5].

We have applied our cw OPOs to mid-infrared spectroscopy, with the focus on sensitive trace gas detection. The high output power makes the OPOs ideal for photo-acoustic spectroscopy, where the detection sensitivity is directly proportional to the optical power. As an example, we have used cantilever-enhanced photoacoustic detection to measure trace amounts of hydrogen cyanide (HCN) and methane (CH4) [1]. While methane has an important role in atmospheric studies, HCN is a highly toxic compound released in many industrial processes and combustion. With our OPO-based photoacoustic spectrometer we have reached detection limits as low as 190 ppt and 65 ppt for HCN and CH4, respectively. Another highly sensitive method that we have applied in our trace gas detection experiments is cavity ring-down spectroscopy. We have developed the first re-entrant off-axis cavity ring-down spectrometer that works in the mid-infrared region [2]. The spectrometer can be used for real-time analysis of trace gases, such as formaldehyde, which is a common indoor pollutant that can cause health problems already at low concentrations. Formaldehyde is also one of the proposed biomarker molecules for medical diagnostics based on breath analysis[3].

Fig. 2. Left: The singly-resonant cw mid-infrared OPO used to realize the method of Fig. 1b. Right: A part of the cavity ring-down spectrometer used in the experiments. A major part of the mid-infrared beam of the OPO (black box in the figure) is frequency doubled and sent to another lab over a fiber link, in order to reference its frequency to an OFC. The beat note between the comb and the frequency-doubled mid-infrared beam is shown on the display, in the background of the figure.

Trace gas detection by laser spectroscopy often relies on spectroscopic databases, such as HITRAN, which lists several parameters of molecular absorption lines, including line strength, center frequency, and broadening parameters. In collaboration with MIKES (the National Metrology Institute of Finland), we use our OPOs in accurate measurements of molecular line data, in order to improve the accuracy of the databases. For this purpose, we have developed a novel spectrometer, which can be used for absolute-frequency spectroscopy in the mid-infrared region [4, 5]. The principle of the spectrometer is shown in Fig. 1. Photographs of the experimental setup are shown in Fig. 2. The output of the cw mid-infrared OPO used for spectroscopy is frequency-doubled, after which it can be locked to a commercial near-infrared frequency comb. The comb is referenced to the SI second for traceable optical frequency measurements. The high output power of the OPO allows us to use saturated absorption spectroscopy either directly [4, 6] or in cavity ring-down measurements [5], so as to improve the resolution of line center determination well below the Doppler limit.

[1] J. Peltola, M. Vainio, T. Hieta, J. Uotila, S. Sinisalo, M. Metsälä, M. Siltanen, and L. Halonen, "High sensitivity trace gas detection by cantilever-enhanced photoacoustic spectroscopy using a mid-infrared cw OPO," Opt. Express 21, 10240-10250 (2013).
[2] J. Peltola, M. Vainio, V. Ulvila, M. Siltanen, M. Metsälä and L. Halonen, "Off-axis re-entrant cavity ring-down spectroscopy with a mid-infrared continuous-wave optical parametric oscillator," Appl. Phys. B 107, 839-847 (2012).
[3] O. Vaittinen, F. Schmidt, M. Metsälä and L. Halonen. "Exhaled breath biomonitoring using laser spectroscopy," Curr. Anal. Chem. 9, 463-475 (2013).
[4] M. Vainio, M. Merimaa, and L. Halonen, "Frequency-comb-referenced molecular spectroscopy in the mid-infrared region," Opt. Lett. 36, 4122-4124 (2011).
[5] J. Peltola, M. Vainio, T. Fordell, T. Hieta, M. Merimaa, and L. Halonen, " Frequency-comb-referenced mid-infrared source for high-precision spectroscopy," Opt. Express (accepted, 2014).
[6] M. Vainio, M. Siltanen, J. Peltola, and L. Halonen, "Grating-cavity cw optical parametric oscillators for high-resolution mid-infrared spectroscopy," Appl. Optics 50, pp. A1-A10 (2011).