Spectral Interleaving with Quantum Cascade Laser Frequency Combs Michele Gianella1, Akshay Nataraj1, Béla Tuzson1, Filippos Kapsalidis2, Mattias Beck2, Stéphane Schilt3, Thomas Südmeyer3, Jérôme Faist2, Andreas Hugi4, Lukas Emmenegger1 1. Laboratory for Air Pollution / Environmental Technology, Empa, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland 2. Institute for Quantum Electronics, ETH Zurich, CH-8093 Zürich, Switzerland 3. Laboratoire Temps-Fréquence, Université de Neuchâtel, Avenue de Bellevaux 51, CH-2000 Neuchâtel, Switzerland 4. IRsweep AG, Laubisrütistrasse 44, CH-8712 Stäfa, Switzerland Introduction Frequency combs based on quantum cascade lasers (QCLs) provide direct access to the mid-infrared spectral range, where the strong and characteristic rovibrational absorption bands of many molecules are located. For gas-phase spectroscopy, however, the large comb spacing (5-15 GHz) of these devices – much larger than the absorption lines of simple molecules at atmospheric pressure or below – is problematic. Phase-sensitive, dual-comb configuration. Free-running QCL frequency comb FC1 (sample comb) probes the sample; FC2 serves to produce multi-heterodyne beat note signals on pho- todetectors, giving access to all optical frequencies simulta- neously, no moving parts required (Fig. 1). Multi-heterodyne beat note signal. Comb lines from FC1 (ν1j) and FC2 (ν2j) produce beat notes (fj= ν1j- ν2j) in measured photocurrent. Sample attenuation and phase shift at ν1j are encoded in beat note amplitude ratio at fj (Fig. 2). Spectral interleaving Spectral resolution of frequency comb measurements is generally limited by the line width of the comb lines (<1 MHz). Spectral sampling period is given by the comb frequency spacing (5-15 GHz typically) and can be reduced by continuous (or step-wise) shift of the frequen- cy comb spectrum (through current modulation) and interleaving (Fig. 3). Demonstration: Methane (CH4) absorption and phase-shift spectrum (at 100 mbar) measured in 120 ms (Fig. 4). Linear trend in phase due to gas cell windows. Spectral resolution: 0.001 cm-1 (30 MHz). Coverage: up to 25 cm-1. Fig. 1. Phase-sensitive dual-comb configuration. Fig. 2. Left: Multi-heterodyne beat note signals generated by the two combs, FC1 and FC2, with and without sample (polypropylene foil in this example). Right: derived transmission spectrum. Fig. 3. Spectral interleaving by continuous or step-wise tuning of the (discrete) frequencies of the comb. Fig. 4. Transmission and phase-shift spectrum of pure methane shown between 1208 and 1221 cm-1 (100 mbar, 25°C, 14 cm path length). The different colours correspond to the range covered by individual comb lines. Applications Potential applications include mid-infrared spectroscopy requiring: - high spectral resolution (<0.001 cm-1) - broadband coverage (>20 cm-1) - 100 ms time resolution (10 ms should be possible with 0.001 cm-1 resolution) Comb line frequencies of free-running sample comb (FC1) are given by 𝜈1𝑗(𝑡) = 𝜈1 0(𝑡) + 𝑗∙Δ𝜈1(𝑡) 𝜈1 0: offset frequency 𝑡: time (laser current) 𝑗: comb line number Δ𝜈1: comb line spacing Fig. 5. Zoom-in of shaded region in Fig. 4 after etalon removal. Examples • time-resolved breath analysis • gas-phase reaction kinetics • … Funding: Swiss National Science Foundation, Bridge Discovery program, Grant 20B2-1_176584 References A. Hugi, G. Villares, S. Blaser, H. C. Liu, and J. Faist, “Mid-infrared frequency comb based on a quantum cascade laser,” Nature 492, 229–233 (2012). G. Villares, A. Hugi, S. Blaser, and J. Faist, “Dual-comb spectroscopy based on quantum-cascade-laser frequency combs,” Nature Communications 5, 1–3 (2014).