Mid-infrared frequency conversion for molecule detection

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Nonlinear laser spectroscopy offers a variety of techniques for sensitive molecule detection. Coherent anti-Stokes Raman-scattering (CARS) is still the work horse for such purposes. The signal yield in CARS benefits from large transition moment between electronic states and two-photon resonance with vibrational states. However, the lasers in CARS are usually very far detuned from single-photon transitions. Moreover, CARS typically requires two laser pulses. Thus, for applications it would be desirable to find an alternative nonlinear technique, which requires only a single laser beam and permits operation at small (or zero) detunings at all single- and multi-photon transitions involved in the frequency conversion process.Third harmonic generation (THG) is the lowest-order frequency conversion process, which occurs in any arbitrary medium. It requires only a single laser. Hence, THG would be a straightforward choice for nonlinear spectroscopy and analytics. This holds in particular true, when we drive THG via molecular “finger-print” vibrational resonances in the mid-infrared – which enhances the signal and provides spectral selectivity. However, the nonlinear process requires intense laser pulses with tunability in the mid-infrared region of vibrational transitions, and narrow spectral bandwidth to maintain selectivity. Such lasers became only available in the last decade, mostly in home-made setups and with spectral bandwidth quite far above the Fourier-transform limit. During the first stage of the project we implemented experimental studies on THG spectroscopy of molecular species, resonantly enhanced by tuning the driving laser to multi-photon transitions between vibrational states. We demonstrated large THG enhancement and spectral selectivity. For the experiments we developed and applied a unique mid-infrared laser system involving a home-made, pulsed amplifier chain for an optical parametric oscillator. The laser system provides tunable nanosecond laser pulses in the mid-infrared with pulse energy in the mJ regime and narrow spectral bandwidth close to the Fourier transform-limit.The aim of this renewal proposal is to extend our previous investigations on resonantly-enhanced THG, develop and investigate novel variants of the approach, and push them towards applications. In particular, we intend to implement mid-infrared THG in molecular species with relevance to combustion analytics and environmental sensing, prove its feasibility also under realistic conditions of such applications, investigate sum-frequency mixing (SFM) as an alternative frequency conversion scheme via rovibrational resonances to yield even larger enhancements of the nonlinear susceptibility, and compare the approaches to conventional CARS spectroscopy with regard to signal yield and particle detection limit. The long term objective is to establish mid-infrared THG and SFM as powerful tools for applied nonlinear spectroscopy and imaging of gaseous mixtures.