Mid-infrared frequency conversion for molecule detection

非线性激光光谱学为敏感分子的检测提供了多种技术。相干反斯托克斯拉曼散射(CARS)仍然是实现这一目的的主要手段。CARS中的信号产额得益于电子态之间的大跃迁矩和振动态的双光子共振。然而，汽车上的激光通常会与单光子跃迁相去甚远。此外，汽车通常需要两个激光脉冲。因此，对于应用来说，希望找到一种替代的非线性技术，它只需要一束激光，并且允许在频率转换过程中所涉及的所有单光子和多光子跃迁时以小(或零)失谐运行。三次谐波产生(THG)是发生在任何任意介质中的最低阶频率转换过程。它只需要一台激光。因此，THG将是非线性光谱学和分析的直接选择。当我们通过分子在中红外的“指纹”振动共振来驱动三倍频时，这一点尤其适用--这增强了信号并提供了光谱选择性。然而，非线性过程需要在振动跃迁的中红外区域具有可调谐的强激光脉冲，并且需要窄的光谱带宽来保持选择性。这种激光器是在过去十年才出现的，大多是自制的，光谱带宽远远高于傅里叶变换的极限。在项目的第一阶段，我们对分子物种的三倍频光谱进行了实验研究，通过将驱动激光调谐到振动态之间的多光子跃迁来共振增强。我们展示了较大的THG增强和光谱选择性。在实验中，我们开发并应用了一种独特的中红外激光系统，其中包括用于光学参量振荡器的自制脉冲放大链。该激光系统在中红外波段提供了可调谐的纳秒激光脉冲，脉冲能量为MJ，光谱带宽接近傅里叶变换极限。这一更新方案的目的是扩展我们以前对共振增强三倍频的研究，开发和研究该方法的新变体，并将其推向应用。特别是，我们打算在与燃烧分析和环境传感相关的分子物种中实现中红外三倍频，在此类应用的现实条件下证明其可行性，研究和频混频(SFM)作为一种替代频率转换方案，通过振动共振产生更大的非线性极化率增强，并在信号产量和粒子检测极限方面与传统的CARS光谱方法进行比较。长期目标是建立中红外三倍频和SFM作为应用非线性光谱学和气体混合物成像的有力工具。

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.