We report a proof-of-principle demonstration of intracavity pump–probe, cavity ringdown (CRD) detection in a three-mirror, traveling-wave cavity. With cavity-enhanced pump power and probe absorption path length, the technique is a generally applicable, high-sensitivity, high-selectivity detection method. In our experiments, the pump radiation is switched off during every other probe ringdown, which allows uncorrelated measurements of analyte and background cavity decay rates. The net, two-color signal from the difference between the pump-on and pump-off decay rates is immune to empty-CRD drifts and spectral overlaps from non-target molecular transitions. The immunity to the ringdown drifts allows longer signal-averaging and, thus, higher detection sensitivity. The ability to compensate for the background absorption enhances the detection selectivity in spectrally congested regions. Our technique is well-suited for trace-detection in the mid-IR region, where pump–probe schemes based on strong rovibrational transitions can be applied. In this work, two-color CRD detection is implemented on a ladder-type, three-level system based on the N₂O, ν₃ = 1 ← 0, P(19) (pump) and ν₃ = 2 ← 1, R(18) (probe), rovibrational transitions. By frequency-locking two-quantum cascade lasers to the p-polarization (pump, Finesse = 5280) and s-polarization (probe, Finesse = 67 700) cavity modes, we achieve high intracavity pump power (36 W) and high probe ringdown rates (>2 kHz). The observed two-color spectra are simulated by a density-matrix, three-level system model that is solved under the constraints of the cavity resonance conditions. In addition to its background compensation capability, experimental flexibility in the selection of pump–probe schemes and signal insensitivity to intracavity laser power are further features that enhance the utility of our technique for mid-IR trace-detection.

中文翻译：

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双色、腔内泵探头、腔衰荡光谱

我们报告了在三镜行波腔中进行腔内泵探头、腔衰荡 (CRD) 检测的原理验证演示。具有腔增强泵浦功率和探头吸收路径长度，该技术是一种普遍适用的、高灵敏度、高选择性的检测方法。在我们的实验中，泵浦辐射在每隔一次探头振铃期间关闭，这允许对分析物和背景空腔衰减率进行不相关的测量。来自泵入和泵出衰减率之间差异的净双色信号不受来自非目标分子跃迁的空 CRD 漂移和光谱重叠的影响。对振铃漂移的免疫允许更长的信号平均时间，因此，更高的检测灵敏度。补偿背景吸收的能力增强了光谱拥挤区域的检测选择性。我们的技术非常适合中红外区域的轨迹检测，其中可以应用基于强振动跃迁的泵浦探针方案。在这项工作中，₂ O，ν ₃ = 1 ← 0，P(19)（泵）和ν ₃ = 2 ← 1，R(18)（探针），振动跃迁。通过将双量子级联激光器锁频到p偏振（泵，Finesse = 5280）和s-极化（探针，Finesse = 67 700）腔模式，我们实现了高腔内泵浦功率（36 W）和高探针衰减率（> 2 kHz）。观察到的双色光谱由密度矩阵、三能级系统模型模拟，该模型在腔谐振条件的约束下求解。除了其背景补偿能力外，泵浦探针方案选择的实验灵活性和对腔内激光功率的信号不敏感性是增强我们技术在中红外跟踪检测中的实用性的进一步特征。