Laser-based mid-infrared (mid-IR) photothermal spectroscopy (PTS) represents a selective, fast, and sensitive analytical technique. Recent developments in laser design permits the coverage of wider spectral regions in combination with higher power, enabling for qualitative reconstruction of broadband absorption features, typical of liquid or solid samples. In this work, we use an external cavity quantum cascade laser (EC-QCL) that emits in pulsed mode in the region between 5.7 and 6.4 µm (1770–1560 cm−1), to measure the absorption spectrum of a thin film of polymethyl methacrylate (PMMA) spin-coated on top of a silicon nitride (Si3N4) micro-ring resonator (MRR). Being the PTS signal inversely proportional to the volume of interaction, in the classical probe–pump dual beam detection scheme, we exploit a Si3N4 transducer coated with PMMA, as a proof-of-principle for an on-chip photothermal sensor. By tuning the probe laser at the inflection point of one resonance, aiming for highest sensitivity, we align the mid-IR beam on top of the ring’s area, in a transversal configuration. To maximize the amplitude of the photoinduced thermal change, we focus the mid-IR light on top of the ring using a Cassegrain reflector enabling for an optimal match between ring size and beam waist of the excitation source. We briefly describe the transducer design and fabrication process, present the experimental setup, and perform an analysis for optimal operational parameters. We comment on the obtained results showing that PTS allows for miniaturized robust sensors opening the path for on-line/in-line monitoring in several industrial processes.

中文翻译：

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使用氮化硅微环谐振器和外腔量子级联激光器测量 PMMA 薄膜的光热光谱片上传感器

基于激光的中红外 (mid-IR) 光热光谱 (PTS) 代表了一种选择性、快速且灵敏的分析技术。激光设计的最新发展允许覆盖更宽的光谱区域并结合更高的功率，从而能够定性重建典型的液体或固体样品的宽带吸收特征。在这项工作中，我们使用外腔量子级联激光器 (EC-QCL)，它在 5.7 至 6.4 µm (1770–1560 cm-1) 范围内以脉冲模式发射−1），测量旋涂在氮化硅（Si3氮4）微环谐振器（MRR）。由于 PTS 信号与相互作用的体积成反比，在经典的探针泵双光束检测方案中，我们利用 Si3氮4涂有 PMMA 的传感器，作为片上光热传感器的原理验证。通过在一个共振的拐点处调整探测激光器，以达到最高灵敏度，我们将中红外光束以横向配置排列在环区域的顶部。为了最大化光致热变化的幅度，我们使用卡塞格伦反射器将中红外光聚焦在环的顶部，从而实现环尺寸和激发源的束腰之间的最佳匹配。我们简要描述了换能器的设计和制造过程，介绍了实验设置，并对最佳操作参数进行了分析。我们对获得的结果进行评论，结果表明 PTS 允许使用小型化的稳健传感器，为多个工业过程中的在线/在线监控开辟道路。