Photothermal spectroscopy (PTS) working in the mid-infrared region is an effective technique for in-situ characterization of the chemical composition of surface contaminants. The sensitivity relies on the way that the laser-induced response of the sample is detected. We present a highly-sensitive PTS assisted with a dual-wavelength Mach-Zehnder interferometer (MZI), MZI-PST in short. The MZI aims to sense all the phase delays taking place at the sample and air when the heat produced by resonance absorption of the contaminant is transferred into its surroundings and further to amplify the total phase delay to a large intensity difference of a probe beam. To guarantee a stable quadrature phase bias of the MZI working in the balanced detection mode, we employ two separate wavelengths, one for sensing and the other for phase bias feedback, to lock the working point to the quadrature point in real time. The MZI is expected to have a 7.8-fold sensitivity enhancement compared with the conventional phase-sensitive PTS in theory. The results of the proof-of-concept experiment on the olive oil contaminated on a wafer surface verify the spectral fidelity and the sensitivity enhancement as well as the capability of photothermal spectral imaging of the MZI-PST.

中文翻译：

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双波长马赫曾德尔干涉仪辅助光热光谱法表征表面污染物

在中红外区工作的光热光谱法（PTS）是一种有效表征表面污染物化学成分的有效技术。灵敏度取决于检测激光诱导的样品响应的方式。我们提出了一种高灵敏度的PTS，并辅之以双波长Mach-Zehnder干涉仪（MZI），即MZI-PST。MZI的目的是当污染物的共振吸收所产生的热量传递到周围环境时，感测在样品和空气上发生的所有相位延迟，并进一步将总相位延迟放大到探测光束的较大强度差。为了确保在平衡检测模式下工作的MZI的正交相位偏置稳定，我们采用了两个独立的波长，一个用于感测，另一个用于相位偏置反馈，实时将工作点锁定在正交点上。理论上，与常规的相敏PTS相比，MZI的灵敏度有望提高7.8倍。对晶片表面污染的橄榄油进行的概念验证实验的结果验证了光谱保真度和灵敏度增强以及MZI-PST的光热光谱成像能力。