Photothermal spectroscopy on-chip sensor for the measurement of a PMMA film using a silicon nitride micro-ring resonator and an external cavity quantum cascade laser

Abstract

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.