Millimeter-scale wide-field mid-infrared photothermal imaging enabled by a broadly tunable picosecond optical parametric oscillator

Abstract: Wide-field mid-infrared photothermal (MIP) imaging enables chemically specific microscopy with submicron spatial resolution but remains fundamentally limited by the trade-off between field of view, mid-infrared pulse energy, and spectral tunability. As a result, current wide-field implementations are typically restricted to fields of view below 200 μm and to either the fingerprint or high-wavenumber spectral regions. Here, we overcome these limitations by developing a wide-field fluorescence-detected mid-infrared photothermal (F-MIP) imaging platform driven by a commercial picosecond optical parametric oscillator (OPO). The system provides pulse energies of up to 360 μJ together with a broad tuning range from 625 to 4327 cm-1, enabling millimeter-scale wide-field imaging in the high-wavenumber regions. We demonstrate a field of view of approximately 1 mm in diameter for fluorescently labeled polystyrene beads while preserving spectral fidelity. Furthermore, the platform enables, to our knowledge, the first wide-field MIP imaging below 900 cm-1. To demonstrate its applicability to biomedical imaging, we performed large-area mosaic imaging of fluorescent tuberculosis-infected tissue sections, providing chemically resolved maps over millimeter-sized sample areas. These results establish broadly tunable OPO-driven F-MIP as a scalable platform for high-throughput vibrational imaging of large biological specimens and advanced materials.