Mid-infrared quantitative phase imaging in aqueous media

Vibrational spectroscopy analysis, such as Fourier-transform infrared spectroscopy (FTIR), is a reference technique for determining the chemical composition of a sample through the vibrational response of molecules. In recent years, the advent of quantum cascade lasers (QCLs) has facilitated the development of multispectral infrared (IR) imaging methods, providing spatially resolved spectral information. However, due to its strong absorption, water remains a major interferent at IR wavelengths, making spectral measurements on hydrated samples challenging. To address this issue, this study explores the potential of phase imaging by two-wave interferometry as a method for analyzing hydrated samples in the mid-infrared (mid-IR) spectral range, using a Mach-Zehnder interferometer (MZI) with QCL sources and a microbolometer array for detection. Using this approach, photoresist patterns could be imaged through 50 µm of water, and their thickness determined with a precision of λ/9. Thus, the wide-field multispectral IR phase imaging developed in this study represents a promising technique for the analysis of aqueous environments, such as the detection of microplastics in water or in vivo diagnosis.