Mid-infrared digital holography imaging in aqueous media

Abstract

Biochemical molecules (nucleic acids, proteins) have their spectral signature in mid-infrared (MIR) due to the vibrational modes of their molecular bonds. Hence, MIR spectroscopy is interesting for identifying molecules in biological tissues. However, water molecules have a very strong absorption in this spectral range and therefore constitute a major interferent for absorption measurements of biochemical species in an aqueous medium. Since a decade, the advent of Quantum Cascade Laser (QCLs), high brightness monochromatic sources, combined with the development of uncooled bolometer matrices, have paved the way for new research in the field of tissue micro-spectroscopy. Recently, MIR absorption imaging turned out to be promising for diagnosis, in particular thanks to QCLs, but still difficult in aqueous media. The aim of our work is to study the interest of phase shift measurement of aqueous samples by dispersion spectroscopy, around known absorption peaks of the species of interest. An imaging setup based on a Mach Zehnder interferometer was implemented, using QCLs sources and a microbolometer matrix, that allows both absorption and dispersion measurements in the 900 to 1660 cm⁻¹ spectral range. It was applied to SU8 photoresist patterns of different thicknesses in a 50 µm-thick cell filled with water. Absorption and phase shift spectra are extracted from images and respectively compared to Fourier transform infrared spectrometer (FTIR) measurements and Kramers Kronig transformation of them. These results with hydrated samples are encouraging and open interesting prospects towards label-free tissue diagnosis.