Enabling Chemical Holography: A Comprehensive Framework and Implementation

Mid-infrared spectroscopic imaging (MIRSI) enables the spatially-resolved identification of molecules and is widely used in fields ranging from biomedical diagnostics to forensics. Current MIRSI technologies measure the sample's extinction coefficient, which is only one component of the complex relative permittivity, and therefore provide incomplete molecular profiles. We propose a new framework and instrument to enable phase-sensitive <i>chemical holography</i> that measures a sample's <i>complex</i> molecular properties at any wavelength, thus overcoming a fundamental limit on molecular specificity. Combining a spatially coherent quantum cascade laser (QCL) source with an interferometer and imaging system can provide a phase-sensitive platform for molecular analysis. This paper describes a theoretical framework for chemical holography and demonstrates benefits for molecular specificity, improved spatial resolution, and greater flexibility. Deep learning is used to solve the inverse scattering problem for chemically heterogeneous samples modeled using Mie theory. Furthermore, we demonstrate new, custom-built instrumentation and experimental results that validate our theoretical framework.