Classification of microplastics in field-collected stream water using a submersible single-shot lensless polarimetric holographic system.

Microplastic pollution in aquatic environments demands field-ready tools that can rapidly detect and discriminate particles in complex aquatic environments. Here, we present a single-shot polarimetric off-axis lensless holographic imaging system integrated into a compact submersible prototype. The system combines a simple off-axis interferometric layout, requiring only one mirror and one beam splitter, with a division-of-focal-plane (DoFP) polarimetric sensor to acquire four polarization-resolved holograms per exposure, enabling Fourier-domain order isolation and complex-field recovery while preserving polarization-dependent contrast. We validate joint phase and polarimetric reconstruction using a birefringent USAF resolution target and demonstrate polymer discrimination directly from raw four-channel hologram patches using a lightweight detection-classification pipeline (no phase reconstruction or handcrafted features), achieving approximately 99% test accuracy across four different polymers. Finally, submerged experiments in unfiltered, field-collected stream water with controlled microplastic spiking confirm accurate detection and time-resolved classification under realistic conditions.