Enabling high-throughput spectroscopy with liquid crystal polarization gratings

Autofluorescence (AF) spectroscopy and imaging are used widely in the field of biomedicine for disease diagnosis and screening. Concentrations of many intrinsic fluorophores share a strict relationship with morphological and functional characteristics of tissue, making AF spectroscopy a powerful tool to directly monitor tissue health. One major challenge with AF imaging is maintaining high signal-to-noise ratios, as emission levels are low due to poor fluorophore quantum efficiencies and low illumination power levels. As a result, maximizing the throughput of the measurement system is critical to mitigate losses. Diffraction gratings are commonly used for spectroscopy for dispersion, but rarely exhibit efficiencies above 80%, limiting the system performance. Liquid crystal polarization gratings (LCPGs) are a relatively new technology that possess extremely high efficiency, typically over 90% for the design wavelength, and in some cases up to 99%, making it an attractive option for AF spectroscopy. However, with unpolarized autofluorescent light, the grating would split the light equally into two orders, only one of which could be collected with a standard detector array. Here, we present the first design and demonstration of a visible light spectrometer using a LCPG. To overcome the loss of 50% of incoming unpolarized light being split into separate orders, we report a novel prism system used to merge the two orders into a single spectrum with minimal degradation of spectral resolution. Our results indicate that that using LCPGs could increase signal levels by up to 20%, significantly improving the performance of spectrometers used for biomedical AF imaging.