Miniaturizing Time Resolved Fluorescence Measurements Using Entangled Photons and On-Chip Photonics - Fluorescent biosensors and microscopes can measure voltage, calcium, neurotransmitters, and other essential endogenous or exogenous biomarkers. The standard practice is to record the change in biosensor fluorescence intensity over time. The signal can be affected by fluctuations in sensor levels between cells and animals, laser power, imaging position, and other unavoidable experimental factors. Time-resolved fluorescence (TRF) and fluorescence lifetime imaging (FLIM) solve this problem by measuring lifetime instead of intensity. TRF allows well-calibrated measurements of biosensor analyte levels by being resistant to intensity fluctuations. TRF also enables new measurement modalities based on the fluorophore’s local environment, ranging from endogenous fluorescence signals to the near-endless continuum of exogenous FRET and lifetime-based biomarkers. However, portable implementation of lifetime-based approaches is hampered by a pulsed laser’s cost, physical size, power draw, and required domain-specific expertise. The proposal uses integrated photonics to create entangled photon sources that enable highly multiplexed, low-cost, low-power, and portable measurement of TRF/FLIM in a universal package. The Cushing lab has discovered that creating entangled photons with integrated photonics can be used for fluorescence lifetime measurements with both quantum and practical advantages. The advantages include improved tuning range (>500 nm) per source, temporal resolution (< 0.1 ps), a CW-like approach that reduces phototoxicity effects, alignment-free operation through a photonic back end, and <1 cm2 physical size – all powered by the equivalent of a mW laser pointer. Entangled TRF, therefore, appears ideal for portable or wearable, miniaturized TRF and FLIM approaches. The technology can be compared to emerging LED-based miniaturized devices, which lack the ability for high wavelength multiplexing without multiple sources nor the time resolution to distinguish multiple biomarkers and their environmental response. By starting with a CMOS-cost-scalable package using thin film photonics, our innovation will significantly improve health equity by making TRF and FLIM accessible to a broader range of biological and medical researchers. The specific aims of the proposal include 1) optimizing the entangled photon source for excitation- wavelength and temporally multiplexed fluorescence lifetime sensing, 2) extending entangled TRF toward shorter wavelength excitations by transitioning from lithium niobate to lithium tantalate, and 3) integrating on-chip photonic elements for a miniaturized TRF architecture. Collaborators specializing in FLIM and TRF biosensors evaluate each stage of the grant with regard to application, including testing in their labs, to ensure realistic criteria are used in addition to photonic metrics. The proposed research is the first step towards a multiplexed platform that brings TRF and FLIM to broader health applications, including portable medical diagnostics and in-vivo or miniaturized sensors, all using the cost scaling of integrated photonics.
