/NASA_TM_2025_80NSSC25K7983

NASA Technical Memorandum NASA/TM–2025-80NSSC25K7983–PS-FLIM-FCS Development of a Picosecond Fluorescence Lifetime Imaging Microscope (ps■FLIM) with Fluorescence Correlation and Lifetime Correlation Spectroscopy for Toroidal Transport Analysis Author: Dr. Clyde Varner II, Assistant Professor of Chemistry, Alabama A&M; University Collaborating Center: NASA Marshall Space Flight Center (EM■32) – Materials and Processes Laboratory Collaborator: Dr. Fredrick Michael, NASA MSFC (EM■32) Distribution Statement A – Approved for public release; distribution is unlimited. Executive Summary This Technical Memorandum describes the development of a picosecond fluorescence lifetime imaging microscope integrated with fluorescence correlation spectroscopy and fluorescence lifetime correlation spectroscopy for analyzing toroidal and chiral transport in Dy3+:CaTiO3 nanoplates. The system enables direct visualization of picosecond scale carrier diffusion, lifetime heterogeneity, and orbital angular momentum driven anisotropy. It complements the Helical Anisotropy and Circular Dichroism and Bayesian nanosecond transient absorption systems developed under NASA Grant 80NSSC25K7983. System Architecture The ps■FLIM microscope integrates a pulsed laser excitation source in the range of 470 to 560 nanometers with a confocal detection path and time correlated single photon counting electronics. The excitation beam may be configured for either Gaussian or toroidal donut mode illumination using a q■plate or spatial light modulator to impart topological charge of plus or minus one or two. A hybrid photomultiplier detector collects fluorescence photons which are histogrammed by arrival time to produce lifetime maps. Each pixel decay curve is modeled as the sum of exponentials I(t) = Σ αi exp(−t/τi). Correlation Spectroscopy Methods Fluorescence correlation spectroscopy extracts temporal fluctuations in fluorescence intensity to yield an autocorrelation function that reports on molecular diffusion and kinetics. The diffusion time and molecular mobility are determined by fitting the autocorrelation function to experimental data. In fluorescence lifetime correlation spectroscopy, the time correlated photon arrival histogram is used as a statistical filter, separating populations with different lifetimes. The combined ps■FLIM and FCS approach distinguishes diffusive, rotational, and lifetime dependent carrier processes with subnanosecond precision. Calibration and Validation Calibration employs Rhodamine 6G and Coumarin 6 as fluorescence lifetime standards with lifetimes of approximately four and two and a half nanoseconds respectively. Diffusion calibration is carried out with fluorescein in water having a diffusion coefficient near three times ten to the minus six square centimeters per second, and with PVA films representing restricted motion. For toroidal studies, Dy3+:CaTiO3 nanoplates are excited using beams with topological charge plus or minus one, and lifetime heterogeneity maps are correlated with the azimuthal diffusion coefficient determined from nanosecond transient absorption. Calibration ensures photon count linearity, temporal response better than thirty picoseconds full width at half maximum, and accurate deconvolution of the instrument response function. Application to Toroidal Transport Under toroidal excitation each photon carries orbital angular momentum that introduces an azimuthal component to the excitation field. In Dy3+:CaTiO3 nanoplates this interaction generates a circulating current density coupled to the toroidal dipole moment. Spatially resolved lifetime maps reveal symmetry dependent relaxation channels, and correlation of lifetime and intensity autocorrelation functions quantifies toroidal anisotropy. The toroidal diffusion tensor and activation energy are determined by comparing correlation derived diffusion times with Bayesian diffusion data from nanosecond transient absorption measurements. Budget Summary Senior personnel include the principal investigator with two months of summer and two months of academic effort, and the co■principal investigator with one month of summer effort for a total of two hundred ten thousand dollars. Two postdoctoral researchers, one domestic for ps■FLIM analysis and one international through the Kyungpook exchange, are budgeted at two hundred forty thousand dollars. Two graduate students receive stipend, tuition, and fringe benefits totaling one hundred sixty thousand dollars. Four undergraduate researchers per year participate through REU style stipends at forty thousand dollars. Equipment including the ps■FLIM module, orbital angular momentum optics, and anisotropy detector is allocated two hundred thousand dollars. Materials and consumables such as Dy doped CaTiO3 precursors and cellulose film reagents total thirty thousand dollars. Travel funds of eighty thousand dollars support collaboration with NASA Marshall, North Carolina A and T, and Kyungpook University. Subawards and consultant costs total one hundred ten thousand dollars including partners at NC A and T, Kyungpook, and Magnitude Instruments. Publication and workshop costs total twenty thousand dollars, and indirect costs at approximately twenty eight percent equal one hundred sixty thousand dollars, giving a total budget of one million dollars over four years. Acknowledgment Supported by NASA Grant 80NSSC25K7983 under the Minority Serving Institution Cooperative Research Framework. The author acknowledges Dr. Fredrick Michael of NASA Marshall Space Flight Center EM■32 for collaboration and optical validation of the ps■FLIM correlation system.