Spatially Filtered Back Focal Plane Imaging for Directional Fluorescence Lifetime Study of Polaritonic States

Back focal plane (BFP) imaging has emerged as a widely used technique for investigating various nanoscale optical devices. The ability to provide the full angular distribution of emitted light has enabled the engineering of precise radiation patterns, enabling new advances in nanophotonics. Continuous improvements in the BFP imaging technique, including wavelength, polarization, and phase-resolved signal detection, have allowed us to gain crucial insights into the various optical and material properties of nanophotonic devices. In this study, we introduce a fluorescence lifetime-resolved BFP imaging configuration, which uses a spatial filtering technique in the Fourier plane to discriminate between different emission directions. Uniform silver film (45 nm) with a PMMA matrix layer of about 20 nm containing Rhodamine 6G fluorescent molecular dye was prepared and measured using total internal reflection ellipsometry (TIRE). A coupled oscillator model was used, and strong coupling was observed with a coupling strength of 160 meV. Time-correlated single-photon counting was used for the estimation of fluorescence lifetime in the sub-nanosecond regime, and a direction-dependent lifetime was observed in the BFP imaging configuration. This modified fluorescence-lifetime-resolved BFP microscopy method is essential for directly correlating the collective quantum dynamics (lifetime/decay rate) with the far-field radiation pattern (angle/coherence). It offers a critical tool for designing and optimizing quantum nanophotonic devices, such as polariton-based components and highly directional single-photon emitters, where controlling both excited-state dynamics and spatial coherence is paramount.