On-Chip Single-Molecule Fluorescence Enhancement via Slotted Gallium Phosphide Nanodisks at Anapole States

Surface-enhanced fluorescence of single emitters lies at the heart of single-molecule DNA sequencers as well as quantum light sources. The state-of-the-art metallic nanoapertures are impaled in a dilemma between enhancing radiative decays and minimizing nonradiative issues such as quenching and resistive heating. Here, lossless gallium phosphide slotted nanodisks are proposed that operate at anapole states by in-plane excitation, precisely aligning the optical "hot spots" with fluidic focuses of zeptomolar sensing volume. Unlike traditional microscopic configurations, this proposed scheme excited via butt-coupled in-plane waveguides not only exhibits averagely 12.4x excitation enhancement growth for a single emitter located in the slot, but also enables precise manipulation of the local density of optical states which gives rise to 13.2x when QY(0) = 0.003. Surprisingly, directional collection from the excitation waveguide appears in the optimal approach rather than the collection from a CMOS camera at the bottom. The overall on-chip fluorescent enhancement is benchmarked with the four dyes' colors, lowering the excitation power to 5.3% of contemporary zero-mode waveguide (ZMW) devices.