Fluorescence Response of Nitrogen-Vacancy Centers in Diamond on Tapered Optical Fiber Tips Decorated with Silver Nanoparticles

Nitrogen-vacancy (NV) color centers in diamond are a promising solid-state quantum sensing platform, but their fluorescence collection is severely constrained by the high refractive index of diamond (n ≈ 2.4), which causes strong total internal reflection at the diamond-air interface and limits the signal-to-noise ratio for practical applications. Here we report the fluorescence response of a tapered optical-fiber probe comprising a CVD-grown nitrogen-doped polycrystalline diamond film and selfassembled silver nanoparticles (Ag NPs). A large-angle tapered tip (~20°, ~370 μm long) is fabricated by hydrofluoric-acid wet etching; an in situ nitrogen-doped diamond thin film is grown by microwave plasma CVD; and a 15 nm Ag film is deposited by sputtering and annealed at 350 °C for 10 min to dewet into a dense array of Ag nanoparticles. A fused-silica witness sample prepared under the same Ag deposition and annealing conditions exhibits a broad absorption band that covers the 532 nm excitation line and extends through the 637 nm zero-phonon line into the NV emission region. Steadystate photoluminescence measurements show an approximately 1.51-fold increase in the detected NV fluorescence intensity at 637 nm after Ag-NP processing. Time-correlated single-photon counting reveals that the intensity-weighted mean fluorescence lifetime changes from 11.01 ± 0.25 ns (bare diamond) to 9.48 ± 0.86 ns (Ag NPs/diamond) across eight independent probes, corresponding to a lifetime-derived total decay-rate enhancement factor Fτ ± 1.17 ± 0.08. Together, the paired measurements show increased detected brightness and faster total decay; however, the radiative and non-radiative contributions remain unresolved, which prohibits exclusive attribution of the observed variations to plasmonic nanoparticle effects. The fabrication route provides a fiber-integrated platform for further optical characterization.