Geometry-driven tunable single-photon coupling into an optical nanofiber cavity

We propose a geometry-driven strategy for achieving tunable single-photon coupling in an optical nanofiber cavity (ONFC), where the resonance wavelength is controlled by tailoring the optical nanofiber (ONF) diameter and the grating pitch. By adjusting these structural parameters, the cavity response can be systematically tuned without modifying the material platform or by using any external control schemes. There is a wide spectral resonance wavelength variation of 60, 106, and 70 nm that can be achieved by varying the ONF diameter in the range of 150 nm, the grating pitch of 60 nm, and the elongation of 400 µm, respectively. The integration of a quantum emitter (QE) within the cavity region retains the geometry-dependent tunability of the system. This enables controlled spectral alignment between the QE emission wavelength and the cavity resonance. For a typical QE placed on the ONFC operating around 640 nm, it facilitates a strong spontaneous emission enhancement with a Purcell factor of ∼20 and a coupling efficiency of ∼94% into the ONF-guided mode. The scattering-limited cavity performance yields a quality factor, finesse, and one-pass power loss of 2930, 104, and 2.9%. Owing to its geometry-controlled design, the proposed device can be readily adapted to different QE wavelengths through appropriate structural optimization, offering a compact and fiber-integrated platform for quantum information and communication applications.