Delivering light to the nanoscale using a flexible and easily integrated fiber platform holds potential in various fields of quantum science and bioscience. However, rigorous optical alignment, sophisticated fabrication process, and low spatial resolution of the fiber-based nanoconcentrators limit the practical applications. Here, a broadband azimuthal plasmon interference nanofocusing technique on a fiber-coupled spiral tip is demonstrated for fiber-based near-field optical nanoimaging. The spiral plasmonic fiber tip fabricated through a robust and reproducible process can reverse the polarization and modulate the mode field of the surface plasmon polaritons in three-dimensionally azimuthal direction, resulting in polarization-insensitive, broad-bandwidth, and azimuthal interference nanofocusing. By integrating this with a basic scanning near-field optical microscopy, a high optical resolution of 31 nm and beyond is realized. The high performance and the easy incorporation with various existing measurement platforms offered by this fiber-based nanofocusing technique have great potential in near-field optics, tip-enhanced Raman spectroscopy, nonlinear spectroscopy, and quantum sensing.

The difficulty of obtaining high-intensity localized light spots for optical probes leads to their lack of good applications in nanoimaging. Here we demonstrate a Fabry–Pérot resonance flat-based plasmonic fiber probe (FPFP). The simulation results show that the probe can obtain a nanofocusing spot at the tip with the radially polarized mode. The Fabry–Pérot interference structure is used to control the plasmon propagation on the surface of the probe, and it effectively improves the local spot intensity at the tip. Furthermore, the experimental results verify that the FPFP (tip curvature radius is 20 nm) prepared by chemical etching method can obtain a nanofocusing spot at the tip. The nanoimaging of the gold slit structure demonstrates the nanoimaging capability of the FPFP, and the 36.9 nm slit width is clearly identified by the FPFP.

The properties of near-field optics have always been the focus of nano-measurement technology. The 11th order effective near-field optical signal with an incident laser wavelength of 1,550 nm is obtained using a platinum-coated optical probe (Pt–Si probe). The experimental results show that the local electric field intensity of the Pt–Si probe is nearly 30 times higher than that of silicon probe (Si probe). Therefore, the highest 7th order near-field optical imaging results are obtained with the Pt–Si probe. Further, near-field optical imaging is performed on samples such as gold grids and carbon nanotubes using the Pt–Si probe. The measurement results show that the high-order signal has the characteristics of less background, higher signal-to-noise ratio, and resolution up to 5.7 nm.