Polarization-insensitive plasmon nanofocusing with broadband interference modulation for optical nanoimaging

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.