Near-field imaging of an ultra-confined optical field on the endface of a nanoslit waveguide.

Generation and characterization of ultra-confined optical fields are of great importance in fundamental optics and optical technologies. A dielectric nanoslit waveguide formed by a coupled nanofiber pair (CNP) can provide such a field openly accessible on its endface, exhibiting extreme confinement down to the sub-nanometer level. However, its direct imaging, which is critical for its characterization and applications, remains challenging, as such a field exists in the near field of the suspended microscale waveguide endface with ultra-small feature sizes. Here, we demonstrate the first near-field imaging of an ultra-confined optical field on the endface of a silica CNP based on a custom-built waveguiding-excited scattering-type scanning near-field optical microscopy system. Using the fiber-taper-assisted waveguiding excitation technique, we excite the nanoslit waveguiding mode with an efficiency of up to 89%. Leveraging the flexible configuration of the piezo-probe for precise engagement on the endface, we resolve the ultra-confined optical near field of the nanoslit mode at a 630-nm wavelength, with a spatial resolution of ∼20 nm under an optimized probe tapping amplitude of 35 nm. The near-field imaging approach shown here enables in situ characterization of and direct access to the ultra-confined optical field on the endface of a nanoslit waveguide, which may lead to a variety of possibilities from nanometer-scale light-matter interaction to optical nanoscopy and atom/molecule manipulation.