Asymmetric Pyramidal Plasmonic Fiber Probe for High-Resolution Near-Field Optical Imaging by Linearly Polarized Excitation

Plasmonic fiber probes have emerged as a powerful solution to overcome the trade-off between nanoscale resolution and optical throughput in traditional aperture-type near-field probes. By converting the incident laser light inside the optical fiber into surface plasmon polaritons on the surface of the metal film of the probe, these probes significantly enhance both signal intensity and resolution in near-field optical measurements. However, current plasmonic probes mainly rely on radially polarized excitation, requiring complex polarization modulation optics and suffering from polarization degradation during propagation. Here, we propose and demonstrate a novel asymmetric pyramidal fiber probe capable of achieving efficient surface plasmon polariton excitation and nano-focusing under linearly polarized illumination. The probe is fabricated via focused ion beam milling, creating an asymmetric apex geometry that breaks the destructive interference condition typically associated with symmetric probes. Experimental characterization of the transmitted and scattered field distributions reveals strong field localization at the tip across a wide range of polarization angles, as confirmed by finite-difference time-domain simulations. Practical near-field optical imaging of a gold nano-porous sample demonstrates a spatial resolution of 32.9 nm, corresponding to λ/20 at 633 nm, demonstrating great promise for applications in super-resolution microscopy. This work opens new avenues for advanced near-field imaging techniques.