Interferometric Near‐Field Scanning Optical Microscopy via Phase Conjugation of a Multimode Fiber Probe

ABSTRACT Interferometric near‐field scanning optical microscopy (NSOM) enables simultaneous measurement of both the amplitude and phase of the optical near field. In its aperture‐type implementation, the local field is collected through a sub‐wavelength aperture and an optical fiber. The choice of this fiber imposes an inherent trade‐off between signal strength and phase fidelity. Single‐mode fiber probes maintain phase fidelity but suffer from low signal intensity, whereas multimode fiber probes offer higher throughput but introduce severe phase distortions. Here, we present phase‐conjugation interferometric NSOM (PCI‐NSOM), which overcomes this limitation. The output field of the multimode fiber is measured by off‐axis holography, and its phase conjugate is displayed on a spatial light modulator, converting the scrambled speckle into a planar wavefront that focuses to a single point in Fourier space. At the focal pixel of the detector, the correction enhances the intensity by up to ∼165‐fold. We demonstrate PCI‐NSOM by imaging surface plasmon polaritons on a gold grating, obtaining accurate phase maps where uncorrected multimode collection fails through modal dispersion and single‐mode collection through a low signal‐to‐noise ratio. PCI‐NSOM thus provides a practical route to high‐sensitivity phase‐resolved nanoscale optical characterization.