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

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.