# A Novel 3D Probe for Near-Field Scanning Microwave Microscopy

## TL;DR

A new 3D probe improves microwave microscopy resolution by enhancing electric-field localization and preserving spatial frequencies.

## Contribution

A novel 3D split-ring resonator probe with vertical metallic bars is introduced to enhance NSMM resolution.

## Key findings

- The 3D probe improves electric-field localization through field singularities and spatial-frequency preservation.
- The fabricated probe successfully imaged a dielectric slab with an air-filled void, matching simulation predictions.
- The convolution-based imaging model confirms the probe's role as a spatial filter enhancing image fidelity.

## Abstract

Near-field scanning microwave microscopy (NSMM) offers the ability to probe local electromagnetic properties beyond the classical Abbe diffraction limit, but achieving high resolution over practical scan areas remains challenging. In this work, we introduce a novel three-dimensional (3D) NSMM probe consisting of a split-ring resonator (SRR) coupled to a microstrip line and loaded with vertically extended metallic bars. The 3D loading enhances electric-field localization in the sensing region by introducing field singularities. Full-wave numerical simulations are used to extract the field-spread function (FSF) of the probe and to quantify how probe geometry, stand-off distance, and bar dimensions control the FSF and its spatial-frequency (k-space) content. An imaging model is then developed in which the NSMM image is represented as a convolution between the object and FSF in one and two dimensions. This framework demonstrates that progressively localized FSFs, obtained through 3D loading and resonator miniaturization, systematically improve image fidelity and preserve higher spatial frequencies. The probe is fabricated using printed circuit board technology (PCB) with vertically attached metallic bars, and its performance is validated by imaging a dielectric slab containing a cylindrical air-filled void. The measured line profiles and two-dimensional images are in good agreement in general characteristics with the convolution-based model, confirming that the proposed 3D SRR-based probe operates as a spatial filter whose engineered near-field distribution governs the achievable resolution in NSMM imaging.