# Near-field imaging beyond the probe aperture limit

## TL;DR

This paper introduces a method to surpass the physical probe size limit in near-field optical microscopy by extracting high-order eigenmodes through a phase-referenced transmission matrix, enabling resolution of features smaller than the probe aperture.

## Contribution

The authors develop a self-interference near-field microscopy technique that reveals high-order near-field eigenmodes, enhancing resolution beyond the probe aperture limit.

## Key findings

- Resolved nano-slots with 50 nm gaps smaller than the probe size
- Successfully extracted high-order eigenmodes using SVD of the FNTM
- Enhanced near-field imaging resolution beyond traditional limits

## Abstract

Near-field scanning optical microscopy has been an indispensable tool for designing, characterizing and understanding the functionalities of diverse nanoscale photonic devices. As the advances in fabrication technology have driven the devices smaller and smaller, the demand has grown steadily for improving its resolving power, which is determined mainly by the size of the probe attached to the scanner. The use of a smaller probe has been a straightforward approach to increase the resolving power, but it cannot be made arbitrarily small in practice due to the steep reduction of the collection efficiency. Here, we develop a method to enhance the resolving power of near-field imaging beyond the limit set by the physical size of the probe aperture. The main working principle is to unveil high-order near-field eigenmodes invisible with conventional near-field microscopy. The destructive interference of near-field waves is induced in these high-order eigenmodes by the locally varying phases, which can reveal subaperture-scale fine structural details. To extract these eigenmodes, we construct a self-interference near-field microscopy system and measure a fully phase-referenced far- to near-field transmission matrix (FNTM) composed of near-field amplitude and phase maps recorded for various angles of far-field illumination. By the singular value decomposition of the measured FNTM, we could extract the antisymmetric mode, quadrupole mode, and other higher-order modes hidden under the lowest-order symmetric mode. This enables us to resolve double and triple nano-slots whose gap size (50 nm) is three times smaller than the diameter of the probe aperture (150 nm). The subaperture near-field mode mapping by the FTNM can be potentially combined with various existing near-field imaging modalities and promote their ability to interrogate local near-field optical waves of nanoscale devices.

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Source: https://tomesphere.com/paper/1903.01691