Probing vectorial near field of light: imaging theory and design principles of nanoprobes

TL;DR

This paper develops a comprehensive imaging theory for nanoprobes in near-field microscopy, enabling better design and understanding of probes for characterizing complex vectorial optical fields at the nanoscale.

Contribution

It introduces a general electromagnetic reciprocity-based theory and design principles for nanoprobes, advancing the analysis and creation of functional near-field probes.

Findings

The theory accurately predicts probe responses to optical magnetism and spin angular momentum.

Numerical analysis validates the theory against experimental measurements.

Provides a new framework for designing probes for various vectorial near-field quantities.

Abstract

Near-field microscopy is widely used for characterizing electromagnetic fields at nanoscale, where nanoprobes afford the opportunity to extract subwavelength optical quantities, including the amplitude, phase, polarization and chirality. However, owing to the complexity of various nanoprobes, a general and intuitive theory is highly needed to assess the vectorial field response of the nanoprobes and interpret the mechanism of the probe-field interaction. Here, we develop a general imaging theory based on the reciprocity of electromagnetism and multipole expansion analysis. The proposed theory closely resembles the multipolar Hamiltonian for light-matter interaction energy, revealing the coupling mechanism of the probe-field interaction. Based on this theory, we introduce a new paradigm for the design of functional nanoprobes by analyzing the reciprocal dipole moments, and establish effective design principles for the imaging of vectorial near fields. Moreover, we numerically analyze the responses of two typical probes, which can quantitatively reproduce and well explain the experimental results of previously reported measurements of optical magnetism and transverse spin angular momentum. Our work provides a powerful tool for the design and analysis of new functional probes that may enable the probing of various physical quantities of the vectorial near field.