Near-Field Mechanical Fingerprints for THz Sensing of 'Hidden' Nanoparticles in Complex Media

TL;DR

This paper demonstrates that near-field mechanical signatures in THz spectroscopy can detect and characterize hidden nanoparticles in complex media with high sensitivity, surpassing traditional far-field methods.

Contribution

The study introduces a novel approach using mechanical variables like forces and torques in THz near-field spectroscopy to identify nanoparticles in complex environments, enhancing detection capabilities.

Findings

Mechanical signatures outperform far-field observables in sensitivity.

Material-specific spectral hotspots enable robust calibration.

Spin torque is modulated by MO-neighbor proximity and magnetic fields.

Abstract

Terahertz (THz) spectroscopy holds transformative potential for non-invasive sensing, yet characterizing individual nanoparticles in complex biological environments remains challenging due to the far-field diffraction limit. While near-field dipolar theory is well established, its application to characterizing/identifying nanoparticles immersed in complex media at THz frequencies is largely unexplored. This work utilizes numerical simulations of magneto-optical (MO) heterodimers -- comprising n-doped Indium Antimonide (n-InSb) and isotropic or birefringent particles (e.g., SiO2, GaSe) -- under counter-propagating, circularly polarized THz illumination. We demonstrate that while far-field observables like absorption cross-sections are often dominated by the MO-active particle, mechanical variables-specifically induced binding forces and spin/orbital torques-exhibit superior sensitivity for detecting "hidden" neighboring components. Because these mechanical signatures depend directly on near-field interactions, they provide higher information density regarding interparticle coupling. Key findings reveal material-specific spectral "hotspots" and "zeros" that serve as robust calibration markers even within dispersive biological surrogates. We show that the spin torque on non-MO particles is significantly modified by MO-neighbor proximity, a phenomenon controllable via static magnetic fields. Furthermore, these variables exhibit high angular sensitivity in perpendicular configurations. Our results provide a roadmap for using optomechanical signatures as high-resolution detectors for in-vivo diagnostics, signal transduction, and low-energy nanocircuit control.