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

TL;DR

Terahertz sensing in complex media is limited by diffraction when characterizing single nanoparticles. The authors propose near-field mechanical fingerprints of magneto-optical dimers, driven by dual counter-propagating THz beams and modulated by a static field $B$, to access interparticle coupling beyond the far-field. They show that near-field observables such as binding force $\Delta$, spin torque $\vec{N}_{spin}$, and orbital torque $\vec{N}_{orb}$ provide higher information density than absorption cross sections $\sigma_{abs}$, including material-specific spectral hotspots and zeros that serve as calibration markers. The work demonstrates controllable interparticle interactions and strong orientation/anisotropy dependence in parallel and perpendicular configurations, suggesting routes to in-vivo THz theragnostics, signal transduction, and low-energy nanocircuit control.

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.

Figures (6)

• Figure 1: (Color online) (a) Schematic of a possible thought setup to study magneto-optical samples consisting of dimers with reduced mobility. The dimers are contained in a cuvette filled with, e.g., viscoelastic gel. The external (static) magnetic field ($\vec{B}$) is generated by Helmholtz coils, and the illumination field at THz is achieved by using appropriate polarization filters. Such beam is built with two counter-propagating plane waves, with wavevectors $k_A$ and $k_B$, both waves possessing left circular polarization (LCP). (b-c) Details of the geometry and composition of the configurations studied in this work; the illuminating beam and $\vec{B}$ are always parallel to the $z$-axis of the cartesian coordinate system. (a) Parallel configuration; or dimer parallel to $z$. (b) Perpendicular configuration, where the dimer is held along the $xy$ plane at $z=0$. The first particle (silvered, $\# 1$) is made of n-doped $InSb$; the other particle (blue, $\#2$) is made of an isotropic or birefrigent material.
• Figure 2: (Color online) Response by a single, $100$ nm, MO particle ($n- InSb$) under the illumination described in Eq. \ref{['eq-E0']}. (a) Map of absorption cross-section's spectra as a function of the external (static) magnetic field. (b) idem -a- for the spin torque exerted on the particle. The embedding medium is "biomimic". Black line highlights the zeros calculated for this observable. (c) Spectra showing the cross-section's resonance peaks for several embedding media. Here, $B= 0.25$ T. (d) Idem -c- for the resonant features in the spin torque.
• Figure 3: (Color online) Response by a MO heterodimer immersed in an average biological medium biomimic (sizes $100$-$50$ nm, both particles made of $n- InSb$, immersed in biomimic, $gap = 10$ nm) under parallel configuration. (a) [b] Absorption cross sections when $B = 0.5$ T [$B = 0$ T]. Black [blue] line for the dimer system [particle $\# 1$ as isolated]. (c) Binding force induced on the dimer when $B$ is on/off. (d) Spin torque exerted on particle $\# 2$; black line when it is coupled to particle $\# 1$, blue when uncoupled (as isolated).
• Figure 4: (Color online) Maps of relevant observables when both the frequency and value of the external magnetic field are swept. The heterodimer consists of a $100$ nm particle ($\#1$) made of $n- InSb$ and a $50$ nm ($\#2$) of $GaSe$ under parallel configuration, with a $10$ nm gap. (a) Absorption cross section. (b) Binding force, and (c) Spin torque exerted on the $GaSe$ particle.
• Figure 5: (Color online) Case of the dimer $\left[n- InSb, SiO_2\right]$ for perpendicular configuration and $B=0.25$ T. The particles' size are $100$ nm and $50$ nm, respectively, with a $10$ nm gap. (a) Absorption cross section; (b) Binding force, (c) z-Spin torque induced on the second particle. (d) z-Orbital torque induced on the system.