Abstract
<title>Abstract</title> <p>Optical torque exerted on an optically bound cluster typically decays with oscillation around zero as the interparticle distance increases. Besides, a homodimer of gold spheres (radius 150 nm) with electric dipole and quadrupole responses, separated by one wavelength, represents the smallest cluster known to achieve stable negative optical torque. Here, we present analytical expressions of optical torque and binding force in a dielectric–metal heterodimer, bound by a circularly polarized plane light, with both electric and magnetic dipolar responses. Our theoretical results show counterintuitively that beyond the Rayleigh limit, the torque increases almost linearly with interparticle distance, a behavior governed by dipolar coupling within the dielectric particle. Meanwhile, small torque fluctuations arise from dipolar hybridization between the two particles. Furthermore, the smallest cluster unit capable of sustaining stable negative torque is reduced to a silicon–silver heterodimer with constituent particle radii below 70 nm. Especially, the minimum and most stable interparticle distance is reduced to 0.4 wavelengths, controlled by a combination of near-field and second-order far-field scattering interactions. Unexpectedly, completely negative torque is achieved across a broad continuous range of interparticle distances, irrespective of the optically bound stability. These findings offer deeper insights into optical torque and light–matter interaction.</p>