Fig. 2. Optical torque acting on rotating particles.

(A to C) Optical response of rotating particles of different geometries, namely, nanorod (A), nanocross (B), and nanodisk (C), to RCP (red) and LCP (blue) incident light. We plot the imaginary part of the polarizability, which is related to the extinction cross section according to σ_ext = (4πω/c) Im {α}. A solid particle such as the nanodisk (C) exhibits different circular dichroism relative to particles with confined electron motion, such as the nanorod (A) and nanocross (B). (D to F) Time-averaged torque M_dr acting on the rotating particles considered in (A) to (C) under linearly polarized illumination. Insets: M_dr versus particle rotating frequency Ω for two typical light frequencies in the cooling and heating regimes. In all cases, damping rates are assumed to be γ = 0.2ω₀ and τ⁻¹ = 0.02ω₀. Apart from the insets, particles are rotating with angular velocity Ω = 0.1ω₀. All frequencies are normalized to the particle resonance frequency ω₀, the polarizability is normalized to ${\mathrm{\alpha }}_0=Q^2/m{\mathrm{\omega }}_0^2$, and the torque is normalized to M₀ = α₀∣E_±∣²/2. We assume a large resonance linewidth for illustration.