Multipolar Pseudochirality-Induced Optical Torque

Abstract

It has been observed that achiral nanoparticles, such as flat helices, may be subjected to an optical torque even when illuminated by normally incident linearly polarized light. However, the origin of this fascinating phenomenon has so far remained mostly unexplained. We therefore propose an exhaustive discussion that provides a clear and rigorous explanation for the existence of such a torque. Using multipolar theory and taking into account nonlocal interactions, we find that this torque stems from multipolar pseudochiral responses that generate both spin and orbital angular momenta. We also show that the nature of these peculiar responses makes them particularly dependent on the asymmetry of the particles. By elucidating the origin of this type of torque, this work may prove instrumental for the design of high-performance nano-rotors.

Figure 1

Problem simplification for different particles subjected to optical forces and torques when illuminated by a z-propagating plane wave.

Figure 2

Lateral force acting on an asymmetric gold particle. (a) Illumination condition. (b) Components of the optical force and Poynting vector S_y = Re{E_zH_x^* – E_x^*H_z}/2. (c) Multipolar decomposition given in terms of the main components of the electric dipole p, the magnetic dipole m, the electric quadrupole , and the magnetic quadrupole . The radiation patterns at the two resonant frequencies are also plotted at the top of (c). The particle dimensions are L_x = 150 nm, L_y = 500 nm, and a thickness of 30 nm.

Figure 3

Optical torque acting on a gold Gammadion particle. (a) Illumination condition. (b) Components of the optical torque and Poynting vector S_y = Re{E_zH_x^* – E_x^*H_z}/2. (c) Multipolar decomposition showing that only the x-component of the electric dipole p is dominant at both resonant frequencies. The particle dimensions are L_x = 90 nm, L₁ = L₂ = 60 nm, and a thickness of 30 nm.

Figure 4

Normalized absolute value of the longitudinal torque, Γ_z, versus wavelength and gammadion shape asymmetry.

Figure 5

Normalized multipolar components versus wavelength and particle asymmetry ratio L₁/L₂. The top and bottom rows correspond to co-polarized and cross-polarized responses, respectively. The plots are normalized by dividing separately each column by the maximum value of either the co- or cross-polarized component.