Optical torque from enhanced scattering by multipolar plasmonic resonance

Abstract

Abstract:We present a theoretical study of the optical angular momentum transfer from a circularly polarized plane wave to thin metal nanoparticles of different rotational symmetries. While absorption has been regarded as the predominant mechanism of torque generation on the nanoscale, we demonstrate numerically how the contribution from scattering can be enhanced by using multipolar plasmon resonance. The multipolar modes in non-circular particles can convert the angular momentum carried by the scattered field and thereby produce scattering-dominant optical torque, while a circularly symmetric particle cannot. Our results show that the optical torque induced by resonant scattering can contribute to 80% of the total optical torque in gold particles. This scattering-dominant torque generation is extremely mode-specific, and deserves to be distinguished from the absorption-dominant mechanism. Our findings might have applications in optical manipulation on the nanoscale as well as new designs in plasmonics and metamaterials.

Optical Manipulation of nanoparticles by simultaneous electric and magnetic field enhancement within diabolo nanoantenna

“…A maximum force appears when the DA resonates and its value is almost 122 times greater than the off-resonance case for a DA immersed in air (@ λ = 1112 nm) and 92 times larger in water (@ λ = 1458 nm). The same phenomenon may exist for other particle geometries providing electromagnetic (electric and/or magnetic) resonances 20 , 21 . In the present case of a resonant DA , a light funneling effect takes place and it contributes to enhance the radiation pressure on the DA and pushes it along the light propagation direction.…”

“…Nevertheless, we have verified that the radiation pressure generated by the DA resonance is as efficient as in the case of the three geometries studied in ref. 20 as shown on Fig. 4a .…”

Computational inverse design of non-intuitive illumination patterns to maximize optical force or torque

“…In Figs.5A-5C, the reference planewave torque spectrum (black dashed line) is identical in all three plots and peaks at both dipole and quadrupole resonances with nearly equal heights (explained in detail in [51]). Illumination-field optimization at dipole resonance and off-resonance achieve a similar 6x-boost at the chosen λ opt value without suppressing the quadrupole resonance.…”

“…We demonstrate our illumination-field optimization framework on the gold nanotriangle illustrated in Fig.1. Our previous work [51] analyzes the optical force and torque on such a particle for circularlypolarized (CP) planewave illumination. CP planewave is a common incident-field choice [52,53,19,20] for torque generation due to its intrinsic spin angular momentum, but we find in our computational optimization that highly optimized field patterns can show 20x improvement of FOM T .…”

Recent Progress in Optical‐Resonance‐Assisted Movement Control of Nanomotors

“…Circularly polarized laser beams, with each photon carrying an inherent spin angular momentum of ± ħ , have been used to rotate plasmonic nanomotors trapped in an optical tweezers . Absorbed photons will transfer its angular momentum completely to the nanomotor, generating the absorption torque, while the scattering torque results from the change of polarization states of the scattered photons . In most cases, both absorption and scattering torques reach their maximum at the plasmon resonance wavelengths.…”