Gain-induced large optical torque in optical twist settings

Inducing a significant optical torque remains a challenging task,since the law of angular momentum conservation implies that one has to harvest a lot of light.Such a problem was partially resolved by using optical twist via strong internal multiple scattering to recycle the photons,and one can induce a large torque per unit of radiation cross section.By using the Maxwell stress tensor and the generalized Lorentz-Mie scattering theory for multi-spheres,we investigate the influence of gain materials in further amplifying optical torque in the optical twist settings.It is found that,when combined with a gain layer,the optical torque of lossy(both in PT-and non-PT-symmetric structures)or lossless(low dielectric materials)clusters at resonance could be one order of magnitude larger than those of a single layer and previous studied plasmonic double layer structures.Moreover,the gain-enhanced large opposite rotations(i.e.,optical twist)of the two layers arise at resonances in these structures.In contrast,in the gain-gain double-layer cluster,optical torques on both layers have no significant increase and the two layers rotate in the same direction at resonances.This work provides an elaborate investigation on the gain media-induced optical twist,which offers more choices for optical micromanipulation.