Optical Twist Induced by Plasmonic Resonance

Abstract

Harvesting light for optical torque is of significant importance, owing to its ability to rotate nano- or micro-objects. Nevertheless, applying a strong optical torque remains a challenging task: angular momentum must conserve but light is limited. A simple argument shows the tendency for two objects with strong mutual scattering or light exchange to exhibit a conspicuously enhanced optical torque without large extinction or absorption cross section. The torque on each object is almost equal but opposite, which we called optical twist. The effect is quite significant for plasmonic particle cluster, but can also be observed in structures with other morphologies. Such approach exhibits an unprecedentedly large torque to light extinction or absorption ratio, enabling limited light to exert a relatively large torque without severe heating. Our work contributes to the understanding of optical torque and introduces a novel way to manipulate the internal degrees of freedom of a structured particle cluster.

Optical pulling and pushing forces exerted on silicon nanospheres with strong coherent interaction between electric and magnetic resonances

“…Si nanoparticles have already been experimentally trapped in an optical tweezers [33]. The investigation of optical torque based on optical force was also carried out [34,35]. In recent years, optical pulling force, which makes particles move against the propagation direction of light in the absence of intensity gradient, has stimulated the interest of researchers working in this field because it is a counter-intuitive phenomenon since the discovery of optical radiation pressure.…”

Gain-induced large optical torque in optical twist settings

“…Angular momentum carried by light can induce optical torque via scattering or absorption, [1][2][3][4][5][6][7][8][9][10] which was first demonstrated by Beth, [11] and provides applications in DNA unfolding, sequencing, and binding, [12][13][14][15] biological molecular motors, [16][17][18] nano-electromechanical systems, [3,[19][20][21][22][23] and so on. However, harvesting light for a large torque is difficult.…”

“…Several ways to obtain a large optical torque have been demonstrated. Researchers can use birefringent particle to enhance the torque by reversing the photon spin angular momentum of the incident circularly polarized light, [24] or use plasmonic motor to increase the extinction cross section via plasmonic resonance, [3] or use double-layer clusters to enhance the torque per unit light energy extinction and absorption rate, [6] etc.…”

“…Double-layer clusters recycle light by resonance light exchange between the two layers, [6] which enhance internal multiple scattering. Thus significant difference between opposite optical torques called optical twist in double layer structures can be induced without extracting a large amount of light from the incident beam.…”

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

“…Recent advances in nanoparticle engineering [7,8] and holographic beam-generation via spatial light modulators (SLMs) [9,10,11,12] and other phase-manipulation techniques [13,14,15,16] have created many new degrees of freedom for engineering light-particle interactions beyond traditional optical tweezers. Enhanced and unusual optical forces and torques can be engineered by designing material objects [17,18,19,20,21] and/or structured illumination, with the latter including "tractor beams" [22,23] and beams carrying optical angular momentum [24,25,26,27]. These increased degrees of freedom pose an interesting design challenge: for a given target object, what is the optimal illumination pattern to produce the strongest optical force or torque?…”