Abstract
This paper aims to maximize optical force or torque on arbitrary micro- and nanoscale objects using numerically optimized structured illumination. By developing a numerical framework for computer-automated design of 3d vector-field illumination, we demonstrate a 20-fold enhancement in optical torque per intensity over circularly polarized plane wave on a model plasmonic particle. The nonconvex optimization is efficiently performed by combining a compact cylindrical Bessel basis representation with a fast boundary element method and a standard derivative-free, local optimization algorithm. We analyze the optimization results for 2000 random initial configurations, discuss the tradeoff between robustness and enhancement, and compare the different effects of multipolar plasmon resonances on enhancing force or torque. All results are obtained using open-source computational software available online.
Highlights
IntroductionWe show how large-scale computational optimization [1, 2, 3] can be used to design superior and nonintuitive structured illumination patterns that achieve 20-fold enhancements (for fixed incident-field intensity) of the optical torque on sub-micron particles, demonstrating the utility of an optimal design approach for the many nanoscience applications that rely on optical actuation of nanoparticles [4, 5, 6].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
We show how large-scale computational optimization [1, 2, 3] can be used to design superior and nonintuitive structured illumination patterns that achieve 20-fold enhancements of the optical torque on sub-micron particles, demonstrating the utility of an optimal design approach for the many nanoscience applications that rely on optical actuation of nanoparticles [4, 5, 6]
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 FOMT
Summary
We show how large-scale computational optimization [1, 2, 3] can be used to design superior and nonintuitive structured illumination patterns that achieve 20-fold enhancements (for fixed incident-field intensity) of the optical torque on sub-micron particles, demonstrating the utility of an optimal design approach for the many nanoscience applications that rely on optical actuation of nanoparticles [4, 5, 6].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? When exploring so many parameters, a large number of scattering problems must be solved efficiently, which requires careful design of the optimization framework
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