  This paper aims to maximize optical force and torque on arbitrary micro- and
nano-scale 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 and torque. All results are
obtained using open-source computational software available online.
