Inhomogeneous and anisotropic particles in optical traps: Physical behaviour and applications

•Theory of optical tweezers, small and extended particles, trap stability.•Computational methods and experimental instrumentation.•Decreasing particle symmetry, from spheres to shaped micro-tools.•Optical anisotropy, liquid crystal droplets and excitable torque wrenches.•Inhomogeneous particles, engineered Fano resonances and optical sail boats.

Beyond the ubiquitous colloidal sphere, optical tweezers are capable of trapping myriad exotic particles with wildly varying geometries and compositions. This simple fact opens up numerous opportunities for micro-manipulation, directed assembly and characterization of novel nanostructures. Furthermore, the mechanical properties of optical tweezers are transformed by their contents. For example, traps capable of measuring, or applying, femto-Newton scale forces with nanometric spatial resolution can be designed. Analogous, if not superior, angular sensitivity can be achieved, enabling the creation of exquisitely sensitive torque wrenches. These capacities, and others, lead to a multitude of novel applications in the meso- and nanosciences. In this article we review experimental and theoretical work on the relationship between particle geometry, composition and trap properties. A range of associated metrological techniques are discussed.