The ability to control the position of a mesoscopic object with nanometric precision is important for the rapid progress of nanoscience. One of the most promising tools to achieve such control is optical tweezers, which trap objects near the focus of a laser beam. However, the drawbacks of conventional tweezers include a trapping volume that is diffraction-limited and significant brownian motion of trapped nanoobjects. Here, we report the first experimental realization of three-dimensional nanometric optical tweezers that are based on nanostructured substrates. Using electromagnetically coupled pairs of gold nanodots in a standard optical tweezers set-up, we create an array of subwavelength plasmonic optical traps that offer a significant increase in trapping efficiency. The nanodot optical near-fields reduce the trapping volume beyond the diffraction limit and quench brownian motion of the trapped nanoparticles by almost an order of magnitude as compared to conventional tweezers operating under the same trapping conditions. Our tweezers achieve nanoscale control of entities at significantly smaller laser powers and open new avenues for nanomanipulation of fragile biological objects.

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Laser trapping of particles in three dimensions can occur as a result of the refraction of strongly focused light through micrometre-sized particles. The use of this effect to produce laser tweezers is extremely common in fields such as biology, but it is only relatively recently that the technique has been applied to liquid crystals (LCs). The possibilities are exciting: droplets of LCs can be trapped, moved and rotated in an isotropic fluid medium, or both particles and defects can be trapped and manipulated within a liquid crystalline medium. This paper considers both the possibilities. The mechanism of transfer of optical angular momentum from circularly polarized light to small droplets of nematic LCs is described. Further, it is shown that droplets of chiral LCs can be made to rotate when illuminated with linearly polarized light and possible mechanisms are discussed. The trapping and manipulation of micrometre-sized particles in an aligned LC medium is used to provide a measure of local shear viscosity coefficients and a unique test of theory at low Ericksen number in LCs.

A detailed study is presented that evaluates the relative importance of wave plate behavior, scattering processes and absorption phenomena in transferring optical torque from circularly polarized light to optically trapped nematic droplets. A wide range of parameters is considered: droplet diameters between 1 and 15 µm, birefringence values from 0.15 to 0.26 and trapping beam powers from 50 mW to 400 mW. Wave plate behavior is verified through the dependence of torque on droplet diameter and material birefringence. The dependence of the magnitude of the torque on material birefringence confirms the additional importance of the scattering mechanism. Absorption processes are found to be negligible

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