Optical trapping for charge detection and spinning

Optical trapping of small particles with laser light is a versatile method for manipulation. In this work two laser trapping experiments are described. In the first experiment the charge of a trapped particle in water is measured with the resolution of the elementary charge, by measuring the displacement in an electric field. An ac electric field is used to exert a sinusoidal force onto the particle, and the amplitude of the oscillation is measured with a quadrant photodetector. This setup has extended the limits of sensitivity, which allows to see discrete steps in the charge of the nanoparticles, even in a highly polar material such as water. In the second experiment micrometer size particles consisting of polymerized chiral liquid crystal start spinning under influence of the polarization of the laser trapping beam. This work consists of experimental results, showing the period of the rotation as a function of the polarization of the laser beam, and simulation results, in which the torque exerted by the laser beam on the chiral particle is calculated. It is observed that some particles can be rotated in both directions when the degree of circular polarization is sufficiently high, while other particles only rotate in one direction.