Motion control of low-index microspheres in liquid based on optical repulsive force of a focused beam array

In the optical tweezers technique a microdielectric object with a higher refractive index than a surrounding liquid is trapped near the focal point of a laser beam, whereas an object with a lower refractive index is subjected to a repulsive force from the focused beam. We demonstrate that the optical repulsive forces are generated with a focused-beam array dynamically formed by a computer-generated hologram displayed on a liquid-crystal spatial light modulator. We also demonstrate that the optical repulsive forces perform a size selection and flow control of hollow glass microspheres.