A stabilizer for single microscopic particles in a quadrupole trap

Light scattered from a single particle suspended in a quadrupole trap is focused on the detector array in a charged coupled diode (CCD) camera. The video signals obtained from two successive rows of sensors framing the center of the image are compared and the difference used to control a superposed vertical field, holding the particle at trap center. The system is effective for liquid and solid particles in the size range from 1 to 10 μm trapped in vacuum or vapor.