Calibration of an optically levitated microsphere as a force sensor

By using an optically trapped and cooled microsphere (bead) as a force sensor, it is predicted to be possible to attain force sensitivity of 10−21 N[1]. This sensitivity is achieved because the force sensor is mechanically decoupled from the environment, which allows for larger quality factors. With this sensitivity, it is possible to test for corrections to Newtonian gravity at 1µm length scales. This thesis describes the experimental setup of optically trapping and cooling a 3µm sphere as well as a technique to calibrate the force sensitivity using applied electric fields. The long term goal of the experiment is to use this force sensor to measure gravity and other forces at short ranges, so calibrating the system is important. We have trapped a 3µm microsphere in an optical trap and have taken it to medium vacuum (2 torr). We have also cooled the bead’s center of mass motion along three axes. We have applied an external force by using an electrical potential and have measured the force exerted on the bead. We have data that shows the force on the bead as a function of voltage, which allows us to find the charge on the beads. The charges on the beads range from zero electrons to upwards of 100. We have evidence that the charge on a bead can be reduced using light from a halogen light