Wide-Range Axial Position Measurement for Jumping Behavior of Optically Trapped Microsphere Near Surface Using Chromatic Confocal Sensor

When a microsphere is trapped near a surface by single-beam gradient force trapping, the standing wave is generated between the microsphere and the surface, where abrupt motion along the optical axis (jumping) is observed corresponding to displacement of the surface. This jumping distance is on the order of a few hundred nanometers. In the vicinity of the surface, intensity of retro-reflected light is increased so that the averaged position of the jumping is shifted up on the order of several micrometers. Therefore wide-range and high-resolution position measurement technique is required. In this article, we proposed to apply a chromatic confocal sensor to measure the axial position of the microsphere in the standing wave. It was experimentally validated that the position of the microsphere could be measured with a resolution of 10 nm and a measuring range of 3 µm.