Measurement of Photophoretic Forces based on Levitating Particles in a Thermophoretic Trap*

We present an experimental platform to study photophoresis based on levitating particles under the illumination of external radiation. The particles are levitated and trapped in a thermophoretic force field in a vacuum cell. Two orthogonal cameras track the three-dimensional motion of the particles. Microspheres, ranging from 5 to 25 μm in radius, are levitated, and they exhibit different motion under illumination: movement in the direction of laser propagation (positive photophoretic force) and opposite the direction of laser propagation (negative photophoretic force). In an effort to understand our observation vis-à-vis existing models of photophoresis, we simulate the radiation field and temperature distribution of levitated spheres and the thermal cell to extract the photophoretic force from the measurement. Our platform allows for long observation time, repeated experiments on the same particle and has the sensitivity reaching 1 pN. This study of illumination-induced dynamics is an essential step towards use of the laser for optical control of mesoscopic particles, which will widen the possibilities of our levitation scheme as a platform for studying force fields in a microgravity environment.