Optical Control of Levitated Particles in A Thermophoretic Trap

We study the dynamics of levitated particles under illumination by a laser. Microspheres ranging from 10 to 50 μm in diameter are levitated and trapped in a thermophoretic force field generated in a vacuum chamber with air pressures between 4 and 15 Torr. The laser heats up and creates a temperature differential in the levitated particles. Momentum exchange with the surrounding gas drives the illuminated particles either in the direction of laser propagation (positive photophoresis) or opposite the direction of laser propagation (negative photophoresis). We report observations of both positive and negative photophoretic forces on levitated particles. To understand our experimental results vis-à-vis existing models of photophoresis, we simulate the radiation field and temperature distribution in levitated spheres to obtain quantitative predictions of the photophoretic and thermophoretic forces. This study of illumination-induced dynamics is a necessary first step towards optical control of levitated particles, which will find applications in studying micron-scale physics and force fields in a microgravity environment.