Volumetric display by movement of particles trapped in a laser via photophoresis

Photophoresis can stably hold opaque microscopic particles in a laser focus surrounded by room air with strength sufficient to enable centimeter-scale patterns to be drawn by sweeping the laser beam. The resulting images rely on visual persistence as laser light scatters from the particle, which is rapidly swept through the 3-D pattern. Control can be maintained while moving the particle with air speeds up to 2 m/s. A desire to greatly increase the sweep speed motivates a re-examination of the fundamentals of photophoresis-based laser-particle traps. Most explanations offered are qualitative, with differing opinions as to whether, for example, asymmetric heating or asymmetric thermal accommodation is primarily at work. Which particles become trapped in the beam is typically based on self-selection, as a variety of particles with possible differing shapes and sizes are offered to the laser focus for capture. Characteristics that make some particles preferred over others are especially relevant. There is broad consensus that structure in the laser focus greatly aids in stable trapping. Nevertheless, it is still possible for even a relatively smooth TEM00 beam to capture and hold particles. Moreover, even in a structured focus (i.e. with aberrations and local intensity minima and maxima), questions remain as to exactly how a particle becomes stably trapped in certain beam locations. A zoomed-in look at trapped particles reveals oscillations or orbits with excursions over tens of microns and accelerations up to 10 gs. We trapped particles in zero-gravity as well as 2-g environments with no noticeable difference in stability.