Levitating Microscopic Particles in a Laser Beam via Photophoresis*

Within the focus of a laser beam, microscopic particles surrounded by air can become trapped by the heating of the particle and the resulting interaction with surrounding air molecules. This photophoretic effect can withstand wind speeds up to 2 meters per second, and thus centimeter-scale patterns can be drawn in mid air by utilizing the strong light scattered from the particle while sweeping the laser beam through space. Motivating the study of this effect is the desire to increase the potential sweep speed of a laser to create larger and more intricate patterns. While it is known that structure in the laser beam, such as that arising from spherical aberration, aids in stable trapping, questions remain as to exactly how a particle becomes stably trapped in a particular beam location, as particles exhibit oscillations or excursions over tens of microns and accelerations up to 10 gs.