Preferred trapping locations of microscopic particles held in a laser beam via photophoresis

It has been known for decades that microscopic dust particles can become trapped near the focus of a continuous laser beam when surrounded by ambient gas such as air. In this photophoretic interaction, the laser heats the particle, which interacts with surrounding gas molecules. Trapped particles typically scatter significant laser light as they are suspended in midair and can be easily observed from the side of the beam. We report on the first on-axis images of photophoretically trapped particles together with the laser-beam profile responsible for the trapping. The radial structure of the laser is recorded using 1:1 imaging, where the 1 W beam must be strongly attenuated without introducing distortion. The trapped particle is weakly illuminated using a different wavelength, chosen to transmit through the filters used to attenuate the laser.