Particles dance in the laser light

A single particle in a focused laser is held stationary at the high-intensity focus. (See Physics Today, December 2018, page 14 .) When two particles share the same optical trap, the results are far more dynamic and surprising. Now Kelken Chang of the University of Gothenburg in Sweden and his colleagues have observed a previously undescribed oscillation state for two optically levitated droplets.

The researchers deposited a pair of glycerol droplets tens of microns in diameter into an upward-pointing laser beam. If the droplets collided head-on during loading, they eventually merged. If they only grazed one another, they settled into a juggling pattern, which was stable for up to half an hour. The particles’ bean-shaped orbits lay in a two-dimensional plane perpendicular to the polarization direction of the light.

To see how the juggling trick works, consider that a particle in the beam is both pushed up by radiation pressure and pulled to the region with highest intensity, which is at the center of the beam. But a transparent particle also refracts the light and alters the beam profile above it. Therefore, when a second particle is injected into the beam (red circle in the figure), the first, lower particle (blue circle) casts a shadow on it and changes what the second particle sees as the region with the highest intensity. The second particle moves to the beam’s edge rather than its center and then falls until it is below the first particle and no longer in shadow. It then gets pulled back to the high-intensity center of the beam and, in turn, casts a shadow on the first particle.