Since Arthur Ashkin's groundbreaking work with optical trapping beginning in 1969, the properties and applications of this field have been studied intensely [1]. Most of the early work with optical trapping was focused on the properties of trapping transparent particles [2]. Research into trapping of opaque particles followed. Initial trap designs for transparent particles using a single gaussian beam proved ineffective with opaque particles. It has since been shown that absorptive particles trap best in dark regions of the laser beam. These dark regions can be caused by abberrations inherent with spherical lenses, doughnut shaped beams shaped by phase plates or holograms from spatial light modulators, hollow beams created by setups with axicon lenses, or a number of other methods.

The forces primarily responsible for stably trapping an opaque particles are known as photophoretic forces. These arise from the transfer of thermal energy stored in the trapped particle to kinetic energy in the gas molecules bouncing off of it.

Works Cited

1. Ashkin A (1970) Phys Rev Lett 24:156–159.
2. Bowman, Richard W., and Miles J. Padgett. “Optical trapping and binding.” Reports on Progress in Physics 76.2 (2013): 026401.