Abstract

Trapping and manipulating particles with light is a technique that has been widely studied for optical tweezers, acting under the gradient force. During the last years there has been a grown in the number of works dedicated to the study of a different type of optical force, capable of trapping and manipulating absorbing macroparticles: the photophoretic force [1, 2]. When an absorbing particle is illuminated, it absorbs part of the energy of the input light field, being this energy redistributed along the particle surface in form of heat. If the particle is inside a medium, e.g. a gas, the radiated heat will increase the kinetic energy and momentum of the particles of that medium and they will transfer their momentum to the absorbing particle. If that particle is illuminated evenly, it will receive an asymmetric transfer of momentum from the gaseous particles and will be pushed away from the light source, being this effect known as photophoretic force. In contrast, if the particle is illuminated symmetrically in all directions, the amount of momentum transfer received from the gas will be also symmetric and, as a consequence, it will remain at a permanent position. In other words, the particle will be 3D trapped. To illuminate a particle symmetrically, the light beam must form an optical bottle, i.e. a region of ideally null intensity surrounded in all directions by regions of higher intensity [3].

© 2015 IEEE