Optothermal trapping of microparticles near an absorbing reflective film with an annular beam

Optothermal manipulation technologies trap and manipulate microparticles under a light-controlled temperature gradient. In this paper, we demonstrate the possibilities of optothermal trap, which is generated by an annular beam irradiating on an absorbing reflective film to capture the microparticles. The particles are subjected to thermal and optical gradient forces. First, we investigate the particle trapping near a non-absorbing reflective surface to understand the action of the optical force of annular beam. The results show that the optical force cannot trap the particles near a reflective surface. Second, an annular beam is generated by Gaussian beam shaping with the aid of axicons, which is then used to irradiate and heat a gold film to create an optothermal trap. The induced thermal gradient and thermal convection can bind the particles in the center of the annular spot. The trapping stiffness of the optothermal trap is 8.1 ± 2.9 fN/μm at a laser power of 100 mW.