How light absorption modifies the radiative force on a microparticle in optical tweezers.

Reflection and refraction of light can be used to trap small dielectric particles in the geometrical optics regime. Absorption of light is usually neglected in theoretical calculations, but it is known that it occurs in the optical trapping of metallic particles. Also, recent experiments with semi-transparent microparticles have shown that absorption of light is important to explain their optically induced oscillations. Here, we propose a generalization of Ashkin's model for the radiative force exerted on a spherical bead, including the contribution due to attenuation/absorption of light in the bulk of the particle. We discuss in detail the balance between refraction, reflection and absorption for different optical parameters and particle sizes. A detailed example is provided in order to clarify how the model can be applied, and it is obtained that the radiative force can either increase or decrease with absorption, depending on the particle size. Our findings contribute to the understanding of optical trapping of light-absorbing particles, and may be used to predict whenever absorption is important in real experiments.