Optical forces in tight spaces: How confinement of light affects its mechanical action

In this paper we revisit the issue of mechanical action of light confined within optical cavities. While the traditional view holds that the modification of the resonant frequencies of the cavities via mechanical motion is the main source of optomechanical interaction, we demonstrate that in the case of cavities with degenerate modes, confinement of light modifies the optical forces in more dramatic ways. Using an analytically solvable model of a nanoparticle interacting with a spherical optical resonator we show that the optomechanical coupling in this case loses its usually conservative nature. We demonstrate that usually conservative part of the optical force, which is not related to photon scattering, acquires non-potential, solenoidal components. We trace this effect to interference between degenerate modes of the resonator whose degeneracy is (partially) lifted by the particle.