All dielectric self-induced back-action trapping.

Strong field gradients in electromagnetic resonators can trap individual nanospheres. Moreover, the presence of the sphere in the resonator can give rise to self-induced back-action (SIBA), which increases the trap stiffness beyond standard dipolar force theory. In this work, we investigate SIBA in a system with a silica sphere of 16 nm in diameter in a dielectric nanocavity with a strongly localized electric field. We analyze the optical forces in a novel framework based on perturbation theory with quasi-normal modes, where modifications to the gradient and scattering forces are found to arise due to the presence of the particle, thus capturing the back-action effect. This gives closed-form expressions with clear insight into the mechanism of SIBA, and provides an efficient method of estimating optical forces when SIBA is present. We compare the results with reference calculations based on the Maxwell Stress Tensor formalism and find good agreement.