Effects of nonlinear polarization in optical trapping

Conventional optical tweezers handle non-absorptive (transparent) particles, and the force exerted to the particle is proportional to the intensity of the trapping laser. Recently, some works on selective trapping/manipulation of absorptive particles based on resonance effects have been reported. Some years ago, our research group found that the use of an ultra-short pulsed laser as a trapping light source shows nonlinear optical effects on trapping behavior for gold nanoparticles under pre-resonant conditions [1]. In this paper, we briefly describe the experimental observation of the phenomenon and interpretation of that.