Optical micromanipulation using nonliner photo-responses

In the present study, we have detected multiphoton absorption force acting on microparticles due to simultaneous twophoton absorption. A polymer microparticle including dye molecules was optically trapped with femtosecond (800 nm, 100 fs) pulse pairs with time-interval Δt in water. Under the condition that Δt > the pulse duration, the particle was trapped with absorbing small number of photons via multiphoton process. The number of photons absorbed via simultaneous two-photon absorption increased by decreasing Δt to overlap the pulse pair, leading to the micromechanical motion of the particle. By tracking the position of the trapped particle, we confirmed that the particles were pushed in the direction of light propagation and the displacement increased with decreasing Δt.