Detection of radiation force due to simultaneous two-photon absorption

In the present study, we have precisely tracked the three-dimensional (3D) motion of single nanoparticles under optical trapping by focused femtosecond laser pulses for detecting radiation force due to simultaneous multiphoton absorption. A 300-nm polymer particle containing dye molecules was optically trapped with pulsed laser (800 nm, 200 fs at the focal point). The 3D motion of the trapped polymer nanoparticle was observed with astigmatism fluorescence imaging method. The trapping point shifted on the optical (Z-) axis towards the propagation direction of the femtosecond laser with increasing laser power. The Z-displacement was dependent on incident laser power and, from which we concluded that the positional shift along the Z-axis can be ascribed to the radiation force due to simultaneous two-photon absorption of the laser pulses by the particle.