Nonlinear optical effects in trapping nanoparticles with femtosecond pulses

The optical trapping technique has been widely used in various areas to manipulate particles, cells, and so forth. The principle of trapping is based on the interaction between optical electric fields and induced linear polarizations. Here we show a novel phenomenon of trapping arising from nonlinear polarization when we trap gold nanoparticles by ultrashort near-infrared laser pulses. That is, the stable trap site is split into two equivalent positions (we call this 'trap split'). The trap positions are aligned along the direction of the incident laser polarization. The dependencies of trap split on the trapping-laser power and wavelength were investigated. The results were successfully interpreted in terms of the nonlinear polarization caused by the femtosecond pulses. This method may give novel applications to micromachining, nanofabrication, and biological samples as well as atomic and molecular trapping at low temperatures.