Novel optical trapping for semiconductor, metal and magnetic particles

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The purpose of this study is to achieve a novel optical trapping technique for metal and semiconductor particles by using a radially polarized laser beam, which polarization distributes in a radial pattern. In the first year, stable oscillation of radially or azimuthally polarized beam was achieved by separating polarization components in a laser cavity by a birefringent crystal. As a initial attempt, a c-cut Nd:YVO_4 crystal was selected as a laser material. When this crystal was end-pumped by semiconductor lasers, the output power of approximately 100 m W with radial polarization was obtained. On the basis of this technique, the output power above 2W was observed when a c-cut, non-doped YVO_4 crystal was inserted into a Nd:YAG laser cavity. In the second year, the experiment of optical trapping was performed. A laser beam was focused using a water-immersion objective with the numerical aperture of 1.2. Glass and polystyrene particles were suspended in a glass vessel filled with water and irradiated by the laser focused laser beam. In order to measurer the longitudinal optical trapping force, glass particles were trapped and then the laser power was decreased. The output power, at which the particle was released from the laser trap, was measured and regarded as a critical power. Similar experiments were done for particles with different sizes. The trapping force with a radially polarized laser beam was founded to be lager than that with a linearly polarized beam. This result agrees with the calculation based on the ray optics and shows the validity of the radially polarized laser beam as a trapping beam. The calculation indicated that the radially polarized laser beam is more effective for particles with large refractive index. This implies the validity of the radially polarized beam for the optical trapping of semiconductor particles.