Manipulation of Birefringent Micro particles using the Angular Momentum of Light in a Fabry-Perot Resonator

Small birefingent particles absorb the spin-angular-momentum of photons in whole or in part and start to rotate or orient in a defined statical way in a polarized beam of light; Laser intensities of some ten milliwatts are sufficient to stabilize them in the grip of optical tweezers and to manipulate them according to the incident polarization. To further reduce this power we investigated the feasibility of placing birefringent particles between the two mirrors of a Fabry-Perot interferometer were the laser beam passes through them several times. Using the Poincaré-sphere method we calculated the influence of mirrors and birefringent plates on the polarization of light and the angular momentum transfer from light to these plates. Experimentally we verified our results by measuring the polarization of different orders of a Fabry-Perot interferometer with birefringent plates inside the resonator. Our results can be applied to Laguerre-Gaussian modes which illuminate screw like shaped microparticles and we show first results of angular momentum transfer to bacteria of the species spirillum.