Spin torque on birefringent "o" and "e" axes in regular rhombohedral calcite crystals

Catherine M. Herne; Natalie A. Cartwright; Johanna R. Levey

doi:10.1117/12.2511989

1 March 2019 Spin torque on birefringent "o" and "e" axes in regular rhombohedral calcite crystals

Catherine M. Herne, Natalie A. Cartwright, Johanna R. Levey

Proceedings Volume 10935, Complex Light and Optical Forces XIII; 109351O (2019) https://doi.org/10.1117/12.2511989
Event: SPIE OPTO, 2019, San Francisco, California, United States

Abstract

We demonstrate the orientation-dependent torque on regular rhombohedral calcite in an optical trap. It is well known that calcite, a birefringent particle, will experience a torque and rotate when tightly trapped at the focus of an elliptically polarized beam due to the transfer of spin angular momentum. Our calcite is grown using a precipitate technique we developed that results in regular crystals approximately 10 μm long on all edges. The regularity of the crystal shape makes it possible to visually identify the optical axis as well as the ordinary (o) and extraordinary (e) polarization axes. When one of our crystals is trapped in an elliptically polarized beam, it first orients itself such that the propagation direction of the beam is along the corner-to-corner optic axis. While in this orientation, the total torque increases and decreases as the crystal rotates, with significant effects at four different locations corresponding to the e and o axes. Current research in this area assumes that there is one crystal axis that is most significant to the motion. We illustrate this axis-dependent calcite rotation at the top of the sample as well as when crystals are trapped three-dimensionally in the middle of the sample fluid, and calculate the torque on the crystal relative to crystal orientation. This work allows us to predict the motion of calcite, giving us an analytical tool for applications such as fluid stirring or as a handle in micro-machines.

Citation Download Citation

Catherine M. Herne, Natalie A. Cartwright, and Johanna R. Levey "Spin torque on birefringent "o" and "e" axes in regular rhombohedral calcite crystals ", Proc. SPIE 10935, Complex Light and Optical Forces XIII, 109351O (1 March 2019); https://doi.org/10.1117/12.2511989

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