Dr. Natalie A. Cartwright Profile

Dr. Natalie A. Cartwright

at State Univ of New York at New Paltz

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Proceedings Article | 1 March 2019 Paper

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

Catherine Herne, Natalie Cartwright, Johanna Levey

Proceedings Volume 10935, 109351O (2019) https://doi.org/10.1117/12.2511989

KEYWORDS: Crystals, Calcite, Polarization, Refraction, Geometrical optics, Optical tweezers, Laser crystals, Calcium, Carbonates, Microscopes

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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.

Proceedings Article | 25 August 2017 Paper

Angle-dependent rotation of calcite in elliptically polarized light

Catherine Herne, Natalie Cartwright, Matthew Cattani, Lucas Tracy

Proceedings Volume 10347, 103472T (2017) https://doi.org/10.1117/12.2273001

KEYWORDS: Crystals, Polarization, Calcite, Motion measurement, Photodiodes, Modes of laser operation, Crystal optics, Optical tweezers, Birefringence

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Calcite crystals trapped in an elliptically polarized laser field exhibit intriguing rotational motion. In this paper, we show measurements of the angle-dependent motion, and discuss how the motion of birefringent calcite can be used to develop a reliable and efficient process for determining the polarization ellipticity and orientation of a laser mode. The crystals experience torque in two ways: from the transfer of spin angular momentum (SAM) from the circular polarization component of the light, and from a torque due to the linear polarization component of the light that acts to align the optic axis of the crystal with the polarization axis of the light. These torques alternatingly compete with and amplify each other, creating an oscillating rotational crystal velocity. We model the behavior as a rigid body in an angle-dependent torque. We experimentally demonstrate the dependence of the rotational velocity on the angular orientation of the crystal by placing the crystals in a sample solution in our trapping region, and observing their behavior under different polarization modes. Measurements are made by acquiring information simultaneously from a quadrant photodiode collecting the driving light after it passes through the sample region, and by imaging the crystal motion onto a camera. We finish by illustrating how to use this model to predict the ellipticity of a laser mode from rotational motion of birefringent crystals.

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