In-situ probing the crystallization dynamics of birefringent vaterite microspheres in a polarization-sensitive optical tweezer

Vaterite microcrystals are widely used functional material in biological, chemical and optical research. The high optical birefringence and the spherical shape of vaterite microparticles provides ideal platform for realizing optically driven micromotors and investigating levitated rotational dynamics. Controlled synthesis of vaterite particles with defined diameter, birefringence and geometric shape is however challenging due to the very high sensitivity of calcium carbonate crystallization process to the reaction conditions. Here we report in-situ observation of the crystallization dynamics of vaterite microcrystals on the single particle level using a polarization-sensitive optical tweezer. Optical images collected by the high numerical aperture objective in the optical tweezer configuration is used to track the instantaneous variation of particle diameter with nanometer resolution, from which the synthesis rate and ultimate diameter of the particle can be retrieved. More importantly, the instantaneous orientation of the optical axis of growing vaterite particle can be explicitly inferred, revealing random fluctuations of the optical axis orientation in the fast and slow time frame. As an application, the technique is used to probe the effect of ultrasonic power on the nuclei process of the vaterite particles, identifying the mechanism of suitable ultrasonic power for the generation of calcium carbonate particles with vaterite crystalline phase and with uniform ultimate diameters. The method unlocks the possibilities of in-situ examination, manipulation and precision synthesis of vast ranges of microcrystals with high temporal and spatial resolution.