Every rigid body has the potential for 6 degrees of freedom: 3 translational and 3 rotational. While optical tweezers have effectively tackled the translational degrees of freedom, the rotational aspects remain challenging to control. The ability to precisely measure and manipulate particles' rotational degrees of freedom holds significant importance in various domains, such as single-cell analysis, drug discovery, and organism studies. In line with airline terminologies, the three rotational degrees of freedom are referred to as yaw (rotation within the plane), roll, and pitch (rotation out of the plane). In this experiment, we employed NaYF4, Yb,Er upconverting particles (UCP) to achieve a full 360-degree rotation (1) within a sample chamber near the gold-coated coverslip. A localized hot spot is generated by directing a laser beam onto the gold surface, resulting in a circulatory convective flow that causes the UCP to rotate in the pitch direction. The UCP possesses shape birefringence, forming four-lobe patterns (2) observable under cross-polarizers. As the particles undergo pitch rotation, the previously symmetrical four-lobe pattern becomes asymmetrical, allowing us to extract information about the pitch angle from the discrepancy between the two halves. Consequently, we can achieve a complete 360-degree rotation of a hexagonal particle and accurately measure the pitch angle with a large detection bandwidth. This particular rotation mode is crucial when the particle is close to the surface, and we can extract information about different surface properties like adhesivity (3), binding rigidity, viscoelasticity, etc.

References:
1.Kumar, Sumeet, et al. "Pitch-rotational manipulation of single cells and particles using single-beam thermo-optical tweezers." Biomedical Optics Express 11.7 (2020): 3555-3566.
2. Roy, Basudev, Avin Ramaiya, and Erik Schaffer. "Determination of pitch rotation in a spherical birefringent microparticle." Journal of Optics 20.3 (2018): 035603.
3. Lokesh, Muruga, et al. "Estimation of rolling work of adhesion at the nanoscale with soft probing using optical tweezers." RSC advances 11.55 (2021): 34636-34642.