Determination of pitch rotation of a hexagonal upconverting particle using emission in the backscattered direction

Abstract Detection of pitch rotation has been shown in an anisotropic particle with the forward scattered light utilizing the interference between scattered and unscattered light. However, the forward scattered light may not be accessible in all cases. Here, we explore an alternate technique to ascertain the pitch rotation by using the fluorescent emission from an upconverting particle (UCP). The particle is trapped using linearly polarized 975 nm light and the visible emission is detected with a video camera. A polarizer is placed in the orthogonal sense to the input polarization, not necessarily making the detection in the orthogonal polarization, as UCP emission is partially polarized. The video image is analyzed for the difference in halves signal which we show to detect the out of plane pitch rotation. We also show that the FFT of the pitch motion detected using the camera matches well with that obtained from the photonic force microscopy.