Axial multi-particle trapping and real-time direct observation

The optical tweezers with the special advantages of non-mechanical contact and the accurate measurement of positions of particles, are a powerful manipulating tool in numerous applications such as in colloidal physics and life science. However, the standard optical tweezers system uses a single objective lens for both trapping and imaging. As a result, the trapping and imaging regions are confined to the volume near the focal plane of the objective lens, making it difficult to track the trapped particles arranged in the axial direction. Therefore, multiple trapping along axial direction remains a challenge. The three-dimensional imaging technology can realize the monitoring of the axial plane, but neither the laser scanning microscopy nor the wide-field imaging technology can meet the requirement of the real-time imaging. To address this issue, we propose a modified axial-plane Gerchberg-Saxton (GS) iterative algorithm based on the Fourier transform in the axial plane. Compared with the direct algorithm such as the Fresnel lens method, the modified axial-plane GS iterative algorithm has a higher modulation efficiency, and the generated axial distribution has a sharper intensity. In theory, the traps generated each have an ideal Gaussian intensity distribution independently, which is proved by the simulation of reconstructed field. With such an iterative algorithm, we can directly create multiple point-trap array arranged along the axial direction. We also develop an axial-imaging scheme. In this scheme, the particles are trapped and a right-angled silver-coated 45 degrees reflector is used to realize axial-plane imaging. The scheme is verified by imaging silica particles in an axial plane and a lateral plane simultaneously. Furthermore, we combine the axial-plane imaging technique with holographic optical tweezers, and demonstrate the simultaneous optical trapping in 2 x 2 trap array and the monitoring of multiple silica particles in the axial plane. The trap stiffness of traps array in axial plane is calibrated by measuring the Brownian motion of the trapped particles in the axial trap array with digital video microscopy. The proposed technique provides a new perspective for optical micromanipulation, and enriches the functionality of optical micromanipulation technology, and thus it will have many applications in biological and physical research.