Axial Multi-particle Trapping And Real-time Direct Observation

The optical tweezers with the special advantage of non-mechanical contact and the accurate measurement of positions of particles,is 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 achieve monitoring of the axial plane(X-Z plane),but the traditional three-dimensional imaging technology is slow and can not meet the requirement of real-time optical tweezers observation.In order to solve these problems in the traditional three-dimensional capture of optical tweezers,we carry out the following theoretical and experimental researches.In order to achieve stable axial multiple optical trapping,the Gerchberg-Saxton(GS)algorithm based on the axial plane Fourier transform is proposed in this thesis,which can directly generate optical trap arrays distributed along the axial direction.To solve the problem of real-time axial observation,we design a scheme for directly observing the sample axial plane based on a 45° reflecting prism.Zemax simulation software is used to simulate the imaging performance of reflective prism systems with different optical parameters,and the optimal imaging parameters of the system has been determined.Based on the Zemax simulation results,we combined the axial plane imaging technology with the holographic optical tweezers,established a set of axial multiple optical trapping and real-time direct observation system,simultaneous observations of the lateral plane and the axial plane and axial multiple optical trapping were carried out in this system.The stiffness of the axial multi-trap was measured by video analysis.The main research work is described as follows:1.We present a GS iterative algorithm based on the axial-plane Fourier transform,this iterative algorithm can be used to directly generate multi-trap arrays distributed along the axial direction.Compared with the algorithm of combination method of Fresnel lens and grating,the iterative algorithm proposed has higher modulation efficiency and modulation accuracy,and each trap generated along the axial distribution has nearly ideal Gaussian intensity distribution.2.We design an axial plane imaging system based on a 45° reflective prism to directly observe the axial plane of the sample.The trapping and imaging regions of the conventional optical tweezers technology are confined to the volume near the focal plane,which causes it is not able to simultaneously observe multiple particles arranged in the axial direction.The axial plane imaging system designed by us can directly image the axial plane of the sample,which adds another observation perspective compared with the traditional optical tweezers and expands the function and application of optical trapping.3.We use Zemax software to simulate the imaging performance of the axial plane imaging system with different optical parameters of reflective prisms,determines the optimal imaging parameters and conducts experimental verification.We compared the point spread function of reflective prisms with three optical glass(FK54,BK7,SF11)and mirror systems,and using Zernike polynomials to analyze the first-order spherical aberration,the first-order coma aberration in X direction and the first-order astigmatism in X direction of the systems.The comprehensive simulation results confirm the optimal imaging parameters of the axial plane imaging system,and we also verify the actual imaging effects of the BK7 coated prism and mirror.4.We combine the axial plane imaging technology with the holographic optical tweezers technology to achieve multiple optical trapping and real-time observation of the axial plane.And the stiffness of the axial multi-trap is calibrated by the video analysis method.We used axial plane GS algorithm to generate the hologram of optical trap array with axial 2×2 distribution and trapped 2×2 silica microspheres array to achieve real-time observation of the trapping process.We use the video analysis method to simultaneously measure the Brownian motion of the captured silica microspheres in the axial multi-trap array,and calibrate the stiffness of the axial multi-trap according to the equipartition theorem.