Development Of Nano Optical Tweezers System And Measurement Of Small Mechanical Quantities

It has been over twenty years since optical tweezers are invented. At first, Optical tweezers are just a simple facility for trap and manipulate micro sized particles. But now, it has become a complex experiment setup consisting of the high precision manipulation and the measurement of small mechanical quantities. Optical tweezers can manipulate many kinds of particles with submicron to tens of microns diameter, including cells, organelles, and polymer beads. Such particles are usually the subject investigated in biology, nanophase materials, and disperse system fields. At present, optical tweezers technique can position and detect the trapped particles in sub nanometer magnitude, and it can precisely measure the small force between sub piconewton to hundreds of piconewtons. The displacements and small forces studied in biology and disperse system are just in such magnitudes. Additionally, because the specimen manipulation by optical tweezers is non-contact, there's mechanical damage on the specimen. All the characters of optical tweezers make it very suitable for being applied in biology and disperse system fields.Along with the development of biology studies, especially in the biological macromolecules researches, there are new requests to optical tweezers technique because it is a very important means in these fields. Our study is just to fulfill these increasing requests. In order to follow this situation, we have begun to upgrade the optical tweezers system in our lab and seeking for more precise and efficient experiment methods. This thesis is mainly about comprehensively upgrading the study ability of optical tweezers technique in our lab. We have designed a new multi-functional experimental setup: nanometer optical tweezers system, introduced or brought forward some experiment methods for different experiment conditions, analyzed the measurement accuracy of these methods, and studied the influence of two parameters, the bandwidth of the acquisition system and the axial deviation of specimen in the experiments, on the measurements of the mechanical quantities.Because optical tweezers setup is the base of the experiment researches using optical tweezers technique, the first step of our job is the design of a new optical tweezers setup. Through the surveys of the setups in other labs and the analysis of our own needs, our design emphasis is finally put on the combination of multi optical tweezers and the upgrade of the accuracies of the manipulation and measurement. In order to study the biological macromolecules and the interaction between the molecules, we have to manipulate several molecules at the same time. We finally choose the project of treble optical tweezers. In the setup, we use several piezoelectric and stepping motor devices. Based on these devices, we design two manipulation modes: initiative manipulation mode and passive manipulation mode. In the experiments, combining these two manipulation modes, we can manipulate three specimens with very high precision and rather big area at the same time.In the experiments, we usually need to measure some small mechanical quantities, for example, the displacements and the forces. Force measurement needs the stiffness of the optical tweezers first, and the stiffness calibration is generally based on the analysis of the specimen movements, so the displacement measurement is actually the foundation of all the mechanical quantities measurements. In our experiment setup, we use two projects to measure the specimen displacements in high precision. One is CCD system associated with correlation calculation of images; the other one is quadrant detector system associated with back scattering illumination. Each of these two projects has some merits and shortcomings, but they can reinforce each other. Combining these projects, the accuracy of the displacement measurement can reach sub nanometer, and the time resolution can reach sub milliseconds. Based on the high precision displacement measurement, we introduce four methods, thermal motion analysis method, drag force method, power spectrum method and periodic driving force method to calibrate the optical trap stiffness. We measure the accuracies of these methods by experiments and analyses, compare the errors of them, discuss the sources of the errors, and finally get the respective appropriate experiment condition of these four methods.The frequency of the Brownian motion of the specimen is quite high; however the bandwidth of any acquisition system is limited and much smaller than that of the Brownian motion. So when an acquisition system is used to measure the track of the Brownian motion, the high frequency signal of the motion will be dropped out. This causes the experimentally measured displacement has a large error. By using Monte-Carlo numerical modeling method, the influence of the insufficient bandwidth of the acquisition system on the displacement measurement and the stiffness calibration using the thermal motion analysis method is studied in the time domain. Through the analysis of the numerical modeling results, we discuss the source that brings on the error and the essential bandwidth when thermal motion method is used.It has been reported a few years ago that when the specimen endures a lateral force, it deviates from the optical trap center not only laterally but also axially. However, this phenomenon and the influence of it on the experiment measurements haven't been studied in detail. We first measure the relation between the axial deviation and the lateral deviation. The result shows that the equilibrium track of the specimen is not in the horizon plane that passes the trap center and the axial position lifts along with the increase of the lateral displacement. Then we calculate the equilibrium track of the specimen by using a ray optics model, the result is consistent with the experiment result. This phenomenon conflicts with the hypothetic premises of drag force method, so it surely brings on errors to lateral force measurement using drag force method. Through further analysis of force field of the optical trap, we can figure out the detailed effect of the axial deviation. When the drag force method is used to calibrate the lateral trap stiffness, the error brought by the axial deviation can be neglect as long as the lateral displacement of the specimen is less than a special value. However, when the drag force method is used to measure the max lateral trapping force, the measured force is actually the escape force, which is much less than the true max lateral trapping force.