The high precision calibration of optical trap stiffness is the foundation of the weak force measurement in optical tweezers system. And the accuracy of the trap stiffness measurement is limited by the bandwidth of the acquisition system. In this article, such an influence is analyzed and discussed. First, the power spectrum of the thermal motion of a trapped bead is analyzed and the true trap stiffness is compared with the stiffness measured by a limited bandwidth acquisition system. Then the stiffness measuring process is simulated using Monte Carlo method when thermal motion analysis method is used to measure the trap stiffness. It is demonstrated that the influence of the bandwidth is related to the trap stiffness and bead diameter. And that the measured trap stiffness is greater than the true value is also demonstrated when the bandwidth of the acquisition system is not sufficient.
It is often necessary to follow the axial movement of a micron particle, such as the one trapped in optical trap, in addition to its radial movement. A new method based on the information entropy is developed for measuring the axial displacement, which is then used to reconsider the drag force method for measuring the radial trapping force and stiffness in an optical tweezers system. It is found that the new equilibrium position of the bead displaces not only radially but also axially when the surrounding viscous fluid flows at constant lateral velocity. The result implies that the trap stiffness measured in such a way is not really for the same horizontal plane. In addition, the measured trajectory of the bead (both radial and axial displacements) shows that the sphere escapes from the optical trap upward in stead of radially when the fluid velocity reaches the critical value. The fact indicates that the escape force is not the maximal radial trapping force as commonly accepted. It also deduced that the axial movement of the bead is one of error sources for trapping force calibration using the drag-force method.
Do-nut Mode Optical Trap is the kind of Optical Trap that the laser distribution in the center part of the beam is approximately zero. Because the trapping effect of an Optical Trap correlates closely with the distribution of optical field, the scattering force of Do-nut Mode Optical Trap is markedly reduced. We realized Do-nut Mode Optical Trap by rebuilding wave front. Then we studied the trapping effect and trap stiffness of Solid Mode Optical Trap and Do-nut Mode Optical Trap in the cases of upright microscope and inverted microscope. We measured the stiffness of the Optical Trap near the focus with Boltzmann statistics method.
The distribution of the optical field of Do-nut Optical Trap is like aura. And the trapping effect changes with the diameter ratio of the aura.
Using optical tweezers we have developed a biomechanical detection system for single molecules detection. This system can control the sample bead which is used as the handle of single molecules with nanometer precision, it also can make quantitative measurements of displacement with nanometer resolution, and measure the Pico-Newton force involved in the dynamical process of the movement of a single biomolecule. We combined two Donut-Mode optical traps with two high-precision observation setups in this system. Experiments described in this paper demonstrate the performance of the system. In our experiments, we monitored the Brownian motion ofpolystyrene beads; as a result we can calculate the optical-trap stiffness. In summary, the system can be used in the research of the dynamical motion of a single biomolecule.