Measuring and modeling forces and micromechanical properties in laser traps

Abstract

Microscopic objects can be trapped with strongly focused laser beams. These traps are called optical tweezers and allow selective manipulation of individual particles or cells. The applied forces can be measured experimentally, but quantification was and still is largely limited to empirically calibrated linear force measurements, without making use of the underlying theory. Since forces in optical tweezers are governed by electromagnetic and hydrodynamic interaction of the trapped particle with the laser beam and the surrounding fluid, respectively, it should be possible to improve on this situation. The aim of this thesis is to use modeling of electromagnetic forces as well as hydrodynamic forces and velocities in combination with experimental measurements of these quantities to develop new applications of optical trapping. The methods developed in this thesis are used to quantify micromechanical properties and are applied to biophysical systems. The first part of the thesis is focused on linear forces. The theory for the interaction of highly focused laser beams with spherical dielectric particles is established. Landscapes of optical trapping are calculated numerically for a wide range of parameters. These landscapes can be used as a guide showing which sets of parameters allow optical trapping. Next, the experimental method for measuring the trap stiffness and the trapping force as well as the position of the particle in the trap is introduced. Robust theoretical modeling and precise trap stiffness measurements are combined to measure the refractive index of single trapped particles. Results are presented for four different materials; one type of material is used as a reference. Force measurements and modeling are then used to characterize the optical force field created by the beam emerging from a lensed optical fiber. The experimental results are compared to beam modeling from far field properties, and conclusions are drawn about the near field of the fiber beam. The established linear force measurements are applied to a biophysical problem, where adhesion forces between substrates and living cells are quantified for a number of substrates and different proteins in the system. The method is a promising substitute for in vivo measurements of cellular adhesion. An alternative form of actuation in laser traps is optical rotation. This method has only recently been established, and quantitative measurements are so far rare. A very important physical property is the coupling between a rotating microscopic particle and the surrounding fluid. Here, this coupling is accessed by measuring the fluid velocity field that is created by the rotating particle. These measurements show the validity of the no-slip boundary condition on the particle's surface, which allows hydrodynamic modeling. They are also necessary for a microrheological application of rotating optical tweezers. In a next step, a biophysical system is investigated. The possibility of applying localized fluid shear stress to living cells is explored. It is shown experimentally that shear stress can locally be applied to cells and that the cells react to the stress. Using finite element methods, a hydrodynamic model of the system is constructed to calculate the shear force at the cell surface. Further applications of rotating optical tweezers are at the moment limited by the objects available for rotation. The last part of this thesis thus explores the fabrication of new objects for rotation that are biocompatible and can be used in simple Gaussian laser beams. Twophoton photopolymerization is used to create microscopic diffractive elements. Rotation of plain microspheres in a linearly polarized Gaussian beam that passes through the element is demonstrated, and the applied torque calculated. Furthermore, an optical rotor is fabricated in a similar fashion. The rotor is 3D trapped and rotated in a linearly polarized Gaussian beam in biocompatible medium. This is a great enhancement over previously fabricated rotors, which had to be rotated in acetone.