Trapping forces for microsized particles by focused laser light

Master thesis in the group of Thomas Franosch

Optical trapping constitutes one of the major tools in biophysics to position and manipulate microsized objects. Conventionally the forces are assumed to be conservative, i.e. there is a potential from which the forces derive. A simplified picture is given by considering the energy gained by pulling a dielectric sphere into the laser focus, which becomes an accurate description for macroscopic spheres. The other limit of small particles is captured by Rayleigh scattering, where the main force is due to induced dipole radiation. The experimental condition is such that the particle's diameter is comparable to the wavelength of light and all approximations become unreliable, in particular in the laser focus. For plane incident wave Mie scattering calculates the distribution of the electromagnetic fields and all forces can be calculated. Yet, then by symmetry the forces are parallel to the light propagation and no trapping is achieved.

The goal of the current project is to develop a theoretical description of the trapping forces for realistic laser profiles starting from Maxwell's equations only. The calculations should be implemented in computer algebra, e.g. matlab or Mathematica, and compared to numerical solutions by the COMSOL package.


The successful candidate should have a strong background in theoretical physics, in particular electrodynamics, and be skilled in computing and willing to learn handling complex calculations by computer algebra. Furthermore, good analytic skills are required to develop the theoretical description of the trapping forces. The new group member should look forward to participate in many discussions within the team as well as with experimentalists.


Thomas Franosch