Lecture Control of optical traps using the helicity and angular momentum of optical beams

We invite you to a lecture by Prof. Gabriel Molina-Terriza from the Material Physics Center (Donostia–San Sebastián, Spain), who will present his current research on optical traps and light manipulation. He will focus on the use of vortex beams and their properties for trapping particles. The event will take place on May 5, 2026, in the ISI CAS, lecture hall, as part of the OP JAK – Top-Level Research project.

Abstract
 In this talk I will present our recent work studying both experimentally and theoretically complex optical traps using vortex beams with circular polarization. These beams typically present a dark spot in the center of symmetry and an annular intensity pattern in the transverse plane. In the small particle regime, it is commonly assumed that the particle will not be trapped it the center and will be confined to the ring of the particle in a non-stationary trap, as the particle will be moving around the beam center due to what is commonly addressed as orbital angular momentum transfer of light. On the other hand, for larger particles, a stable and stationary trap can be found at the intensity minimum of the optical trap. Surprisingly, this trapping condition can offer higher forces in some situations than the usual Gaussian beam trapping. 
I will show an analytical, numerical and experimental analysis of this kind of traps, which shows that this unusual trapping conditions are due to the multipolar modes that large particles can support. Our model is based on a non-traditional expansion of electromagnetic fields using helicity multipolar modes. We will show that this kind of expansion is better suited to the typical experimental conditions and also, that it can provide useful physical insight in all optical trapping regimes. In this way, we can explain the higher optical forces that can be obtained with higher order optical vortex beams. Moreover, we predict new experimental regimes that can be of interest to the optical trapping community.