Many-body feedback cooling : particle manipulation through far-field wavefront shaping

Abstract

In this thesis a new way of cooling dielectric particles with optical forces will be presented. Without tracking particle positions, the collective motion inside a system gets monitored from the far-field by measuring the time evolution of the system's scattering matrix. This information alone enables us to shape the input field, so that its interaction with the particles counteracts or enhances their motion optimally. We formulate an underlying theory based on a scalar field approximation of the electromagnetic field. The particle positions serve as an optical potential for the field and resulting field configurations generate forces upon the particles in the direction of higher field intensity. The macroscopic kinetic energy change of the particle system is connected with changes in the scattering matrix, encoded in a new linear operator, which provides optimal cooling states through solving a simple eigenvalue problem. We test the field-matter interactions in simulations using a 2D-multimode waveguide. The influence of randomized as well as of constant input fields on the particle motion will be compared to the strongly damped motion, induced by the optimal cooling states. We characterize our cooling procedure by increasing the particle number, starting with a single particle. We confirm the robustness of our method by testing it on a ten particle system for challenging scenarios, like incomplete far-field information, absorption and complex particle geometries. In addition, simulations with realistic values are made and results are compared with existing cooling approaches in the mesoscopic regime.