Optimizing free-space light-matter interaction by mode matching
Marianne Bader
This thesis describes an experiment to optimize the interaction of light and a single ion in free space. The key to efficient coupling consists in mode-matching the incident electromagnetic field to the radiative properties of the ion. For perfect coupling, the incident mode has to match the emission pattern of the ion. In free space, this requires illumination of the ion with an incoming dipole wave. In the present work, such a situation is realized experimentally. To this end, a radially polarized doughnut mode is focused with a deep parabolic mirror while the ion is localized in the focal point of the parabolic mirror by means of a trap of special geometry. This thesis reports first on the implementation of the two basic components of the experiment: the ion trapping in the focal point of the parabolic mirror and the generation of the required doughnut mode. Then, the coupling efficiency of the combined system is analyzed by means of saturation measurements. Characterization of our setup reveals aberrations of the parabolic mirror which prevent perfect coupling. However, the measured coupling efficiency of (7.2 ± 0.2)% is to date among the highest values reported for coupling experiments with a single atom in free space. A special aim of our group is the full excitation of an ion by a single photon. This requires, in addition to the above mentioned spatial mode matching, temporal shaping of the incident light. Perfect excitation can be achieved if the single-photon wavepacket shows an exponentially rising envelope with a time constant matching the lifetime of the excited state of the ion. The present work demonstrates the power of temporal mode matching using a model system: an empty optical cavity. Both, light inside a cavity and atomic excitation, decay exponentially. Thus, there is an analogy between atomic excitation and field energy inside a cavity. The task of exciting an ion translates to the task of coupling a classical light pulse into a cavity. This thesis confirms experimentally that almost the entire energy of a rising exponential pulse can be coupled into a cavity, provided the pulse time constant matches the cavity lifetime.
•Jan 1, 2016•0 citations