The interaction strength of light and a single atomic dipole is determined by the electric field component parallel to the transition dipole moment. In free space, this field component is maximized when a pure electric-dipole wave is incident onto the atom.
We attempt this optimum coupling scenario by focusing a suitable mode with a deep parabolic mirror onto a Yb ion. The ion is positioned in the mirror's focus by means of a dedicated ion trap. Efficient coupling of the focused field and the ion is tested in e.g. measuring the phase shift imprinted onto a coherent state or by exciting the ion with faint light pulses. For the latter experiment, one has to take guidance from time-reversal symmetry arguments which require the excitation pulses to have an exponentially rising envelop.
Efficient coupling of light to solid-state quantum targets by focusing from full solid angle necessitates mounting the targets without degrading the quality of the focal spot. We solve this difficulty by trapping nano-particles with optical tweezers. In proof-of-principle experiments we trap colloidal dot-in-rod nano particles. Furthermore, the deep parabolic mirror facilitates the efficient collection of photons emerging from a quantum emitter.
Our optical dipole trap is generated in the same way as the dipole modes employed in our coupling experiments, i.e. by focusing with a deep parabolic mirror. As a result, the trap stiffness and the depth of the trapping potential approach the optimum figures obtainable for focusing in free space. This makes our set-up also interesting for opto-mechanical experiments.
The very tight focusing encountered when illuminating a deep parabolic mirror with a suitable light mode is not captured by the standard approximations usually applied in non-linear optics. Moreover, there exist no experimental investigations of non-linear optics under such extreme focusing conditions at all.
We investigate third-harmonic generation in noble gases under such boundary conditions, testing the borders of validity of the predictions of non-linear optics in the paraxial regime and looking for new effects.