Optimal spatial mode distributions for the coupling of a single photon to an atomic dipole transition in free space

This study presents the field distributions which achieve maximization for several optical focusing devices. The overlap of these spatial mode distributions, which sets an upper limit for the field strength parallel to the atomic dipole at the focus, is also discussed. Results of this work and the perturbation theory are utilized to predict the maximum achievable absorption efficiencies for single photons. By using fundamental Gaussian modes and doughnut modes, maximum absorption efficiencies result to 84% and 92%, respectively.