Experimental optimization of the fiber coupling efficiency of GaAs quantum dot-based photon sources

We present an efficient experimental method to optimize the combined extraction efficiencies and the far-field emission patterns of solid state-based single and entangled photon pair sources for efficient coupling to single mode fibers. This method is demonstrated for emitters based on droplet etched GaAs quantum dot nanomembranes attached to gallium phosphide solid immersion lenses using an adhesive layer of poly(methyl methacrylate). By varying the thickness of the latter, the optimization of both the extraction efficiency and the far-field emission pattern for single mode fiber coupling is facilitated. The applied method of far-field characterization is validated by benchmarking it against direct measurements of the single mode fiber coupling efficiency. Using this scheme, devices with a more than 150-fold enhanced free-space intensity compared to an unprocessed sample as well as a fiber coupling efficiency of 64% are achieved. In addition, the optimized device has been employed for on-demand generation of maximally entanglement photon pairs using two-photon excitation of the quantum dot bi-exciton exciton cascade. This universal approach for experimental optimization can be applied to other photonic nanostructures, including circular Bragg grating and micropillar cavities as well as monolithic microlenses.