Multimode nanobeam photonic crystal cavities for Purcell enhanced quantum dot emission

Epitaxial III–V semiconductor quantum dots (QDs) integrated with nanophotonic structures are promising on-demand sources of indistinguishable single photons for quantum photonic circuits. Close proximity of QDs to etched sidewalls in such structures, however, may induce excitonic linewidth broadening, reducing photon indistinguishability. Here, we design and demonstrate GaAs photonic crystal cavities based on multimode nanobeam waveguides that maximize QD separation from etched surfaces beyond an empirically determined threshold that suppresses spectral broadening, while enabling QD access through higher-order waveguide modes. Although multimode propagation adds design complexity, simulations predict quality factors Q≈103 and mode volumes V/(λ/n)3≈2 for Purcell radiative rate enhancements of Fp≈100. Fabricated devices containing QD ensembles exhibit resonances consistent with these predictions, and single-QD measurements yield Fp<5 for 11 randomly located emitters. Monte Carlo simulations of spatially dependent Fp distributions indicate that slow carrier capture and relaxation dynamics, rather than QD placement, primarily limit the observation of higher Purcell factors. These results highlight the potential of our cavities for integrating epitaxial QDs while clarifying key constraints on observation of radiative rate enhancement.