Fabrication and characterization of shape- and topology-optimized optical cavities with deep sub-wavelength confinement for interfacing with colloidal quantum dots

We employ a combined shape- and topology-optimization strategy to design manufacturable two-dimensional photonic crystal-based optical nanocavities that confine light to length scales well below the resonance wavelength. We present details of the design strategy as well as scanning electron micrographs of the fabricated indium phosphide cavities with a compact footprint of approximately $4.5\lambda \times 4.5\lambda$, which feature gaps on the order of 10 nm and a theoretical mode volume in the gap center below ${0.1(\lambda /2n_{\mathrm{air}})}^3$. Subsequent optical characterization of the far-field emission as well as Purcell-enhanced photoluminescence from the cavities with and without spin-coated colloidal quantum dots are compared to numerical simulations and reveal an interesting interplay of two spectrally adjacent modes. The results corroborate the potential of the fabrication process for ensuring high yield and reliable performance as well as the viability of the material platform for exploring light-matter interaction with colloidal QDs.