Unidirectional frequency conversion in microring resonators for on-chip frequency-multiplexed single-photon sources

Microring resonators are attractive for low-power frequency conversion via Bragg-scattering four-wave-mixing due to their comb-like resonance spectrum. However, conversion efficiency is limited to 50% due to the equal probability of up- and down-conversion. Here, we demonstrate how two coupled microrings enable highly directional conversion between the spectral modes of one of the rings. An extinction between up- and down-conversion of more than 40 dB is experimentally observed. Based on this method, we propose a design for on-chip multiplexed single-photon sources that allow localized frequency modes to be converted into propagating continuous-mode photon wave packets using a single operation. The key is that frequency conversion works as a switch on both spatial and spectral degrees of freedom of photons if the microring is interferometrically coupled to a bus waveguide. Our numerical results show 99% conversion efficiency into a propagating mode with a wave packet having a 90% overlap with a Gaussian for a ratio between intrinsic and coupling quality factors of 400

Grating-assisted superresolution of slow waves in Fourier space

We present a far-field optical technique allowing measurements of the dispersion relation of electromagnetic fields propagating under the light cone in photonic nanostructures. It relies on the use of a one-dimensional grating to probe the evanescent tail of the guided field in combination with a high-numerical-aperture Fourier- space imaging setup. A high-resolution spectroscopy of the far-field emission diagram allows us to accurately and efficiently determine the dispersion curve and the group-index dispersion of planar photonic crystal waveguides operating in the slow-light regime