Integrating suspended tapered photonic waveguides with thermal alkali vapour

Abstract

In this work, we demonstrate - for the first time, to the best of our knowledge - electromagnetically induced transparency (EIT) on a fully integrated, chip-scale platform based on nanoscale tapered suspended waveguides surrounded by hot rubidium vapour. We further show that these waveguides enable the excitation of rubidium atoms to Rydberg states, and implement an electric readout scheme to detect these atoms by their ionisation currents. The fabrication workflow for standard ridge waveguides made of silicon nitride (Si3N4) is optimised, leading to significantly improved optical quality. Building on this, different techniques for locally suspending photonic structures out of Si3N4 are developed, allowing us to retain the waveguide interconnects between the structures on a conventional SiO2 layer for full chip integration. The use of a sacrificial silicon layer proves to be advantageous for suspending resonator structures, as this method does not change the optical properties of the device, as opposed to other established suspension methods. With these fabrication techniques, high quality suspended tapered waveguides are realised. Spectroscopy measurements on the D2 transition of rubidium confirm that the Doppler and transit time broadening are significantly reduced compared to standard ridge waveguides, owing to the extended evanescent field of the tapered geometry. After this, EIT is demonstrated on the tapered waveguides using a three level ladder scheme, benefiting from the longer atomic interaction times enabled by the extended evanescent field. The experimental results are compared to numerical simulations, showing good agreement. Finally, the excitation to Rydberg levels in the evanescent field of the tapered waveguide is investigated. An electrically contacted vapour cell is designed to detect the current from excited Rydberg atoms, ionised through collisions with ground-state atoms or perturber gases in the cell. Clear Rydberg signals are observed and compared with simulations modelling excitation and ionisation in the confined geometry of the evanescent field. The results indicate that the excitation can indeed occur within the guided mode and that the corresponding ionisation currents are of the same order of magnitude as those measured experimentally. However, strong scattering of the guided light is also observed at the transitions to the underetched regions, suggesting that a fraction of the Rydberg excitation may originate from this scattered light rather than from the evanescent field alone. Further investigations are ongoing at the time of writing to quantify this contribution.