"Fat" Nanobeam-Waveguide Resonator for Suppressing Spectral Diffusion in InGaAs Quantum Dots

Abstract: The coherent coupling of photons to solid state emitters is one of the building blocks for future quantum technology. However, one of the main sources for non-perfect or even strongly reduced coherence of the photons is charge noise which introduces energy shifts during or in between the photon emission events. In particular solid-state emitters in nanostructures suffer from such spectral diffusion as surface charge defects from nanofabrication are often only tens of nanometers away from the emitter. In this thesis, a novel type of light–matter interface, a GaAs-based nanobeam cavity, has been designed and tested to couple emission from self-assembled InAs quantum dot (QD). A wide nanobeam cavity with a modulated outline has been designed and optimized, which possess a high intrinsic quality factor for the fundamental optical mode (10^5 for 450 nm wide cavity and 6'10^4 for 600 nm wide cavity), and is expected to reduce spectral diffusion of the QD originated from the etched defects. First, a Finite Element (Comsol) simulation was used for the design, and secondly, the fabricated design was tested at room and cryogenic temperatures. When the QD was coupled to the wide nanobeam cavity inside cryostat, a fast decay rate of 19 ± 2 ns^(-1) is measured, resulting in a lifetime of ~ 52.6 ps. Comparing with the lifetime of QDs in bulk material, the spontaneous emission lifetime, i.e. Purcell factor, is derived to be ~ 36 under a non-resonant excitation scheme. A lifetime of 52.6 ps corresponds to a lifetime-limited linedwidth of 8.37 GHz. Compared to typical non-Purcell enhanced linewidths of about 200 MHz, a line-width of 8.37 GHz strongly relaxes the sensitivity to charge noise introduced by the environment. Furthermore, and not tested here, the additional distance to surface charges, possible due to the novel "fat" cavity design, should furthermore reduce the induced spectral diffusion as observed before in non-resonant nanostructures.