Efficient Integration of Quantum Emitters in Laser Written Optical Channel Waveguides in Silica

Silica or Silica on Silicon is a established optical integration platform [1], while the integrated Laser- written circuits' technology [2] offers additionally high customizability, and flexibility for rapid prototyping. Such integrated circuits could provide certain advantages for the development of robust and customizable quantum computers, although a critical issue to their required scalability is the integration capability of multiple single photon sources. While the main class of single photon sources is based on quantum dots embedded in semiconductor nanowires (NWQD) [3] allowing their deterministic integration, such hybrid circuits have been demonstrated only in high refractive index platforms like silicon or silicon nitride, due the modal characteristics compatibility to NWQD. It was shown recently [1] the restricting limitations of coupling in silica waveguides due to the low refractive index contrast leading to coupling performance <5%. A new light coupling scheme [4] proposed here overcomes this limitation, providing means for light coupling >60%. The scheme is based on the incorporation of an optical microsphere between the nanowire and the waveguide, which can be properly optimized and arranged in terms of: size (d), refractive index (n), and the distance of the microsphere between the nanowire (L) and waveguide (D). Given a suitable optical arrangement, the illuminated by the NWQD microsphere produces a photonic nanojet which excites efficiently the guided eigenmodes of the input channel waveguide facilitating the light coupling by the nanowire. The feasibility of the proposed method (Fig. 1) is demonstrated by considering readily available Alumina or Barium Titanate microspheres with diameter range 4μm – 12μm, leading to coupling efficiencies higher than 60%. The method is adequately tolerant [5] in displacements, misalignments, and imperfections and is feasible, as we show here, by current state of the art techniques like Focussed Ion Beam (FIB) for enabling NWQD-microsphere-waveguide alignment. The proposed method could enable the on-chip integration of multiple single photon emitters' in silica, and is expected to have a high impact in scalable photonic integrated circuits for quantum computing or sensing applications.