Single-Photon Emission from Individual Nanophotonic-Integrated Colloidal Quantum Dots

Solution processible colloidal quantum dots hold great promise for realizing single-photon sources embedded into scalable quantum technology platforms. However, the high-yield integration of large numbers of individually addressable colloidal quantum dots in a photonic circuit has remained an outstanding challenge. Here, we report on integrating individual colloidal coreshell quantum dots (CQDs) into a nanophotonic network that allows for excitation and efficient collection of single-photons via separate waveguide channels. An iterative electron beam lithography process provides a viable method to position single emitters at predefined positions on a chip with yield that approaches unity. Our work moves beyond the bulk optic paradigm of confocal microscopy and paves the way for supplying chip-scale quantum networks with single photons from large numbers of simultaneously controllable quantum emitters. Introduction Nanoscale solid-state single-photon emitters are of central importance for experiments in quantum optics and realizations of quantum technologies. Several emitter systems are currently being considered for such purposes, with epitaxial quantum dots showing leading performance regarding purity, indistinguishability, and stability, while presenting challenges in growth and integration with other quantum technology platforms. Colloidal quantum dots (CQDs) also show great promise as single-photon sources, allowing tunable emission wavelengths, possibly even into the telecom range, through well controlled variations in geometry and material composition. Improvements in photon purity, indistinguishability and stability were recently achieved through advances in optimizing the shell composition and thickness as well as smoothing the interface potential between core and shell in CQD synthesis or coupling to nanocavities. Importantly, CQDs can be processed in solution therewith offering tremendous flexibility and scalability for integrating them with a broad range of material platforms and nanostructures. To study systems aspects of complex quantum optical ensembles and realize practical applications of quantum technologies, large numbers of quantum emitters will have to be integrated into a network that allows for simultaneous optical addressing and manipulation. Photonic integrated circuits are well suited for this task because they provide reproducible replication of compact functional units, low channel loss and cost-efficient fabrication on monolithic chips. However, embedding large numbers of quantum emitters with high placement accuracy in a reconfigurable nanophotonic network as well as efficient interfaces between quantum emitters and low-loss waveguides remain challenging. Moreover, high levels of intrinsic photoluminescence in the visible spectral range plague most established wide-band transparent waveguides made from high refractive index dielectrics, such as silicon nitride, which hampers experiments at the single-photon level. While extensive experimental efforts with single molecules, color centers in diamond, as well as self-assembled and colloidal quantum dots have been undertaken, photonic integrated circuits with large numbers of embedded individually addressable single photon sources have so far remained elusive. Here we demonstrate single-photon emission from individual CQDs into nanophotonic circuits at room temperature and show how the approach can be scaled up to larger system size. We employ colloidal CdSeTe/ZnS core-shell quantum dots as single-photon sources that can be processed in solution, thus allowing for applying them in large numbers to lithographically patterned dies or even at the wafer-scale. We eliminate intrinsic material photoluminescence from dielectric waveguides upon optical excitation of CQDs by realizing nanophotonic circuits from tantalum pentoxide (Ta2O5) on insulator (SiO2) thin films that benefit experiments with single photons through ultra-low fluorescent background in addition to low transmission loss (-1 dB/cm). Efficient and broadband interfaces between individual quantum emitters and nanophotonic circuits are achieved by positioning CQDs inside circular holes at the intersection of Ta2O5-waveguides for excitation and fluorescence collection, both of which only support a single transverse electric mode. Low loss and low fluorescent 3D optical interconnects between the waveguides and optical fibers further allow for efficiently extracting light from a chip and verifying the single-photon characteristics by recording the second-order autocorrelation function g(τ), which shows antibunching. By utilizing a multilayer lithography procedure with high overlay accuracy, we are able to iteratively fill vacant sites on a chip with CQDs while passivating occupied sites, thus providing a high-yield solution for integrating single-photon sources with nanophotonic circuits. Our approach paves the way for equipping scalable photonic integrated circuits with individually and simultaneously addressable single-photon sources. In combination with efficient optical interconnects such chips with massively parallelized integrated single-photon emitters will benefit applications in quantum communication , e.g. for outperforming laser-based quantum key distribution schemes. Figure 1 FDTD numerical simulations of the waveguide crossing. a) Schematic illustration of the device layout considered for 3D numerical simulations. A dipole emitter representing a single CQD is placed inside the hole. At each side of the collection waveguide, monitors (orange planes) are positioned to determine the point vector flux inside of the waveguide. Additionally, another monitor is placed above the waveguide intersection, which represents the collection into an objective lens with 0.9 numerical aperture (see supplementary information). b) Coupling efficiency into the waveguide mode as a function of the waveguide hole radius (x/y/z-axis (blue, green, orange) oriented dipole – collected into the waveguide (solid) or the confocal microscope objective lens (dashed)). c) Coupling efficiency into the waveguide mode as a function of the hole depth in the waveguide. Dipole orientation along the x/y/z-axis (blue, green, orange) results in the corresponding collection efficiency into the waveguide (solid) or the confocal microscope objective lens (dashed). d) Coupling efficiency determined at right monitor in dependence of emitter position at the bottom of a 25 nm radius hole (100 nm depth) for a dipole oriented along the y-direction. Due to the symmetry of the simulated area, the result for the left monitor is mirrored vertically. CQD – waveguide coupling We here consider the coupling of a CQD positioned at the intersection of two Ta2O5waveguides, one for optical excitation with a 532 nm wavelength laser and one for fluorescence collection in the 650 – 750 nm spectral range, as shown in Fig. 1 a. While efficient optical excitation of the CQD can conveniently be achieved by setting appropriate laser power levels, the collection of photons will depend on positioning the quantum dot with respect to the waveguide mode. We perform finite-difference time-domain (FDTD) simulations for placing a single CQD within a hole inside the collection waveguide, as shown in Fig. 1 a. We systematically vary the hole radius, the hole depth and the position of the CQD within the hole to optimize the coupling conditions, as shown in Fig. 1 b – d. The CQD is here represented by a point dipole source emitting at a wavelength of 705 nm, embedded into spheres representing core and shell. Firstly, we consider a CQD at the center of a 100 nm deep hole and calculate the optical power at two monitor planes extending somewhat beyond the 100 nm high, 700 nm wide collection waveguide, as shown in Fig. 1 a. We then derive the coupling efficiency into the waveguide from the sum of optical powers in both monitors normalized to the total emitted optical power. Upon varying the hole radius, we find a maximum overall coupling efficiency of 47% for a dipole emitter oriented along the y-direction and a 25 nm hole radius, as shown in Fig. 1 b. Remarkably, the coupling efficiency only changes marginally (45 % – 47 %) when varying the hole radius in the 0 – 75 nm range, highlighting the robustness of the geometry against fabrication imperfections. As expected, dipole emitters oriented along the xand z-directions show no appreciable coupling into the guided optical mode. Moreover, we determine the collection efficiency into a 0.9 numerical aperture (NA) confocal microscope objective by placing a corresponding monitor plane above the waveguide intersection. Here, it is possible to collect light from dipole emitters oriented along xand y-directions with maximal efficiencies of 19% and 15%, respectively, for hole radii below 10 nm. Larger hole radii reduce the collection efficiency in confocal microscopy down to 15% (x-direction) and 12% (y-direction), as shown in Fig. 1 b. Secondly, we assess the influence of the hole depth, which could be controlled in a precisely timed etching process during device fabrication. In Fig. 1 c we consider a CQD at the center of a hole with a fixed radius of 25 nm and find the optimal coupling efficiency into the collection waveguide for hole depths in the 90 – 100 nm range, i.e. when etching (almost) through the entire waveguide (100 nm height), down to the buried oxide layer. Reducing the hole depth, the coupling efficiency gradually decrease to 28 % at 0 nm depth, i.e. the emitter lies on top of the waveguide. The preference for placing CQD closer to the bottom of the waveguide rather than the top reflects the higher refractive index of the substrate as compared to that of air surrounding the waveguide otherwise. Lastly, we study the influence of the CQD position inside a 25 nm radius hole on the achievable coupling efficiency. Fig. 1 d shows the coupling efficiency when only considering one of the two power monitors,