Single-Photon Emission from Silicon-Vacancy Color Centers in Polycrystalline Diamond Membranes

Color centers in diamond are being studied as potential single-photon sources for quantum devices. As a single- photon source, the silicon-vacancy (SiV) color center has demonstrated the most promise among them. Even at room temperature and at higher temperatures, it shows strong emission in the zero-phonon line (ZPL). Color centers must be embedded in thin, free-standing diamond membranes with thicknesses ranging from a few micrometers to hundreds of nanometers for scalable heterostructure devices, integrated quantum photonics, and hybrid quantum systems. Although it makes sense to use the best single-crystal diamond for this application, creating single-crystal diamond membranes presents many technological difficulties. In contrast to cutting-edge material platforms like silicon technology, single-crystal diamonds cannot be grown on non-diamond substrates, and their physical and chemical characteristics make micro- and nanostructuring more challenging. However, selective (wet-)etching of substrates supporting nanocrystalline diamond seeds can easily produce free-standing polycrystalline diamond membranes. They have not yet found widespread use in integrated quantum photonics and quantum optics, though. Crystallographic flaws, particularly non-diamond carbon phases in the host matrix, are responsible for this. Specifically, the sp2 hybridized carbon in grain boundaries and centers lowers the emitters' quantum efficiency. Furthermore, it is challenging to distinguish individual color centers in polycrystalline membranes due to background photoluminescence. Recently, we demonstrated a single-photon source based on SiV color centers embedded in a polycrystalline diamond membrane. The spectrum of the SiV located in the grain center shows a Lorentzian-like emission profile However, color centers located within or near the grain boundaries exhibit background contributions and spectral broadening. This behavior is primarily due to sp2-hybridized and disordered carbon phases, which introduce electronic states into the bandgap and contribute to the characteristic luminescence of grain boundaries in polycrystalline diamond. Nevertheless, the ZPL emission spectrum in both cases is broader compared to bulk diamond. This broadening is mainly attributed to stress and crystal imperfections. Time-resolved spectroscopy revealed that the SiV centers in the grain centers exhibit a deconvoluted excited- state lifetime of 1.1 ns. Furthermore, second-order correlation measurements using Hanbury Brown-Twiss intensity interferometry confirm photon antibunching, with g(2)(0) ≈ 0.04 under continuous-wave excitation. This verifies the detection of single SiV centers in the grain centers. Our results demonstrate the potential of polycrystalline diamond for use in hybrid quantum systems, quantum photonics and quantum nano-optics.