Novel single-photon emitters

True single-photon emitters that operate at room temperature are essential components of the interdisciplinary field of quantum-information processing (QIP).¹ In the schemes proposed to implement QIP, photons are employed for information transfer, which in turn highlights the need for new sources of quantum light that meet the criteria of emerging technologies. Triggered single-photon emitters can be generated from trapped ions or atoms, quantum dots, or single molecules. However, unprecedentedly photostable single-photon emission can be achieved from color centers in diamond, a unique material that can host a variety of impurities in its crystal lattice.² Some are characterized by radiative decay leading to photoluminescence, allowing single-photon extraction with high brightness. Room-temperature operation and optical single-spin readout of such single emitters³ has prompted suboptical imaging resolution,^4,5 which could contribute favorably to advances in nano-imaging and major nanofabrication approaches.

Diamond-based single-photon emitters have been known for more than a decade. Mainstream devices include the famous nitrogen vacancy, nickel-related centers, a silicon vacancy, and carbon-related emitters. We discovered and engineered novel color centers based on chromium (Cr) impurities.⁶ We created the emitters in both single-crystal⁷ and nanodiamonds (NDs),⁸ thus providing controllable methods to explore their fundamental physical structure and tailor photophysical properties to specific applications.

The Cr centers in NDs are fabricated by growing crystals on a sapphire substrate using a microwave-assisted chemical-vapor-deposition method.⁹ The Cr-related centers are generated through diffusion of Cr into the crystals from the sapphire substrate during growth. The Cr emitters can be divided into two categories, including one where the emitter exhibits two-level-system behavior⁶ and one where they typically possess a three-level system and exhibit bunching in their photon statistics (which is attributed to the existence of a metastable state). The emission is typically in the region of ∼750nm. Two-level emitters have a spectral width of ∼11nm, while three-level emitters exhibit a narrower spectral width of 4nm. One of the most remarkable properties of Cr emitters is their extreme brightness, with emission at saturation approaching 3×10⁶counts/s.

Figure 1 shows the emission spectra of various color centers in diamond and a scanning-electron-microscopy image of NDs. In our work, we considered two second-order correlation functions, g⁽²⁾(τ), of Cr emitters embedded in NDs, measured at different laser-excitation powers (see Figure 2). Lack of correlation counts at zero delay time is a signature of single-photon emission. For the two-level system, the function exhibits a simple exponential behavior for timescales equivalent to the fluorescence decay rate of the excited state, with an exponential asymptotic value of unity. The three-level system, on the other hand, contains two exponential components, each with a characteristic time constant. Depending on the transition rates from the excited to the shelving state and from the shelving back to the ground state, g⁽²⁾(τ) can increase beyond unity for times longer than the radiative lifetime, before going to the asymptote.

Figure 1. (left) Typical photoluminescence spectra in arbitrary (arb.) units from known color centers in diamond (black nitrogen vacancy, red silicon vacancy, and blue nickel centers) and chromium (Cr) centers (purple and green). The Cr centers' emission is in the region between 740 and 770nm. (right) Scanning-electron-microscopy image of diamond nanocrystals grown by chemical-vapor deposition.

Figure 2. Typical g⁽²⁾(τ)correlation-function measurements with a Hanbury-Brown and Twiss interferometer in nanodiamonds for different emitters with wavelengths of 740–770nm. (a) Two- and (b) three-level-system emitters. The correlation functions have been displaced for clarity. P_sat: Saturation power.

We engineered the Cr centers in single-crystal diamond by ion implantation of chromium and oxygen, accelerated to 50 and 19.5keV, respectively. Cr-related emitters in bulk diamond all exhibit a three-level system. Fabrication of the center in single-crystal diamond is significant to gain a better understanding of the fundamental properties of the emitters, which are presently unknown. We performed direct emission-dipole imaging in diamond Cr centers. By combining emission-dipole and polarization measurements, we established that the absorption and emission dipoles in Cr are not parallel. In particular, they are typically orthogonal in single crystals. By positioning the emitters at different distances from the diamond-air interface, we measured a quantum efficiency for the Cr-related emitters of ∼30%.¹⁰ Further work will aim to employ the linear-dipole emission property of these centers for single-particle tracking in biological imaging (which is crucial to achieve optimal dipole-type coupling to cavities) and enable mode matching to plasmonic waveguides in integrated devices. Further measurements should also provide more details on the symmetry of the center and provide vital information toward identification of its chemical structure.

Diamond single-photon emitters are at the frontier of QIP and usher the development of novel quantum technologies. In particular, NDs containing Cr-related emitters could be integrated by nanomanipulation in hybrid systems such as photonic-crystal cavities, photonic fibers, plasmonics, and metamaterials,¹¹ opening up an enormous amount of novel research and systems in nanophotonics. On the other hand, bright Cr emitters in bulk crystal could provide the ultimate platform for development of integrated photonic components on a single chip (based on sculpting solid immersion lenses) or diamond nanowire antennas on demand. This could dramatically enhance the collection efficiency of the emitted photons. We envisage that optical collection would lead to higher emission rates in the GHz regime, while a redefinition of the luminous-intensity unit, the candela,¹² could be based on the fundamental entity (the photon).

This work was supported by the Australian Research Council, the International Science Linkages Program of the Australian Department of Innovation, Industry, Science, and Research, and the European project ‘Engineered Quantum Information in Nanostructured Diamond’ (EQUIND).