Silicon-vacancy Centers Research Articles

Phonon-Assisted Photoluminescence Up-Conversion of Silicon-Vacancy Centers in Diamond.

Phonon-assisted anti-Stokes photoluminescence (ASPL) up-conversion lies at the heart of optical refrigeration in solids. The thermal energy contained in the lattice vibrations is taken away by the emitted anti-Stokes photons' ASPL process, resulting in laser cooling of solids. To date, net laser cooling of solids is limited in rare-earth (RE)-doped crystals, glasses, and direct band gap semiconductors. Searching more solid materials with efficient phonon-assisted photoluminescence up-conversion is important to enrich optical refrigeration research. Here, we demonstrate the phonon-assisted PL up-conversion process from the silicon vacancy (SiV) center in diamond for the first time by studying ASPL spectra for the dependence of temperature, laser power, and excitation energy. Although net cooling has not been observed, our results show that net laser cooling might be eventually achieved in diamond by improving the external quantum efficiency to higher than 95%. Our work provides a promising route to investigate the laser cooling effect in diamond.

Accuracy of the Heyd-Scuseria-Ernzerhof hybrid functional to describe many-electron interactions and charge localization in semiconductors

Hybrid functionals, which mix a fraction of Hartree-Fock (HF) exchange with local or semilocal exchange, have become increasingly popular in quantum chemistry and computational materials science. Here, we assess the accuracy of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional to describe many-electron interactions and charge localization in semiconductors. We perform diffusion quantum Monte Carlo (DMC) calculations to obtain the accurate ground-state spin densities of the negatively charged (SiV)$^-$ and the neutral (SiV)$^0$ silicon-vacancy center in diamond, and of the cubic silicon carbide (3C-SiC) with an extra electron. We compare our DMC results with those obtained with the HSE functional and find a good agreement between both methods for (SiV)$^-$ and (SiV)$^0$, whereas the correct description of 3C-SiC with an extra electron crucially depends on the amount of HF exchange included in the functional. Also, we examine the case of the neutral Cd vacancy in CdTe, for which we assess the performance of HSE against the many-body \emph{GW} approximation for the description of the position of the defect states in the band gap.

Photon-mediated interactions between quantum emitters in a diamond nanocavity.

Photon-mediated interactions between quantum systems are essential for realizing quantum networks and scalable quantum information processing. We demonstrate such interactions between pairs of silicon-vacancy (SiV) color centers coupled to a diamond nanophotonic cavity. When the optical transitions of the two color centers are tuned into resonance, the coupling to the common cavity mode results in a coherent interaction between them, leading to spectrally resolved superradiant and subradiant states. We use the electronic spin degrees of freedom of the SiV centers to control these optically mediated interactions. Such controlled interactions will be crucial in developing cavity-mediated quantum gates between spin qubits and for realizing scalable quantum network nodes.

Cavity-Enhanced Raman Emission from a Single Color Center in a Solid

We demonstrate cavity-enhanced Raman emission from a single atomic defect in a solid. Our platform is a single silicon-vacancy center in diamond coupled with a monolithic diamond photonic crystal cavity. The cavity enables an unprecedented frequency tuning range of the Raman emission (100GHz) that significantly exceeds the spectral inhomogeneity of silicon-vacancy centers in diamond nanostructures. We also show that the cavity selectively suppresses the phonon-induced spontaneous emission that degrades the efficiency of Raman photon generation. Our results pave the way towards photon-mediated many-body interactions between solid-state quantum emitters in a nanophotonic platform.

Man-made synthesis of ultrafine photoluminescent nanodiamonds containing less than three silicon-vacancy colour centres

Silicon-vacancy (SiV) centred diamond exhibits near infrared emission at 738 nm with narrower zero-phonon line and shorter lifetime comparative with nitrogen-vacancy (NV) centre, considered as a promising room-temperature single-photon source. However, unlike NV centred nanodiamonds easily obtained by detonation, man-made synthesis of ultrafine SiV nanodiamonds is a key challenge. Here, we provide a controllably method for synthesizing nanodiamonds with one or few SiV emitters by reducing their size to the near minimum via oxygen plasma treatments. The size of nanodiamond is uniformly reduced from 11 to 1.7 nm under different treatment durations. It is found that nanodiamonds of a size between 2.1 and 4 nm contain fewer than three emitters. Size dependence of microstructure and photoluminescence suggests that SiV centres exist in the outer shell of nanodiamonds with diameter larger than ∼2 nm. This material has significant applications in nanophotonics platform, quantum communications and quantum photonics.

Effect of the solvent-catalyst composition on diamond crystallization in the Mg-Ge-C system

Crystallization of diamond in the Mg-Ge-C system has been studied at 7.5 GPa and 1800 °C with the catalyst composition ranging from pure Mg to pure Ge. It is found that with increasing Ge content of the solvent-catalyst, the degree of graphite-to-diamond conversion gradually decreases from 92 to 16%, which is a consequence of the low solubility of carbon in the germanium melt. A decrease in the Mg/Ge ratio leads to a change in the diamond growth form from cube to octahedron and a drastic increase in the number of diamond nucleation centers. The spectral properties of the crystallized diamonds are studied by photoluminescence techniques. The correlations between the Ge content of the Mg-Ge-C system and the luminescence characteristics of the diamond crystals caused by the germanium-vacancy, silicon-vacancy, and nitrogen-vacancy optical centers are established. The results obtained in this study demonstrate that by changing the catalyst composition in the Mg-Ge-C system, it is possible to control the design of the diamond crystals and synthesize isometric, pyramidal, prismatic or needle-like crystals containing Ge-V centers.

Vibrational modes of negatively charged silicon-vacancy centers in diamond from ab initio calculations

Silicon-vacancy (SiV) center in diamond is a photoluminescence (PL) center with a characteristic zero-phonon line energy at 1.681 eV that acts as a solid-state single photon source and, potentially, as a quantum bit. The majority of the luminescence intensity appears in the zero-phonon line; nevertheless, about 30\% of the intensity manifests in the phonon sideband. Since phonons play an essential role in the operation of this system, it is of importance to understand the vibrational properties of the SiV center in detail. To this end, we carry out density functional theory calculations of dilute SiV centers by embedding the defect in supercells of a size of a few thousand atoms. We find that there exist two well-pronounced quasi-local vibrational modes (resonances) with $A_{2u}$ and $E_u$ symmetries, corresponding to the vibration of the Si atom along and perpendicular to the defect symmetry axis, respectively. Isotopic shifts of these modes explain the isotopic shifts of prominent vibronic features in the experimental SiV PL spectrum. Moreover, calculations show that the vibrational frequency of the $A_{2u}$ mode increases by about 30\% in the excited state with respect to the ground state, while the frequency of the $E_u$ mode increases by about 5\%. These changes explain experimentally observed isotopic shifts of the zero-phonon line energy. We also emphasize possible dangers of extracting isotopic shifts of vibrational resonances from finite-size supercell calculations, and instead propose a method to do this correctly.

Observation of an environmentally insensitive solid-state spin defect in diamond.

Engineering coherent systems is a central goal of quantum science. Color centers in diamond are a promising approach, with the potential to combine the coherence of atoms with the scalability of a solid-state platform. We report a color center that shows insensitivity to environmental decoherence caused by phonons and electric field noise: the neutral charge state of silicon vacancy (SiV0). Through careful materials engineering, we achieved >80% conversion of implanted silicon to SiV0 SiV0 exhibits spin-lattice relaxation times approaching 1 minute and coherence times approaching 1 second. Its optical properties are very favorable, with ~90% of its emission into the zero-phonon line and near-transform-limited optical linewidths. These combined properties make SiV0 a promising defect for quantum network applications.

In search of the right diamond defect

Quantum Information Certain defects in diamond are among the most promising physical implementations of qubits, the building blocks of quantum computers. However, identifying a defect with balanced properties is tricky: Nitrogen vacancy centers have a long lifetime but comparatively poor optical properties, whereas negatively charged silicon vacancy centers have the opposite characteristics. Rose et al. used careful materials engineering to stabilize the neutral charge state of silicon vacancy centers and found that they combine long coherence times with excellent optical properties. Science , this issue p. [60][1] [1]: /lookup/doi/10.1126/science.aao0290

Tin-vacancy in diamonds for luminescent thermometry

Color centers in diamonds have shown promising potential for luminescent thermometry. So far, the nitrogen-vacancy (NV) color center has demonstrated a high sensitivity for optical temperature monitoring in biological systems. However, the NV center requires microwave excitation which can cause unwanted heating, and the NV is also sensitive to non-axial magnetic fields, both of which can result in inaccurate temperature measurements. To overcome this drawback, the silicon-vacancy (SiV) and germanium-vacancy (GeV) color centers in diamonds have recently been explored and have shown good optical temperature sensitivity owing to the temperature dependent wavelength optical zero-phonon line. Here, we report optical temperature measurements using the recently discovered tin-vacancy (SnV) color center in diamond and show sensitivity better than 0.2 K in 10 s integration time. Also, we compare the relative merits of SnV with respect to SiV and GeV for luminescent thermometry. These results illustrate that there are likely to be many future options for nanoscale thermometry using diamonds.

Diamond nano-pyramids with narrow linewidth SiV centers for quantum technologies

Color centers in diamond are versatile solid state atomic-like systems suitable for quantum technological applications. In particular, the negatively charged silicon vacancy center (SiV) exhibits a narrow photoluminescence (PL) line and lifetime-limited linewidth in a bulk at cryogenic temperature. We present a low-temperature study of chemical vapour deposition (CVD)-grown diamond nano-pyramids containing SiV centers. The PL spectra feature a bulk-like zero-phonon line with ensembles of SiV centers, with a linewidth below 10 GHz which demonstrates very low crystal strain for such a nano-object.

Strain engineering of the silicon-vacancy center in diamond

We control the electronic structure of the silicon-vacancy (SiV) color-center in diamond by changing its static strain environment with a nano-electro-mechanical system. This allows deterministic and local tuning of SiV optical and spin transition frequencies over a wide range, an essential step towards multi-qubit networks. In the process, we infer the strain Hamiltonian of the SiV revealing large strain susceptibilities of order 1 PHz/strain for the electronic orbital states. We identify regimes where the spin-orbit interaction results in a large strain suseptibility of order 100 THz/strain for spin transitions, and propose an experiment where the SiV spin is strongly coupled to a nanomechanical resonator.

Phonon Networks with Silicon-Vacancy Centers in Diamond Waveguides.

We propose and analyze a novel realization of a solid-state quantum network, where separated silicon-vacancy centers are coupled via the phonon modes of a quasi-one-dimensional diamond waveguide. In our approach, quantum states encoded in long-lived electronic spin states can be converted into propagating phonon wave packets and be reabsorbed efficiently by a distant defect center. Our analysis shows that under realistic conditions, this approach enables the implementation of high-fidelity, scalable quantum communication protocols within chip-scale spin-qubit networks. Apart from quantum information processing, this setup constitutes a novel waveguide QED platform, where strong-coupling effects between solid-state defects and individual propagating phonons can be explored at the quantum level.

All-optical nanoscale thermometry with silicon-vacancy centers in diamond

We demonstrate an all-optical thermometer based on an ensemble of silicon-vacancy centers (SiVs) in diamond by utilizing the sensitivity of the zero-phonon line wavelength to temperature, Δλ/ΔT=0.0124(2) nm K–1 [6.8(1) GHz K–1]. Using SiVs in bulk diamond, we achieve 70 mK precision at room temperature with a temperature uncertainty σT=360 mK/Hz. Finally, we use SiVs in 200 nm nanodiamonds as local temperature probes with 521 mK/Hz uncertainty and achieve sub-Kelvin precision. These properties deviate by less than 1% between nanodiamonds, enabling calibration-free thermometry for sensing and control of complex nanoscale systems.

Feasibility of efficient room-temperature solid-state sources of indistinguishable single photons using ultrasmall mode volume cavities

Highly efficient sources of indistinguishable single photons that can operate at room temperature would be very beneficial for many applications in quantum technology. We show that the implementation of such sources is a realistic goal using solid-state emitters and ultrasmall mode volume cavities. We derive and analyze an expression for photon indistinguishability that accounts for relevant detrimental effects, such as plasmon-induced quenching and pure-dephasing. We then provide the general cavity and emitter conditions required to achieve efficient indistinguishable photon emission, and also discuss constraints due to phonon sideband emission. Using these conditions, we propose that a nanodiamond negatively charged silicon-vacancy center combined with a plasmonic-Fabry-Perot hybrid cavity is an excellent candidate system.

Diamond in a Nanopocket: A New Route to a Strong Purcell Effect.

Light emission from the color centers in diamonds can be significantly enhanced by their interaction with optical microcavities. In the conventional chip-based hybrid approach, nanodiamonds are placed directly on the surface of microcavity chips created using fabrication-matured material platforms. However, the achievable enhancement due to the Purcell effect is limited because of the evanescent interaction between the electrical field of the cavity and the nanodiamond. Here, we propose and statistically analyze a diamond in a nanopocket structure as a new route to achieve a high enhancement of light emission from the color center in the nanodiamond, placed in an optical microcavity. We demonstrate that by creating a nanopocket within the photonic crystal L3 cavity and placing the nanodiamond in, a significant and a robust control over the local density of states can be obtained. The antinodes of the electric field relocate to the nanosized air gaps within the nanopocket, between the nanodiamond and the microcavity. This creates an elevated and uniform electric field across the nanodiamond that is less sensitive to perturbations in the shape and orientation of the nanodiamond. Using a silicon nitride photonic crystal L3 cavity and aiming at silicon-vacancy and nitrogen-vacancy color centers in diamond, we performed a statistical analysis of light emission, assuming random positions of color centers and dipole moment orientations. We showed that in cavities with experimentally feasible quality factors, the diamond in the nanopocket structure produces Purcell factor distributions with mean and median that are tenfold larger compared to what can be achieved when the diamond is on the surface of the microcavity.

Fluorescence and Raman Spectroscopy of Doped Nanodiamonds

Raman and fluorescence spectroscopic techniques were used to study doped nanodiamonds synthesized at high pressure and high temperature (HPHT technique) and by chemical vapor deposition from the gas phase (CVD technique). For the CVD diamonds, a hundred-fold increase in fluorescence intensity of the silicon-vacancy centers normalized to the volume of the probe material was observed with an increase in synthesized diamond particle diameter from 150 to 300 nm. Graphitization temperature upon heating in the air significantly lower than for detonation nanodiamonds was found for the boron-doped HPHT nanodiamonds.

Molecular dynamics simulation of silicon ion implantation into diamond and subsequent annealing

Ion implantation is one of the best methods to manufacture silicon-vacancy (SiV) centers in diamond, which can be used as qubits. In this work, molecular dynamics (MD) simulation was conducted to analyze the damage evolution and distribution during the process of silicon ion implantation into bulk diamond and subsequent annealing. Tersoff-ZBL (Ziegler-Biersack-Littmark) potential was used to describe the atomic interaction. Identify Diamond Structure (IDS) and Wigner-Seitz defect analysis methods were used to calculate damages and vacancies. After 2393 K annealing, about 42.5% of ion induced IDS damages were recovered. During the temperature cooling down from 2393 K to 293 K, the movements of silicon atoms along the implantation direction were sensitive to the temperature variation, while vacancies were almost insensitive. MD simulation is helpful to illustrate the ion implant induced damages’ dynamic evolution and Si-V related defects, which can assist a deeper understanding of SiV center’s manufacturing.

Excited-state lifetime measurement of silicon vacancy centers in diamond by single-photon frequency upconversion

We report a method with high time resolution to measure the excited-state lifetime of silicon vacancy centers in bulk diamond avoiding timing jitter from the single-photon detectors. Frequency upconversion of the fluorescence emitted from silicon vacancy centers was achieved from 738 nm to 436 nm via sum frequency generation with a short pump pulse. The excited-state lifetime can be obtained by measuring the intensity of upconverted light while the pump delay changes. As a probe, a pump laser with pulse duration of 11 ps provided a high temporal resolution of the measurement. The lifetime extracted from the pump–probe curve was 0.755 ns, which was comparable to the timing jitter of the single-photon detectors.

On-Demand Generation of Neutral and Negatively Charged Silicon-Vacancy Centers in Diamond.

Point defects in wide-band-gap semiconductors are emerging as versatile resources for nanoscale sensing and quantum information science, but our understanding of the photoionization dynamics is presently incomplete. Here, we use two-color confocal microscopy to investigate the dynamics of charge in type 1b diamond hosting nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers. By examining the nonlocal fluorescence patterns emerging from local laser excitation, we show that, in the simultaneous presence of photogenerated electrons and holes, SiV (NV) centers selectively transform into the negative (neutral) charge state. Unlike NVs, 532nm illumination ionizes SiV^{-} via a single-photon process, thus hinting at a comparatively shallower ground state. In particular, slower ionization rates at longer wavelengths suggest the latter lies approximately ∼1.9 eV below the conduction band minimum. Building on the above observations, we demonstrate on-demand SiV and NV charge initialization over large areas via green laser illumination of variable intensity.