Neutral Silicon-Vacancy Center in Diamond: Spin Polarization and Lifetimes

B. L. Green, S. Mottishaw, B. G. Breeze, A. M. Edmonds, U. F. S. D’Haenens-Johansson, M. W. Doherty, S. D. Williams, D. J. Twitchen, and M. E. Newton

Phys. Rev. Lett. 119, 096402 – Published 31 August 2017

Abstract

We demonstrate optical spin polarization of the neutrally charged silicon-vacancy defect in diamond ( ${SiV}^{0}$ ), an $S = 1$ defect which emits with a zero-phonon line at 946 nm. The spin polarization is found to be most efficient under resonant excitation, but nonzero at below-resonant energies. We measure an ensemble spin coherence time $T_{2} > 100 μ s$ at low-temperature, and a spin relaxation limit of $T_{1} > 25 s$ . Optical spin-state initialization around 946 nm allows independent initialization of ${SiV}^{0}$ and ${NV}^{-}$ within the same optically addressed volume, and ${SiV}^{0}$ emits within the telecoms down-conversion band to 1550 nm: when combined with its high Debye-Waller factor, our initial results suggest that ${SiV}^{0}$ is a promising candidate for a long-range quantum communication technology.

Received 31 May 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.096402

Physics Subject Headings (PhySH)

Defects Nitrogen vacancy centers in diamond Spin polarization

Diamond

Electron paramagnetic resonance

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

B. L. Green^1,*, S. Mottishaw¹, B. G. Breeze¹, A. M. Edmonds², U. F. S. D’Haenens-Johansson³, M. W. Doherty⁴, S. D. Williams², D. J. Twitchen², and M. E. Newton^1,†

¹Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
²Element Six Limited, Global Innovation Centre, Fermi Avenue OX11 0QR, United Kingdom
³Gemological Institute of America, 50 W 47th Street, New York, New York 10036, USA
⁴Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Australian Capital Territory 0200, Australia

^*Corresponding Author. b.green@warwick.ac.uk
^†m.e.newton@warwick.ac.uk

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Vol. 119, Iss. 9 — 1 September 2017

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Images

Figure 1
(a) The structure of the silicon-vacancy defect in the diamond lattice: the silicon adopts a bond-center location, leading to $D_{3 d}$ symmetry ( $⟨ 111 ⟩$ axis) in both the neutral and negative charge states [16]. (b) Photoluminescence spectrum of ${SiV}^{0}$ (zero phonon line at 946 nm) at a temperature of 80 K in sample B. The feature at 976 nm is related to ${SiV}^{0}$ and may indicate local strain [11].
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Figure 2
(a) Schematic of optically pumped EPR measurements. Light from a laser is delivered via optical fiber to the sample mounted into the cryostat. cw measurements performed using lasers at 532, 830, 915 nm and a tuneable 915–985 nm. The 532 nm laser is switched using an acousto-optic modulator (AOM) for pulsed EPR measurements. Components of the EPR spectrometer are in blue. (b) EPR spectra of the $m_{s} = 0 \leftrightarrow + 1$ (left) and $m_{s} = 0 \leftrightarrow - 1$ (right) transitions of ${SiV}^{0}$ in sample A, collected with applied magnetic field $B$ within 2° of $⟨ 111 ⟩$ . Upper spectra collected at room temperature; lower spectra collected at a sample temperature of 10 K during optical pumping with 80 mW ( $10.2 W {cm}^{- 2}$ ) at 830 nm. The opposite phase of the low- and high-field lines indicate enhanced absorption and emission, respectively: we achieve a bulk polarization of $ξ = 4.2 %$ . Optically pumped spectra have been offset for clarity; magnetic field given for room temperature measurement. Inset: schematic of ground-state energy levels with nominal spin-polarized populations illustrating the origin of enhanced absorption and emission.
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Figure 3
Spin polarization of sample A measured by a two-pulse Hahn echo while sweeping the incident laser wavelength at an optical power of 9 mW. The spin polarization decreases before reaching a sharp peak at the ZPL wavelength. Above-ZPL excitation continues to generate spin polarization, with a sharp signal decrease observed when the laser is switched off. Inset: high-resolution scans over the ZPL indicate that the ZPL is split by approximately 0.4 nm ( $\approx 134 GHz$ ).
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Figure 4
(a) Pulse sequence used to measure $T_{1}$ . ${SiV}^{0}$ is spin polarized by optical pumping at 532 nm to increase the signal strength, then an echo-detected inversion-recovery measurement is performed to measure $T_{1}$ : the inversion pulse is included to enable multistage phase cycling. (b) Echo-detected inversion-recovery measurement at 17 K, yielding a longitudinal spin lifetime of 18(2) s. (c) Temperature dependence of $T_{1}$ in ${SiV}^{0}$ (sample B, dots) and ${NV}^{-}$ (triangles, two different concentrations—taken from [27]). The ${SiV}^{0}$ spin-lattice relaxation lifetime $T_{1}$ depends strongly on temperature in this sample. At low temperatures, $T_{1} > 25 s$ . Crosses obtained via an indirect measurement of $T_{1}$ using linewidth (see [17]); dashed line is a fit to different temperature-dependent relaxation processes (see text).
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Figure 5
$T_{2}$ decoherence lifetimes measured by echo decay at 96 and 27 K in sample B. Inset: comparison of $T_{1}$ and $T_{2}$ spin lifetimes. The measured $T_{2}$ is effectively limited by $T_{2} \leq 2 T_{1}$ at room temperature and 96 K, but has reached a non- $T_{1}$ -limited value of $103 μ s$ at 27 K.
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Figure 6
Possible mechanisms for generation of spin polarization into the GS $m_{s} = 0$ state based on ISC within ${SiV}^{0}$ . Optical transitions are given as solid arrows; non-radiative transitions are dotted. The molecular orbital configuration for each state is given in brackets, spin-orbit (SO) states are given and spin-spin effects have been neglected. (a) Spin polarization occurs by spin-orbit coupling between the ES and a singlet state; below-ZPL polarization is generated by a dipole-allowed transition from the GS to the singlet which becomes weakly allowed due to SO mixing in the GS. (b) Spin polarization is generated by ISC from a third triplet state between the GS and ES. ZPL dipole transitions between the GS and the ${{}^{3}A}_{2 u}$ are forbidden, but can be driven into the vibronic sideband by emission of $A_{2 g}$ phonons. In both cases, the given singlet characters are examples; however, there can be no dipole transition between singlets in order for the upper singlet to be an effective shelving state, as observed in temperature-dependent PL measurements [10]. In comparison to the well-understood model for ${NV}^{-}$ [34], our model must also account for spin polarization at below-resonant energies.
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