All-optical hyperpolarization of electron and nuclear spins in diamond B. L. Green,∗ B. G. Breeze, G. J. Rees, J. V. Hanna, and M. E. Newton† Low thermal polarization of nuclear spins is a primary sensitivity limitation for nuclear magnetic resonance and imaging. Several methods can increase nuclear polarization but rely on low temperature, high magnetic fields or microwave pumping. Here we demonstrate optically pumped (microwave-free) nuclear spin polarization of 13 C and 15 N in 15 N-doped diamond pumped with monochromatic light from 750–375 nm. Long nuclear spin lifetimes of 30 minutes are observed in the paramagnetic system 15 N0s , with 15 N polarization enhancements up to −2000 above thermal equilibrium. Nuclear spin polarization is shown to diffuse to bulk 13 C, leading to NMR enhancements of −200 at room temperature and −500 at 240 K. A mechanism based on multiple-spin mixing at coupled defect pairs is proposed. PACS numbers: 76.30-v, 76.70.Dx (a)
(b)
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dark N0s hyperfine-coupled 13C
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Diamond has emerged as a host crystal for a number of functional defects: the negatively-charged nitrogen vacancy center (NV− ) in diamond has attracted great interest due to its potential for use in nano-sensing of electric [1] and magnetic fields [2–5], remote temperature measurement [6–8], quantum simulation [9], and quantum information processing [10, 11]. Additionally, the negatively-charged silicon vacancy center (SiV− ) has recently been gaining attention in complementary areas such as quantum photonics [12–14] and quantum key distribution [15, 16]. NV− applications arise primarily from its opticallypumped electron spin polarization (approaching 100 %), spin-dependent fluorescence and long spin coherence in ambient conditions. Simultaneous optical and microwave irradiation of NV− centers transfers some electron polarization to nuclear polarization (dynamic nuclear polarization (DNP)) via the hyperfine interaction [17, 18]. The enhancement of nuclear polarization is of great importance to nuclear magnetic resonance (NMR) experiments, where the primary sensitivity limit is due to the small thermal population differences of nuclear spin levels. The development of a general purpose nuclear hyperpolarization technique at arbitrary fields would enable measurement of previously-inaccessible biomolecules and reaction dynamics while decreasing routine NMR measurement times by orders of magnitude [19]. Several successful approaches to DNP have been taken; however, the majority are limited to specific fields [20–22], low temperatures [23, 24], specific molecules [25], or require microwave irradiation of the sample [17, 25]. Microwave-free opticallypumped DNP (OPDNP) of a diamond containing a high concentration of NV− has been demonstrated [18, 26]; however, the mechanism is not well-understood. In this Letter we demonstrate the electronic spin polarization of two paramagnetic nitrogen centers, Ns 0 (substitutional nitrogen, Fig. 1(a)) and N3 V0 (vacancy with three nearest-neighbor N), in a 15 N-doped synthetic diamond with an NV− concentration 103 less than Ns 0 . The electron spin polarization is transferred to proximal 13 C
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arXiv:1610.03823v1 [cond-mat.mes-hall] 12 Oct 2016
Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom
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FIG. 1. (a) Atomic structures of Ns (left), NV and N3 V. In all cases the unpaired electron probability density is localized primarily in the carbon orbitals (gray). (b) EPR spectra collected without (top) and with illumination by 80 mW of light at 532 nm (2.33 eV) with the sample at 85 K and the external magnetic field Bk|h1 1 1i. The two visible systems are 15 N0s (nitrogen hyperfine transitions numbered) and 15 N3 V0 : inversion of the lines under illumination indicates electron spin polarization, and the change in relative intensity of different lines is due to nuclear spin polarization. Panel highlights nuclear polarization of 15 N3 V0 and 13 C coupled to 15 N0s . (c) Single shot 13 C NMR spectra at 7.04 T. Illuminated spectra were collected following illumination at 520 nm (2.38 eV); the dark spectrum was collected after 86 h at field. Inset: room temperature bulk polarization build-up, collected using the given pulse sequence.
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FIG. 2. (a) Dependence of EPR enhancement η on laser wavelength for each of the 15 N0s hyperfines at 85 K (labeled as Fig. 1(b)). Measurements taken at 80 mW optical power at the sample. (b) EPR enhancement as a function of power at 520 nm and 50 K. (c) Build-up and decay of electron polarization at 50 K when illumination is switched on and off, respectively.
and 15 N nuclei through anisotropic hyperfine interactions, with local 15 N polarization enhancement of > 2000 over thermal equilibrium. Nuclear spin polarization is shown to diffuse to the bulk 13 C, leading to microwavefree OPDNP enhancements of −200 at room temperature and −500 at 240 K. Sample – The sample was grown by the high temperature high temperature (HPHT) method described in [1], with approximate concentrations of 80 ppm 15 N0s and 5 ppm 14 Ns 0 . The sample was treated with high energy (4.5 MeV) electron irradiation and HPHT annealing at 1900 ◦C to produce 1.6 ppm 15 N3 V0 , 20 ppm 15 N0s and 40 ppm N-N nearest-neighbor pairs. See Supplemental Material for further detail [28]. Results – Ns 0 and N3 V0 centers in diamond each possess a h1 1 1i C3v symmetry axis [Fig. 1(a)], and thus have four symmetry-related orientations. Both centers are S = 1/2 in the ground state (GS). The use of 15 N (I = 1/2) during synthesis greatly simplifies the electron paramagnetic resonance (EPR) spectra [Fig. 1(b)] compared to 14 N (I = 1) due to the lack of nuclear quadrupole interactions [3, 28]. At temperatures below approximately 120 K, in-situ optical illumination results in electron spin polarization of both paramagnetic centers in field-parallel and nonfield-parallel orientations [Fig. 1(b)]. The constituent 15 N nuclei are spin polarized, as are proximal 13 C (1.1 % abundance). The spin polarization mechanism is orientation-dependent [28], and most efficient with Bkh1 1 1i (symmetry axis of one orientation). EPR enhancements η = (Ilight − Idark )/Idark up to a factor of η = −3 were measured using 150 mW at 532 nm (2.33 eV)
and a sample temperature of 50 K. The polarization excitation mechanism is highly broadband, with electron and nuclear enhancements measured for 750–375 nm (1.65–3.31 eV) [Fig. 2(a)]. The polarization saturates at high optical powers [Fig 2(b)]: it is postulated that this can be accounted for primarily by a mixture of sample heating and photoionization of Ns 0 . 15 N nuclear polarization persists after optical excitation is removed, and is strongest in the field-parallel orientation where mS , mI are eigenstates of the Ns 0 spin system [Fig 3]. The difference in relaxation timescales for the electron and nuclei allows the 15 N spin to be indirectly read-out using the electron. Immediately following the removal of illumination the ratio of observed nuclear polarization to thermal equilibrium, 15 N , was measured as −2000, corresponding to ≈ 1/3 of electron thermal polarization: sequential measurement of the Ns 0 spectrum 15 reveals a nuclear lifetime N T1 = 30(1) min. Single-shot 13 C NMR measurements collected with the sample under in-situ optical illumination at 520 nm (2.38 eV) indicate that the nuclear spin polarization extends beyond the local nuclei and into the bulk [Fig.1(c)]. The characteristic time for this process is 94 min, too slow for an electronic process, and hence is proposed to be mediated by nuclear spin diffusion from the polarized shell around the paramagnetic centers. Bulk OPDNP enhancements of 13 C = −200 were measured at room temperature, and 13 C > −500 at 240 K, leading to experimental speedup factors of 40, 000 and 250, 000, respectively. An additional factor of 4 is gained by the 13 reduction in spin-lattice relaxation ( C) T1,dark > 8 h to 13 ( C) T1,light > 1.5 h. Discussion – There have been several reports of OPDNP in diamond. Typically, microwaves are used to transfer spin polarization from NV− centers to bulk nuclei by driving zero-, double- or multiple-quantum transitions through either thermal mixing or the solid effect [30, 31]. We are aware of only two reports (from the same group) that study all-optical diamond DNP [18, 26]: in both cases the effect is attributed to polarization transfer from NV− . The NV− concentration in the present sample is below EPR detection limits (≈ 10 ppb), even when measured under illuminated (spin-polarized) conditions. Optically-pumped measurements of four other samples, both 14 N- and 15 N-doped with a range of NV− concentrations [see Table I in [28]] failed to locate another sample which exhibited any detectable electronic polarization: thus we do not attribute the present mechanism to NV− . The accepted electronic structure of Ns 0 places only one level (of a1 symmetry) in the band gap: thermoconductivity measurements give the ionization threshold at 1.7 eV, whereas photoionization is subject to a substantial Stokes shift and starts at approximately 1.9–2.2 eV [5, 6]. Similarly, the GS of N3 V0 has only one hole (also a1 symmetry, see [28] for detail), but two excited states
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FIG. 3. (a) EPR spectrum taken approximately 30 s after illumination is switched off. Field-parallel hyperfine transitions of 15 N0s 1 & 4 correspond to |mI i = +1/2 and |mI i = −1/2, respectively: intensity difference is due to 15 N nuclear polarization. Dotted line indicates equilibrium intensity of transitions 1 & 4. (b) Nuclear polarization of field-parallel 15 N0s hyperfines (1 & 4) as a function of time: equilibrium is reached with a characteristic lifetime of 31(1) min at 50 K. A nuclear polarization of 15 N ≈ −2000 over thermal equilibrium is observed. The other pair of hyperfines equilibrate with a lifetime of 42(3) min.
inside the band gap at 3.0 and 3.6 eV [34]. Ns is therefore incapable of internally generating high spin (S > 1/2) states, and we expect the optical threshold for N3 V to be greater than 3.0 eV, contrary to the ≈ 2.0 eV observed here [Fig. 2(a)]: this limitation precludes the typical internal singlet-triplet intersystem crossing and level anticrossing polarization mechanisms seen in diamond and SiC [22, 35, 36]. Both Ns 0 (including 15 N0s [37]) and N3 V0 have been studied extensively under optical excitation [12, 39], and no spin polarization of either system has been reported. The other high-abundance defects in this sample (N2 , N4 V) have no reported optical transitions below 4 eV; and the optical absorption spectrum of this sample contains only Ns 0 and N3 V0 [28]. Finally, the simultaneous observation of spin polarization in two well-characterized systems suggests a common mechanism. We conclude that the phenomenon reported here is likely a result of multiple interacting spin systems, rather than being intrinsic to either system. The data allow us to place constraints on the spin polarization mechanism: we suppose the same mechanism is responsible for polarization at both 0.34 and 7.04 T, and therefore is insensitive to magnetic field-strength. Additionally, the mechanism must be capable of spin polarizing electrons and nuclei in multiple systems simultaneously. Standard DNP mechanisms require microwave saturation of either the allowed (∆mS = ±1; ∆mI = 0) or forbidden (∆mS = ±1; ∆mI = ±1) transitions to transfer electron polarization to the nuclei [30]. One possible
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FIG. 4. (a) Possible model for the spin polarization mechanism. Generic state labels are given for spin singlets |Si and triplets |T i; subscripts denote the ms value. We note that infrared absorption measurements of this sample indicate that Ns 0 (Ns + ) increases (decreases) on optical illumination. (b) Difference frequencies generated by the “allowed” electron transitions of a 15 N0s –15 N3 V0 pair for Bkh1 1 1i at 0.34 (red, ω13 C = 3.64 MHz) and 7.04 T (blue, ω13 C = 75.3 MHz) with an isotropic dipolar coupling of 0.5 MHz: stronger couplings will increase the number of frequencies generated and enhance polarization transfer. 13 C hyperfine couplings have been neglected from the model.
microwave-free mechanism, well documented in liquids and avian navigation [40, 41], is the spin-correlated radial pair, which can generate electron and nuclear spin polarization in solids through electron-electron-nuclear three spin mixing (TSM) [20] or differential decay (DD) [43]. Initially, two spatially proximal systems are in an equilibrium singlet state [Fig 4(a)]. They are photoexcited to a (singlet) excited state, with electron transfer between systems occurring during or immediately after photoexcitation. The TSM phenomenon occurs during evolution of the spin-correlated pair in the excited state — small fluctuations in the magnetic field local to each spin causes the relative phase to oscillate between inphase (spin triplet |T0 i) and antiphase (spin singlet |S0 i) configurations. This requires anisotropic spin interactions (Zeeman g, hyperfine A, and dipolar J ), and enables transfer of the electron polarization to nuclei by pseudosecular (off-diagonal) terms of the hyperfine interaction. The electron-nuclear transfer is most efficient when twice the difference in the electron Larmor frequencies simultaneously matches twice the nuclear Larmor and hyperfine frequencies (the double matching condition 2|∆ωS | = 2|ωI | = |A|) [20]. Spin polarization can also be generated by the incoherent differential decay mechanism: here, the two decay rates from the excited state must be different i.e. γ1 6= γ2 [Fig. 4(a)] [44]. In this case the double matching con-
4 dition above is relaxed to 2|ωI | = |A| [20]. At 0.34 T and 7.04 T, ω13 C = 3.64 and 75.3 MHz, respectively. The g and hyperfine values for 15 N3 V0 and 15 N0s [28] are such that a large number of frequencies between 0 and 100 MHz are generated at both field strengths [Fig 4(b)]. The proposed model is sensitive to both the spatial proximity of paramagnetic centers, and also to the spin Hamiltonian parameters of the centers (i.e. the ‘type’ of center). Statistical modelling of relative positions at the present concentrations indicates that between 5 and 20 % of defect center pairs have a separation of 1.7–4.7 nm [28], corresponding to dipolar coupling frequencies of 0.5–10 MHz. This distribution of dipolar couplings will yield a population of centers which are difficult to observe in EPR due to a combination of large linewidths and lock-in detection, but will generate additional resonance frequencies (and hence ∆ωS ), increasing the probability of meeting the polarization transfer matching conditions above. Additionally, the small difference in g-values between the two defects means these conditions will be met for a large range (approx. 0.3 to > 14 T) of magnetic field strengths [28]. The efficiency of the polarization mechanism is difficult to estimate: in our measurements, 40 % polarization of 5 % population is indistinguishable from 10 % polarization of 20 % population. The sample under study is highly inhomogeneous, with at least three optically distinguishable nitrogen concentrations [28], and two distinct concentrations visible in EPR spectra (determined by multiple simultaneous linewidths). If the polarization mechanism is dependent on interaction between Ns 0 and N3 V then we expect it to occur in only the higher nitrogen sectors (upper limit 80 % of the sample). At room temperature no electron polarization is visible in the EPR spectra, and the upper limit on 13 C polarization is therefore given by the ratio of the Boltzmann polarizations ∝ µe /µ13 C ≈ 2600: enhancements of −200 correspond to an effective homogeneous efficiency of approximately 8 %. Conclusion – Our results show that optical pumping can induce electron and nuclear polarization in two paramagnetic systems in diamond with very low NV− concentration. NMR measurements with in-situ illumination show that the nuclear polarization diffuses out to the bulk 13 C, leading to OPDNP enhancements of up to −500 at 240 K. The two systems involved, 15 N0s and 15 N3 V0 , possess only S = 1/2 and S = 0 states, and hence the standard internal triplet intersystem crossing or level anticrossing mechanisms for solid-state polarization [22, 36] cannot be responsible here. We have proposed a model based on the solid-state spin-correlated radical pair mechanism [40, 41, 43]. Further experimental and theoretical work is required to test the model. With knowledge of the mechanism, engineered synthetic nanodiamonds with concentrations designed to maximize the bulk nuclear polarization would provide a general
platform for optical hyperpolarization of a target sample, enabling study of new biological and dynamical systems without the requirement for sample shuttling, low temperature or microwave irradiation. The authors thank H. Fedder, M. W. Doherty, A. Gali, M. W. Dale and C. J. Wedge for helpful discussions. We acknowledge funding from the Engineering and Physical Sciences Research Council (EP/M013243/1 and EP/J500045/1) and the Gemological Institute of America.
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Corresponding Author;
[email protected] Corresponding Author;
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S1
All-optical hyperpolarization of electron and nuclear spins in diamond: Supplemental Material PRODUCTION OF
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N3 V
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Absorption coefficient (cm )
The 15 N-enriched sample (figure S1) used for EPR and optical studies was grown using the technique described in [S1]. Post-synthesis, the sample contained mean substitutional nitrogen concentrations of [15 N0s ] = 80(2) ppm and [14 Ns 0 ] = 4(3) ppm, respectively. The sample was neutron irradiated to a dose of 5 × 1017 neutrons cm−2 and subsequently annealed under a non-oxidizing atmosphere for 15 h at 1500 ◦C, before finally being annealed under high pressure at a nominal temperature of 1900 ◦C for 1 h. This processing regime generated a total concentration of [15 N3 V0 ] = 1.6(2) ppm and substitutional nitrogen concentrations of 20 ppm [15 N0s ] and 5 ppm [15 N+ s ], respectively. The sample was polished in order to remove the seed crystal and to provide a flat reference face (within 1◦ of h1 1 0i). Inhomogeneities in the uptake of nitrogen during growth are visible in the sample when viewed under a microscope (figure S1).
1 mm
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FIG. S1. (a) Photograph of sample used for this study. Nitrogen inhomogeneity is evident by the variation in yellow color saturation in the different growth sectors. (b) UV-Vis absorption spectrum of the sample at 80 K.
TABLE S1. Summary of the samples tested for the presence of electron or nuclear polarization under the same experimental conditions as the primary sample (sample 1). Sample Enrichment 14 N :15 N 1 5 :95 2 5 :95 3 5 :95 4 15 :85 5 100 :0
Defect concentration (ppm) 0/+ Ns NV− N3 V0 N02 N4 V0 25
