Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Microwave-Assisted Cross-Polarization of Nuclear Spin Ensembles from Optically Pumped Nitrogen-Vacancy Centers in Diamond F. Shagieva,* S. Zaiser, P. Neumann, D. B. R. Dasari, R. Stöhr, A. Denisenko, R. Reuter, C. A. Meriles,† and J. Wrachtrup Institute for Quantum Science and Technology (IQST), University of Stuttgart, Third Institute of Physics, Stuttgart 70569, Germany S Supporting Information *
ABSTRACT: The ability to optically initialize the electronic spin of the nitrogenvacancy (NV) center in diamond has long been considered a valuable resource to enhance the polarization of neighboring nuclei, but efficient polarization transfer to spin species outside the diamond crystal has proven challenging. Here we demonstrate variable-magnetic-field, microwave-enabled cross-polarization from the NV electronic spin to protons in a model viscous fluid in contact with the diamond surface. Further, slight changes in the cross-relaxation rate as a function of the wait time between successive repetitions of the transfer protocol suggest slower molecular dynamics near the diamond surface compared to that in bulk. This observation is consistent with present models of the microscopic structure of a fluid and can be exploited to estimate the diffusion coefficient near a solid−liquid interface, of importance in colloid science. KEYWORDS: NV center in diamond, dynamic nuclear polarization, NOVEL, spin-locking, hyperpolarization, color centers in diamond, Hartmann−Hahn condition, external spin bath polarization, proton hyperpolarization
T
ature 13C spin polarization at low3−8 (i.e., TCP, and it remains constant for any further increases in TR. However, at faster diffusion rates (TD ≪ TCP), the cross-polarization rate averages to zero, thereby reducing the transfer efficiency for any TR. Hence, regardless of the repetition rate, high diffusive fluids such as water cannot be polarized efficiently through this technique without the formation of a near-surface layer featuring sufficiently slow molecular dynamics. The discussion above can be formally extended to nuclear (H) spins in solids if T(H) D is replaced by the spin diffusion time TS ≈ (rd − rs)2/DS, where DS ≈ (μ0/4π)(0.1γH2ℏ/rH−H) is the spin diffusion coefficient32 and rH−H is the internuclear distance. This process, however, is inherently slow: Even for strongly coupled nuclear systems such as protons, we calculate DS ≈ 2 × 10−4 nm2/μs, orders of magnitude smaller than the selfdiffusion coefficient of most fluids. The rapid polarization of neighboring nuclei thus prevents further transfer, the so-called “blockade”, hence leading to negligible NOVEL contrast. Naturally, the contrast can be made nonzero by periodically cycling the sign of the starting NV− spin, though at the expense of a net polarization gain in the nuclear spin reservoir. By the same token, nonzero contrast is expected in the limit of exceptionally short nuclear spin−lattice relaxation times, precisely the condition throughout our NOVEL transfer experiments to protons in a PMMA film (Section 4 in the Supporting Information). In summary, we demonstrated microwave-assisted crosspolarization from optically pumped shallow NVs to protons in
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b00925. Creation of the NV centers; numerical simulation of the detection volume; bare diamond measurements; laser ablation of PMMA; power stability of the amplifier; effect of inhomogeneous couplings and spin-diffusion on the polarization dynamics of NV electron spin (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
F. Shagieva: 0000-0003-4881-8365 P. Neumann: 0000-0003-2146-0412 C. A. Meriles: 0000-0003-2197-1474 Present Address †
CUNYCity College of New York, New York, 10031 New York, United States.
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DOI: 10.1021/acs.nanolett.8b00925 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters Author Contributions
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P.N. conceived of the presented idea. F.S. carried out the experiment. A.D. and R.R. contributed to the sample preparation. S.Z., P.N., C.A.M., and J.W. supervised the project. D.B.R.D. and C.A.M. developed the theory and performed the numerical and analytical simulations. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support from the Max Planck Society, the DFG, ERC Advanced Grant SMeL, BW-Stiftung, and BMBF. F.S. expresses the gratitude to Jochen Scheuer for the help with the Qudi software suite installation35 and Nabeel Aslam and Matthias Pfender for fruitful discussions. C.A.M. acknowledges support from the National Science Foundation through grants NSF-1619896 and NSF-1401632, from the NSF CREST-IDEALS HRD-1547830, and from Research Corporation for Science Advancement through a FRED Award. The authors dedicate this work to the memory of Professor Charles Pence Slichter.
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DOI: 10.1021/acs.nanolett.8b00925 Nano Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.nanolett.8b00925 Nano Lett. XXXX, XXX, XXX−XXX
