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Physical review. B, Condensed matter

Paramagnetic resonance of photoexcited N-Vdefects in diamond. I. Level anticrossing in the 3 Aground state

Xing-Fei He, Neil B. Manson, Peter Fisk

Published Apr 1, 1993• 88 citations
Publisher

Abstract

Raman-heterodyne-detected paramagnetic resonance has been used to study the level anticrossing in the $^{3}$A state of the N-V defect in diamond. The electron-paramagnetic-resonance (EPR) frequencies are well accounted for by a triplet-spin Hamiltonian. Comparison of the EPR spectra with the calculated transition dipole strengths indicates that optical pumping determines, to a large extent, the Raman-heterodyne-signal intensity. Spin alignment in the $^{3}$A ground state under optical pumping leads to the collapse of the EPR signal at the level anticrossing, where the population is equalized due to the mixing of the spin states. This is substantiated by transient nutation and spin-echo measurements, from which the dipole moment and dephasing time are examined. Laser detuning within the 638-nm zero-phonon line shows that the Raman signal is only detectable when the laser frequency is tuned to the low-energy part of the optical line.

DOI
10.1103/physrevb.47.8809

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Quantum Electronics and Laser Science Conference•May 12, 1991•0 citations
Raman-heterodyne-detected paramagnetic resonance has been used to study the level anticrossing in the 3^{3}3A state of the N-V defect in diamond. The electron-paramagnetic-resonance (EPR) frequencies are well accounted for by a triplet-spin Hamiltonian. Comparison of the EPR spectra with the calculated transition dipole strengths indicates that optical pumping determines, to a large extent, the Raman-heterodyne-signal intensity. Spin alignment in the 3^{3}3A ground state under optical pumping leads to the collapse of the EPR signal at the level anticrossing, where the population is equalized due to the mixing of the spin states. This is substantiated by transient nutation and spin-echo measurements, from which the dipole moment and dephasing time are examined. Laser detuning within the 638-nm zero-phonon line shows that the Raman signal is only detectable when the laser frequency is tuned to the low-energy part of the optical line.
This thesis describes the Raman heterodyne detection of magnetic resonance in the ³A ground state of the nitrogen-vacancy C'-1· V) centre in diamond. The recently developed theory of Raman heterodyne spectroscopy was tested and compared with the spectra measured on the N-V centre. Among those results, the electron-nuclear double resonance (ENDOR), nuclear-nuclear double resonance (double NMR), Autler-Townes effect, coherence transfer and Zeeman quantum beat were observed for the first time using such techniques. Raman heterodyne techniques were also used to investigate the level anticrossing, hyperfine and nuclear quadrupole interactions, and spin relaxations of the N-V centre with high sensitivity and precision. This thesis is comprised of three parts. Part one includes Chapters 1 to 3, providing introductory accounts. Chapter l is an introduction to the N-V colour centre, which describes the defect configuration, the spin Hamiltonian and the calculations of energy levels and wave functions of the ³A ground state. Chapter 2 briefly describes the theory of Raman heterodyne spectroscopy - a coherent optical radio-frequency (RF) double resonance technique, in particular the Raman heterodyne signals at weak and intense RF fields. Chapter 3 deals with experimental details and techniques utilized in the studies. Part two consists of Chapters 4 and 5, where the Raman heterodyne theory is compared with experimental results. Chapter 4 presents the Raman heterodyne detected nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) lineshapes compared with theoretical results at various RF power levels, laser intensities and laser frequencies within the 6380A zero-phonon line. The double resonance spectra obtained by Raman heterodyne techniques are discussed in Chapter 5, where attention is paid to the analysis of their spectral profiles depending on the detection scheme. The application of Raman heterodyne techniques to the studies of magnetic resonance of the N-V centre is described in part three, consisting of Chapters 6 to 9. The EPR measurements are described in Chapter 6, which were used to study various interactions near the level anticrossing, in particular the spin alignment. The hyperfine and nuclear quadrupole interactions probed by the NMR, ENDOR and hole burning measurements are described in Chapter 7. From these measurements, the hyperfine and quadrupole parameters in the spin Hamiltonian were fully determined. Chapter 8 deals with the Autler-Townes effect in the NMR and EPR transitions. By measuring the Autler-Townes spittings, the magnitudes of matrix elements of the NMR transitions were examined and compared. Chapter 9 presents the coherent spin transient properties, including nutations, echoes, Zeeman beating and coherence transfer. The dephasing time and homogeneous linewidths of the NMR and EPR transitions were then determined from the echo measurements. Finally, the thesis is summarized in Chapter 10.
Upon the cw optical excitation of N-V centers in diamond, a spin alignment in the ensemble of nonexcited N-V defects in the electron spin-triplet ground state is induced. When the diamond sample is subjected to magnetic fields of suitable directions and strengths, cross relaxation (CR) and level anticrossing are found to affect the ground-state spin alignment as well as the intensity of the emission due to the photoexcited N-V centers. In this paper, a study of the dynamics involved in the cross-relaxation and autocrossing processes is reported, utilizing techniques for the optical detection of spin-coherent transients. A major finding is that when CR conditions prevail, the stimulated spin-echo and spin-locking signals decay at a rate about three times faster than in the situation in which CR does not occur. The results are accounted for by considering the magnetic dipole-dipole interactions within the ensemble of N-V centers in the triplet ground state in a memory function approach.
The Autler–Townes effect in nuclear magnetic resonance (NMR) and electron paramagnetic resonance has been observed using recently developed Raman heterodyne techniques. The measurements were carried out on the nitrogen-vacancy color center in diamond, where the 3A↔3E optical transition was driven by a single mode dye laser at 638 nm while the magnetic transitions in the 3A ground state were driven by two rf fields. The measured splittings in the NMR lines were found to be in good agreement with theoretical predictions. Using the Autler–Townes effect, the magnitude of the dipole moments of the mI=0↔±1 (mS=0) transitions in the ground state was determined, and found to be the same. Comparison of this latter result with the NMR spectra shows that Raman heterodyne signal intensities are determined by population factors.
Doubly resonant Raman electron paramagnetic transitions in ruby (Al2{}_{2}2​O3{}_{3}3​:Cr3+{}^{3+}3+) have been investigated using dye laser excitation tuned across the range of the Zeeman components of its well-known R1{R}_{1}R1​ emission line. With magnetic field B normal to c, the optic axis of ruby, it is seen that the Raman electron paramagnetic resonance (EPR) lines exhibit significant intensity enhancements due to the simultaneous occurrence of ``in resonance'' and ``out resonance,'' as visualized in the Kramers-Heisenberg formalism of inelastic light scattering. The specific Raman EPR features observed in the present study, together with their resonances, however, differ markedly from those observed in our previous investigation with $\mathbf{B}\ensuremath{\parallel}\mathbf{c}$. These differences can be traced to level mixing effects within the Zeeman multiplet of the 4A2{}^{4}{A}_{2}4A2​ (ground) state on the one hand, and the negligible value of ${g}_{\ensuremath{\perp}}$ for the $\mathrm{E\ifmmode \bar{}\else \={}\fi{}}$ (excited) state of the R1{R}_{1}R1​ emission line on the other. Furthermore, we note that with B\ensuremath{\perp}c and under resonant conditions, the experimentally observed \ensuremath{\Delta}mmm === \ifmmode\pm\else\textpm\fi{}1 pair of Stokes and anti-Stokes Raman lines are not connected by time-reversal symmetry.
The nitrogen-vacancy center in diamond exhibits a zero-phonon transition art 637 nm that has been assigned as an A→E transition at a site with trigonal symmetry.1 Paramagnetic resonance studies revealed a triplet feature that was originally assigned to an excited state.2 More recently, it was argued that the triplet signal actually arose from the ground state,3,4We employ a new method of ultra-high-resolution nearly degenerate four-wave-mixing (NDFWM) spectroscopy, as well as persistent hole burning and electron paramagnetic resonance, to measure the metastable spin singlet excited-state lifetime and the g round-state spin-lattice relaxation time and also to prove that the ground state is indeed a triplet.