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Physical review. B, Condensed matter

Cross-relaxation dynamics of optically excitedN-Vcenters in diamond

Eric van Oort, Max Glasbeek

Published Oct 1, 1989• 54 citations
Publisher

Abstract

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.

DOI
10.1103/physrevb.40.6509

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Optics and Spectroscopy•Jun 1, 2003•42 citations
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.
In brown diamond, the long-lived emission at 2.818 eV is known to be due to a localized center in the photoexcited triplet state. In this paper, the emission intensity of the 2.818-eV center is studied as a function of the strength of an externally applied magnetic field. The cross-relaxation (CR) with N-V centers, P1{\mathit{P}}_{1}P1​ centers, and g=2 doublet spins of unknown origin, as well as level anticrossing (LAC), is found to affect the phosphorescence intensity for certain magnetic-field strengths and orientations. The CR dynamics is studied optically by measuring the phosphorescence transients induced by microwave pulses resonant with a spin transition in the photoexcited triplet state. Under CR conditions, spin population relaxation in the excited triplet state becomes faster and deviates from a single-exponential recovery behavior. The triplet-state kinetics including CR effects is studied theoretically, and a double-exponential recovery is predicted. Based on an analysis of the kinetics, the experimental microwave recovery transients under the CR conditions are satisfactorily simulated. The CR rate is estimated to be (5.0\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}102{10}^{2}102 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the CR with the N-V center and (8.0\ifmmode\pm\else\textpm\fi{}2.5)\ifmmode\times\else\texttimes\fi{}102{10}^{2}102 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the CR with the g=2 doublet species.
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.
A new phase-modulation technique for nonlinear laser spectroscopy is applied with a relative resolving power in the sub-Hz range to measure fundamental relaxation processes of the N-V center in diamond. Complementary EPR experiments versus temperature establish the spin character of the ground state in the absence of optical illumination and show that spin-lattice decay occurs via two-phonon processes involving the dominant vibrational mode. The combined results permit deduction of reliable fine-structure assignments for three states of the center and accurate values for zero-field intersystem crossing and spin-lattice relaxation rates from linewidths of individual resonances in the four-wave-mixing spectrum.
We present an experimental study of the longitudinal electron-spin relaxation of ensembles of negatively charged nitrogen-vacancy (NV−) centers in diamond. The measurements were performed with samples having different NV− concentrations and at different temperatures and magnetic fields. We found that the relaxation rate T1−1T_{1}^{-1}T1−1​ increases when transition frequencies in NV− centers with different orientations become degenerate and interpret this as cross-relaxation caused by dipole-dipole interaction.
The negatively charged nitrogen vacancy ($\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$) defect in diamond serves as a popular platform for manipulating and exploiting long-lived coherent spin dynamics at room temperature combined with optical readout. The required spin polarization of the spin triplet 3A2^{3}A_{2}3A2​ electronic ground state occurs through a cycle of repetitious optical photoexcitation events to the 3E^{3}E3E electronic excited state that is accompanied by a series of electronic transitions to a 1A1^{1}A_{1}1A1​ and a 1E^{1}E1E electronic state, and back to the 3A2^{3}A_{2}3A2​ state. The timescales of these transitions are largely known, yet for the relaxation time of the $^{1}A_{1}\ensuremath{\rightarrow}^{1}E$ infrared transition, which predominantly occurs via nonradiative recombination, only an upper limit of 1 ns could be determined so far. Here, we employ ultrafast transient absorption spectroscopy to probe the dynamics of the nonradiative relaxation from the 1A1^{1}A_{1}1A1​ to the 1E^{1}E1E state after photoexcitation of the 3E^{3}E3E state and find a relaxation time of 100 ps at a temperature of 78 K.
The spin-selective photokinetics of a single matrix-isolated impurity molecule with a triplet-triplet optical transition, T 0–T 1, is considered and the manifestations of the photokinetics in the fluorescence excitation spectra and intensity autocorrelation functions g (2)(τ) of the molecule undergoing narrow-band optical excitation is studied to resolve the fine structure of the transition. The rates of intersystem crossings (ISCs) T 1→S→T 0 to and from a nonradiating singlet state S of the molecule and the rate of population relaxation among the ground (T 0) state sublevels can be obtained from the spectra and g (2)(τ) using the analytical expressions obtained. New experiments on an individual NV defect center in nanocrystals of diamond, where, for the first time, the fine structure of its triplet-triplet 3 A-3 E zero-phonon optical transition (~637 nm) at 1.4 K was resolved, are interpreted. It is concluded that the rate of the ISC transition from the m S =0 sublevel of the excited 3 E state to the singlet 1 A state (~1 kHz) is much slower than the rates from the m S =±1 substates, while the rates of ISC transitions to different m S substates of the ground 3 A state are close to each other (~1 Hz). As a result, only the optical transition between m S =0 sublevels in the 3 A-3 E manifold contributes strongly to the fluorescence. The experimentally observed double-exponential decay of the g (2)(τ) function is explained by the two pathways available to the center for it to leave the S state: (i) the S→ T 0(m S )=0) transition and (ii) the S→T 0(m S =±1) transitions followed by the slow spin-lattice relaxation T 0(m S =±1)→T 0(m S =0) (rate ~0.1 Hz). The work is important for studies where the NV center is used as a single photon source or for quantum information processing.