We applied a new phase-modulation technique for nonlinear laser spectroscopy with sub-Hz relative resolving power to measure fundamental relaxation processes of the N-V center in diamond. Complementary EPR experiments versus temperature establish the ground-state spin character 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. PACS numbers: 71.55.Ht, 42.65.Ma, 76.30.Mi, 78.50.Ec Nearly degenerate four-wave-mixing (NDFWM) spectroscopy with 1-Hz resolution based on acousto-optic frequency-modulation techniques has previously been applied to frequency-domain measurements of slow relaxation processes in impurity-doped solids [1] and pointdefect systems [2]. This coherent spectroscopy has proven very useful for precise measurements of decay processes too slow for eA'ective signal averaging in real time.Here we introduce a new optical technique for the performance of NDFWM with much higher resolution (10 mHz), and apply it in concert with double-cavity EPR experiments to resolve the controversy over the energylevel scheme of the N-V center in diamond and to study system dynamics.We provide evidence for a hypothesized metastable state and demonstrate that precise measurements of intersystem crossing and spin-lattice relaxation rates can be obtained from linewidth measurements of individual, ultranarrow resonances in the fourwave-mixing spectrum recorded with sub-Hz resolution.The N-V center is a product of irradiation and annealing processes in diamond crystals containing nitrogen [3] and exhibits a zero-phonon line at 637 nm assigned by uniaxial stress measurements[4] to an 2 E electricdipole transition at a site of trigonal symmetry. It consists of substitutional-nitrogen-vacancy pairs oriented along equivalent (111) directions, and exhibits triplet spin resonance which was first attributed to a metastable excited state [5] because it required optical illumination in the N-V absorption band. This conclusion and even the existence of a metastable state was challenged recently in a series of papers reporting hole burning [6], optically detected magnetic resonance [7], and Raman heterodyne experiments [g], which indirectly suggested that the triplet state occurs in the ground-state manifold rather than the metastable manifold.However, these experiments were performed with optical illumination of the centers, and are consistent with alternative explanations based either on absorption at 637 nm by a metastable excited triplet population or on a third possible energy-level structure of the center. To examine these possibilities, the current work was undertaken to determine fine-3A 1E 'A -'E= I» nm g (1a& b) I 'A (c) FIG. I. Possible energy level schemes of the N-...

Quantum-beat spectroscopy has been used to observe excited states of the N-V center in diamond. For the 1.945-eV optical transition, direct evidence is presented for the existence of GHz-scale fine structure, together with a much larger 46-cm Ϫ1 level splitting in the E state. An interference effect observed in transient fourwave-mixing response is explained with a polarization selection rule involving Zeeman coherence among magnetic sublevels. Also, detailed dephasing measurements versus temperature and wavelength have identified the decay mechanisms operative among the various states. A comparison of these results with ab initio calculations of excited electronic structure and interactions based on several multielectron models supports the conclusion that the N-V center is a neutral, two-electron center governed by a strong Jahn-Teller effect and weak spin-spin interactions.

New satellite features and antiholes in the persistent hole-burning spectrum of N-V centers in diamond, as well as their dependences on applied electric fields and frequency within the inhomogeneous absorption line, are reported. These results, together with reassignments of spin states of this center, permit an understanding of the origin of the satellite holes as well as of possible mechanisms for the persistent hole-burning phenomenon itself. In addition we report narrow optical interference fringes in heterodyne-detected spectra of persistent spectral holes in the N-V defect center in diamond and discuss a recent suggestion for high-resolution Ramseyfringe hole-burning spectroscopy of solids based on phase-separated fields.

A new method for two-beam coupling measurements based on acousto-optic modulation with subhertz tuning capability is applied to study excited-state dynamics of the nitrogen-vacancy center in diamond. Results yield fundamental decay constants and the nonlinear refractive index and are in good agreement with the calculated dual grating response to resonances in the microscopic lattice relaxation.In recent years two-beam coupling has become established as a precise method for investigating nonlinear refractive indices of saturable absorbers. 1Intersecting laser beams with finite detuning are typically used to generate slowly moving optical gratings in absorptive solids with phases that lead those of refractive-index changes caused by slow excited-state dynamics. Energy transfer between pump and probe waves then occurs, with detuningdependent gain. This method has been employed to study nonlinearities in Cr-doped laser materials 2 with simple impurity energy levels and hostdependent nonlinearities of relevance to high-power tunable Cr-laser performance.Here we introduce a method that furnishes twobeam coupling spectra versus detuning in seconds, and we use it to study simple impurity centers in diamond. With it, relaxation processes of the nitrogen-vacancy (N-V) color center, for which the energy-level structure was determined only recently, 3 are characterized. By extending two-beam coupling theory to incorporate two separate decay processes contributing to saturation, we obtain good agreement between complex changes in signal intensity and detuning, as well as consistency with earlier determinations of N-V relaxation times. We also report estimates of real and imaginary parts of the nonlinear index.The N-V center consists of a N-V complex in diamond, 4 one of many point defects 5 that need to be characterized carefully to evaluate potential applications' of this emerging wide-gap semiconductor. The electronic structure 3 and absorption spectrum of the center are shown in Fig. 1(a). As indicated by three-pulse photon echo data in Fig. 1(b), the 3 E state decays rapidly in 13.3 ± 0.2 ns, revealing that this decay channel cannot contribute to optical saturation at low intensities. 6 However, intersystem crossing from the 'A to the ground state (r = 265 ms) and the spin-lattice relaxation between the 3 A ground-state components (X = 1.17 ms) are extremely slow. 3 Hence in the N-V center we expect two physically distinct processes to contribute to low-intensity saturation behavior at wavelengths near the first resonance, unlike the singlesusceptibility due to intersystem crossing and spincomponent saturation in Cr-doped solids. Moreover, the response times of these contributions should have a one-to-one correspondence with resonances observed in nearly degenerate four-wave mixing (NDFWM), which probes the microscopic susceptibility directly. 3 To account for contributions from multiple components, we extended previous expressions' for pump and probe wave intensities I, and I2, respectively, as follows:(1)Here gR = /3n ...

A new method was used to fabricate nanometer-scale structures in Si for photoluminescence studies. Helium ions were implanted to form a dense subsurface layer of small cavities (1–16 nm diameter). Implanted specimens subjected to annealing in a variety of atmospheres yielded no detectable photoluminescence. However, implantation combined with electrochemical anodization produced a substantial blueshift relative to anodization alone. This blueshift is consistent with the quantum confinement model of photoluminescence in porous silicon.