Intersystem crossing of F_2 color centers studied by four-wave mixing spectroscopy

We report the first observation to our knowledge of continuous-wave four-wave mixing and phase conjugation using color centers. The resonance in γ-irradiated LiF at room temperature is shown to arise from saturated absorption involving intersystem crossing of F2 centers and has a width that reveals the excited-triplet to ground-state-singlet decay rate. This is confirmed by direct measurement of the nearly degenerate four-wave mixing spectrum in the visible spectral region using a jitter-free technique. The observed Lorentzian linewidth of 4.70 Hz (FWHM) is in excellent quantitative agreement with the independently measured phosphorescent decay rate of the lowest triplet levels and a calculation of the spectral response of this three-level spin system owing to decay of ground-state spatial hole burning. The linewidth is predicted to be strongly temperature dependent and indeed narrows to 2.1 Hz on cooling to only −25°C.