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Optics Letters

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

Stephen Colby Rand

Published Mar 1, 1986• 11 citations

Abstract

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.

DOI: 10.1364/ol.11.000135

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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.

We report the observation of cw four-wave mixing and phase conjugation in the UV on the N3 color-center resonance in diamond. Using a jitter-free technique, the nearly degenerate four-wave mixing spectrum has been measured and the ultranarrow, 250-Hz-wide resonance analyzed to understand the metastable-state decay process responsible for the low quantum efficiency and large nonlinear susceptibility of this aggregated nitrogen structure. The results establish the existence and lifetime of a new excited state of the defect.

Luminescence and absorption spectra of LiF crystals with F2− color centers (CCs) were studied in the 14–300 K temperature range. The Huang–Rhys factors measured from absorption and emission spectra were equal in magnitude and calculated to be 3.2 and 2.72 at 77 and 300 K, respectively. It was shown that below 40 K inhomogeneous broadening of the zero phonon line of the F2− CC prevails over homogeneous broadening. Inhomogeneous broadening of the zero phonon line was observed to vary from 5 to 15 cm−1 for different crystal preparations. Persistent spectral hole burning stable at room temperature was realized in LiF:F2− crystals.

We present results showing narrow resonant behavior in a collisionless four-wave mixing interaction in atomic sodium. In a two-level system the excited state radiatively decays to the ground state at rate ${\ensuremath{\gamma}}_{\mathrm{sp}}$, and the two states decay to the reservoir at ${\ensuremath{\gamma}}_{1}$ and ${\ensuremath{\gamma}}_{2}$. If ${\ensuremath{\gamma}}_{1}$ and ${\ensuremath{\gamma}}_{2}$ equal the transit time, the resonance width is determined by the total decay rate out of state 2. In the presence of additional excited-state decay to another state, a narrow resonance with width ${\ensuremath{\gamma}}_{1}$ is oberved as a result of the effect of the additional excited-state decay on the magnitude of the ground-state probability amplitude.

Two-frequency four-wave mixing in a dye is demonstrated, in which degenerate four-wave mixing is performed simultaneously within the absorption and emission frequency spectra, with two separate sets of beams. The coupling between the two bands gives rise to backward output waves at both frequencies, which are proportional to the phase conjugate of a single probe which can be at either of the two frequencies. Forward-propagating waves which are not conjugated are also generated at both frequencies. The frequency shifted outputs are tunable over the bandwidth of the spectrum in which they lie. A theoretical analysis is performed which is based on a four-level system model of the ground and first excited singlet states in a dye. Resulting are coupled equations, which are solved to yield the efficiencies of the various outputs. Experiments give results in good agreement with the theory.