Persistent vibrational spectral hole burning and dephasing properties of orientationally aligned CN- defects in cesium halide crystals

Persistent spectral hole burning was used to study the dephasing mechanism of vibrationally excited ${\mathrm{K}}^{+}$:${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ pair defects in cesium chloride crystals. The ${\mathrm{K}}^{+}$:${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ pair defects are formed by statistical association of ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{K}}^{+}$ doping impurities. They occur in two nonequivalent oppositely oriented 〈111〉 configurations (${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$:${\mathrm{K}}^{+}$ or ${\mathrm{NC}}^{\mathrm{\ensuremath{-}}}$:${\mathrm{K}}^{+}$) with corresponding spectrally separated transitions. Continuous optical excitation of either configuration in its second-harmonic transition (near 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) with a narrow-linewidth color-center laser leads to 180\ifmmode^\circ\else\textdegree\fi{} ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ flips in spectrally selected ${\mathrm{K}}^{+}$:${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ defects. This produces, with efficiencies in the ${10}^{\mathrm{\ensuremath{-}}5}$ range, a spectral hole in the laser-excited transition and a corresponding (slightly broadened) antihole in the other transition.The spectral holes were burned, and both the holes and antiholes were probed with the same single-mode-tunable color-center laser. Measurements of the holewidth under variation of defect concentration and crystal temperature reveal that the excited-state dephasing of the ${\mathrm{K}}^{+}$:${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ pair defects is strongly influenced by the motional behavior (tunneling, hindered rotation) of the isolated, i.e., not ${\mathrm{K}}^{+}$-associated, ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ molecules. The low-temperature (T=5 K) residual holewidth (as low as 20 MHz) is attributed to an elastic dipole-dipole interaction between the excited ${\mathrm{K}}^{+}$:${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ defect and the isolated ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ molecules. We discuss this mechanism in an elastic-continuum model where the isolated ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ molecules, which tunnel rapidly at this temperature between their eight 〈111〉 equilibrium orientations, are the source of fluctuating strain fields in the crystal and cause ``dynamic strain broadening.'' The holewidth varies linearly with the concentration of the isolated ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ molecules, in agreement with the model. Under temperature increase the homogeneous linewidth broadens with a ${\mathit{T}}^{2}$ dependence up to the \ensuremath{\sim}40 K thermal-stability limit of the spectral holes. This ${\mathit{T}}^{2}$ behavior far below the Debye temperature (${\mathit{T}}_{\mathit{D}}$=165 K for CsCl) indicates a non-Debye effective phonon density of states with a large number of low-frequency modes participating in the dephasing of the ${\mathrm{K}}^{+}$:${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ defects. In fact, the experimental data are consistent with theory for Raman scattering of phonons peaking in their density of states in the 10-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range. Candidates for this excitation are hindered-rotor levels of isolated ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ molecules or possibly an internal low-frequency mode of the ${\mathrm{K}}^{+}$:${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ defect itself.