We have observed persistent spectral hole burning in a family of centers in CaF2:Pr3+:D− and SrF2:Pr3+:D− crystals in which the Pr3+ ion has multiple D−-ion near neighbors. The hole burning is caused by light-induced D- motion that leads to persistent changes in the local crystal field of the Pr3+ ion. Depending on the center and the temperature, side-hole or antihole structure is observed that has its origin in the second-order hyper-fine interaction. The variation in the hyperfine splittings between centers is consistent with the separation between the ground state and the first excited state.
Persistent spectral hole-burning is extensively studied in many materials. In inorganic materials doped with rare earth ions, we have mainly two mechanisms for hole-burning; one is optical pumping to hyperfine sublevels in rare earth ions, and the other is optically induced rearrangement of local structure around optical centers. In crystals we often observe the hole due to the former mechanism while the hole due to the latter is reported mainly in glasses. Up to now, there seems to be only little study about the correlation between the mechanism of hole-burning and sample-quality or defects. Recently we reported that the character of holes in Y2O3:Pr3+ depends on the quality of samples [1]. In this paper we study the origin of these holes in Y2O3 with a well-controlled manner. For this purpose, we applied hole-burning spectroscopy to various Y2O3:Pr3+(0.2mol%) samples doped with other metallic ions.
Optical hole burning has been observed in the 5941-A transition of Pr(3+) in a charge-compensated tetragonal site of CaF(2) . The Pr(3+) ground state is doubly degenerate and shows a large first-order hyperfine splitting, which is clearly resolved because of the very narrow inhomogeneous linewidth of 650 MHz. The hole burning involves a new mechanism, in which optically induced spin flips of neighboring nuclei (here (19)F) shift the optical transition frequency outside its homogeneous linewidth. This mechanism was confirmed by optical rf double resonance.
With a ring dye laser persistent spectral hole-burning was measured by fluorescence excitation spectroscopy in the zero-phonon lines of the 2llg ↔ 2llu transition in KI:S2 − and of the 3H4 → 3P0 transition of a defect center with three D− ions in SrF2:Pr3+:D−and also in the zero-phonon line at 462.5 nm of a color center in neutron-irradiated and annealed α-Al2O3. In all three lines holes with a width in the order of 1 GHz could be obtained at 1.5 K with single frequency laser. With broad-band laser in KI:S2 − and SrF2:Pr3+:D− spectral redistribution within the full range of inhomogeneous broadening could be observed which are explained by a photophysical model based on barrier crossing in asymmetric double-well-potentials. In the case of sapphire a photobleaching process is dominant which is effective also for nonselective excitation, and the holes are thermoresistant up to 300 K.
Persistent and time-resolved transient holes were burnt into the
1D2${\ensuremath{\leftarrow}}^{3}$
H4 transition of
Pr3+ and the
5D0${\ensuremath{\leftarrow}}^{7}$
F0 transition of
Eu3+ in silicate glasses. The holewidths were studied as a function of burning-fluence density, wavelength, and temperature between 0.4 and 12 K. Transient holes arising from optical pumping of ground-state nuclear hyperfine levels were found for both ions at T3 K. Their lifetime decreases with increasing temperature. They are accompanied by symmetrically placed antiholes. For
Eu3+, two pairs of antiholes were found which are attributed to the two europium isotopes,
151Eu and
153Eu. For
Pr3+, an additional mechanism leading to persistent hole burning dominates above 3 K. The holewidth follows a ${\mathit{T}}^{(1.3\ifmmode\pm\else\textpm\fi{}0.1)}$ dependence for
Pr3+ and a ${\mathit{T}}^{(1.0\ifmmode\pm\else\textpm\fi{}0.1)}$ dependence for
Eu3+. The temperature dependence is the same as that of the contribution of the two-level systems to the specific heat measured on the same samples and in the same temperature range. The quadrupole splittings of the
7F0 ground state and the crystal-field splittings of the
7F1 and
7F2 levels of
Eu3+ as a function of excitation wavelength indicate that the strength of the noncubic component of the crystal field varies monotonically for excitation across the
5D0${\ensuremath{\leftarrow}}^{7}$
F0 absorption band, falling to zero at the long-wavelength edge. The hole width shows a similar variation with excitation frequency. No influence of the experimental time scale on the hole width was detected for either sample between ${10}^{\mathrm{\ensuremath{-}}2}$ and
102 s.