Search

In ion-doped solids there is a nonlinear refractive index n_2 that is due to the polarizability difference Δα between excited and ground states, the so-called population-lens (PL) effect. In addition, the thermal-lens (TL) effect is particularly important in fluoride materials, owing to their low thermal conductivity. We performed time-resolved Z-scan and mode-mismatched TL measurements at λ=488 nm in SrAlF_5:Cr^+3. In this crystal the PL effect is faster than that of the TL, owing to its relatively short metastable lifetime (93 μs), and therefore we could temporally discriminate between these two contributions to the nonlinear refractive index. For the PL effect we measured n_2=(6.6+1.7i)×10^−11 cm^2/W^−1 and calculated Δα=3.1×10^−26 cm^3 and Δσ=?1.7×10^−20 cm^2. From the TL measurements we obtained the thermal diffusivity D=6.5?10^−3 cm^2 s^−1 and estimated the thermal conductivity K=1.7×10^−2 W cm^−1 K^−1 and ds/dT=−8.5?10^−7 K^−1.

We report how the photorefractive properties of cobalt-doped barium titanate (BaTiO_3) are influenced by deep and shallow traps. The light-induced dark decay of the space-charge field has two distinct time constants. The faster decay is due to the thermalization of charges from shallow traps, whereas the longer decay is that from deep traps formed when BaTiO_3 is doped with cobalt. This dopant substantially decreases the dark conductivity of BaTiO_3 and thus increases the dark storage time. The power coefficient of the sublinear intensity dependence of the photorefractive response time progressively increases with the cobalt-dopant concentration. Apparently cobalt doping of the deep level eliminates the effect from the shallow level in BaTiO_3. Although cobalt-doped BaTiO_3 has an increased effective trap density and thus an increased electrooptic gain, the response time and absorption are also increased, which thus decreases the photorefractive sensitivity.

Room-temperature tunable-laser action has been observed in Cr-doped SrAlF_5. The measured tuning range is 852–947 nm in a quasi-cw laser mode. The effect of nonequivalent Cr sites in SrAlF_5 is studied from the absorption and the excitation spectra. A broad absorption band peaked near 750 nm does not contribute to the Cr emission and appears to be associated with a nonradiative site.

Variably valent iron impurities have been suggested as the source of photorefractive charge carriers in BaTiO_3. High-purity BaTiO_3 crystals were grown with transition-metal impurity levels below the 10^16 cm^−3 baseline estimated for photorefractivity. Iron-doped crystals were grown with concentrations of 5, 50, 250, 500, 750, and 1000 parts in 10^6 of iron. Changes in iron valence monitored by optical absorption in the P_O2 range 1–10^−4 atm were found to be consistent with a defect-chemical model indicating Fe^3+ and Fe^4+ to be the stable valence states in this range. Photorefractive characterization of high-purity BaTiO_3 suggests that variably valent iron ions are not the dominant photorefractive species in these crystals, whereas the role of iron centers in doped crystals is complicated by large absorption losses.

Room-temperature polarized reflectivity measurements are reported for single-crystal La_2CuO_4 for the (E ⊥ ĉ) and (E || ĉ) polarizations, where ĉ denotes the tetragonal axis. A Kramers–Krönig analysis is performed to obtain the frequency-dependent dielectric functions. Frequencies for the seven zone-center IR-active phonons are obtained and the longitudinal-optic–transverse-optic splittings are used to evaluate the dynamic charge on the oxygen atoms. The reflectivity spectrum for the (E ⊥ ĉ) polarization is decidedly metallic and is fitted accordingly with an overdamped Drude model and a plasma frequency of 95 meV. In the high-frequency region of the spectrum, selection rules are observed for allowed and forbidden electronic transitions. The importance of this study to the interpretation of optical measurements on high-Tc, polycrystalline pellets of La_2CuO_4 and related materials is also discussed.

A series of cobalt-doped barium titanate (BaTiO_3) crystals were grown; these crystals have the highest beam-coupling gain reported to date, 7.9 cm^−1, measured by using the smallest electro-optic coefficient of BaTiO_3. The intensity dependence of the absorption and the gain indicate that there are at least two active photorefractive species. These samples show light-induced absorption and negative absorptive coupling, which is indicative of crystals with deep and shallow traps. However, doping with cobalt diminishes the intensity dependence of the electro-optic gain and increases the electro-optic and absorptive coupling, which indicates that cobalt is a deep-level photorefractive dopant. The higher gain is attributed to an increase of the total effective trap density and to an increase in the deep- and shallow-trap intensity-dependent factor η(I), with higher cobalt doping.

Energy-transfer processes (including upconversion) are quantitatively studied in a Tm:Yb:BaY_2F_8 laser crystal. Several energy-transfer, cross-relaxation, and upconversion parameters are determined. We prove that at excitation of the crystal via Yb ions, the Tm levels ^3F_4, ^3H_4, ^3F_3, 2, ^1G_4, and ^3P_0 are populated via Yb–Tm interaction, and the level ^1D_2 is likely populated owing to ^3F_3 Tm–^3F_3 Tm upconversion. The possibility of resonant excited-state absorption at 706 nm and resonant energy-transfer upconversion populating the level ^1D_2 are discussed. The possibility of avalanche pumping of Tm:Yb:BaY_2F_8 at 706 nm is shown.