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Physical review. B, Condensed matter

Line shift of two-photon transitions of Eu2+ in CaF2 as a function of temperature

Liderio Citrangulo Ioriatti Junior, Jarbas Caiado de Castro, Luiz A. O. Nunes, Sérgio Carlos Zílio

Published Oct 15, 1988• 1 citation
Publisher

Abstract

Line shifts of two-photon $^{8}\mathrm{S}_{7/2}$${\ensuremath{\rightarrow}}^{6}$${\mathrm{P}}_{7/2}$ transitions in ${\mathrm{Eu}}^{2+}$-doped ${\mathrm{CaF}}_{2}$ are studied as a function of temperature. They show a linear dependence on the occupation number of LO phonons (${n}_{\mathrm{LO})}$ of the ${\mathrm{CaF}}_{2}$ lattice. The proportionality constant agrees better than 10% with the theoretical value found previously. The role played by LO and TO phonons in the line shift and broadening mechanisms is discussed.

DOI
10.1103/physrevb.38.7839

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Line shifts of two-photon ^{8}\mathrm{S}_{7/2}$${\ensuremath{\rightarrow}}^{6}$${\mathrm{P}}_{7/2} transitions in Eu2+{\mathrm{Eu}}^{2+}Eu2+-doped CaF2{\mathrm{CaF}}_{2}CaF2​ are studied as a function of temperature. They show a linear dependence on the occupation number of LO phonons (nLO){n}_{\mathrm{LO})}nLO)​ of the CaF2{\mathrm{CaF}}_{2}CaF2​ lattice. The proportionality constant agrees better than 10% with the theoretical value found previously. The role played by LO and TO phonons in the line shift and broadening mechanisms is discussed.
Two-photon absorption linewidths of 8S7/2^{8}\mathrm{S}_{7/2}8S7/2​${\ensuremath{\rightarrow}}^{6}$P7/2{\mathrm{P}}_{7/2}P7/2​ transitions in Eu2+{\mathrm{Eu}}^{2+}Eu2+-doped CaF2{\mathrm{CaF}}_{2}CaF2​ are studied as a function of the temperature. Linewidths are shown to have a linear dependence on the occupation number of LO phonons (nLO{n}_{\mathrm{LO}}nLO​) of the CaF2{\mathrm{CaF}}_{2}CaF2​ lattice. This result is explained by means of the different polarizabilities of the ground and the excited states, which give rise to elastic phonon scattering. By using the polarizability of Eu2+{\mathrm{Eu}}^{2+}Eu2+ in EuF2{\mathrm{EuF}}_{2}EuF2​ we predict a slope for the linewidth \ensuremath{\Gamma}(T) versus nLO(T{n}_{\mathrm{LO}(\mathrm{T}}nLO(T​) curve, which is in fairly good agreement with the experimental data. The linewidth at T close to zero is ascribed to inelastic phonon scattering which causes the 4f7{f}^{7}f7-4${f}^{6}$5d configuration mixing. The theory also predicts a line shift proportional to the occupation number.
Intraconfiguration ${4f}^{7}\ensuremath{\rightarrow}{4f}^{7}$ transitions of Eu2+{\mathrm{Eu}}^{2+}Eu2+ in KMgF3{\mathrm{KMgF}}_{3}KMgF3​ have been investigated by two-photon excitation (TPE) spectroscopy at 10 K. Emission bands due to the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{P}_{7/2}}}^{8}{S}_{7/2}$ and ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{I}_{7/2}}}^{8}{S}_{7/2}$ transitions are obtained by two-photon excitation into the 6IJ{}^{6}{I}_{J}6IJ​ (J=7/2,9/2,17/2,11/2,15/2,13/2)(J=7/2,9/2,17/2,11/2,15/2,13/2)(J=7/2,9/2,17/2,11/2,15/2,13/2) and 6DJ{}^{6}{D}_{J}6DJ​ (J=1/2,7/2,3/2,5/2)(J=1/2,7/2,3/2,5/2)(J=1/2,7/2,3/2,5/2) states. The TPE spectrum of the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{I}_{7/2}}}^{8}{S}_{7/2}$ emission is observed to be different from that of the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{P}_{7/2}}}^{8}{S}_{7/2}$ emission. This is explained by the relative positions of the lowest crystal-field component of the 6I7/2{}^{6}{I}_{7/2}6I7/2​ level and the lowest level of the 4f65d{4f}^{6}5d4f65d configuration for cubic-site and non-cubic-site Eu2+{\mathrm{Eu}}^{2+}Eu2+ ions. The observed TPE spectra are satisfactorily interpreted by a suggestion that the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{I}_{7/2}}}^{8}{S}_{7/2}$ emission is produced by only cubic-site Eu2+,{\mathrm{Eu}}^{2+},Eu2+, while the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{P}_{7/2}}}^{8}{S}_{7/2}$ emission is produced by both cubic-site Eu2+{\mathrm{Eu}}^{2+}Eu2+ and non-cubic-site Eu2+.{\mathrm{Eu}}^{2+}.Eu2+.
Abstract The technique of two‐photon‐absorption spectroscopy with polarized laser‐light is applied to localized excitations of 4f‐electrons in crystals. The relevant selection rules are discussed. A method is outlined for the unique determination of the relative magnitude of partial transition‐probabilities with different types of symmetry using linear and circular polarization of the incident light. The anisotropy of the two‐photon absorption has been measured in single crystals of cubic CaF2:Eu2+ at five different wavenumbers. The relative intensities of transitions belonging to the three different types of symmetry active in two‐photon absorption are constant in the range of the e‐band (26,000 to 29,000 cm−1) but differ from these values at 37,700 cm−1 in the range between the e‐ and the t‐band. This result is discussed in the frame of existing interpretations of the 4f65d‐configuration of Eu2+ in CaF2.
It is demonstrated that persistent hole burning in the zero-phonon line of Eu2+ in SLs of CaF2:Eu/CdF2 with uniformly doped CaF2 layers is mainly due to tunneling of electrons from the 4f65d excited state of Eu2 to the conduction band of adjacent CdF2 layer. However in selectively doped suoerlattices (SLs), the two-step photoionization process dominates in the formation of spectral holes. Two mechanisms of spectral hole broadening were discussed. These are the Stark-shifting of the Eu2 f-d transition energy induced by internal electric field and phonon assisted transitions.
The linewidths and line positions of 4I15/2 to 4I13/2 absorption transitions of trivalent erbium in CsCdBr₃ and CaF₂ were measured as a function of temperature from approximately 10 K to 300 K. By comparing the temperature dependence of these transitions with theoretical models of electron-phonon coupling the primary mechanism involved were determined. For Er3+ doped CaF₂ the Raman scattering processes dominated the line broadening, although the single-phonon direct processes were significant enough to drastically alter the values of the Raman scattering electron-phonon coupling constant. In comparison, the Er³⁺ doped CsCdBr₃ had negligible Raman scattering contributing to the line broadening. This is likely due to the exceedingly low Debye temperature and consequently low number of phonons, reducing the likelihood of two-phonon processes relative to single-phonon processes. The results were then analysed in terms of the bond length with the ligands and compared with other studies showing that as the bond length gets shorter the electron-phonon coupling constant associated with Raman scattering is expected to get smaller. To explain the line shifts in CaF₂:Er³⁺ it is necessary to treat the Debye temperature as a parameter. This indicates that the phonon modes causing the line shifts are different from those causing the line broadening, and in most of the cases are vibronic processes rather than Raman processes. In the case of CsCdBr₃:Er³⁺ there is very little difference between the fits using the fixed Debye temperature and a varied Debye temperature in over half of the cases examined. Given the distribution of phonon modes, it is likely that this small difference arises because combinations of these processes are of similar intensity, meaning that a combination of Raman and optical phonon modes are likely causing the line shift.
We have measured the temperature dependence of the linewidths and positions for a number of transitions in the spectrum of LaF3:Nd3+. As the temperature increases, all lines broaden because of relaxation and Raman scattering of phonons. Relative to the ground state, all observed levels shift to lower energies as the temperature increases. Most lines show the usual shifts to the red. All these effects are qualitatively attributable to lattice vibrations. In general, the level shifts increase for the higher levels so that, ordinarily, line shifts are to the red with increasing temperatures. However, because of irregularities of the magnitudes of the level shifts, a few lines show shifts to the blue with increasing temperature, over some temperature ranges.