J. Heber

Electronically resonant optical cross relaxation in YAG: Tb3+

Tb3+ ions in solids usually show a phonon-assisted optical cross relaxation between the levels 5D3 → 5D4 and 7F6 → 7F1,2. In the Y-Al-garnet (YAG) we encounter the very rare and interesting case of an electronically resonant process within the homogeneous widths of the involved crystal field levels. So direct phonon assistance is not needed. The time-resolved donor and acceptor fluorescences were analyzed by the Inokuti-Hirayama model. At low temperatures a dipole-quadrupole interaction was found to be responsible for the cross relaxation whereas at room temperature dipole-dipole interaction prevails. These findings are verified by the selection rules for the electronic transitions involved.

Interaction between excitons and rare-earth ions

Energy transfer between rare-earth (RE) ions and excitons in semiconductors has already been known for some time. In this paper we would like to direct your attention to the interaction between excitons and RE ions in ionic crystals as used, e.g. for laser applications. A suitable model substance to study these effects is RE-doped CsCdBr3. The reasons are: (i) the low phonon frequencies due to the heavy ion masses (ℏωmax<200 cm−1); (ii) metastable excitonic states in the visible spectral range; (iii) strong exciton–ion interaction due to covalent overlap of the wavefunctions. Due to the low phonon frequencies, multiphonon processes of the RE ions are reduced drastically and ion–exciton processes can be studied in more detail. The following processes were found: (i) exciton-mediated nonradiative relaxations. These processes can bridge much larger energy gaps than direct multiphonon relaxation and are more efficient. The reason is that the nonradiative multiphonon relaxation of the RE ion is dominated by a fast multistep one-phonon relaxation of the exciton by energy transfer; (ii) exciton-mediated quantum upconversion. This effect is based on a cooperative energy transfer from two excited RE ions to an exciton and a subsequent back transfer to a single ion. This process is much faster than upconversion by phonon-assisted cross relaxation between two excited RE ions; (iii) exciton-induced changes in the crystal-field splitting of RE ions. Energy levels of RE ions in resonance with the excitons show crystal-field splittings which cannot be described by the parameters suitable for the other levels. We propose an increased covalent overlap between the wavefunction of the RE ion and of the exciton-forming ligands due to hybridization as an explanation for this effect.

Electronic states and interactions in pure and rare-earth doped CsCdBr3

Abstract CsCdBr3 adopts the pseudo-one-dimensional CsNiCl3 structure. It consists of chains of face-sharing [CdBr6]4- -octahedra separated by parallel chains of Cs+ ions. Numerical calculations show that it possesses a narrow isolated conduction band of Cd s-electrons. The well-known symmetry of this band allows the analysis of its optical transitions from and to the different branches of the valence band by symmetry selection rules. The symmetries of the branches can be determined by means of the corresponding LCAO molecular orbitals. The analysis of the optical and near UV spectra shows that the “optical” band gap exceeds significantly that of energy. Thus there is a number of optically inactive or “silent” electronic states within the optical band gap. These states are responsible for a number of surprising effects at dopant rare-earth ions as e.g. fast nonradiative transitions, fast quantum upconversion, and resonantly enhanced crystal-field splittings.

Band structure and excitons in CsCdBr3

Abstract CsCdBr 3 has a very unique electronic band structure [J. Kubler, private communication]. The conduction band is split up into two parts: a lower isolated and very narrow band (Δ E ∼4000 cm −1 ) and higher lying ones. Just below the lowest conduction band we observe an absorption band of excitons localized at lattice defects. Both of them, the low-lying conduction band as well as the localized excitons, play an important role in the fast and very efficient energy transfer within the crystal. This can be shown by analyzing the decay dynamics of the fluorescence of the dopant RE-ions and of the localized and lattice excitons (excitons of the perfect lattice). Also, excitation into either the excitons or the RE-ion clearly shows an energy transfer back and forth between both of them. We deem this mechanism to be the most important one for the observed upconversion processes in CsCdBr 3 .

DIRECT OBSERVATION OF MIGRATION OF OPTICAL EXCITATION ENERGY IN YAG : TB3+

Abstract In the Y-Al-garnet (YAG) the rare-earth ions can occupy six different sites which are crystallographically equivalent but have different orientations of the local symmetry axes relative to the cubic unit cell. Knowing the symmetries of the crystal field states one can excite ions on selected sites only by a proper choice of polarization and direction of the exciting light beam. Energy migration can then be observed directly by measuring the time-resolved fluorescence originating from the not-directly excited ions. The experimental results were analyzed by the Inokuti-Hirayama model of energy transfer extended by Huber for back transfer. At low temperatures the energy migration probability shows an activation energy of about 67 cm -1 . Above 80 K the probability decreases again.

Submillimeter electron-nuclear excitation spectra in CsCdBr3:Ln3+ (Ln=Tm, Ho) crystals

The ESR spectra of single and pair impurity centers of thulium and holmium ions in CsCdBr3:Tm3+ and CsCdBr3:Ho3+ crystals are measured in the frequency range 160-400 GHz. Analysis of the characteristic features of the hyperfine structure of the ESR lines and analysis of the variations in the spectra as a function of the temperature and external magnetic field shows that the Ln3+ ions substitute for Cd2+ ions and predominantly form symmetric pair centers of the type Ln3+-(vacancy at a neighboring Cd2+ site)-Ln3+. The ESR spectra of CsCdBr3: Ln3+ crystals are used to make a positive identification of the optical spectra of selective laser excitation.

Up-conversion in Pr3+ : CsCdBr3

Abstract For reasons of charge compensation only two definite types of Pr 3+ pairs are formed in Pr 3+ : CsCdBr 3 independent of concentration. Only the pair with the Pr 3+ ions as direct neighbors shows efficient fast up-conversion. The dynamics of this up-conversion process is studied by time-resolved spectroscopy.

The rare-earth centers in CsCdBr3

Abstract CsCdBr 3 crystallizes in the quasi-linear CsNiCl 3 structure. It possesses linear chains of [CdBr 6 ] 4− octahedra separated by parallel chains of Cs + . Trivalent rare-earth (RE) ions substitute for the divalent Cd ions. The need for charge compensation leads to a number of RE centers, the most prominent of which is the symmetric pair center RE 3+ –(Cd vacancy)–RE 3+ . Madelung calculations were performed for a number of different centers giving a ranking for their chance of realization. Experimental evidence from optical spectroscopy and submillimeter ESR is given for the most likely centers. The symmetric pair center is of special interest for cooperative phenomena of RE 3+ ions. This includes direct ion–ion interactions and interactions with or via the electronic excitations of the host lattice. Some experiments are discussed.

Absence or existence of intrinsic optical bistability of coupled ion pairs in solids

Abstract There has been a number of papers [2] , [3] , [4] , [8] published which deal with the prediction of intrinsic optical bistability of coupled ion pairs in a coherent radiation field. This coupling can be different, either electromagnetic near-field coupling of the transition dipoles or other more general interactions. In the last year Malyshev et al. published a paper in which the authors deny the existence of this bistability. In this paper the reason is discussed for this discrepancy, which seems to be due to different approaches of introducing the external phase destroying perturbations. The bistability is based on an inversion-dependent resonance frequency of coupled ions which can generally be important for phase-sensitive effects.

Interaction Between Pr3☎ Pairs and Excitons in CsCdBr3

CsCdBr3 consists of linear chains of [CdBr6]4- octahedra separated by Cs☎ ions. The excited states of the covalently bound [CdBr6]4- octahedra form the lowest excitonic states of the crystal lattice. The trivalent Pr ions substitute for the divalent Cd ions. For reasons of charge compensation in the linear Cd2☎ chains, pair centers of the form Pr3☎-(Cd vacancy)-Pr3☎ are predominantly formed. In CsCdBr3 the optical band gap is wider than the electronic one. This allows to study the interaction between Pr3☎ ions and excitons by means of optical spectroscopy. The interaction leads to quantum up- and down-conversion generating one photon from two lower ones in energy and vice versa. Related experiments are discussed.