R. Demirbilek

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.

Optical Spectra, Properties and First Principles Computations of Ba(Fe, Nb)O3 and Pb(Fe, Nb)O3

Optical absorption in the IR region has been recorded and first-principles computations have been done for some Fe contining perovskites. The IR absorption reveals a broad peak at about 11000 cm−1. First principles computations established that BaFe1/2 Nb1/2O3 is not ferroelectric, but PbFe1/2Nb1/2O3 does have a ferroelectric instability. These data confirm that the large dielectric permittivity found in BaFe1/2Nb1/2O3 is not due to a ferroelectric phase transition but rather because of extrinsic effects.

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 .

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.

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.

Investigation of Far Infrared Transitions of Ce3+ in Congruent SBN Crystals

Three relatively sharp FIR absorption bands of Ce3+ in Strontium Barium Niobate (SBN) are observed in the 2000–3000 cm−1 region. A subband structure of these bands indicates three slightly inequivalent positions of the Ce3+ in the SBN crystal lattice. The concentration and temperature dependence of these bands is investigated.

Doping-dependent properties in photorefractive congruent SrxBa1-xNb2O6:Ce, Cr crystals

The photorefractive properties of Sr x Ba1-x Nb2O6 (SBN, congruent composition x = 0.61) can be enhanced by doping with suitable polyvalent ions like Ce or Cr. We report on optical absorption measurements in the UV-FIR region and on dielectric measurements in the temperature range 2–400 K as a function of the dopant concentration. The addition of a third doping component, photorefractive not active Li-ions, did not raise the ferroelectric phase transition temperature T c .