A. Petraru

Resonant electron-phonon coupling of Yb3+ in YAG

Abstract An analysis of Yb3+ spectra in YAG in terms of the splitting of the zero-phonon lines in the case of near resonance between Stark levels and sharp peaks in the phonon spectrum is presented. The theoretical relations allow estimation of the position of the electronic level and vibronic coupling strength from the intensity ratio and spectral shifts. A new energy level scheme for the 2F5/2 Yb3+ multiplet in YAG is proposed.

ELECTRON-PHONON COUPLING FOR HEAVY RE3+ IONS IN CRYSTALS

Abstract The near resonant effects of electron-phonon coupling in the optical spectra of rare earth (RE) ions are analysed. A theoretical treatment that could describe effects such as splittings or temperature shifts and broadening of zero-phonon lines is presented. The main factors that determine the magnitudes of the near resonant effects are the strength of electron–phonon coupling and the peaks in the phonon density in the resonance region. Experimental data refer to heavy RE 3+ ions, i.e. Tm 3+ and Yb 3+ in Y 3 Al 5 O 12 (YAG), known as important laser systems. The fitting of experimental data with theory allows the identification of the electronic levels with a good approximation. The obtained data concerning the strength of the electron–phonon coupling show the tendency of increasing from Tm 3+ to Yb 3+ .

Inhomogeneous broadening by point defects of RE3+ optical lines in YAG

Most of the RE3+ optical lines in YAG, at low temperature and concentrations, present two interesting features: a multisite structure and Lorenz lineshape of the inhomogeneous broadened lines. The satellite structure of RE3+ ions optical spectra in high temperature YAG present a group of P satellite lines of almost equal intensity that was assigned to nonstoichiometric occupation of Al3+ octahedral sites (a) by Y3+. Up to now, no reliable explanation of the anisotropic effects of Y3+(a) on RE3+ spectra was given. In this paper an attempt is made to explain the multisite structure and lineshapes in terms of the strain field (induced by point defects) theory. The model is illustrated for the absorption line 4I15/2 implies 4S3/2 of Er3+ (1.at.%) doped in YAG.

Spectroscopy and energy transfer characteristics of Nd3+ in CNGG

The garnets with disordered structure such as calcium niobium gallium garnet (CNGG) doped with Nd3+ are interesting materials for diode pumping. Spectroscopic investigations on Nd3+:CNGG have revealed a multisite structure and this is reflected in laser emission characteristics. The purpose of this paper is to present several new spectral aspects, a modeling for the main nonequivalent centers, energy transfer characteristics and an attempt to connect the laser characteristics to centers structure. Samples were grown by Czochralski method with Nd3+ from 0.1 to 5 at.% and the high resolution spectral and decay measurements were performed at 10 K. Similar considerations were made in the case of Nd3+ doped in calcium lithium niobium gallium garnet (CLNGG).

Growth and spectral characteristics of Nd3+ in calcium lithium niobium gallium garnet (CLNGG) crystals

Laser crystals with disordered structure present a series of advantages for diode pumping or short pulse generation as compared with ordered crystal. Such a system proved to be calcium lithium niobium gallium garnet (CLNGG) doped with Nd3+. The papers present the growth and x-ray analysis of specially doped CLNGG. Very few spectroscopic data on this system have been previously published. The paper present low temperature absorption spectra, site selective excitation and lifetime measurements of Nd3+ in CLNGG. At least four distinct nonequivalent centers are observed, the spectral lines for every center presenting a disordered Gaussian shape. based on structural, spectral static and emission dynamics data an attempt of modeling the structure of these centers is proposed.

Upconversion violet emission in diluted Er:YAG crystals

In this paper we discuss the upconversion processes responsible for violet luminescence from the Erbium levels 2P3/2 and 2H29/2 in diluted Er:YAG crystals, excited with pulsed as well with cw lasers. Besides the general need for new spectroscopic data concerning laser materials, the interest for this study is generated by the fact that energy levels of very different quantum efficiencies can produce, function of the pumping conditions, fluorescence spectra of comparable intensities or even with reversed intensity ratio.

Optical phonon effects on linewidth of several laser active ions in YAG

An analysis of low temperature lineshapes for various absorption lines of several RE3+ ions from the beginning Nd3+ and end of lanthanide series Er3+, Tm3+ embedded in the same crystal - YAG known as important laser systems for 1-3 micrometers emission is presented. New aspects of the dependence of electron-phonon coupling on RE3+ ions and matrix are observed from the analysis of many transitions of these ions at low temperature. These data show that eh electron-phonon coupling presents a symmetric behavior in the lanthanide series, larger toward beginning and end. With only very few exceptions, less than 2 percent, the broadenings or splittings of zero-phonon liens could be connected to optical Raman phonons of undoped lattice. That means that the parity of 4fn states could still be considered a good parent quantum number, even if the local symmetry has no inversion, and the near resonant coupling involves the even part of the electron-phonon coupling operator. Besides, the one phono relaxation involve the undoped lattice phonons. For ions with small spin-orbit coupling, the vibronic contribution to the line intensity given by ir phonons through a Van Vleck mechanism could be important for spin forbidden transitions.

Spectroscopy of Yb3+codoped Pr3+:YAG laser crystals

The sensitization with Yb3+ of other rare earth ions emission is intensively used in the last years for increasing excitation efficiency of infrared (diode laser) pumped solid state lasers. It proved as a good sensitizer for Pr3+ emission in various matrices. The spectral investigations for the elucidation of the nature of multisite structure of Pr3+ in YAG and of the Yb3+ co-doping on Pr3+:YAG are presented in this paper. In Pr3+:YAG two types of multisites have been observed: perturbed Pr3+ sites by nonstoichiometric defects and strongly coupled Pr3+ - Pr3+ near neighbor pairs. Pr3+ spectra in Yb3+, Pr3+:YAG show new lines assigned to near neighbor Pr3+ - Yb3+ pairs and Pr3+ - Yb3+ - Yb3+ triads.

Energy transfer processes in Cr3+, Nd3+:YAG

The effects of co-doping with Cr3+ on the absorption and emission characteristics of double doped Cr3+, Nd3+:YAG (yttrium aluminum garnet) are analyzed. The multisite structure due to co-doping (samples with a large range of Cr3+ content were used) is separated from that in single doped crystals. At least seven new lines have been separated in high resolution spectra and a tentative assignment in terms of structural data is proposed. These data suggest that the energy transfer data should be analyzed in terms of inhomogeneous system. However, the measurements indicate strong migration effects. The global donor and acceptor data at 300 K show a linear dependence on donor concentration, but the decays cannot be described in terms of published theoretical models.