F. Jaque

Comparative study of the singularity in the optical properties of congruent doped LiNbO3 crystals

Abstract A comparison of the optical properties such as optical absorption band edge, ordinary and extraordinary refractive indices, powder second harmonic generation and the non-linear coefficient, d33, of congruent LiNbO3 crystals doped with di-, tri- and hexa-valent cations is reported. It has been found that the referred parameters show a clear concentration singularity depending on the valency of the cation. A charge compensation mechanism is proposed to explain the role of the cation. The experimental data of no and ne are described by a generalised Sellmeier equation considering the different ions sites in the crystal.

Optical spectroscopy of Cr3+ ions in stoichiometric LiNbO3 crystals and co-doped with MgO

Abstract The optical spectroscopy of Cr 3+ ions in near stoichiometric LiNbO 3 crystals, ‘pure’ and co-doped with MgO, has been investigated. In the near stoichiometric LiNbO 3 :Cr (0.2%) optical absorption and emission spectra show the usual features of broad bands and narrow lines typical of congruent samples with the an additional of a narrow emission lines centred at 738xa0nm and has been ascribed to the zero-phonon line of the 4 T 2 → 4 A 2 transition. The concentration equilibrium between the two type of centres is strongly displaced towards the [Cr] Nb centre permitting thus to obtain with accuracy the parameters of the broad bands. The R-lines associated with the [Cr] Nb centre is only observable in low-temperature emission spectrum. The presence of Fano anti-resonance lines has been observed to be more pronounced in the near stoichiometric than in congruent samples.

Luminescence of the Cr3+ R-lines in pure and MgO co-doped near stoichiometric LiNbO3:Cr crystals

Abstract The photoluminescence property of pure and MgO co-doped near stoichiometric LiNbO3:Cr crystals has been investigated. It has been found that the presence of low Mg ion content in the crystals induces the coexistence of two types of Cr3+ sites; Cr3+ ion substituting Li+ and Nb5+ sites. In MgO co-doped samples, four narrow emission bands have been found and three of them are correlated with the broad absorption bands associated to the Cr3+ ions located in Nb5+ site ([Cr]Nb centres). The observation of the zero-phonon line at 13xa0540 cm−1 only in pure LiNbO3:Cr crystals of near stoichiometric composition suggests it could be used as a probe to monitor the composition of LiNbO3 crystals.

Nd3+—Cr3+ pairs in Gd3Sc2Ga3O12 garnet crystals

Abstract Several crystals of Gd 3 Sc 2 Ga 3 O 12 (GSGG) with different concentrations of Nd 3+ and Cr 3+ ions have been grown using the Czochralski technique. Optical absorption and site-selective luminescence spectra have been measured in the wavelength range 350–900 nm at low temperatures (15–77 K). These spectral reveal the presence of Nd 3+ ions in two different crystal field sites. Some Nd 3+ ions substitute for Gd 3+ ions in unit cells in which the central octahedral site is occupied by Sc 3+ ions. However, in a significant fraction of unit cells containing Nd 3+ the central octahedral site is occupied by Cr 3+ ions. These centres are labelled Nd 3+ (Sc 3+ and Nd 3+ (Cr 3+ ), respectively. The spectroscopic characteristics of these perturbed Nd 3+ centres are reported and their relevance to the enhanced Nd 3+ laser efficiency of Cr 3+ :Nd 3+ :GSGG relative to Nd 3+ :GSGG is discussed.

The role of the Mg2+ ions in Cr3+ spectroscopy for near-stoichiometric LiNbO3 crystals

The optical spectroscopy of Cr3+ ions doped into near-stoichiometric LiNbO3 crystals, pure and co-doped with MgO, has been investigated. In the near-stoichiometric LiNbO3:Cr(0.2 mol%):Mg(2 mol%) crystal, the optical spectra resemble those previously observed for congruent LiNbO3:Cr:MgO samples when the total MgO content exceeds the 4.6 mol% threshold. The coexistence of two types of Cr3+ centre ([Cr]Li and [Cr]Nb) characterized the optical and luminescence spectra of this sample. The concentration equilibrium between the two types of centre is strongly displaced towards the [Cr3+ ] Nb centre, permitting us to obtain with accuracy the parameters of the broad bands. The R-line associated with the [Cr]Nb centre is only observable in the low-temperature emission spectrum. The Fano anti-resonance lines present have been observed to be more pronounced for the near-stoichiometric samples than for congruent ones.

The effect of the ferroelectric domain walls in the scanning near field optical microscopy response of periodically poled Ba2NaNb5O15 and LiNbO3 crystals

A study of Ba(2)NaNb(5)O(15) and LiNbO(3) crystals with periodic ferroelectric domain structures using the scanning near field optical microscopy technique is reported. Optical contrast is observed in the regions of ferroelectric domain boundaries and it is analysed using beam propagation method modelling. This reveals that the optical contrast, a consequence of changes in the refractive index, is not due to variation of the waveguide-coupling efficiency, and supports the hypothesis that it is associated with the domain array, which is related to the size of the domain.

Dependence of the refractive indices in LiNbO3:Cr crystals doped with HfO2

The dependence of the refractive indices, absorption edge and optical damage in congruent HfO2-doped LiNbO3 crystal are reported. A small amount of Cr3+ ions (0.1 mol%) was introduced in the doped crystals as optical probe. The experimental data indicates that the extraordinary and ordinary indices depend on the Hf4+ ions concentration showing a singularly for a dopant level of ~ 2.7 mol%. The dependence of the absorption edge also shows a transition region at this Hf4+concentration. The distortion of the transmitted light through samples doped with different concentrations of Hf4+ ions indicates that an inhibition of optical damages is achieved at above 3 mol% of dopant. A charge compensation mechanism is proposed to explain the role of the Hf4+ ions for the observed experimental result.

Optical spectroscopy of Cr3+ ions in LiF single crystals

This paper reports on the luminescence spectrum, its excitation-wavelength dependence and decaytime of Cr3+:LiF crystals. In contrast to Al2O3 and MgO, where the Cr3+ luminescence at low temperature is due to the R-line and its vibronic sideband, luminescence from Cr3+:LiF is assigned to the broad-band 4T2→4A2 transition. A periodic structure in the vibronic sidebands of this transition occurs with energy separations of ca. 60 meV. The linear polarisation of the vibronic sidebands shows that their radiative transition takes place from the lowest excited level of the 4T2 state, i.e. 4B1, into the 4A2 ground state of Cr3+ ions in orthorhombic symmetry sites. The crystal-field parameters calculated from the experimental data, and luminescence decay-time of the two polarized configurations of the 4B1(4T2)→4A2 transition are also reported.

Optical properties of WO3 doped congruent LiNbO3 crystals

Abstract The dependence of the optical absorption band edge, ordinary and extraordinary refractive index, powder second harmonic generation and the non-linear coefficient, d 33 , of congruent LiNbO 3 crystals doped with WO 3 are reported. It has been found that the referred parameters show a clear singularity for a concentration in the range of 1.5–2%. In comparison with previous works on congruent LiNbO 3 crystals doped with divalent and trivalent ions a charge compensation mechanism is proposed to explain the role of the cation. The experimental data of n o and n e are described by a generalised Sellmeier equation considering the different ions sites in the sample.