J. F. H. Nicholls

Characterization of Cr3+ centres in LiNbO3 using fluorescence line narrowing

The optical spectroscopic properties of Cr3+ in LiNbO3 have been investigated using optical absorption, fluorescence, fluorescence line narrowing (FLN), selective excitation and radiative lifetime measurements. The present results are compared with data obtained using other techniques including electron spin resonance (ESR) and electron nuclear double resonance (ENDOR), and shown to be consistent with there being five Cr3+ centres. The predominant centre is due to substitution at the Li+ site, which provides the weak crystal field in which Cr3+ ions emit into a broad 4T2 to 4A2 band. Using FLN, two centres experiencing a stronger crystal field are shown to be due to substitution at this same site, probably perturbed by the location of Nb5+ antisites and Nb5+ vacancies in the next-nearest-neighbour cation shell. The addition of Mg2+ ions to LiNbO3 appears not to create new Cr3+ complexes: rather it modifies the nature of the disorder, thereby changing the relative concentrations of the different centres. To this extent it becomes possible to identify two other lightly occupied Cr3+ centres where substitution is at the Nb5+ site. This total of five Cr3+ centres in LiNbO3 gives a consistent interpretation of all the available spectroscopic evidence, including ESR and ENDOR, while pointing to the difficulties of theoretical work in sites where the differences in the energy levels of Cr3+ ions are too small to be distinguished by the available models.

Accurate determination of the indices of refraction of nonlinear optical crystals.

Two techniques are described for measurement of the indices of refraction of novel optical crystals. First a modification is presented of the traditional measurement of the angle of minimum deviation of a refracted beam and applied to lead tetraborate (PTB). The second method applies polarized Michelson interferometry to the principal refractive indices of RbNbB(2)O(6) (RNB). Both PTB and RNB are nonlinear optical crystals that have been recently synthesized and grown for the first time.

The nonlinear optical properties of the XYB2O6 family of compounds

Abstract The mixed niobate–borate crystals KNbB2O6 and RbNbB2O6 are monoclinic with space group Pna21 and point group m. Some of their properties are favourable for applications in non-linear optics including exceptional resistance to laser damage, UV cut-off at wavelengths below 290 nm and large d-coefficients. Although they have refractive indices that are similar to KTiPO5 their birefringences are smaller than anticipated for structures containing planar borate rings and distorted NbO6 octahedra, leading to large acceptance angles for second harmonic generation (SHG). Type 1 phase matching has been observed at 532 nm in RbNbB2O6. The birefringence of KNbB2O6 is smaller than that of RbNbB2O6, and KNbB2O6 is not phase-matchable at this wavelength. Nevertheless, it is possible to tune the birefringence of KNbB2O6 by replacing some K+ ions by the larger Rb+ ions so that non-critical phase matching may be possible at room temperature.

Site selective upconversion in Nd3+:KLiYF5

Abstract Site selective upconversion of Nd3+:KLiYF5 is reported in dilute (0.2% molar) and concentrated (3.4% molar) crystals. On the basis of site selective excitation spectroscopy, the mechanism for upconversion is determined as being due to the sequential absorption of two photons. The excitation spectrum is shown to be composed of features resonant with each of the two stages of excitation. The dilute sample shows site selective upconversion fluorescence throughout the excitation range. For the more concentrated sample, energy transfer amongst the lower states equalises the population of the sites when shelved in the metastable 4 F 3 2 state so that site selectivity only occurs during the second absorption stage.

Spectroscopy and lasing performance of Nd:KLiYF5 and KLiGdF5

Polarized spectroscopic studies are reported of Nd3+ doped KLiYF5 and Nd3+ doped KLiGdF5--a new anisotropic mixed fluoride. Emission in the 4F3/2 yields 4I11/2 channel is strongly polarized. Explanations of the spectroscopic behavior are presented in terms of the local rare earth environment. Lifetime measurements of the 4F3/2 state of Nd3+ ions in these monoclinic hosts and the dependence of fluorescence lifetime on dopant ion concentration are reported. Low threshold pulsed laser action was realized in both hosts with high efficiency, pumping with a long pulse Cr:LiSAF laser tuned to 797 nm.

On the lack of influence of disorder in Cr3+-doped LiSr0.8Ca0.2AlF6

Optical and electron spin resonance spectroscopies have been used to investigate the thermal stability of lasers based on Cr3+:LiSr0.8Ca0.2AlF6, a material in which the onset of critically inhibiting nonradiative decay at temperatures below 300 K might be expected based on results from other Cr3+-activated disordered gain media. The influence of disorder on the Cr3+ environment is shown to be very small in LiSr0.8Ca0.2AlF6 so that the 4T2→4A2 fluorescence transition is not broadened relative to that in LiSrAlF6 and the onset of significant nonradiative decay is also held above room temperature.

KNB (KNbB2O6) and its isomorphs: a new family of nonlinear optical materials

The new crystal KNB (KNbB2O6) has been discovered and the XYB2O6 family of isomorphs (where X equals Cs, Rb, K, Y equals Nb or Ta) has been identified as a potential structure for non-linear optical applications. KNB was grown using a resistance heated TSSG scheme in an excess of B2O3 and K2O. It was found that KNB has orthorhombic symmetry with space group Pna21, unit cell parameters a equals 31.236(4), b equals 7.315(2), c equals 9.210(4) angstroms, and is isostructural with RNB (RbNbB2O6), which was also grown. The non-linear properties of both members were assessed and discussions are presented on the origin of the non-linearity in this family of compounds.