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.

In this paper a simple method is described for measuring the principal refractive indices of crystals using a Michelson interferometer and a single principal section of the material. This work formed the basis of the final year BSc project in laser physics and optoelectronics for one of the authors. The refractive indices of a sapphire crystal obtained by this technique are accurate to the third decimal place. The procedure is then used to obtain the three principal refractive indices of a new biaxial crystal, potassium niobium borate, whose refractive indices were previously unknown.

We report for the first time the growth of large size high quality single crystals of KYF4 (KYF) by TSSG method using the conventional weight-feed-back automatic diameter control Czochralski puller. We have made major revision of the KF -YF3 phase diagram and showed that KYP melts peritectically. The crystals can accommodate large size variations of the dopants. We were able to dope it with a large number of rare earth elements. The crystal has long fluorescence lifetime and weak phonon energy making it ideal as upconversion laser host.

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