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2014 On étudie la signification d'un paramètre scalaire de champ cristallin defini précédemment [4, 5]. On montre pourquoi il peut être souvent utilisé comme une indication de l'amplitude totale d'une levée de dégénerescence Stark pour un terme J donné, quelle que soit la symétrie cristalline du site de l'ion étudié. On donne ainsi le moyen d'utiliser les tables de paramètres Bkq existantes pour comparer des cristaux de structures et symétries de sites différents, et prédire les écartements Stark maxima. Réciproquement, ceux-ci donnent un moyen simple pour comparer la force du champ cristallin de différents cristaux.

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We have developed a device based upon the concept of two-photon up-conversion to use a part of the IR photons otherwise lost by transparency in a GaAs cell. An ultra thin GaAs cell has been fabricated using the technique of epitaxial lift-off (ELO). This thin cell is placed on top of a 100 µ m thick vitroceramic doped with Yb3+ and Er3+. The two photon upconversion process involved here is based on sequential absorption and energy transfer of two IR photons from Yb3+ to Er3+, which then emit one photon in the green. This green light then produces a photoresponse in the GaAs cell. This cell coupled to the vitroceramic was lighted by an Ti-sapphire IR laser at 1.391 eV, a photon energy below the band gap of GaAs, with an input power able to reach ∼1 W. The GaAs cell photoresponse increases quadratically with the input excitation. For an input excitation of 1 W at 1.39 eV on a 0.039 cm2 substrate-free GaAs cell, the measured efficiency was 2.5%.