For the first time two-photon luminescence excitation (TPLE) of Eu3+ in CaF2 is carried out and the TPLE frequency spectrum of the Fo-* G2 transition in the tetragonal center of a CaF2 cubic lattice is measured. Two-photon luminescence excitation and one-photon luminescence excitation (OPLE) are .compared, The integrated TPLE efficiency-to-OPLE efficiency ratio is measured 1 2 to be (1.6 2 0 . 3 )~l O -l~The measurements were made a t 77 K on a CaF2:Eu3+ sample (type I1 in accordance with /l/) with Eu3+ concentration of 0.2 wto/o. The available excitation source was a tunable pulsed dye laser irradiating linearly polarized light with a spectral linewidth of 0.025 nm. Dye laser operation was multimode but rehtive fluctuations of the luminescence yield did not exceed * 15 % through all measurements, this fact shows strong evidence for a sufficient stability of the exciting radiation mode structure. Recording of excited luminescence was made at the wavelength h = 593 nm ( Do" F i transition). The available experimental set-up was a somewhat modified one described in /2/.The TPLE frequency spectrum at the F o 4 G2 transition excitation is shown in Fig, la; the OPLE spectrum, obtained under the same experimental conditions except for the second harmonic linear polarization which was orthogonal to the fundamental harmonic linear polarization-of the exciting radiation, is shown in Fig. l
The results are reported of the first study of the 5D, manifold of Eu3+ in the noninversion and inversion centres of YAG using the method of two-photonluminescence excitation. Based on the number of Stark components and the two-photon polarization dependences it is shown that the set of Stark components of 5D multiplet corresponds to tetragonal local symmetry. The components of 5D, manifold are identified and the crystal field parameters are found for the noninversion centre. The europium ion distribution over the centres is determined showing that a t least 60% of the ions are in the inversion centre.
An effect has been revealed associated with the l a s e r field action, which can be conventionally called Stark level reversible deformation and which was observed in the one-photon luminescence excitation (OPLE) spectrum of the europium transition 7F0+5G2 in fluorite. Its essence lies in such reversible OPLE spectrum changes as component frequency displacement, component relative intensity redistribution and integrated intensity reduction of the spectrum by a factor not l e s s than 2.5.The effect was revealed during two-photon luminescence excitation (TPLE)investigations under conditions described in /I/. In addition, it is noteworthy that the recorded luminescence of the 5D0-47F1 transition was selected by a grating monochromator with a slit spectral width of 2 0 nm. Thus, the total transition luminescence was recorded. The excitation and observation conditions of the OPLE spectrum are schematically shown in Fig. 1. The small circle (shaded) with the a r e a S1 corresponds to the cross-section of the sample region which was laser-field effected by a pulse series with a mean peak intensity per series of about 50 MW cm'2 The large circle with the a r e a S2 corresponds to the cross-section of the
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