Direct imaging of rare-earth ion clusters inYb:CaF2

Abstract

The existence and the identification of only one or several coparticipating luminescent Yb 3+ centers in the heavily doped Yb:CaF 2 laser crystals which are considered in the development of several high intensity laser chains have been examined first by using two complementary and original experimental approaches, i.e., registration of low temperature site-selective laser excitation spectra related to near-infrared and visible cooperative emission processes, on the one hand, and direct imaging at the atomic scale of isolated ions and clusters using a high-resolution scanning transmission electron microscope in the high angle annular dark-field mode, on the other hand, and then correlating the data with simple crystal field calculations. As a consequence, and although all the experimental details could not be accounted for quantitatively, a good overall correlation was found between the experimental and the theoretical data. The results show that at the investigated dopant concentrations, Yb:CaF 2 should be considered as a multisite system whose luminescent and lasing properties are dominated by a series of Yb 3+ clusters ranging from dimers to tetramers. Hexameric luminescent centers may be dominant at really high dopant concentrations (likely above 20 at. %), as was originally proposed, but certainly not at the intermediate dopant concentrations which are considered for the laser application, i.e., between about 0.5 and 10 at. %.

Spatio‐Temporal Characterization of Pump‐Induced Wavefront Aberrations in Yb3 + ‐Doped Materials

“…Due to the replacement ratio of 2 Yb‐ions for 3 Ca‐ions, the face‐centered‐cubic lattice structure is considerably disturbed by the doping . This results in, e.g., a strongly doping‐dependent thermal conductivity, and thus, can also influence the internal stress distribution of the pumped material.…”

Diode-pumped Yb,Y:CaF2 laser mode-locked by monolayer graphene

“…However, trivalent rare-earth ion doped CaF 2 single crystals tend to form clusters even in low doping concentrations, which could affect the luminescent properties (absorption, emission and lifetime) and quantum efficiency [6][7][8][9]. In order to break the clusters, one effective approach is to dope the charge compensating ions or buffer ions into a single CaF 2 crystal.…”

Probability density function of the rate of energy transfer from luminescent ions to a random distribution of traps in the static limit

“…Hence, there is a significant interest in understanding and controlling the spatial distribution of ions and traps inside a host material to optimize the compromise between efficient energy transfer and high quantum luminescence efficiency. For example, the formation of Yb 3 ion clusters and their size distribution inside a CaF 2 host directly affect the spectroscopic properties of this material [9]; yet, the direct demonstration of the clustering of Yb 3 ions inside such a host has proven very difficult so far [10,11].…”

“…They involve the measurement of the decay of luminescence intensity [5,12,13], the decay of fluorescence polarization anisotropy for organic molecular systems [14][15][16], the time-dependent emission spectra by spectral energy transfer in inhomogeneously broadened lines using the time-resolved fluorescence line narrowing [17][18][19], or site-selective laser excitation spectroscopy of cooperative luminescence [11]. These techniques are useful to shed light on the distribution of luminescent ions or molecules inside a material, but they often require a model describing the excitation probability of a collection of ions in the presence of traps and energy migration.…”