Direct imaging of rare-earth ion clusters in $\mathrm{Yb}:{\mathrm{CaF}}_2$

The existence and the identification of only one or several coparticipating luminescent ${\mathrm{Yb}}^{3+}$ centers in the heavily doped $\mathrm{Yb}:{\mathrm{CaF}}_2$ 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, $\mathrm{Yb}:{\mathrm{CaF}}_2$ should be considered as a multisite system whose luminescent and lasing properties are dominated by a series of ${\mathrm{Yb}}^{3+}$ 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. %.

Figure 1

Nonlinear cooperative luminescence process in which two nearby excited ${\mathrm{Yb}}^{3+}$ ions couple to each other to emit one short-wavelength photon per each two excited ions from a virtual emitting state located at twice their excitation energy.

Figure 2

Photoluminescence excitation and emission spectra obtained by monitoring near-infrared emissions peaking at 984.6, 986.6, and 991.25 nm and by exciting the samples at 981.4, 980.5, and 979.4 nm.

Figure 3

Visible cooperative emission spectra registered around 490 nm (half the wavelength of the main near-infrared emission peak around 980 nm) after excitation at 980.5 and 980.7 nm.

Figure 4

Photoluminescence excitation spectra registered around 980 nm by monitoring visible cooperative emissions peaking around 490.3 nm (a) and 490.6 nm (b). Also reported, the absorption spectrum registered in the same spectral range (c) and the sum of the two previous excitation spectra (d).

Figure 5

STEM-HAADF images (10 nm × 10 nm) acquired along a $\langle 001\rangle$ zone axis (a) for the pure ${\mathrm{CaF}}_2$ and (b) for the $5\%\mathrm{Yb}:{\mathrm{CaF}}_2$ sample. The white rectangles bordered by arrows indicate the areas where intensity profiles have been recorded. (c) Comparison of intensity profiles recorded along $\langle 100\rangle$ directions and averaged on 0.3 nm of width for the pure and for the doped ${\mathrm{CaF}}_2$. The arrows show local increase of intensity along some Ca columns (for more clarity, the profiles have been shifted vertically).

Figure 6

SVD analysis applied on Fig. 5. (a) Experimental HAADF image of the Yb-doped $5\%\mathrm{Yb}:{\mathrm{CaF}}_2$ viewed along [001], after smoothing to reduce noise. (b) Lattice contribution of image (a) from SVD (1–8), also smoothed. (c) Dopants contribution of image (a) from SVD (9–480), also smoothed.

Figure 7

Intensity maps calculated from the HAADF images shown in Fig. 5, (a) for the pure ${\mathrm{CaF}}_2$ and (b) for the $5\%\mathrm{Yb}:{\mathrm{CaF}}_2$ samples. The colored disks reflect the mean intensity integrated along the $\langle 001\rangle$ Ca column detected on the image (intensity values are expressed in × $10{}^4$ counts).

Figure 8

(a) Image reconstruction of $5\%\mathrm{Yb}:{\mathrm{CaF}}_2$ using SVD (9–480) coupled with the mean intensity evaluation (image size is 10 nm × 10 nm). The small red circles indicate the positions of the brighter atomic columns (mean intensities $>4.92\times {10}^4$ counts). The most frequently observed patterns are surrounded by a white line. (b) Low-pass filtered and interpolated HAADF images showing the typical patterns identified in Fig. 8. All the images (1.2 nm × 1.2 nm size) are displayed in nonlinear intensity scale.

Figure 9

Comparison of experimental STEM-HAADF images obtained for the $0.8\%\mathrm{Yb}:{\mathrm{CaF}}_2$ sample (left) with theoretical clusters model (middle) and image simulations (right) projected along $\langle 001\rangle$ directions. Experimental images, displayed in nonlinear intensity scale, have been low-pass filtered to reduce noise and interpolated to add pixels. The experimental image dimensions are 1.17 × $1.17 \mathrm{nm}{}^2$. On projections, the green and larger circles correspond to Yb in substitution on a Ca site, the blue and smaller circles represent F, and the red circles correspond to Ca. In some cluster projections, two Yb can be projected along the same column (see indication). On the experimental and simulated images, the intensity increases from black to yellow.

Figure 10

A few projections of clusters along $\langle 001\rangle$ directions (• = ${\mathrm{Ca}}^{2+}$, • = ${\mathrm{Yb}}^{3+}$, • = $F^{-}$).