## 3 Hydrothermal Crystal Growth
### Recent Results For Sesquioxide Laser Materials  
#### 3.1.1 Yb:Lu\({}_{2}\)O\({}_{3}\) Absorption Spectroscopy  
Our group has recently obtained cryogenic absorption cross-section data for Yb:Lu\({}_{2}\)O\({}_{3}\), with the results submitted for publication [119]. Figure 35 shows a portion of that data at 298 K and 80 K. These data were obtained with high spectral resolution down to 0.1 nm, and clearly show that the zero-phonon line is still not resolved at 80 K. While the absorption band around 900 nm is resolved at all temperatures, and we see the usual progression of increased cross-section values as temperature is lowered, the bands at 950 nm and 977 nm are not completely resolved, even at 0.1 nm resolution. Our analysis shows that the zero-phonon linewidth is \(<\)0.1 nm and likely below 0.05 nm. A similar situation has been found in Yb:YAG where the zero-phonon linewidth was initially measured to be \(<\)0.1 nm [27], and a later measurement yielded a value of 0.05 nm [3]. Due to the narrow linewidth, it is unlikely that the lower quantum defect associated with the 977 nm line can be obtained in practice, since the linewidth needed for efficient optical pumping is beyond even the capabilities of recently developed volume-Bragg-grating (VBG) narrowed diode laser sources. For the 950 nm band, however, there is an increased cross-section at 80 K, however, when higher-resolution measurements are available the increase will likely be larger than shown in Figure 35.  
Figure 34: Highly faceted, optically clear, as-grown Yb:Lu\({}_{2}\)O\({}_{3}\) single crystal grown at 700 \({}^{\circ}\)C with a 30 degree thermal gradient. Crystal dimensions are 6 \(\times\) 6 \(\times\) 4 mm\({}^{3}\). Figure 35: Absorption cross-section as a function of wavelength for a hydrothermally-grown Yb\({}^{3+}\):Lu\({}_{2}\)O\({}_{3}\) laser crystal at 80 K (blue) and 298 K (red). 1.2 Sm:YAG and Sm:LuAG For The Suppression of Parasitic Oscillations and Amplified Spontaneous Emission  
We have investigated the hydrothermal growth of both Sm:YAG and Sm:LuAG. Sm is a well-known 1064 nm absorber, and has been used as Sm:Glass and Sm:YAG for some time as a suppressor of parasitic oscillations and amplified spontaneous emission (ASE). Figure 36 shows a side view of a Nd:YAG laser rod with an external Sm:YAG cladding layer on the barrel. In the absence of the Sm:YAG coating, it is well-known that a polished barrel will result in the oscillation of transverse straight-across surface mode, ring modes, and longitudinal (bulk) parasitic oscillation modes that will clamp the laser gain. In some cases, a surface parasitic located at the pump beam entrance face will have the lowest threshold. To eliminate parasitics and minimize ASE, such rods usually have ground barrels. Grinding of course results in sub-surface fractures that limit the power handling capability of the rod. A highly-doped layer of Sm:YAG can be used to suppress the parasitics and minimize ASE losses. Our interest in these materials began in 2012 when we demonstrated, for the first time, that we could grow epitaxial-like layers of Sm:YAG on Nd:YAG substrates [120]. A more recent example of this is shown in Figure 37. A Nd-doped rectangular cm sized laser crystal was used, and a highly doped layer of Sm:YAG was successfully grown on all six external surfaces. After layer growth, the end faces of the crystal are ground to remove the Sm:YAG, and polished and dielectrically-coated to allow 1064.2 nm lasing. In practice, this type of layer may be grown on any Nd:YAG laser crystal, and can be used to suppress parasitics and ASE. Previously, other researchers have reported Sm:YAG clad ceramic Nd:YAG rods [121; 122] and disks [123]. In our work, we are primarily interested in providing a method that can be use on commonly used Czochralski-grown single crystals, without using expensive methods like diffusion-bonding. Figures 38 and 39 show the absorption cross-sections as a function of wavelength for Sm:YAG and Sm:LuAG. We are particularly interested in the region near 1064 nm.