## 3 Mid-Infrared Fe: ZnSe and Cr: ZnSe Solid-State Lasers  
Taking transition metal doped II-VI chalcogenides crystallized group sulfide crystals as gain media is an important means to realize mid-infrared laser. The two typical laser materials are Fe: ZnSe and Cr: ZnSe crystals. Characteristics descriptions of Fe: ZnSe and Cr: ZnSe crystals are shown in Figure 6.  
Fe: ZnSe is a four-energy level structure. When Fe\({}^{2+}\) is doped into ZnSe, Zn\({}^{2+}\) in the center of tetrahedron will be replaced. The ground state energy level \({}^{5}\)D of the outermost electron \({}^{3}\)d\({}_{6}\) splits into duplex degenerate states \({}^{5}\)E and triple-degenerate states \({}^{5}\)T\({}_{2}\) under the action of a crystal field [55]. Then the one-step orbital spin coupling splits the \({}^{5}\)T\({}_{2}\) state into three energy bands and the second-order orbital spin coupling splits the \({}^{5}\)E state into five energy levels. The energy level diagram is shown as Figure 7. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Crystal** & **Crystal System** & **Point Group** & **Nonlinear Coefficient/pm-V-1** & **Transparency Range/\(\mu\)m** & **Thermal Conductivity/W:m**–1**–**K–1** & **Damage Threshold/GW:cm**\({}^{2}\) \\ \hline \multirow{2}{*}{\begin{tabular}{c} LiNO\({}_{3}\) \\ PdPNL \\ MgO-PPLN \\ KTP \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} trigonal system \\ 3 m \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} 3 m \\ \(d_{31}=4.3\) d\({}_{30}=27.2\) \\ \(d_{32}=27.2\) \\ \(d_{33}=14.8\) \\ \(d_{35}=1.9\) \\ \end{tabular} } & 0.35–4.5 & 5.6 & 0.2 \\  & & & & 0.33–5.5 & 5 & 0.3 \\  & & & & 0.36–5 & 4.4 & 0.6 \\  & & & & 0.35–4.5 & 0.4 & 1.5 \\  & & & & 0.43–4.2 & 2.8 \\  & & & & 0.43–4.2 & 2.8 \\  & & & & 0.43–10.6 & 2 \\  & & & & 0.43–10.6 & 2 \\ AgGaSe\({}_{2}\) & tetragonal system & 42 m & \(d_{30}=39.5\) & 0.73–18 & 1 & 0.04 \\ AgGaS\({}_{2}\) & tetragonal system & 42 m & \(d_{30}=13.4\) & 0.53–13 & 1.5 & 0.04 \\ ZGP & tetragonal system & 42 m & \(d_{30}=75\) & 0.74–12 & 35 & 30 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Properties of mid-infrared nonlinear crystals mentioned above. Figure 6: Characteristics descriptions of Fe: ZnSe and Cr: ZnSe crystals. Figure 7: Diagram of Fe: ZnSe energy level. Cr: ZnSe is a four-energy level structure. Under the action of a pump light, Cr\({}^{2+}\) in the ground state of \({}^{5}\)T\({}_{2}\) transits to the vibrational levels of excited state \({}^{5}\)E, and because there is no other energy level above the \({}^{5}\)E excited state level, therefore, there is almost no excited state absorption process for Cr\({}^{2+}\)[56]. The energy level diagram is shown as Figure 8.  
The absorption peak of Fe: ZnSe crystal is near 3 \(\upmu\)m at room temperature. Additionally, the emission peak is near 4.3 \(\upmu\)m. Take note that the absorption characteristics of Fe: ZnSe crystal varies greatly with temperature, as shown in Figure 9. The absorption cross sections of Fe: ZnSe crystal are greatly at 14 K. Additionally, the absorption cross section will become lower while, at the same time, the absorption range will become wider at 300 K. From the emission spectrum of Fe: ZnSe, the material emission spectrum range is 3-5 \(\upmu\)m [1]. Figure 8: Diagram of Cr: ZnSe energy level. Figure 9: Absorption and emission spectrum of Fe: ZnSe crystal. Cr: ZnSe has a relatively wide absorption band, at 1.5-2 \(\upmu\)m; as shown in Figure 10, the absorption peak is around 1.75 \(\upmu\)m. The emission spectroscopy is 2-3 \(\upmu\)m, and the emission peak is about 2.45 \(\upmu\)m [56]. It can be seen from Figure 10 that it is not a good choice to use the Cr: ZnSe crystal to achieve a laser output above 3 \(\upmu\)m, because, although the crystal has emission at 3 \(\upmu\)m, its gain is relatively low.