## 2 Bulk Crystal Characteristics
### Q-Switching Laser Performance  
As mentioned earlier, the Yb\({}^{3+}\)-doped ScBO\({}_{3}\) crystal has an excellent energy storage capacity, which is favorable for Q-switching pulse generation with moderate or even high output energy. Meanwhile, the passive Q-switching properties were characterized when utilizing the polished and uncoated Yb\({}^{3+}\)-ScBO\({}_{3}\) laser crystal and black phosphorous (BP) as the saturable absorber [33]. This crystal sample was incised along the \(a\) axis and enclosed in a water-cooled Cu block at 2 \({}^{\circ}\)C. A plane-concave cavity was applied, which was the same as that of the CW laser configuration described earlier. The CW laser could be discovered without the BP optical switcher, as shown in Figure 8. When BP was inserted into the cavity, the pulsed laser was generated with the threshold of 3.3 W, the slope efficiency of 5%, and the highest average output power was 43 mW, as illustrated in Figure 9a. The obvious threshold diversity between the pulsed lasers and CW was observed by means of the small emission cross-section of the crystal. In the meantime, the threshold was increased owing to the reflection loss, which was attributed to the coating-less wafer. The pump power versus the repetition frequency and pulse width are shown in Figure 9b. Under a pump power of 3.4 W, the pulse width was 1393 ns and the repetition frequency was 20 kHz. When the absorption pump power was 4.15 W, the pulse width was 495.5 ns, and the repetition frequency was 30.6 kHz. With the increasing power of the pump, the trends of the pulse width and repetition frequency were opposite. The above phenomenon is a characteristic of passive Q-switching lasers [53, 54]. Under an average output power of 43 mW, the maximal pulse energy was 1.4 \(\upmu\)J, which was notably higher than the acquired pulse energy in the BP-modulated Yb\({}^{3+}\)-CYA crystal. The energy storage capacity was also notably better than that of Yb\({}^{3+}\)-CYA [55, 56], because of the smaller emission cross-section and longer fluorescence lifetime. The inset of Figure 9c shows a  
Figure 8: (**a**) Pump power versus average output power and (**b**) laser spectra [33]. Figure 9: (**a**) Pump power versus average pulsed output power, (**b**) pump power as a function of repetition frequency and pulse width, respectively, and (**c**) single pulse. Inset: pulsed train [33]. characteristic pulsed train with a repetition frequency of 29 kHz. The narrowest pulse width was determined to be 495.5 ns, as shown in Figure 9c. The beam size for the BP sample was determined to be 181.9 \(\upmu\)m, based on the ABCD matrix [57]. In the meantime, the beam properties, with a size of 225.5 \(\upmu\)m and a M\({}^{2}\) factor of 1.2, were evaluated using a CCD (CinCam COMS-1201). Therefore, stable passive Q-switching was achieved, indicating that moderate- and high-energy pulses can be achieved in this gain medium, under both active and passive Q-switching regimes. Moreover, self-Q-switching operation was also demonstrated in Yb\({}^{3+}\):ScBO\({}_{3}\) crystal under the same diode pump configuration, but with a plane-plane cavity configuration. Here, a laser sample with dimensions of 2 \(\times\) 3 \(\times\) 3 mm\({}^{3}\) and the output couplers with transmittances of 1% and 3% were utilized. Figure 10 shows the relevant characteristics of the output beam, including the output power versus the absorbed pump power (Figure 10a), the laser spectra (Figure 10b), and the beam profile (inset of Figure 10b). As the transmittance of the output coupler increased from 1% to 3%, the threshold of the absorption pump power increased from 0.81 W to 1.26 W, the maximal output power increased from 312 mW to 332 mW, and the corresponding slope efficiency increased from 27.9% to 39.8%. The slope efficiency and output power were improved compared with the study on the Q-switching process modulated by BP [33], because both the laser crystal and cavity design were optimized. The laser spectra of the output lasers are exhibited in Figure 10b.