Review of the Yb3+:ScBO3 Laser Crystal Growth, Characterization, and Laser Applications

Passive Q-switching is an effective approach for generating pulsed lasers, owing to its compact and additional modulation-free design. However, to compare favorably with active Q-switching and multi-stage amplification, the output energy needs to be enhanced for practical applications. Kramers Ytterbium ion (Yb3+)-doped borate crystals, with their excellent energy storage capacity, have been proven to be high-potential laser gain mediums for achieving pulsed lasers with moderate and high output energy using passive Q-switching technology. In this study, the growth, characterization, and laser generation of one Yb3+-doped borate crystal, the Yb3+:ScBO3 crystal, are systematically reviewed. The continuous-wave and passive Q-switching laser characteristics are presented in detail, and the self-pulsations derived from intrinsic ground-state reabsorption are also demonstrated. The specific characteristics and experiments confirm the potential of the Yb3+:ScBO3 crystal for future pulsed laser applications with moderate or even high energy output.


Introduction
Pulsed lasers with typical single-pulse energy at the microjoule scale play vital roles in ranging, remote sensing, micro-machining, medical treatment, and other applications [1][2][3][4][5][6]. Generally, the commonly used passive Q-switching technology has been of great interest because it has a compact structure and no need for additional modulation and control systems, compared with active Q-switching technology, multi-stage amplification, etc. [7][8][9][10][11][12]. However, the output energy of passive Q-switching still needs to be improved to allow its use in practical applications. Based on the solid-state laser theory and the passive Q-switching rate equation, a laser gain medium with a high energy storage capacity is beneficial for moderate-and high-energy pulsed laser output [13,14]. Such a laser gain medium possesses desirable emission characteristics, such as a long fluorescence lifetime, small emission cross-section, and broad fluorescence spectra. The laser crystal, an important component of various laser gain media, is composed of doping ions and host materials. The comprehensive emission equities of the crystal are ascertained by the electronic structure and crystal field [15,16]. Therefore, laser crystals with favorable doping ions and host materials are desired for passive Q-switching laser applications.
In 1993, the spectral characteristics of Yb 3+ :ScBO 3 crystal were reported, with an emission cross-section of~0.1 × 10 −20 cm 2 and a fluorescence lifetime of~5 ms, indicating that it can be an ideal gain medium for obtaining Q-switching lasers with moderate or high pulse energy [32]. However, the growth difficulties of Yb 3+ :ScBO 3 crystal with its high optical quality have hindered further progress in laser applications. Most recently, the first optical grade Yb 3+ :ScBO 3 crystal was successfully grown and both CW and passive Q-switching lasers have been realized [33,34]. In addition, the growth method, characteristics, and laser performance of Yb 3+ :ScBO 3 crystal has been systematically reviewed. The self-Q-switching characteristics derived from intrinsic ground-state reabsorption are also discussed. All results presented a high potential gain medium for moderate-or even high-energy pulsed laser generation using both active and passive Q-switching technologies.

Crystal Structure
The ScBO 3 crystal remains a trigonal crystal system, a -3m1 point group, and a R -3c space group. As exhibited in Figure 1a, the Sc atoms and O atoms are coordinated to form ScO 6 octahedra, which are connected to each other to form a pore structure. The B atoms are located in the pores and coordinate with the O atoms to form BO 3 tetrahedra. In addition, the effective ionic radii of Yb 3+ and Sc 3+ are 0.868 and 0.745 Å [35], respectively, which is beneficial, even for the Yb 3+ -dropped and lattice pattern distortion in Yb 3+ :ScBO 3 crystals.
In 1993, the spectral characteristics of Yb 3+ :ScBO3 crystal were reported, with an emission cross-section of ~0.1 × 10 −20 cm 2 and a fluorescence lifetime of ~5 ms, indicating that it can be an ideal gain medium for obtaining Q-switching lasers with moderate or high pulse energy [32]. However, the growth difficulties of Yb 3+ :ScBO3 crystal with its high optical quality have hindered further progress in laser applications. Most recently, the first optical grade Yb 3+ :ScBO3 crystal was successfully grown and both CW and passive Q-switching lasers have been realized [33,34]. In addition, the growth method, characteristics, and laser performance of Yb 3+ :ScBO3 crystal has been systematically reviewed. The self-Qswitching characteristics derived from intrinsic ground-state reabsorption are also discussed. All results presented a high potential gain medium for moderate-or even highenergy pulsed laser generation using both active and passive Q-switching technologies.

Crystal Structure
The ScBO3 crystal remains a trigonal crystal system, a 3 m1 point group, and a R3 c space group. As exhibited in Figure 1a, the Sc atoms and O atoms are coordinated to form ScO6 octahedra, which are connected to each other to form a pore structure. The B atoms are located in the pores and coordinate with the O atoms to form BO3 tetrahedra. In addition, the effective ionic radii of Yb 3+ and Sc 3+ are 0.868 and 0.745 Å [35], respectively, which is beneficial, even for the Yb 3+ -dropped and lattice pattern distortion in Yb 3+ :ScBO3 crystals.
The diffraction indices of ScBO3 crystal, measured by X-ray powder diffraction, matched well with standard PDF#79-0097, as exhibited in Figure 1b. After calculations, the cell parameters of ScBO3 were obtained: a = b = 4.776 Å and c = 15.405 Å . When doped with the Yb 3+ ion, the cell parameters and crystal structure apparently do not adjust compared with undoped ScBO3 crystal [36]. Here, the buoyancy method was applied for measuring crystal density, and the result was 3.787 g/cm 3 , which corresponded with the theoretical density of 3.812 g/cm 3 .  The diffraction indices of ScBO 3 crystal, measured by X-ray powder diffraction, matched well with standard PDF#79-0097, as exhibited in Figure 1b. After calculations, the cell parameters of ScBO 3 were obtained: a = b = 4.776 Å and c = 15.405 Å. When doped with the Yb 3+ ion, the cell parameters and crystal structure apparently do not adjust compared with undoped ScBO 3 crystal [36]. Here, the buoyancy method was applied for measuring crystal density, and the result was 3.787 g/cm 3 , which corresponded with the theoretical density of 3.812 g/cm 3 .

Crystal Growth
ScBO 3 crystals are usually grown utilizing the Czochralski method due to the congruent characteristics of this crystal [37]. However, difficulties, such as the intense volatilization of B 2 O 3 , not only lead to segregation of the composition, but also give rise to instability in growth, which greatly affects the optical quality of the crystals [37]. Here, a 10.0 at.% Yb 3+ -doped ScBO 3 single crystal with a 20-mm-diameter was successfully grown using an argon atmosphere in an iridium crucible. The starting materials, Yb 2 O 3 , Sc 2 O 3 , and H 3 BO 3 , were weighed according to the chemical formula Yb 0.1 Sc 0.9 BO 3 . Generally, an additional 3wt% H 3 BO 3 was put in the raw ingredients during the batching process, which was used to compensate for B 2 O 3 volatilization during crystal growth and polycrystalline material preparation. The equation for the chemical reaction is as follows: A mullite brick with an appropriate thickness was used in the furnace to create a large temperature gradient to avoid constitutional supercooling and reduce volatilization. First, the seed was a platinum wire to obtain polycrystal with plenty of cracks, as shown in Figure 2a. To diminish the cracking, the seed was cut from the bulk polycrystal along c-axis. Meanwhile, a micro-convex solid-liquid interface was achieved by optimizing the temperature field and rotation speed, thereby balancing the natural convection and forced convection during the growth process. Although some subsidiary white matter took shape owing to the volatilization, the samples for laser experiments (as presented in the inset of Figure 2b) could be extracted from this optical-quality crystal (Figure 2b). Polycrystalline material was used as the standard to measure and calculate the density of elemental Yb 3+ in this crystal. The effective distribution coefficient of Yb 3+ was ascertained as 0.95.

Crystal Growth
ScBO3 crystals are usually grown utilizing the Czochralski method due to the congruent characteristics of this crystal [37]. However, difficulties, such as the intense volatilization of B2O3, not only lead to segregation of the composition, but also give rise to instability in growth, which greatly affects the optical quality of the crystals [37]. Here, a 10.0 at.% Yb 3+ -doped ScBO3 single crystal with a 20-mm-diameter was successfully grown using an argon atmosphere in an iridium crucible. The starting materials, Yb2O3, Sc2O3, and H3BO3, were weighed according to the chemical formula Yb0.1Sc0.9BO3. Generally, an additional 3wt% H3BO3 was put in the raw ingredients during the batching process, which was used to compensate for B2O3 volatilization during crystal growth and polycrystalline material preparation. The equation for the chemical reaction is as follows: A mullite brick with an appropriate thickness was used in the furnace to create a large temperature gradient to avoid constitutional supercooling and reduce volatilization. First, the seed was a platinum wire to obtain polycrystal with plenty of cracks, as shown in Figure 2a. To diminish the cracking, the seed was cut from the bulk polycrystal along c-axis. Meanwhile, a micro-convex solid-liquid interface was achieved by optimizing the temperature field and rotation speed, thereby balancing the natural convection and forced convection during the growth process. Although some subsidiary white matter took shape owing to the volatilization, the samples for laser experiments (as presented in the inset of Figure 2b) could be extracted from this optical-quality crystal (Figure 2b). Polycrystalline material was used as the standard to measure and calculate the density of elemental Yb 3+ in this crystal. The effective distribution coefficient of Yb 3+ was ascertained as 0.95.

Thermal Characteristics, Laser Damage Threshold and Mechanical Characteristics
Laser performance and crystal growth are usually affected by thermal expansion, specific heat, and so on [38]. Laser crystals are liable to break during the growth process, usually owing to obvious anisotropic thermal expansion, low thermal conductivity, and low specific heat. Meanwhile, the crystal may have a significant thermal effect during the laser experiment process due to a large temperature gradient.
The specific heat capacity was measured and then calculated using a thermal analyzer and software produced by Perkin-Elmer Co., Waltham, MA, USA. The specific heat (Cp) of Yb 3+ :ScBO3 increased with temperature, and the median was 0.73 J·g −1 ·K −1 at 330 K, as shown in Figure 3a [36,39]. Because of a large specific heat, compared with Yb 3+ :YCOB (0.75 J·g −1 ·K −1 ) and Yb 3+ :YVO4 (0.62 J·g −1 ·K −1 ), Yb 3+ :ScBO3 will not be extremely influenced by the heat caused during the laser emission action and a very large damage threshold can be forecast.

Thermal Characteristics, Laser Damage Threshold and Mechanical Characteristics
Laser performance and crystal growth are usually affected by thermal expansion, specific heat, and so on [38]. Laser crystals are liable to break during the growth process, usually owing to obvious anisotropic thermal expansion, low thermal conductivity, and low specific heat. Meanwhile, the crystal may have a significant thermal effect during the laser experiment process due to a large temperature gradient.
The specific heat capacity was measured and then calculated using a thermal analyzer and software produced by Perkin-Elmer Co., Waltham, MA, USA. The specific heat (C p ) of Yb 3+ :ScBO 3 increased with temperature, and the median was 0.73 J·g −1 ·K −1 at 330 K, as shown in Figure 3a [36,39]. Because of a large specific heat, compared with Yb 3+ :YCOB (0.75 J·g −1 ·K −1 ) and Yb 3+ :YVO 4 (0.62 J·g −1 ·K −1 ), Yb 3+ :ScBO 3 will not be extremely influenced by the heat caused during the laser emission action and a very large damage threshold can be forecast.

Spectroscopic Characteristics
A spectrophotometer, with a spectral sharpness of separation of 0.2 nm, was used to determine the room temperature (RT) absorption spectra. The cutting orientation of the sample was along the a axis. An Edinburgh Instruments FLS920 fluorescence spectrometer was utilized to determine both the room and cryogenic (79 K) fluorescence properties, and the thickness of the sample was 0.5 mm. The absorption spectra and RT fluorescence of Yb 3+ :ScBO3 are presented in Figure 4a. The main absorption peak was at 964.8 nm. Its fullwidth at half maximal (FWHM) was determined to be 19.86 nm, and the absorption crosssection was determined to be 0.13 × 10 −20 cm 2 . Diode laser sources are appropriate for the wide absorption bandwidth. At 79 K, the zero-photon line was at 964.8 nm, as exhibited in Figure 4a. Figure 4b exhibits the energy levels of the Yb 3+ ions in the ScBO3 crystal [43].
The cross-section of the RT emission was determined using the following formula [32,44]: where I (λ) represents the emission intensity, n represents the refractive index, and τrad represents the radiative lifetime. As exhibited in Figure 4a, the strongest emission peak (λpeak) was at 1021 nm. Meanwhile, the corresponding emission cross-section (σem) was determined to be 0.12 × 10 −20 cm 2 , which was significantly larger than that of Yb 3+ :YAG [45] and Yb 3+ :YGG [46,47]. After testing and fitting, it was determined that the fluorescence lifetime at room temperature was 5.25 ms [48], as shown in Figure 5. Both the exponential and linear fitted decay curves provided the single exponential trend, as shown in Figure 5 [49]. Table S1 lists the passive Q-switching performance and corresponding spectral and thermal characteristics of commonly used Yb 3+ -doped crystals, such as Yb 3+ :YAG, Yb 3+ :YGG, Yb 3+ :KGW, Yb 3+ :YCOB, and Yb 3+ :GdCOB. Among them, the Yb 3+ :ScBO3 crystal possesses A laser flash apparatus was used to take stock of the thermal diffusivity; the findings are shown in Figure 3b, which shows a plot of the thermal diffusivity (λ ij ) of Yb 3+ :ScBO 3 at temperatures ranging from 303.15 K to 774.15 K.
In view of the following equation, the thermal conductivity (k) was determined: when the temperature increased at above temperatures range, the a-axis thermal conductivity increased from 3.61 W·m −1 ·K −1 to 3.86 W·m −1 ·k −1 , and the c-axis thermal conductivity also increased from 2.98 W·m −1 ·K −1 to 3.36 W·m −1 ·K −1 (as presented in Figure 3c). Owing to the tendency of the thermal conductivity to rise with rising temperature, this crystal can be utilized in generating medium lasers.
The α ij of one crystal, which is a meristic second-order tensor, represents the thermal expansion coefficient [40]. Since Yb 3+ :ScBO 3 appertain a trigonal system, on account of the principle of Neumann, the thermal expansion coefficient tensor is exhibited to be: According to the crystal structure, Yb 3+ :ScBO 3 has two thermal expansion coefficients, α 11 and α 33 , which are independent. The measurements of the thermal expansion tensor of this crystal were utilized a thermal mechanical analyzer (Diamond TMA). Moreover, the thermal expansion curves of Yb 3+ :ScBO 3 , plotted versus temperature, are presented in Figure 3d. The two aforementioned thermal expansions remained virtually linear. Then, the thermal expansion coefficients along these two axes were separately determined to be 1.05 × 10 −6 K −1 for α 11 and 10.24 × 10 −6 K −1 for α 33 . The considerable anisotropy in terms Appl. Sci. 2021, 11, 10879 5 of 14 of thermal expansion may lead to cracking during the crystal growth process. The density at diverse temperatures for Yb 3+ :ScBO 3 was determined utilized the following formula: where theoretical density ρ 0 is 3.812 g/cm 3 . The data were fitted to a linear formula, as shown in the result: ρ = (−5 × 10 −5 )T + 3.83.
To measure the laser damage threshold, a Q-switched pulsed laser with an emission wavelength of 1064 nm was utilized. Then, the damage threshold of the Yb 3+ :ScBO 3 crystal was 509 MW/cm 2 . The threshold value of Yb 3+ :ScBO 3 was almost half that of Yb 3+ :YVO 4 and Yb 3+ :YAG.
For the purpose of acquiring the hardness of the sample, a digital microhardness tester was utilized. The hardness was determined using the following equation: where H v is the Vickers hardness, d is trace length along the diagonal, and P is the load.
The Vickers hardness of the as-grown crystal along the a, b, and c axes was tested to be 7.05, 6.85, and 10.67 GPa, respectively. The hardness along the a and b axes was anisotropic with comparable values. The Yb 3+ :ScBO 3 crystal had modest hardness, which was smaller than that of YAG (12.7 GPa) [41] and bigger than that of YVO 4 (4.8 GPa) [42]. Thus, the Yb 3+ :ScBO 3 crystal may be incised efficiently.

Spectroscopic Characteristics
A spectrophotometer, with a spectral sharpness of separation of 0.2 nm, was used to determine the room temperature (RT) absorption spectra. The cutting orientation of the sample was along the a axis. An Edinburgh Instruments FLS920 fluorescence spectrometer was utilized to determine both the room and cryogenic (79 K) fluorescence properties, and the thickness of the sample was 0.5 mm. The absorption spectra and RT fluorescence of Yb 3+ :ScBO 3 are presented in Figure 4a. The main absorption peak was at 964.8 nm. Its full-width at half maximal (FWHM) was determined to be 19.86 nm, and the absorption cross-section was determined to be 0.13 × 10 −20 cm 2 . Diode laser sources are appropriate for the wide absorption bandwidth. At 79 K, the zero-photon line was at 964.8 nm, as exhibited in Figure 4a. Figure 4b exhibits the energy levels of the Yb 3+ ions in the ScBO 3 crystal [43].
Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 14 the smallest emission cross-section, the longest fluorescence lifetime, and a moderate fluorescence linewidth, indicating its excellent potential for achieving pulsed lasers with moderate, or even high, pulse energy.  The cross-section of the RT emission was determined using the following formula [32,44]: where I (λ) represents the emission intensity, n represents the refractive index, and τ rad represents the radiative lifetime. As exhibited in Figure 4a, the strongest emission peak (λ peak ) was at 1021 nm. Meanwhile, the corresponding emission cross-section (σ em ) was determined to be 0.12 × 10 −20 cm 2 , which was significantly larger than that of Yb 3+ :YAG [45] and Yb 3+ :YGG [46,47]. After testing and fitting, it was determined that the fluorescence lifetime at room temperature was 5.25 ms [48], as shown in Figure 5. Both the exponential and linear fitted decay curves provided the single exponential trend, as shown in Figure 5 [49]. Table S1 lists the passive Q-switching performance and corresponding spectral and thermal characteristics of commonly used Yb 3+ -doped crystals, such as Yb 3+ :YAG, Yb 3+ :YGG, Yb 3+ :KGW, Yb 3+ :YCOB, and Yb 3+ :GdCOB. Among them, the Yb 3+ :ScBO 3 crystal possesses the smallest emission cross-section, the longest fluorescence lifetime, and a moderate fluorescence linewidth, indicating its excellent potential for achieving pulsed lasers with moderate, or even high, pulse energy.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 14 the smallest emission cross-section, the longest fluorescence lifetime, and a moderate fluorescence linewidth, indicating its excellent potential for achieving pulsed lasers with moderate, or even high, pulse energy.

Continuous Laser Performance
The effective gain cross-section (σg) of Yb 3+ :ScBO3 was determined using the following formula before the lasing experiments, as presented in Figure 6: where is the proportion of the active ions excited to the upper level. The least value of (1021 nm) is determined using the following formula: The positive part of σg is presented in Figure 6 as different values of β. When = 0.75, the positive extent, which resembles Yb 3+ :YAG (950-1080 nm) [50], Yb 3+ :CYB (960-1100 nm), and Yb 3+ :CaGB (960-1100 nm), was 970-1100 nm [47,51]. The excellent characteristics of these crystal are suitable for tunable and femtosecond lasers. When β = 0.75, Yb 3+ :ScBO3 could, in theory, back a pulse continuation of 65 fs. The correlation between

Continuous Laser Performance
The effective gain cross-section (σ g ) of Yb 3+ :ScBO 3 was determined using the following formula before the lasing experiments, as presented in Figure 6: where β is the proportion of the active ions excited to the upper level. The least value of β min (1021 nm) is determined using the following formula: β min (1021 nm) = σ abs (1021 nm) σ abs (1021 nm) + σ em (1021 nm) = 0.1 (8) the gain bandwidth (Δν) and the shortest pulse duration (tFWHM) was used to determine the pulse continuation [52]: The experimental configuration is depicted in Figure 7 and a plane-concave cavity, which was 42 mm in length, was applied for CW laser generation. The emission wavelength of the pump source was 976 nm and the crystal was cut along the a axis, uncoated, and burnished. Furthermore, the input coupler had a high reflectance, from 1000 to 1100 nm, and a transmittance of over 99.5% at 950-990 nm. The transmittance from 1000-1100 nm of the output coupler was 5%.
A CW laser has been demonstrated and the corresponding laser performance is exhibited in Figure 8. It indicates that the threshold was 2.38 W. Under a 4.9-W pump power, the output power rose to 167 mW, and the slope efficiency was 9% at a wavelength of 1.063 μm. The light-to-light conversion efficiencies, calculated from the data above, are 4% and 9%, respectively. The laser performance can be greatly enhanced by utilizing a higher transmission of the output couplers, mode matching, coating conditions, etc.  The positive part of σ g is presented in Figure 6 as different values of β. When β = 0.75, the positive extent, which resembles Yb 3+ :YAG (950-1080 nm) [50], Yb 3+ :CYB (960-1100 nm), and Yb 3+ :CaGB (960-1100 nm), was 970-1100 nm [47,51]. The excellent characteristics of these crystal are suitable for tunable and femtosecond lasers. When β = 0.75, Yb 3+ :ScBO 3 could, in theory, back a pulse continuation of 65 fs. The correlation between the gain bandwidth (∆ν) and the shortest pulse duration (t FWHM ) was used to determine the pulse continuation [52]: The experimental configuration is depicted in Figure 7 and a plane-concave cavity, which was 42 mm in length, was applied for CW laser generation. The emission wavelength of the pump source was 976 nm and the crystal was cut along the a axis, uncoated, and burnished. Furthermore, the input coupler had a high reflectance, from 1000 to 1100 nm, and a transmittance of over 99.5% at 950-990 nm. The transmittance from 1000-1100 nm of the output coupler was 5%.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 14 the gain bandwidth (Δν) and the shortest pulse duration (tFWHM) was used to determine the pulse continuation [52]: ⋅ Δ = 0.315 (9) The experimental configuration is depicted in Figure 7 and a plane-concave cavity, which was 42 mm in length, was applied for CW laser generation. The emission wavelength of the pump source was 976 nm and the crystal was cut along the a axis, uncoated, and burnished. Furthermore, the input coupler had a high reflectance, from 1000 to 1100 nm, and a transmittance of over 99.5% at 950-990 nm. The transmittance from 1000-1100 nm of the output coupler was 5%.
A CW laser has been demonstrated and the corresponding laser performance is exhibited in Figure 8. It indicates that the threshold was 2.38 W. Under a 4.9-W pump power, the output power rose to 167 mW, and the slope efficiency was 9% at a wavelength of 1.063 μm. The light-to-light conversion efficiencies, calculated from the data above, are 4% and 9%, respectively. The laser performance can be greatly enhanced by utilizing a higher transmission of the output couplers, mode matching, coating conditions, etc.  A CW laser has been demonstrated and the corresponding laser performance is exhibited in Figure 8. It indicates that the threshold was 2.38 W. Under a 4.9-W pump power, the output power rose to 167 mW, and the slope efficiency was 9% at a wavelength of 1.063 µm. The light-to-light conversion efficiencies, calculated from the data above, are 4% and 9%, respectively. The laser performance can be greatly enhanced by utilizing a higher transmission of the output couplers, mode matching, coating conditions, etc. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 14

Q-Switching Laser Performance
As mentioned earlier, the Yb 3+ -doped ScBO3 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+ :ScBO3 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 °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 μJ, 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 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 μm, based on the ABCD matrix [57]. In the meantime, the beam properties, with a size of 225.5 μm 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.

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 • 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. Moreover, self-Q-switching operation was also demonstrated in Yb 3+ :ScBO3 crystal under the same diode pump configuration, but with a plane-plane cavity configuration.
Here, a laser sample with dimensions of 2 × 3 × 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. The center wavelength is 1063.9 nm and the laser profile was detected by CCD (CinCam COMS-1201), which is shown in the inset of Figure 10b and is close to the fundamental 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 µJ, 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 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 µm, based on the ABCD matrix [57]. In the meantime, the beam properties, with a size of 225.5 µm 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 × 3 × 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. The center wavelength is 1063.9 nm and the laser profile was detected by CCD (CinCam COMS-1201), which is shown in the inset of Figure 10b and is close to the fundamental transverse electromagnetic mode (TEM00). Moreover, self-Q-switching operation was also demonstrated in Yb 3+ :ScBO3 crystal under the same diode pump configuration, but with a plane-plane cavity configuration.
Here, a laser sample with dimensions of 2 × 3 × 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. The center wavelength is 1063.9 nm and the laser profile was detected by CCD (CinCam COMS-1201), which is shown in the inset of Figure 10b and is close to the fundamental transverse electromagnetic mode (TEM00). As presented in Figure 11, the temporal characteristics of self-Q-switching pulses, which are the pump power versus the repetition frequency and pulse width, were determined with an oscillator combined with a silicon detector. Because the pump power improved from 0.81 to 2 W, the pulse width was shortened from 1674 to 541.8 ns; however, when using the 1% output coupler, the repetition frequency presented the opposite trend. In the meantime, the absorbed pump power improved from 1.26 to 2 W and the pulse As presented in Figure 11, the temporal characteristics of self-Q-switching pulses, which are the pump power versus the repetition frequency and pulse width, were determined with an oscillator combined with a silicon detector. Because the pump power improved from 0.81 to 2 W, the pulse width was shortened from 1674 to 541.8 ns; however, when using the 1% output coupler, the repetition frequency presented the opposite trend. In the meantime, the absorbed pump power improved from 1.26 to 2 W and the pulse width reduced from 1329 to 731.1 ns when using the transmittance of the 3% output coupler. Here, the repetition frequency also revealed the opposite trend. These trends are typical in passive Q-switching lasers [53,54].
widths, mentioned earlier, are shown in Figure 11c,d, and the relevant pulse trains are shown in the insets. The pulse energy can be calculated from the values of the output power and the repetition frequency. If combined with the pulse width, the peak power can also be obtained. When using the 1% output coupler, the maximal peak power was 9.8 W and the maximal pulse energy was determined to be 5.3 μJ. The maximal peak power was 10.2 W, and the maximal pulse energy was 7.3 μJ as using the 3% transmittance output coupler. The nonlinear optical characteristics of the Yb 3+ :ScBO3 crystal were determined using a Z-scan test. Figure 12 shows the transmittance versus input energy intensity of the crystal when the Yb 3+ :ScBO3 crystal was at a wavelength of 1064 nm. The data were fitted utilizing the equation below [58]: where A represents the normalized parameter, δα represents the saturable absorption, I represents the incident intensity, and Is represents the saturation intensity. Furthermore, transmittance and intensity have a nonlinear relationship. Through fitting, the modulation depth of the Yb 3+ :ScBO3 sample was 17.77%. In addition, the absolute modulation depth was 10.2% and the saturation intensity was 4.1 × 10 8 W/m 2 at 1064 nm. As shown in Figure 11a,b, because the absorption pump power increased, the repetition frequency and the pulse width curves flattened. For the 1% and 3% output couplers, the shortest pulse widths were 541.8 ns and 731.1 ns, and the maximal repetition frequencies were 46.1 kHz and 38.8 kHz, respectively. The single pulses with minimum pulse widths, mentioned earlier, are shown in Figure 11c,d, and the relevant pulse trains are shown in the insets. The pulse energy can be calculated from the values of the output power and the repetition frequency. If combined with the pulse width, the peak power can also be obtained. When using the 1% output coupler, the maximal peak power was 9.8 W and the maximal pulse energy was determined to be 5.3 µJ. The maximal peak power was 10.2 W, and the maximal pulse energy was 7.3 µJ as using the 3% transmittance output coupler.
The nonlinear optical characteristics of the Yb 3+ :ScBO 3 crystal were determined using a Z-scan test. Figure 12 shows the transmittance versus input energy intensity of the crystal when the Yb 3+ :ScBO 3 crystal was at a wavelength of 1064 nm. The data were fitted utilizing the equation below [58]: where A represents the normalized parameter, δα represents the saturable absorption, I represents the incident intensity, and I s represents the saturation intensity. Furthermore, transmittance and intensity have a nonlinear relationship. Through fitting, the modulation depth of the Yb 3+ :ScBO 3 sample was 17.77%. In addition, the absolute modulation depth was 10.2% and the saturation intensity was 4.1 × 10 8 W/m 2 at 1064 nm. As previously reported, the Yb 3+ :ScBO3 crystal was expected to have strong inversion population accumulation owing to its large excited-state lifetime and small emission cross-section [34]. Hence, the self-Q-switching process in the Yb 3+ :ScBO3 crystal can be attributed to the ground-state reabsorption, which is the saturable nature of the reabsorption loss at the lower level, 2 F7/2 [59][60][61]. All the results contribute to an understanding of the self-Q-switching in gain mediums with strong reabsorption effects, and aid in the development of the highly compact pulsed laser systems.

Conclusions
The growth, structure, physical characteristics, and laser performance of Yb 3+ :ScBO3 crystal with an excellent energy storage capacity are comprehensively reviewed, for both CW and passive Q-switching laser characteristics. After partially overcoming difficulties, such as the intense volatilization of B2O3, a bulk crystal was triumphantly grown using the Czochralski technique. However, the optical quality is still required to be improved since there are lots of inclusions, as presented above, although the Czochralski method has been proved to be feasible. The thermal properties of the Yb 3+ :ScBO3 crystal, consisting og thermal conductivity and specific heat and so on, were characterized. In addition, the Yb 3+ :ScBO3 crystal was found to possess a small emission cross-section of 0.12 × 10 −20 cm 2 and a long fluorescence lifetime of 5.2 ms, which is beneficial for generating moderate and high-energy pulsed lasers. A CW laser, with a maximal output power of 167 mW and a slope efficiency of 9% under the wavelength of 1.063 μm, was first obtained in the Yb 3+ :ScBO3 crystal, utilizing a plane-concave cavity configuration. Then, a passive Qswitching laser in this crystal was achieved utilizing BP as the optical modulator, with a minimum pulse width of 495.5 ns and a maximal pulse energy of 1.4 μJ. The consequences show the feasibility of Yb 3+ :ScBO3 crystal achieving pulsed lasers with moderate or high energy. Finally, the self-Q-switching and nonlinear optical characteristics of this crystal were evaluated and the mechanism was attributed to ground state re-absorption. The preferable spectroscopic characteristics and preliminary laser performances indicate that the as-grown Yb 3+ :ScBO3 crystal would be a high-potential laser gain medium in establishing compact pulsed laser systems with moderate or even high energy output. Future research will be dedicated to the optimization of the crystal growth in the aspects of establishing the thermal field with small gradient and searching for the optimal growth parameters, including pulling and rotation rates, etc. In the meantime, the actively Q-switched pulsed lasers with high energy output are also expected in the Yb 3+ :ScBO3 crystal.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1. Passively Q-switching performance and corresponding spectral and thermal properties in commonly used Yb 3+ -doped crystals. As previously reported, the Yb 3+ :ScBO 3 crystal was expected to have strong inversion population accumulation owing to its large excited-state lifetime and small emission cross-section [34]. Hence, the self-Q-switching process in the Yb 3+ :ScBO 3 crystal can be attributed to the ground-state reabsorption, which is the saturable nature of the reabsorption loss at the lower level, 2 F 7/2 [59][60][61]. All the results contribute to an understanding of the self-Q-switching in gain mediums with strong reabsorption effects, and aid in the development of the highly compact pulsed laser systems.

Conclusions
The growth, structure, physical characteristics, and laser performance of Yb 3+ :ScBO 3 crystal with an excellent energy storage capacity are comprehensively reviewed, for both CW and passive Q-switching laser characteristics. After partially overcoming difficulties, such as the intense volatilization of B 2 O 3 , a bulk crystal was triumphantly grown using the Czochralski technique. However, the optical quality is still required to be improved since there are lots of inclusions, as presented above, although the Czochralski method has been proved to be feasible. The thermal properties of the Yb 3+ :ScBO 3 crystal, consisting og thermal conductivity and specific heat and so on, were characterized. In addition, the Yb 3+ :ScBO 3 crystal was found to possess a small emission cross-section of 0.12 × 10 −20 cm 2 and a long fluorescence lifetime of 5.2 ms, which is beneficial for generating moderate and high-energy pulsed lasers. A CW laser, with a maximal output power of 167 mW and a slope efficiency of 9% under the wavelength of 1.063 µm, was first obtained in the Yb 3+ :ScBO 3 crystal, utilizing a plane-concave cavity configuration. Then, a passive Q-switching laser in this crystal was achieved utilizing BP as the optical modulator, with a minimum pulse width of 495.5 ns and a maximal pulse energy of 1.4 µJ. The consequences show the feasibility of Yb 3+ :ScBO 3 crystal achieving pulsed lasers with moderate or high energy. Finally, the self-Q-switching and nonlinear optical characteristics of this crystal were evaluated and the mechanism was attributed to ground state re-absorption. The preferable spectroscopic characteristics and preliminary laser performances indicate that the as-grown Yb 3+ :ScBO 3 crystal would be a high-potential laser gain medium in establishing compact pulsed laser systems with moderate or even high energy output. Future research will be dedicated to the optimization of the crystal growth in the aspects of establishing the thermal field with small gradient and searching for the optimal growth parameters, including pulling and rotation rates, etc. In the meantime, the actively Q-switched pulsed lasers with high energy output are also expected in the Yb 3+ :ScBO 3 crystal.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/app112210879/s1, Table S1. Passively Q-switching performance and corresponding spectral and thermal properties in commonly used Yb 3+ -doped crystals. Data Availability Statement: All the data reported in the paper are not publicly available at this time but maybe provided upon reasonable request.

Conflicts of Interest:
The authors declare no competing financial interest.