Qiu Li

Zhangli Shi

Qingsong Song

Jie Tian

Jian Liu

Qingguo Wang
email: wqingguo2013@163.com

Xiaodong Xu

Jun Xu

Kheirreddine Lebbou
email: kheirreddine.lebbou@univ-lyon1.fr

The micro-pulling-down growth of Tm:LuAG and Tm,Pr:LuAG crystals and optical properties

Keywords: Tm, Pr: Lu 3 Al 5 O 12, Micro-pulling-down (μ-PD) method, Near-infrared laser

Tm 3+ doped and Tm 3+ , Pr 3+ co-doped Lu 3 Al 5 O 12 (LuAG) single crystals have been successfully grown by the micro-pulling-down (μ-PD) method. The absorption spectra, fluorescence spectra and lifetimes were investigated at room temperature. In the absorption spectra, the transition centered at 782 nm matches well with the GaAlAs LDs laser pumping. The incorporation of Pr 3+ into Tm 3+ -doped LuAG crystals significantly enhanced the intensity of 1.5 μm emission and decreased the fluorescence lifetime of the lower energy level for 1.5 μm emission from 10.427 ms to 2.603 ms. The results indicate that Tm,Pr:LuAG crystal can be promising candidate for near infrared 1.5 μm laser materials.

Introduction

Laser sources operating at 1.5 μm region have attracted much attention due to the use for remote sensing, biomedical systems, optical subscriber networks, telecommunication industry, and so on [START_REF] Zhang | Enhanced emission of the 1.5-1.67 μm fluorescence in Er 3+ ,Ce 3+ -codoped Lu 3 Al 5 O 12 crystal[END_REF][START_REF] Shi | Role of PbO substitution by Bi 2 O 3 on 1.47 μm luminescence properties of Tm 3+ /Tb 3+ -doped Bi 2 O 3 -GeO 2 -Ga 2 O 3 glass[END_REF][START_REF] Burns | Optimization of Er, Yb:YCOB for CW laser operation[END_REF]. Using an AlGaAs diode laser as pump source, trivalent thulium (Tm 3+ ) can provide laser oscillation at this specific wavelength through the 3 H 4 → 3 F 4 transition channel. However, the transition of Tm 3+ that generates emission around 1.5 μm is considered to be self-terminating, for the lifetime of 3 F 4 energy level is much longer than that of 3 H 4 energy level [START_REF] Percival | Highly efficient 1.064 μm upconversion pumped 1[END_REF]. In order to deal with the unfavorable population bottleneck and depopulate the 3 F 4 state efficiently, co-doping sensitive ions like Tb 3+ , Ho 3+ and Pr 3+ ions were proposed to quench the terminal 3 F 4 level and increase the emission intensity at around 1.5 μm by means of energy transfer [START_REF] Zhang | Enhanced 1.4 µm emissions of Tm 3+ via Tb 3+ deactivation in (Gd 0.5 Lu 0.5 ) 2 SiO 5 crystal[END_REF][START_REF] Rosenblatt | Diode-Pumped Room-Temperature cw 1.45 µm Tm,Tb:YLF laser, Advanced Solid State Lasers[END_REF][START_REF] Ermeneux | Growth and fluorescence properties of Tm 3+ doped YVO 4 and Y 2 O 3 single crystals[END_REF][START_REF] Stoneman | Continuous-wave 1.50-µm thulium cascade laser[END_REF][START_REF] Mazurak | Optical properties of the Tm 3+ and energy transfer between Tm 3+ →Pr 3+ ions in P 2 O 5 -CaO-SrO-BaO phosphate glass[END_REF].

Lu 3 Al 5 O 12 (LuAG) is a promising laser host material due to its relatively high thermal conductivity, chemical and physical stability [START_REF] Sugiyama | Scintillation properties of Tm-doped Lu 3 Al 5 O 12 single crystals[END_REF]. Tm:LuAG crystal can be easily grown by the Czochralski method [START_REF] Xu | Crystal growth, spectroscopic and laser properties of Tm:LuAG crystal[END_REF] and micro-pulling-down method [START_REF] Song | Laser Operation In a Tm:LuAG Crystal Grown By the Micro-pulling-down Technique[END_REF]. In 1995, A diode-pumped Tm:LuAG laser was demonstrated with a relatively low optical-to-optical efficiency of 7.3% [START_REF] Barnes | Diode-pumped, room-temperature Tm:LuAG laser[END_REF]. For continuous-wave operation, a maximum output power of 4.91 W was reported with a slope efficiency of 25.3 % [START_REF] Wu | Diode-pumped Tm:LuAG laser at room temperature[END_REF]. To the best of our knowledge, there has no research about the growth and performance of Tm,Pr:LuAG crystal.

In this work, Tm,Pr:LuAG crystal was grown by the micro-pulling-down method. The spectroscopic properties of Tm,Pr:LuAG crystal were reported and Pr 3+ ion was demonstrated to be an efficient deactivated ion for Tm 3+ ion by effective energy transfer from Tm 3+ : 3 F 4 level to Pr 3+ : 3 F 2 level.

Experiments 2.1 Crystal growth

The 2.0 at.% Tm 3+ , 0.2 at.% Pr 3+ co-doped LuAG and 2.0 at.% Tm 3+ single doped LuAG single crystals were grown by the μ-PD method [START_REF] Xue | Spectroscopic and laser properties of Tm:CNGG crytsals grown by he micropulling-down method[END_REF][START_REF] Xu | Spectroscopic characteristics of Dy 3+ -doped Y 3 Al 5 O 12 (YAG) and Y 3 ScAl 4 O 12 (YSAG) garent single crystals grown by the micro-pulling-down method[END_REF]. The Tm 2 O 3 , Lu 2 O 3 , Al 2 O 3 and Pr 6 O 11 powders with purity of 99.999% were prepared as raw materials. After being mixed, the mixtures were shaped into a rod and pressed by cold isostatic, then sintered at 1300°C for 12 h in a muffle furnace. A <111> oriented LuAG crystal was used as seed and the pulling rate was 0.3 mm/min. The growth atmosphere was high-purity flowing argon. Fig. 1 shows the obtained Tm:LuAG and Tm,Pr:LuAG crystals with diameter of about 2 mm. The crystals are transparent and colorless with homogeneous diameter and free of macroscopic defects.

Spectral measurements

The Tm:LuAG and Tm,Pr:LuAG crystal samples were cut from the as-grown crystals and two surfaces were polished for measurements. The absorption spectra of the samples were performed by a spectrometer (Lambda 900, Perkin-Elmer UV-VIS-NIR) in the wavelength range of 400-2100 nm. An Edinburgh Instruments FLS 980 spectrophotometer was used to measure the emission spectra and decay curves under the excitation of 790 nm. All the measurements of the samples above were taken at room temperature.

Results and discussion

Absorption spectra

The room temperature absorption spectra of Tm:LuAG and Tm,Pr:LuAG single crystals in the wavelength range of 400-2000 nm are shown in Fig. 2. The five absorption bands of Tm:LuAG crystal centered at 460, 681, 782, 1178 and 1632 nm correspond to the transitions from the ground state 3 H 6 to the excited states 1 D 2 , 1 G 4 , 3 F 2,3 , 3 H 4 , 3 H 5 and 3 F 4 , respectively. The absorption band centered at around 1472 nm, corresponding to the 3 H 4 → 3 F 4 + 3 F 3 transition of Pr 3+ ions, is also clearly in the absorption spectrum of Tm,Pr:LuAG crystal. As is known to all, the transition at 782 nm matches well with AlGaAs LDs pumping sources. The absorption coefficient at 782 nm of Tm:LuAG and Tm,Pr:LuAG was calculated to be 1.23 cm -1 and 1.30 cm -1 , respectively. Besides, the full width at half maximum (FWHM) of absorption band at 782 nm was 11.4 nm and 15.0 nm, respectively. The FWHM is larger than the value of Tm:LYSO (8nm at 791nm [START_REF] Di | Growth, crystal structure and optical study of Tm:LuYSiO 5 single crystal[END_REF]), Tm:YAP (3.9 nm at 777 nm [START_REF] Guillemot | Continuous-wave Tm:YAlO 3 laser at ∼2.3 μm[END_REF]) and Tm:KY 3 F 10 (3.8nm at 778 nm [START_REF] Guillemot | Close look on cubic Tm:KY 3 F 10 crystal for highly efficient lasing on the 3 H 4 → 3 H 5 transition[END_REF]). The absorption bands of Tm:LuAG and Tm,Pr:LuAG crystals around 782 nm are wide enough to steadily absorb the incident pump power.

Fluorescence spectra

Fig. 3 shows the fluorescence spectra of Tm:LuAG and Tm,Pr:LuAG crystals under 790 nm excitation at room temperature. Both samples have similar emission bands around 1431 nm, which correspond to the 3 H 4 → 3 F 4 transition of Tm 3+ ions. As can be seen from Fig. 3, the intensity of the emission line around 1431nm increased after the introduction of Pr 3+ ions. The enhanced fluorescence emission of the 3 H 4 → 3 F 4 transition can justify that the introduction of Pr 3+ ions can depopulate the Tm 3+ : 3 F 4 level effectively. The results suggest that Tm,Pr:LuAG crystal could be a promising material for 1.5 μm laser generation.

fluorescence lifetime

The room-temperature decay curves of 3 H 4 and 3 F 4 levels of Tm:LuAG and Tm, Pr:LuAG crystals excited by 790 nm laser are shown in Fig. 4. For Tm:LuAG crystal, the fluorescence lifetimes of 3 H 4 and 3 F 4 level were measured to be 378 μs and 10.427 ms, respectively. It can be easily found that the fluorescence lifetime of the 3 F 4 terminal level is much longer than that of the 3 H 4 initial level. Therefore the laser around 1.5 μm is normally self-terminating. In order to depopulate the lower level through the energy transfer process, we introduce Pr 3+ ions into Tm:LuAG crystal. Fig. 5 shows the energy level scheme of Tm 3+ and Pr 3+ ions in LuAG crystal. Under a 790 nm LD excitation, the Tm 3+ ions are excited from ground state 3 H 6 to the excited state 3 H 4 . The fluorescence emission at 1.5 μm is due to the Tm 3+ : 3 H 4 → 3 F 4 transition. For the 1.8 μm emission, the population of 3 F 4 level is based on the cross relaxation (CR) Tm 3+ : 3 H 4 + 3 H 6 → 3 F 4 + 3 F 4 , and the cross relaxation process increased lifetime of 3 F 4 level [START_REF] Sasikala | Photoluminescence properties of singly doped Tm 3+ and co-doped Tm 3+ /Tb 3+ ions in tellurite glasses[END_REF]. Since the Pr 3+ : 3 F 2 level is adjacent to the Tm 3+ : 3 F 4 level, an effective energy transfer (ET) Tm 3+ : 3 F 4 + Pr 3+ : 3 H 4 → Tm 3+ : 3 H 6 + Pr 3+ : 3 F 2 will depopulate the Tm 3+ : 3 F 4 level. To verify the mechanism, the lifetimes of 3 H 4 and 3 F 4 levels of Tm,Pr:LuAG crystal are compared with those of Tm:LuAG crystal. By co-doping Pr 3+ ions into Tm:LuAG crystals, the lifetime of 3 F 4 level decreased greatly to 2.603 ms, while the lifetime of 3 H 4 level experienced a slight decrease from 378 μs to 346 μs. The lifetime of 3 H 4 level of Tm,Pr:LuAG crystal is much higher than the value of 0.5% Tm, 1% Tb:YVO 4 (50 μs [START_REF] Ermeneux | Growth and fluorescence properties of Tm 3+ doped YVO 4 and Y 2 O 3 single crystals[END_REF], 0.5% Tm:GdScO 3 (265 μs [START_REF] Li | Crystal growth, spectroscopic characteristics, and Judd-Ofelt analysis of Tm:GdScO 3[END_REF]), 1% Tm:KGd(WO 4 ) 2 (175 μs [START_REF] Güell | Blue luminescence in Tm 3+ -doped KGd(WO 4 ) 2 single crystals[END_REF]) and 1.5% Tm:YAG (270 μs [START_REF] Sudesh | Spectroscopy, modeling, and laser operation of thulium-doped crystals at 2.3 μm[END_REF]). To further investigate the effect of Pr 3+ ions, the energy transfer efficiency was calculated according to the flowing formula:

(1) where and are the fluorescence lifetimes of Tm:LuAG and Tm,Pr:LuAG crystals, respectively. The measured fluorescence lifetimes τ and calculated energy transfer efficiency η of Tm:LuAG and Tm,Pr:LuAG crystals are listed in Table 1. The he energy transfer efficiency from Tm 3+ : 3 F 4 level to Pr 3+ : 3 F 2 level was calculated to be75.0%. The results indicate that the co-doping of Pr 3+ ions can effectively depopulate 3 F 4 lower level, while has little influence on the 3 H 4 upper level. Therefore, the co-doping of Pr 3+ ions into Tm 3+ doped crystals can be used as an effective way for 1.5 μm laser operation.

Conclusion

In conclusion, Tm 3+ and Tm 3+ ,Pr 3+ co-doped LuAG single crystals were successfully grown by the μ-PD method. Compared with Tm:LuAG crystal, the absorption coefficient and FWHM at 782 nm of Tm 3+ ,Pr 3+ co-doped LuAG crystal is higher. It was also demonstrated that after the introduction of Pr 3+ ions, the intensity of 1.5 μm emission increased and the fluorescence lifetime of Tm 3+ : 3 F 4 level decreased greatly from 10.427 ms to 2.603 ms, while the lifetime of Tm 3+ : 3 H 4 level changed little from 378 μs to 346 μs. Pr 3+ ions can efficiently depopulate the 3 F 4 lower level, while having little influence on the 3 H 4 upper level of Tm 3+ ions. All the results show that Tm,Pr:LuAG crystal could be a potential laser material for eye-safe 1.5 μm laser operation.

Fig. 1 .

 1 Photographs of the as-grown 2.0 at.% Tm:LuAG (a) and 2.0 at.% Tm, 0.2 at.% Pr:LuAG (b) single crystals.

Fig. 2 .

 2 Fig. 2. Room temperature absorption spectra of Tm:LuAG and Tm,Pr:LuAG crystals.

Fig. 3 .

 3 Fig. 3. Room temperature fluorescence spectra of Tm:LuAG and Tm,Pr:LuAG crystals.

4 Fig. 4 .Fig. 5 .

 445 Fig.4. Fluorescence decay curves of the 3 H 4 and 3 F 4 mainfold of Tm:LuAG and Tm,Pr:LuAG crystals.

Table 1

 1 Fluorescence lifetimes τ and fluorescence quantum efficiencies η for the Tm:LuAG and Tm, Pr:LuAG crystals.

	Tm:LuAG	τ(μs) 3 H 4	τ(ms) 3 F 4	η 3 F 4
	2%Tm	378	10.427
	2%Tm, 0.2%Pr	346	2.603	75.0%

Acknowledgements

This work is partially supported by National Natural Science Foundation of China (No. 61621001) and "Qinglan Project" of the Young and Middle-aged Academic Leader of Jiangsu College and University.


Yuxin Pan

Bei Liu

Jian Liu

Qingsong Song

Jie Xu

Dongzhen Li

Peng Liu

Jie Ma

Xiaodong Xu

Hui Lin

Jun Xu
email: xujun@mail.shcnc.ac.cn

Kheirreddine Lebbou

Polarized spectral properties and laser operation of Nd:SrAl 12 O 19 crystal

Keywords: Nd:SRA, Czochralski method, Judd-Ofelt theory, Continuous-wave laser

Nd:SrAl 11 O 19 (Nd:SRA) crystal has been grown by the Czochralski method. Polarized absorption spectra, polarized fluorescence spectra and fluorescence lifetime were investigated. For σpolarization, the peak absorption cross section at 798 nm is 2.08×10 -20 cm 2 with full width at half maximum (FWHM) of 10.2 nm and the peak emission cross section is 3.36×10 -20 cm 2 at 1048.5 nm with FWHM of 4.80 nm. The Judd-Ofelt parameters Ω 2,4,6 were obtained to be 0.51×10 -20 cm 2 , 2.08×10 -20 cm 2 , and 2.12×10 -20 cm 2 , respectively. Continuous-wave laser operation of the a-cut and ccut Nd:SRA sample has been demonstrated. The maximum output power of 8.33 W was achieved from c-cut sample at an absorbed power of 27.73 W, corresponding to a slope efficiency of 31.3%.

Introduction

Laser sources operating at 1.0 μm wavelength region have attracted much attention due to their important applications, such as biology, medical detection, optical communication and scientific research [START_REF] Kane | Coherent laser radar at 1.06 μm using Nd:YAG lasers[END_REF][START_REF] Washio | Neodymium-doped solid-state lasers and their applications to materials processing[END_REF][START_REF] Moncorge | Nd doped crystals for medical laser applications[END_REF]. In general, 1.0 μm wavelength laser was realized in trivalent Yb 3+ -and Nd 3+ -doped laser materials. Compared with Yb 3+ -doped gain materials, Nd 3+ -doped materials have relatively large emission cross section and negligible re-absorption loss due to their four-level operation. Among Nd 3+doped laser crystals, Nd:YAG crystal has been proved to be excellent materials because of good physical and chemical properties. However, Nd:YAG crystal is limited to low Nd 3+ doping concentration, narrow absorption and emission bandwidth, which causes a strong sensitivity to the pumping wavelength of AlGaAs laser diodes and limits the pulse width when used for mode-locked lasers. Nd 3+ -doped disordered crystals are promising in realizing mode-locked femtosecond laser, because they possess better thermal conductivity than Nd:glass and broader emission spectra than Nd:YAG crystal. In 2009, Xie et al. generated a passively mode-locked Nd:CLNGG disordered crystal laser with the pulse of 900 fs [START_REF] Xie | Subpicosecond pulse generation from a Nd:CLNGG disordered crystal laser[END_REF]. Qin et al. demonstrated Nd,Y:CaF 2 disordered crystal laser with pulse duration as short as 103 fs [START_REF] Qin | Generation of 103 fs mode-locked pulses by a gain linewidth-variable Nd,Y:CaF2 disordered crystal[END_REF]. Ma et al. achieved stable continuous-wave mode-locked pulses as short as 79 fs from Nd:Ca 3 La 2 (BO 3 ) 4 disordered crystal [START_REF] Ma | Sub-80 femtosecond pulses generation from a diode-pumped mode-locked Nd:Ca3La2(BO3)4 disordered crystal laser[END_REF].

SrAl 11 O 19 (SRA) crystallizes in the magneto-plumbite structure with the space group P6 3 /mmc [START_REF] Lindop | Refined structure of Sr0.6Al2O3[END_REF][START_REF] Merkle | Spectroscopy and laser operation of Pr,Mg:SrAl12O19[END_REF][START_REF] Liu | Growth and red-orange emission of Sm 3+ doped SrAl12O19 single crystals[END_REF]. SRA crystal exhibits an excellent thermal conductivity of 11 W/m•K and high Moh's hardness of 9 [START_REF] Fechner | Crystal growth, spectroscopy, and diode pumped laser performance of Pr,Mg:SrAl12O19[END_REF]. When it is doped with Nd 3+ ion to form Nd:SRA, Nd 3+ ion will replace the Sr 2+ ion and charge compensation is achieved by substituting Mg 2+ ion for Al 3+ ion, which causes a complex and disordered lattice field in SRA crystal. In 1997, Verdun et al. reported the optical properties including absorption spectra and fluorescence lifetime of Nd 3+ -doped SrAl 12 O 19 crystal grown by the laserheated pedestal-growth method [START_REF] Verdum | Optical properties of Nd 3+ in single crystal SrAl12O19[END_REF]. Recently, we demonstrated a diode-pumped Nd:SRA Q-switched laser for the first time. A high repetition rate of 201 kHz and a Q-switched pulse of 346 ns were obtained, with the peak power of 1.87 W and single pulse energy of 0.65 µJ [START_REF] Zhao | High repetition rate passively Qswitched laser on Nd:SRA at 1049 nm with MXene Ti3C2Tx[END_REF].

In this work, Nd:SRA crystal has been grown by the Czochralski method. The polarized spectral of Nd:SRA crystal were studied. Using 800 nm laser diode as pumping source, the continuous-wave laser operation of Nd:SRA crystal was demonstrated.

Crystal growth

Nd:SRA crystal with Nd 3+ concentration of 5 at.% was grown using the Czochralski method. The MgO, SrCO 3 , Al 2 O 3 and Nd 2 O 3 powders with purity of 99.999% were used as starting raw materials. They were dried and weighed according to a definite molar ratio. Due to the incongruent melting behavior, an excess of SrCO 3 is required to obtain SRA single phase [START_REF] Gantis | Sr-Al2O3 system[END_REF]. After the compounds were thoroughly mixed, they were pressed into pellet and then sintered at 1300 ℃ for 20 h in air atmosphere. The charge was then loaded into an iridium crucible for crystal growth. The growth direction was along the crystalline a-axis with the pulling rate of 1mm/h and rotation rate of 10-20 rpm. High-purity nitrogen gas was introduced in the growth chamber as a protective atmosphere. Fig. 1 shows the asgrown Nd:SRA crystal with the size of Φ28×40 mm 3 . The crystal boule is transparent pink color. Some inclusions at the end (Tail) of the boule were observed.

In order to confirm the structure of the as-grow Nd:SRA crystal, room temperature powder X-ray diffraction (XRD, Bruker-D2, Germany) at a scan width of 0.05° with a range 2θ = 10-80° was performed. The XRD patterns of Nd:SRA crystal is displayed in Fig. 2. The diffraction peaks of the sample match well with the SRA standard card (JCPDS 80-1195). The data reveals that the as-grown Nd:SRA crystal possesses a hexagonal system with the space group of P6 3 /mmc. The cell parameters of Nd:SRA crystal were measured to be a=b=5.5638(3)Å, c=21.98558(5)Å. The Nd 3+ ions concentration in Nd:SRA crystal was measured with an inductively coupled plasma atomic emission spectrometry (ICP-AES). The segregation coefficient of Nd 3+ ions in Nd:SRA crystal was calculated to be arround 1.35.

Spectral properties

The crystal sample for spectroscopic measurements was cut from the as-grown Nd:SRA crystal, and the surfaces perpendicular to the < 100 > growth axis were polished. The selected Nd 3+ dopant concentration has been chosen to avoid strong emission quenching. The polarized absorption spectra were measured with a UV/VIS/NIR spectrophotometer (Model Cary-5000, Varian, USA) at room temperature. The fluorescence spectra and the fluorescence lifetime were recorded with an Edinburg Instrument FLS980 fluorescence spectrometer under 796 nm excitation. All the measurements were taken at room temperature. Fig. 3 presents the polarized absorption spectra of Nd:SRA crystal in the wavelength region of 300-1000 nm. All the absorption bands corresponding to transitions from the 4 I 9/2 ground state to the various excited states of Nd 3+ ions are marked. The absorption spectra show strong polarization dependence due to the anisotropy of the crystal, and the absorption cross-section of the σ polarization is much higher than that of the π polarization. The absorption cross sections were calculated to be 2.08×10 -20 cm 2 at 798 nm for σ polarization and 0.21×10 -20 cm 2 at 794 nm for π polarization, with full width at half maximum (FWHM) of 10.2 nm and 23.4 nm, respectively. The relatively large FWHM permits high conversion efficiency when pumped by AlGaAs laser diode. Besides, the absorption cross section for σ polarization is comparable with that of Nd:Ca 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (2.18×10 -20 cm 2 at 792 nm [START_REF] Pan | Spectroscopic properties and continuous-wave laser operation of Nd:Ca0.7La0.3Mg0.3Al11.7O19[END_REF]), but is much larger than the value of Nd:LaMgAl 11 O 19 (1.7×10 -20 cm 2 at 795 nm [START_REF] Pan | Growth, spectral properties, and diode-pumped laser operation of a Nd 3+ -doped LaMgAl11O19 crystal[END_REF]) and Nd:Sr 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (8.6×10 -21 cm 2 at 792 nm [START_REF] Zheng | Diode-pumped laser operation at 1053 and 900 nm in Sr1-xLax-yNdyMgxAl12-xO19 (Nd:ASL) single crystal[END_REF]). The Judd-Ofelt theory [START_REF] Judd | Optical absorption intensities of rare-earth ions[END_REF][START_REF] Ofelt | Intensities of crystal spectra of rare-earth ions[END_REF], which is the most popular and useful method for evaluating spectroscopic parameters of rare earth ions in materials, was applied in our paper. The reduced matrix elements used for absorption and emission transitions can be found in references [START_REF] Carnall | Energy levels in the Trivalent Lanthanide Aqua Ions. I. Pr 3+ , Nd 3+ , Pm 3+ , Sm 3+ , Dy 3+ , Ho 3+ , Er 3+ , and Tm 3+[END_REF] and [START_REF] Kaminskii | Spectroscopy of a new laser garnet Lu3Sc2Ga3O12:Nd 3+ . Intensity luminescence characteristics, stimulated emission, and full set of squared reduced-matrix elements |<‖U (t) ‖>| 2 for Nd 3+ ions[END_REF], respectively. The J-O intensity parameters Ω 2 , Ω 4 and Ω 6 are listed in Table 1. The effective J-O intensity parameters were calculated by Ω= (2Ω σ + Ω π )/3, and the Ω 2,4,6 were obtained to be 0.51×10 -20 cm 2 , 2.08×10 -20 cm 2 , and 2.12×10 -20 cm 2 , respectively. In general, Ω 2 is covalency-dependent parameter related to contravalency of the Nd 3+ ion sites. Compared with Nd:YAG (Ω 2,4,6 = 0.2, 2.7, and 5.0×10 -20 cm 2 [START_REF] Krupke | Radiative transition probabilities within 4 F3 ground configuration of Nd-YAG[END_REF]), Nd:SRA has a relatively large Ω 2 value, which indicates that the covalency of Nd:SRA is higher than that of Nd:YAG.

The spontaneous transition rate, branching ratios and the radiative lifetime of 4 F 3/2 → 4 I 9/2 , 4 F 3/2 → 4 I 11/2 , 4 F 3/2 → 4 I 13/2 and 4 F 3/2 → 4 I 15/2 transitions of Nd:SRA crystal are given in Table 2. We can see that the branching ratio of the 4 F 3/2 → 4 I 9/2 transition is smaller than that of the 4 F 3/2 → 4 I 11/2 transition for σ polarization, but larger than that of the 4 (500 μs at 792 nm [START_REF] Zheng | Diode-pumped laser operation at 1053 and 900 nm in Sr1-xLax-yNdyMgxAl12-xO19 (Nd:ASL) single crystal[END_REF]), Nd:KGd(WO 4 ) 2 (111 μs [START_REF] Chen | Polarized spectroscopic properties of Nd 3+ -doped KGd(WO4)2 single crystal[END_REF]), Nd:Bi 4 Ge 3 O 12 (293 µs [START_REF] Li | Optical properties of Nd:Bi4Ge3O12 crystals grown by the micro-pulling-down method[END_REF]), and Nd:BaBi 2 (MoO 4 ) 4 (115 µs [START_REF] Lebedev | Growth and spectroscopic studies of Nd 3+ -doped BaBi2(MoO4)4 crystal[END_REF]). The results indicate that Nd:SRA crystal has a higher energy storge ability. The polarized fluorescence spectra of Nd:SRA crystal in the range of 850-1450 nm was recorded under 796 nm excitation. The stimulated emission cross section can be calculated from the fluorescence spectra using the Füchtbauere Ladenburg (F-L) formula [START_REF] Aull | Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections[END_REF]. Fig. 4 presents the stimulated emission cross section for each polarization, and three emission bands are assigned to the transitions from 4 F 3/2 energy level to 4 I 9/2 , 4 I 11/2 and 4 I 13/2 energy level, respectively. The emission cross section of the σ polarization is larger than that of the π polarization. The peak emission cross section was calculated to be 3.36×10 -20 cm 2 for σ polarization and 0.65×10 -20 cm 2 for π polarization both at 1048.5 nm, respectively, with FWHM of 4.80 and 8.25 nm. The emission cross section of 4 [START_REF] Zheng | Diode-pumped laser operation at 1053 and 900 nm in Sr1-xLax-yNdyMgxAl12-xO19 (Nd:ASL) single crystal[END_REF]), Nd:CaYAlO4 (129 µs [START_REF] Li | Polarized spectral properties of Nd 3+ ions in CaYAlO4 crystal[END_REF]), Nd:GdNbO 4 (178 µs [START_REF] Ding | Crystal growth, spectral properties, and continuous wave laser operation of Nd:GdNbO4[END_REF]) and Nd:YAG (248 µs [START_REF] Singh | Stimulated-emission cross section and fluorescent quantum efficiency of Nd 3+ in yttrium aluminum garnet at room temperature[END_REF]). According to the radiative lifetime from Table . 2, the luminescent quantum efficiency of the 4 F 3/2 level was calculated to be 79.0% by the equation of η=τ f /τ rad . The results show that Nd:SRA crystal is a promising gain medium for solid-state laser.

Laser properties

Samples used in the laser experiments were cut along a and c crystalline axis with dimensions of 3 × 3 × 6 mm 3 . The LD-pumped continuous-wave laser performance of Nd:SRA was carried out in a simple plano-plano resonator. The schematic diagram of the setup is the same as in [START_REF] Pan | Spectroscopic properties and continuous-wave laser operation of Nd:Ca0.7La0.3Mg0.3Al11.7O19[END_REF]. The pump source was a fiber-coupled 800 nm AlGaAs diode laser with a core diameter of about 200 μm and a numerical aperture of 0.22. Three different output couplers (OCs), with transmissions of 5%, 10% and 15% at 1049 nm, were used in the experiments.

Fig. 6(a) shows the output power as a function of absorbed pump power of a-cut Nd:SRA laser. The threshold absorbed pump powers were 0.04, 0.099 and 0.05W, respectively, for the OCs with transmissions of 5%, 10% and 15%. When the output coupler transmission was 10%, a maximum output of 4.315 W was achieved at the absorbed pump power of 15.41 W, corresponding to an opticalto-optical conversion efficiency of 28.0% and a slope efficiency of 28.8%. The laser spectrum registered at maximum output power is shown in Fig. 6(b) with peak wavelength of about 1049.48 nm. The dependence of the output power on the absorption pump power of c-cut Nd:SRA laser is illustrated in Fig. 7(a). Using the output coupler with transmission of 5%, a maximum output power reached about 7.19 W with the threshold of about 0.061 W and the slope efficiency of about 26.8%. Using the output coupler with transmission of 10%, a maximum output power up to 8.25 W was achieved with the threshold of 0.032 W of absorbed power, which led to a slope efficiency of about 30.5%. Using the output coupler with transmission of 15%, the maximum output power increased to 8.33 W with the threshold of about 0.091 W. The slope efficiency was linearly fitted to be about 31.3% and the corresponding optical-to-optical conversion efficiency was 30.0%. The maximum output power of Nd:RSA is larger than that of Nd:Ca 0.7 La 0. [START_REF] Zheng | Diode-pumped laser operation at 1053 and 900 nm in Sr1-xLax-yNdyMgxAl12-xO19 (Nd:ASL) single crystal[END_REF]). At maximum output power, the peak wavelength was measured to be 1049.28 nm (see Fig. 7(b)). There was no pump saturation in our experiments, which indicates the output power can be further scaled with high pump power. corresponding laser spectrum achieved at maximum output power

Conclusions

In conclusion, Nd:SRA crystal has been grown from the melt by Czochralski method. The crystal crystallizes in the hexagonal system with space group P6 3 /mmc. The cell parameters of Nd:SRA crystal were measured to be a=b=5.5638Å, c=21.98558Å. Polarized absorption spectra, polarized fluorescence spectra and fluorescence lifetime were investigated. The peak absorption cross-section was 2.08×10 -20 cm 2 at 798nm for σ polarization, with the FWHM of 10.18 nm. The effective intensity parameters Ω 2,4,6 were obtained to be 0.51×10 -20 cm 2 , 2.08×10 -20 cm 2 , and 2.12×10 -20 cm 2 , respectively. The peak emission cross section was 3.36×10 -20 cm 2 for σ-polarization at 1048.5 nm, with a FWHM of 4.80 nm. The radiative and fluorescence lifetime are 552 and 436 μs, respectively, resulting in a quantum efficiency of 79.0%. Continuous-wave laser operation of Nd:SRA under 800nm laser diode has been demonstrated. For a-cut sample, the maximum output power of 4.315 W was obtained at an absorbed power of 15.41 W with the T OC = 10%, corresponding to a slope efficiency of 28.8%. For c-cut sample, the maximum output power of 8.33 W was obtained at an absorbed power of 27.73 W with the T OC = 15%, corresponding to a slope efficiency of 31.3%.

Fig. 1 .

 1 Fig. 1. As-grown single crystal ingot of Nd:SRA.

Fig. 2 .

 2 Fig. 2. Room temperature X-ray diffraction pattern of Nd:SRA crystal and standard pattern of SRA.

Fig. 3 .

 3 Fig. 3. Polarized absorption spectra of Nd:SRA crystal at room temperature.

  F 3/2 → 4 I 11/2 transition for π polarization. The radiative lifetime of 4 F 3/2 energy level was calculated to be 552 μs, which is much longer than the value of Nd:Ca 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (364 μs [14]), Nd:LaMgAl 11 O 19 (401 μs [15), Nd:Sr 0.7 La 0.3 Mg 0.3 Al 11.7 O 19

  F 3/2 → 4 I 11/2 transition for σ polarization is smaller than the value of Nd:Ca 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (4.01×10 -21 cm 2 at 1052 nm [14]) and Nd:LaMgAl 11 O 19 (4.08×10 -20 cm 2 at 1054.5 nm [15]).

Fig. 4 .

 4 Fig. 4. Polarized emission spectra of Nd:SRA crystal excited by 796 nm at room temperature Fig. 5 shows the fluorescence decay curve of the 4 F 3/2 multiplet. The decay curve shows a single exponential decaying behavior. The fluorescence lifetime was fitted to be 436 µs, which is much longer than that of Nd:Ca 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (333 μs [14]), Nd:LaMgAl 11 O 19 (321 μs [15), Nd:Sr 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (372 μs[START_REF] Zheng | Diode-pumped laser operation at 1053 and 900 nm in Sr1-xLax-yNdyMgxAl12-xO19 (Nd:ASL) single crystal[END_REF]), Nd:CaYAlO4 (129 µs[START_REF] Li | Polarized spectral properties of Nd 3+ ions in CaYAlO4 crystal[END_REF]), Nd:GdNbO 4 (178 µs[START_REF] Ding | Crystal growth, spectral properties, and continuous wave laser operation of Nd:GdNbO4[END_REF]) and

Fig. 5 .

 5 Fig. 5. The fluorescence decay curve of the 4 F 3/2 manifold of Nd:SRA

Fig. 6 (

 6 Fig. 6 (a)Output powers versus absorbed pump powers of doped-pumped a-cut Nd:SRA laser and (b) corresponding laser spectrum achieved at maximum output power

Fig. 7 (

 7 Fig. 7 (a)Output powers versus absorbed pump powers of doped-pumped c-cut Nd:SRA laser and (b) corresponding laser spectrum achieved at maximum output power

Table 1 .

 1 The J-O intensity parameters of Nd:SRA crystal.

	Intensity parameter		Ω (10 -20 cm 2 )
		σ-polarization	π-polarization	Ω= (2Ω σ + Ω π )/3
	Ω 2	0.72	0.10	0.51
	Ω 4	2.59	1.07	2.08
	Ω 6	2.93	0.51	2.12

Table 2 .

 2 The spontaneous transition rate, branching ratios and radiative lifetime for different transition levels of Nd:SRA crystal.

	Transitions	σ-polarization	π-polarization
	4 F 3/2 →	A σ (S -1 )	β σ (%)	A π (S -1 )	β π (%)
	4 I 9/2 4 I 11/2 4 I 13/2 4 I 15/2	997.12 1153.31 229.22 4.88	41.82 48.37 9.61 0.20	363.48 254.77 40.19 0.84	55.13 38.64 6.10 0.13
	Radiative lifetime (μs)		τ rad = 552

  3 Mg 0.3 Al 11.7 O 19 (5.40 W [14]), Nd:LaMgAl 11 O 19 (1.71 W [15), Nd:Sr 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (6.9 W [16]), but the slope efficiency of Nd:SRA is lower than that of Nd:Ca 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (47.7% [14]), Nd:LaMgAl 11 O 19 (40.4% [15), Nd:Sr 0.7 La 0.3 Mg 0.3 Al 11.7 O 19 (50%

Funding

National Natural Science Foundation of China (No. 61621001) and "Qinglan Project" of the Young and Middle-aged Academic Leader of Jiangsu College and University.

Disclosures. The authors declare no conflicts of interest.