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# Diode-pumped mode-locked Yb:YScO3 laser

Longxin Liu, Jian Liu, Zhiyong Liang, Zhengxin Jiang, Chuang Yang, Chong

Xu, Jie Ma, Xu Xiaodong, Kheirreddine Lebbou, Hai-Yu Nie, et al.

► To cite this version:

Longxin Liu, Jian Liu, Zhiyong Liang, Zhengxin Jiang, Chuang Yang, et al., "Diode-pumped mode-locked Yb:YScO3 laser," Optics Express, 2025, 33 (4), pp.7508. 10.1364/OE.549309. hal-04996668

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Submitted on 19 Mar 2025

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# Diode-pumped mode-locked Yb:YScO₃ laser

LONGXIN LIU,<fnref n="1" /> JIAN LIU,<fnref n="1" />,<fnref n="5" /> ZHIYONG LIANG,<fnref n="1" /> ZHENGXIN JIANG,<fnref n="1" />
CHUANG YANG,<fnref n="1" /> CHONG XU,<fnref n="1" /> JIE MA,<fnref n="1" /> XU XIAODONG,<fnref n="1" />,<fnref n="6" />
KHEIRREDDINE LEBBOU,<fnref n="2" /> HAI-YU NIE,<fnref n="3" /> GE ZHANG,<fnref n="3" /> WEIDONG
CHEN,<fnref n="3" /> ID AND JUN XU<fnref n="4" />

**Abstract:** To the best of our knowledge, we report on the mode-locking of the mixed sesquioxide single crystal Yb:YScO₃ for the first time. The spectra of Yb<fnref n="3" />⁺ ions in this crystal were studied at both room temperature and low temperature. A continuous-wave laser output at 1080 nm was achieved using a fiber-coupled semiconductor laser, with a pump wavelength of 976 nm, in the Yb:YScO₃ crystal, yielding a maximum output power of 7.45 W and a slope efficiency of 52.3%. By implementing a SESAM for initiating and stabilizing the mode-locked operation, the Yb:YScO₃ laser produced soliton pulses as short as 48 fs at 1056.4 nm, with an average output power of 45 mW at a pulse repetition rate of ~ 71.55 MHz. These results suggest that Yb:YScO₃ crystals are promising candidates for high-power ultrashort pulse lasers.

© 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

## 1. Introduction

Rare earth scandate crystals (ReScO₃), known for their high refractive index and excellent transparency, have a wide range of applications in optics [1–4]. They are frequently employed in the development of luminescent materials and optical devices. These crystals exhibit exceptional thermal and chemical stability, enabling them to retain superior performance under both high-temperature and high-pressure conditions [5,6]. Additionally, these ternary oxides possess a high dielectric constant, low leakage current, a substantial thermal budget, and significant optical energy capabilities [7–12].

Among the many ultrafast laser media, those doped with rare-earth ions Nd<fnref n="3" />⁺ and Yb<fnref n="3" />⁺ are particularly attractive in the 1-µm band. Nd<fnref n="3" />⁺ laser crystals exhibit excellent thermodynamic properties and a large emission cross-section, but their fluorescence spectrum is relatively narrow. As a result, they are commonly used in high-power picosecond lasers and amplifiers. In contrast, Yb<fnref n="3" />⁺-doped gain media offer significant advantages for femtosecond pulsed laser generation. The broad optical spectrum of Yb<fnref n="3" />⁺ allows for the generation of ultrashort pulses down to sub-100 femtoseconds. In the ongoing quest for shorter pulses, considerable efforts have been made to extend the wavelength range covered by ultrashort pulses. Recent research has demonstrated the generation of lasers beyond the fluorescence spectral range of 400 nm in Yb<fnref n="3" />⁺-doped YCOB crystals, utilizing a multiphonon process where electron jumps are accompanied by the emission or absorption of multiple phonons [13]. Additionally, a transition from a zero-phonon line laser at 1064 nm to a phonon-pumped laser at 1176 nm has been achieved in Nd:YVO₄ lasers

<footnote n="1">¹Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China</footnote>

<footnote n="2">²Institut Lumière Matière, UMR5306 Université Lyon1-CNRS, Université de Lyon, Lyon 69622, Villeurbanne Cedex, France</footnote>

<footnote n="3">³Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 350002 Fuzhou, China</footnote>

<footnote n="4">⁴School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China</footnote>

<footnote n="5">⁵jianliu586@jsnu.edu.cn</footnote>

<footnote n="6">⁶xdxu79@jsnu.edu.cn</footnote>
by coexciting electron jumps with both photon and phonon pumping [14]. This breakthrough enables low-threshold laser output at high temperatures, paving the way for the development of new types of lasers.

YScO₃ crystallizes in a bixbyite-type cubic structure characterized by disorder due to a random distribution of Y³⁺ and Sc³⁺ cations [15]. The disordered crystal structure results in broader and smoother spectra, attributed to inhomogeneous spectral broadening, which facilitates the generation of ultrashort pulses. Ideally, these disordered crystals should exhibit favorable thermal, thermo-optic, and thermo-mechanical properties to support power scaling and enable high peak power pulse output. Consequently, YScO₃ shows significant potential for ultrafast lasers. Various dopants have been incorporated into YScO₃ crystals, including Nd³⁺ [16], Gd³⁺ [17], Tm³⁺ [18,19], Cr³⁺ [15], Er³⁺ [20], and Yb³⁺ [21]. These sesquioxides are notable candidates for high-power laser systems alongside Yb, owing to their robust emission gain, excellent thermo-mechanical resilience, and broad emission bandwidth. However, a significant challenge in utilizing these materials is their high melting point, which complicates crystal growth processes. Recently, Christian Kränkel et al. demonstrated that approaching a stoichiometric ratio close to 1:1 can lower the melting point to approximately 2200 °C, thereby enabling crystal growth via the conventional Czochralski (Cz) method [22].

In this work, Yb³⁺-doped YScO₃ crystal was successfully grown using the Czochralski (Cz) method. Its spectral and laser properties were characterized and continuous-wave laser operation at 1 µm were demonstrated. Furthermore, Yb:YScO₃ crystal was used to generate 47 fs pulses from a passive mode-locked laser.

## 2. Crystal growth and structure

The Yb:YScO₃ crystal was grown using the Czochralski method. Yb₂O₃, Y₂O₃, and Sc₂O₃ powders with a purity of 99.99% (5N) were prepared as raw materials and weighed according to the formula Yb₀.₀₃Y₀.₄₈₅Sc₀.₄₈₅O₃. The mixed powders were pressed into bulks and then sintered in the air at 1400°C for 24 h. The treated blocks were then positioned within iridium crucibles under a nitrogen atmosphere for crystal growth. The pulling speed ranged between 0.5 and 1.5 mm/h, with a rotational speed of approximately 15 rpm. High-purity nitrogen gas was introduced as a protective atmosphere. The crystals were cooled to room temperature at a rate of 40 °C/h. Figure 1 shows the XRD patterns of 3 at.% Yb:YScO₃ crystals compared to the standard card of YScO₃ crystal (PDF#004-002-5314). According to X-ray powder diffraction data, the Yb³⁺-doped yttrium scandate adopts a bixbyite-type cubic structure (space group Ia, lattice parameter α = 10.1445 Å). This reduction in lattice parameters (pure YScO₃ lattice parameter α = 10.2001 Å) is attributed to the substitution of Yb³⁺ with a smaller cation radius in place of Y³⁺.

<figure><img src="image_3.png" /><figcaption>Fig. 1. The XRD patterns of Yb:YScO₃ single crystal.</figcaption></figure>
### 3. Spectroscopic properties of Yb:YScO₃

The segregation coefficient of Yb³⁺ in the as-grown Yb:YScO₃ crystal can be calculated using the formula [23]: $k_m = C_t/C_o$. Where $C_t$ is the Yb³⁺ concentration at the growth starting position in the crystals, and $C_o$ is the initial Yb³⁺ concentration in the melt. The segregation coefficient of Yb³⁺ was found to be 0.96. The concentration of Yb³⁺ in the samples for spectroscopic measurements can be determined using the equation [24]: $C_s = C_0k_m(1-g)^{k_m-1}$. Where g is the crystallized fraction of the melt, $k_m$ is the segregation coefficient and $C_0$ is the initial Yb³⁺ concentration in the melt. The calculated value of $C_s$ is equal to 2.89%. For spectral measurement, the sample was cut from the as-grown crystal and optically polished to a thickness of about 1 mm. A spectrophotometer (Lambda 900, PerkinElmer UV-VIS-NIR) with a step size of 1 nm was used to determine absorption spectra in the region of 850–1100 nm at room temperature. The Edinburgh Fluorescence Spectrophotometer (FLS1000) was used to measure the emission spectra and fluorescence lifetime under 905 nm excitation at room and low temperature. Subsequently, the volume concentration can be calculated to be $4.46 \times 10^{-20} \text{ cm}^{-3}$. Figure 2(a) depicts the absorption cross section ($\sigma_{abs}$) of the crystal at room temperature across wavelengths ranging from 850 to 1100 nm. From the absorption cross sections curves of Yb:YScO₃ in Fig. 2, it can be seen that the extreme values of the absorption cross sections are located at 900, 946, and 975 nm, with the corresponding cross-section values, are $0.9 \times 10^{-20}$, $1.44 \times 10^{-20}$ and $3.83 \times 10^{-20} \text{ cm}^2$, respectively. The peak absorption cross section is comparable to that of Yb:YAB ($3.40 \times 10^{-20} \text{ cm}^2$ at 975 nm) [25]. Therefore, Yb:YScO₃ is attractive for the pumping of commercial high-power InGaAs laser diodes at 976 nm.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (a) Absorption cross section versus wavelength measured at room temperature for Yb:YScO₃. (b) low and room temperature emission spectra, low temperature absorption spectra. (c) the stark splitting of the Yb³⁺ multiplets in the YScO₃ crystal.</figcaption></figure>

To investigate the Stark splitting of the ground and excited states of the Yb³⁺ ions in YScO₃, we conducted measurements of both room temperature (RT) and low temperature (LT) spectra. The spectra obtained are displayed in Fig. 2(b). At T = 77 K, the emission peak is sharp and distinct, with the most intense peak observed at 1037 nm. In contrast, at T = 300 K, the spectra become smoother, showing significant changes near 976 nm and 1037 nm. This suggests that when the Yb:YScO₃ crystal is at high temperature, the reabsorption of the corresponding energy level transitions or lattice vibrations have a more serious effect on the energy level transitions corresponding to these two wavelengths.
According to the assignment of the electronic transitions, we plotted the scheme of energy levels of Yb³⁺ ions in YScO₃, see Fig. 2(c). The comparison shows that the zero-phonon line of Yb³⁺ should be located near 976 nm. Calculations reveal that the Yb³⁺ ion in YScO₃ crystals exhibits a ground state energy level splitting of 1004 cm⁻¹ for ²F₇/₂ and 865 cm⁻¹ for the excited state energy level ²F₅/₂. The substantial 1004 cm⁻¹ splitting of the ²F₇/₂ multiplet supports a favorable quasi-three-level operation scheme. This configuration suggests a lower population density in the lower laser level, thereby reducing reabsorption losses.

The stimulated emission cross section ($\sigma_{SE}$) of the $^2F_{5/2} \rightarrow ^2F_{7/2}$ transition of Yb$^{3+}$ was estimated with the reciprocity method (RM) [26] and the Füchtbauer–Ladenburg (F-L) [27] formula. where $\tau_{rad}$ is the radiative lifetime of the emitting state ($^2F_{5/2}$), which is estimated to be 0.63 ms using the modified reciprocity method [11].

In Fig. 3(a), a comparison of the emission cross section curves obtained by the two different methods reveals that the results calculated by the FL are substantially smaller than those calculated by the RM, particularly in the wavelength range below 1100 nm. The value obtained by using the FL formula becomes very small and nearly zero. In practice, there are certain problems with the RM method, where experimental and computational errors in measuring crystal transmission spectra can introduce deviations in the calculated absorption cross sections. These deviations can cause significant dispersion in the calculated emission cross-section, particularly in the longer wavelength regions beyond the zero-phonon line. The $\sigma_{SE}$ of RM at 1039 nm is $1.74 \times 10^{-20}$ cm². By the FL, $\sigma_{SE}$ gives a fairly consistent value, $1.31 \times 10^{-20}$ cm², for the same peak. Another $\sigma_{SE}$ amounts to $0.82 \times 10^{-20}$ cm² at 1084 nm with a FWHM of the emission peak of 34 nm. It is even broader than other ytterbium-doped mixed sesquioxides ever reported, as shown in Table 1. The spectral properties of the Yb:YScO₃ crystal reveal that this crystal is promising for ultrashort pulse laser generation around 1 µm.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (a) The stimulated emission cross section of Yb:YScO₃ crystal. (b) Fluorescence decay curves of the $^2F_{5/2}$ multiplet of Yb:YScO₃ crystal. (c) Gain cross section of Yb:YScO₃ crystal.</figcaption></figure>

The fluorescence decay curves are shown in Fig. 3(b). By a single-exponential fitting, the room temperature fluorescence lifetime $\tau$ was found to be 1.20 ms, which is slightly longer than the estimated $\tau_{rad}$ and low temperature fluorescence ($\tau = 1.07$ ms) due to the reabsorption effect.

The emission bandwidth is a crucial parameter for evaluating the suitability of a material for ultrashort pulse laser applications. In Yb³⁺-doped crystals, it is more reasonable to use the
<table><caption>Table 1. Comparison of emission properties in different ytterbium-doped mixed sesquioxides</caption><thead><tr><th>Crystal</th><th>σ<sub>SE</sub>/<br>10<sup>-20</sup> cm<sup>2</sup></th><th>λ<sub>SE</sub>/<br>nm</th><th>FWHM/<br>nm</th><th>References</th></tr></thead><tbody><tr><td rowspan="2">Y<sub>2</sub>O<sub>3</sub></td><td>0.56/</td><td>1030/</td><td>10/</td><td rowspan="2">[28]</td></tr><tr><td>0.08</td><td>1075</td><td>15</td></tr><tr><td>Sc<sub>2</sub>O<sub>3</sub></td><td>1.3</td><td>1041</td><td>12</td><td>[21,29]</td></tr><tr><td>Lu<sub>2</sub>O<sub>3</sub></td><td>1.5</td><td>1033</td><td>13</td><td>[30]</td></tr><tr><td>LuScO<sub>3</sub></td><td>0.9</td><td>1039</td><td>22</td><td>[21,31]</td></tr><tr><td rowspan="2">YScO<sub>3</sub></td><td>1.31/</td><td>1039/</td><td>22/</td><td rowspan="2">This work</td></tr><tr><td>0.82</td><td>1084</td><td>34</td></tr></tbody></table>

FWHM of the gain curve for comparison, as there is an overlap between the absorption and emission spectra. The gain cross section of Yb:YScO₃ crystal is displayed in Fig. 3(c). As can be seen, the σₗ increased obviously with the increase of the inversion ratio. At low inversion ratio (β = 0.02), the local peak is positive only around 1083 nm. When the inversion ratio is higher than 0.06, two positive peaks are observed at 1039 and 1084 nm, respectively. The value of the FWHM for the Yb:YScO₃ is 65 nm when β = 0.18. These results indicate the great potential of Yb:YScO₃ for realizing high-power ultrashort pulse lasers.

4. Continuous-wave (CW) operation and laser set-up

Figure 4(a) shows the experimental setup of the Yb:YScO₃ crystal laser test. The pump source is a fiber-coupled laser diode (K976AG1RN, BWT) with an emission wavelength of 976 nm, a fiber core diameter of 105 µm, a numerical aperture of 0.22, and a maximum output power of 100W. The pump beam is focused into the crystal using a collimating lens ($f₁ = 50$ mm), followed by a focusing mirror ($f₂ = 75$ mm), resulting in a spot size of approximately 157.5 µm. The Yb:YScO₃ crystals are wrapped in indium foil and placed in a water-cooled copper block with cooling water maintained at 20°C to remove heat. Three planar couplers with different transmissions ($T_{OC} = 2.4\%$, 5%, and 10%) are used to optimize the output efficiency. The flat-flat

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** (a) Schematic of the Yb:YScO₃ crystal laser. (b) Output power versus absorbed pump power for different output couplers. (c) The corresponding optical spectra of the Yb:YScO₃ at different OCs at an absorbed pumping power of 12.3W.</figcaption></figure>
cavity is 6 mm long. The maximum output power was achieved when the *Toc* was 5%, with a pump absorbed power of 16.59 W, yielding an output power of 7.45 W and a corresponding slope efficiency of 52.3%. Figure 4(c) shows the spectrum of the Yb:YScO₃ continuous-wave laser with the central wavelength of 1083 nm, measured at an intermediate power level for the Yb:YScO₃ lasers. The Yb:YScO₃ crystal's effective gain cross section governs the laser emission wavelength, with an increasing transmittance corresponding to a higher laser loss. Typically, laser oscillation directions exhibit a preference for long-wave lengths. At a *Toc* of 5%, the laser emission wavelength is in the range of 1077.2-1092.1 nm. The situation for *Toc* = 10% proved to be quite different, where the laser oscillations occurred in two separate spectral regions, with their central or peak emission wavelength located, respectively, at 1039.5 nm and 1083.2 nm.

**5. Mode-locked laser operation**

The experimental set-up of the diode-pumped mode-locked Yb:YScO₃ laser is shown in Fig. 5.

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Experimental setup of the diode-pumped SESAM ML Yb:YScO₃ laser: LD: fiber-coupled laser diode; L₁: aspherical lens; L₂: spherical lens; M₁-M₃: curved mirrors (*R*OC = -100 mm); DM₁ and DM₂: flat dispersive mirrors; OC: output coupler; SESAM: SEMiconductor Saturable Absorber Mirror.</figcaption></figure>

The laser sample having a dimension of 3 mm × 3 mm in aperture and 2.5-mm thick, which was polished both sides to laser-grade quality with good parallelism and left uncoated. It was mounted in a copper holder without active cooling and positioned at Brewster's angle between two concave folding mirrors, M₁ and M₂ (radius of curvature, R<sub>OC</sub> = -100 mm), in an astigmatically compensated X-folded standing-wave cavity. The pump source was a fiber-coupled InGaAs laser diode, providing a maximum incident power of 1.34 W at 976 nm (unpolarized radiation). Its emission wavelength was stabilized by a fiber Bragg grating (FBG), resulting in an emission linewidth (FWHM) of ~0.2 nm. The measured pump beam quality factor (M²) at the maximum output power was ~1.02. Collimation and focusing into the laser crystal through the pump mirror (M₁) were realized using an aspherical lens L₁ (f = 26 mm) and a spherical lens L₂ (f = 75 mm). This resulted in a beam waist (radius) of 14.6 µm × 32 µm in the sagittal and tangential planes, respectively. A commercial SESAM (BATOP, GmbH) was implemented as saturable absorber (SA) to initiate and stabilize ML operation. An additonal curved mirror M₃ (R<sub>OC</sub> = -100 mm) was used for creating a secondary beam waist on the SESAM with a beam radius of ~77 µm for efficient bleaching. Its parameters at ~1 µm were as follows: modulation depth of 0.6%, non-saturable loss of 0.4%, saturation fluence of 70 µJ/cm² and time constant of ~1 ps. In order to compensate for the material dispersion from the laser crystal and to balance the self-phase modulation (SPM), two flat dispersive mirrors (DM₁ = DM₂ = -150 fs²) were implemented in the other cavity arm for soliton-like pulse shaping. The physical cavity length of the diode-pumped ML Yb:YScO₃ laser was 2.096 m, corresponding to a pulse repetition rate of ~71.55 MHz.

ML operation was initiated and stabilized by implemented a SESAM. Two DMs were introduced into the laser cavity, providing an overall negative GDD of -1800 fs² for soliton-like pulse shaping, refer to Fig. 5. The spectral and temporal characteristics of the ML Yb:YScO₃ laser are depicted in Fig. 6. Using a 1.6% OC, the ML laser delivered pulses with a spectral bandwidth of
27.5 nm (FWHM) at a central wavelength of 1056.4 nm, assuming a sech²-shaped spectral profile, see Fig. 6(a). The second-harmonic generation (SHG)-based background-free intensity autocorrelation trace (AC) was well-fitted with a sech²-shaped temporal profile, resulting in an estimated pulse duration of 48 fs (FWHM) and a time-bandwidth-product (TBP) of 0.355, slightly higher compared to the Fourier-transform-limited value (0.315), see Fig. 6(b). An average output power of 45 mW was obtained for an absorbed pump power of 559 mW, with a pulse repetition rate of 71.55 MHz, corresponding to a peak power of 11.6 kW.

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Diode-pumped SESAM ML Yb:YScO₃ laser with T<sub>OC</sub> = 1.6%. (a) Laser spectrum and (b) Recorded SHG-based autocorrelation trace.</figcaption></figure>

The stability of the ML operation was confirmed by recording the radio-frequency (RF) spectra of the shortest pulses across various frequency ranges, as shown in Fig. 7. The first beat note at 71.55 MHz exhibited a substantial extinction ratio, exceeding 80 dBc above the carrier, see Fig. 7(a). Additionally, the uniform harmonics measured over a 1-GHz frequency span highlighted the exceptional stability of the single-pulse ML operation, see Fig. 7(b).

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** RF spectra of the Diode-pumped SESAM ML Yb:YScO₃ laser with T<sub>OC</sub> = 1.6%: (a) Fundamental beat note at 71.55 MHz recorded with a resolution bandwidth (RBW) of 200 Hz, and (b) Harmonics on a 1-GHz frequency span recorded with an RBW of 100 kHz.</figcaption></figure>
## 6. Conclusion

To conclude, we demonstrate spectroscopic investigations and diode-pumped laser operation of Yb:YScO₃ crystals grown by the Czochralski method. The absorption cross section of Yb:YScO₃ crystal at 976 nm was calculated to be 3.82 × 10⁻²⁰ cm², which indicates the crystal can be efficiently pumped with InGaAs laser. The maximum σ<sub>SE</sub> value for Yb:YScO₃ is 1.31 × 10⁻²⁰ cm² at 1039 nm. The σ<sub>g</sub> value of the FWHM for the Yb:YScO₃ is 65 nm when the inversion ratio is higher than 0.18. Using a 976-nm diode laser as the pump source, highly efficient laser operation was achieved in a simple flat-flat resonator with 5% output coupling. The system generated a maximum output power of 7.45 W, with an optical-to-optical efficiency of 32% and a slope efficiency of 52.3%. ML performance of the Yb:YScO₃ laser was investigated by using a commercial SESAM as mode locker. Pumping with a spatially single-mode, fiber-coupled laser diode at 976 nm, Yb:YScO₃ laser generated soliton pulses as short as 48 fs at 1056.4 nm with an average output power of 45 mW and a pulse repetition rate of 71.55 MHz. Through further appropriately managing the intracavity dispersion and mode-locked pulse intensity, shorter pulses could be generated directly from Yb³⁺-doped mode-locked YScO₃ lasers. All of these results show that the Yb:YScO₃ crystal exhibits attractive broad emission bands, which make it promising for ultrashort pulse generation and high-power lasers.

**Funding.** National Natural Science Foundation of China (61975071); Natural Science Foundation of Jiangsu Province (BK20230685).

**Disclosures.** The authors declare no conflicts of interest.

**Data availability.** Data underlying the results presented in this paper are not publicly available.

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