Chinese Optics Letters, Volume. 23, Issue 7, 071403(2025)

Spectroscopic characterization and efficient tunable lasers of Ho:BaF2 single crystals

Xinyu Qian1... Ning Zhang1,2, Wudi Wang1, Qingsong Song1, Yanyan Xue3,**, Song Wang4, Feng Wu1, Chenbo Zhang1, Huili Tang1, Xiaodong Xu2, Yongguang Zhao2,5, Qingguo Wang1,* and Jun Xu1 |Show fewer author(s)
Author Affiliations
  • 1MOE Key Laboratory of Advanced Micro-Structured Materials, School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China
  • 2Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
  • 3State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China
  • 4School of Physical Sciences, University of California, Irvine CA 92697, America
  • 5State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, China
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    A high-quality 1% (atom fraction) Ho3+:BaF2 crystal was successfully grown using the temperature gradient technique (TGT). The optical properties of the crystal were investigated, and continuous-wave (CW) laser operation of Ho3+ ions in the 2 µm range was successfully demonstrated for the first time in the BaF2 crystal, to the best of our knowledge. Spectral parameters such as Ωt (t = 2, 4, 6) and radiative lifetimes were calculated and studied by the Judd–Ofelt (J–O) theory. The quality factor was calculated to be Q = 6.60 × 10-20 cm2·ms, and the full width at half-maximum (FWHM) was fitted to be 134.5 nm, indicating that the Ho:BaF2 crystal has a low laser threshold and broadband tunability. A maximum output power of 1.5 W and a slope efficiency of 29.3% were achieved by the 1908 nm fiber laser as the pumping source, with a relatively low threshold of 399 mW. Additionally, the Ho:BaF2 crystal achieved a tunable laser output with a bandwidth of 166.4 nm, which is the widest as reported for other 2 µm band Ho-doped fluoride crystals to the best of our knowledge. These results suggest that the Ho:BaF2 crystal has the potential to achieve femtosecond ultrafast pulse laser output through mode-locking operation.

    Keywords

    1. Introduction

    In recent years, mid-infrared lasers around the 2μm wavelength band have garnered significant attention due to their extensive application prospects in fields such as military countermeasures, medical treatment, atmospheric monitoring, and space exploration[13]. Currently, 2μm lasers can be primarily achieved through three types of laser gain media: Tm-doped[4], Ho-doped[5], and Tm/Ho co-doped systems[6]. Among these, Tm3+ ions can promote cross relaxation through high-concentration doping, which has enabled high slope efficiency and high-power laser output at 1.9μm. As a result, Tm3+-doped lasers have increasingly moved toward practical applications[79]. However, it is noteworthy that the emission wavelength of Tm3+ ions is relatively shorter than that of Ho3+ ions. The I57I58 transition in Ho3+ ions can produce longer wavelength laser output in the 2.01 to 2.13 µm range (avoiding the characteristic absorption of water molecules), which can be achieved through sensitization of Tm3+ ions and resonant pumping with a Tm-doped laser[1012]. Beyond the 2 µm range, Ho3+ ions can also generate multi-wavelength laser output across the near-infrared to mid- and far-infrared regions, including at 1.2 µm (I56I58)[13,14], 2.9 µm (I56I57)[15,16], and 3.9 µm (I55I56)[17].

    A continuous-wave (CW) 2μm wavelength laser from Ho3+ ions was first reported in 1965[18]. With the growing demand for ultrafast lasers in the 2μm range for applications such as chirped-pulse amplification systems, higher-harmonic terahertz generation, high-resolution molecular spectroscopy, and synchronously pumped optical parametric oscillators[1921], Ho-doped and Tm/Ho co-doped laser gain media have become research hotspots. Currently, Ho3+ ions have achieved sub-picosecond and femtosecond laser outputs in various crystals, including oxides, fluorides, and perovskite structures. For example, a mode-locked ultrafast laser with a semiconductor saturable absorber mirror (SESAM) of Tm,Ho:CALYO crystals can be achieved with an average output power of 27 mW, a pulse width of 87 fs, and a repetition rate of 80.45 MHz by a pumped 800nm titanium:sapphire laser[22]. This was the first reported ultrafast laser at the hundred-femtosecond level. The shortest pulse width so far was achieved in calcium aluminate crystal, with a pulse duration of 46 fs and an average power of 121 mW[23]. In the perovskite-structured GdScO3 crystal, Ho3+ ions have generated a 72 fs pulsed duration laser at the central wavelength of 2078 nm. Meanwhile, the CW tunable bandwidth reached up to 213 nm[24]. Alkaline Earth fluoride crystals, due to their low phonon energy, broad transmission range, low growth temperature, and large crystal growth potential, have emerged as ideal host materials for Ho3+ doping. Both CaF2 and SrF2 fluoride crystals had achieved ultrafast laser output[25,26]. In SrF2 crystals, a steady-state passively mode-locked (ML) system produced 246 fs pulses, which was the shortest pulse duration recorded in fluoride crystals, while the CW output power and tunable range of this system were 0.58 W and 144.2 nm, respectively[26]. To the best of our knowledge, there has not been any report on Ho3+-doped lasers in BaF2 crystals.

    In this paper, a high-quality Ho:BaF2 crystal was grown by the temperature gradient technique (TGT). The crystal structure and spectral properties were investigated, and various spectral parameters were carefully analyzed and compared. In a CW laser experiment, a maximum output power of 1.5 W with a maximum slope efficiency of 29.3% was achieved at central wavelengths of 2085 and 2073 nm. This is the first reported laser operation of Ho3+ ions in the 2μm range in the BaF2 crystal. Additionally, the wavelength of the Ho:BaF2 CW laser could be tuned using a Lyot filter with a tuning range of 166 nm, which is currently the widest tuning width among Ho3+-doped fluoride crystals, to the best of our knowledge.

    2. Experiment

    A high-quality 1% (atom fraction) Ho:BaF2 crystal was grown by the TGT. High-purity (99.99%) BaF2 and HoF3 powders were accurately weighed according to the chemical formula, and 1% (mass fraction) PbF2 powder was added as an oxygen scavenger. The raw material powders were heated in a porous graphite crucible in a vacuum state (<103Pa). The process of crystal growth was as follows: First, the crucible was heated to 1350°C and maintained about 12 h to melt the raw materials. Subsequently, it was cooled down to 1200°C in 150 h to achieve equal diameter crystal growth. Finally, the crucible was cooled down to room temperature for 72 h before removing the crystal. A high vacuum was maintained inside the furnace chamber throughout the crystal growth process. Figure 1 shows the as-grown and polished 1% (atom fraction) Ho:BaF2 crystal.

    Photographs of (a) the as-grown Ho:BaF2 crystal and (b) processed and polished Ho:BaF2 crystals.

    Figure 1.Photographs of (a) the as-grown Ho:BaF2 crystal and (b) processed and polished Ho:BaF2 crystals.

    Ho:BaF2 samples with a size of 3mm×3mm×15mm were cut and polished for luminescence and laser experimentation. The crystalline phase and structure of the grown crystal were measured by powder X-ray diffraction (XRD, Bruker-D8, Germany) with steps of 0.02°. The actual concentration of Ho3+ ions was tested by an inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 725, USA). The room temperature absorption spectra of the samples were measured by a UV-VIS-NIR spectrophotometer (Agilent Cary-5000, USA) with steps of 1 nm. The room temperature luminescence spectra and fluorescence decay curves were measured by the FLS-1000 fluorescence spectrometer (Edinburgh Company, British), using 640 nm laser diodes (LDs) as the pumping source.

    In the laser experiment, an X-shaped optical resonator was used. Figure 2 shows the schematic diagram of the CW and tunable laser cavity using the 1% (atom fraction) Ho:BaF2 crystal. The crystal was wrapped in indium foil and securely mounted on a copper water-cooled holder, with the cooling water maintained at 14°C to mitigate thermal effects during laser operation. A Tm-doped fiber laser centered at 1908 nm with a maximum output power of 19.5 W was used as the pumping source. The pumping light was focused onto the crystal by the spherical lens with a focal length of f=12.5mm and a laser spot diameter of 22 µm. Mirrors M1, M2, and M3 are all coated with high-reflectivity coatings. Specifically, the curvature radius of planar concave mirrors M1 and M2 was Roc=150mm. A Lyot filter was placed before the plane-wedged output coupler (OC) to enable tunable laser operation. The transmittances of the plane-wedged OC Toc are 0.5%, 1%, 1.5%, 3%, 5%, and 10%, respectively.

    Schematic of the Ho:BaF2 crystal laser. M1, M2, folding mirrors; M3, high reflective mirror; LF, Lyot filter inserted at Brewster’s angle; OC, output coupler.

    Figure 2.Schematic of the Ho:BaF2 crystal laser. M1, M2, folding mirrors; M3, high reflective mirror; LF, Lyot filter inserted at Brewster’s angle; OC, output coupler.

    3. Results and Discussion

    3.1. XRD and ICP-OES Results

    The room temperature XRD pattern of Ho:BaF2 powder is shown in Fig. 3. The XRD results indicate that all diffraction peaks match well with the undoped BaF2 (PDF#85-1341), and no diffraction peaks of other phases or impurities are observed. The sample retains the cubic structure of BaF2, and the calculated lattice parameter is a=b=c=6.193. This result is slightly smaller than that of undoped BaF2 (a=b=c=6.196), which can be attributed to the smaller ionic radius of Ho3+ ions (r=1.015, C.N.=8) compared to Ba2+ ions (r=1.42Å, C.N.=8). The actual doping concentration of Ho3+ ions in the BaF2 crystal was determined to be 0.91% by an inductively coupled plasma optical emission spectrometer (ICP-OES), which was close to the original nominal concentration (XHo=1%).

    Room temperature XRD pattern of the powdered Ho:BaF2 crystal.

    Figure 3.Room temperature XRD pattern of the powdered Ho:BaF2 crystal.

    3.2. Spectral characterization

    Figure 4 shows the room temperature absorption spectra of the Ho:BaF2 crystal. In the range of 400–2200 nm, there are six primary absorption bands, corresponding to energy transitions from the ground state I58 level to the G55, F51+G56, S52+F54, F55, I56, and I57 levels. For the I58F55 transition, the absorption cross-section reaches 5.33×1021cm2 with a full width at half-maximum (FWHM) of 11.77 nm centered at 637 nm. For the I58I56 transition, the absorption cross-section reaches 1.50×1021cm2 with an FWHM of 48.19 nm at 1157 nm. For the I58I57 transition, the absorption cross-section reaches a maximum of 3.51×1021cm2 at 1951 nm, while the FWHM of this absorption band is 112.30 nm, which is larger than those of Ho:YAG, Ho:Lu2O3, and Ho:KY3F10[2729]. The broadband absorption of the I58I57 transition in Ho:BaF2 crystals indicates compatibility with a wider range of pump sources, such as LDs and Tm-doped fiber lasers[30,31]. Additionally, the broadband absorption spectrum (FWHM=112.3nm for I58I57) enhances compatibility with diverse pump sources, such as LDs and Tm-doped fiber lasers, thereby improving pump efficiency and simplifying optical design[32].

    Room temperature absorption cross-section of the Ho:BaF2 crystal.

    Figure 4.Room temperature absorption cross-section of the Ho:BaF2 crystal.

    The radiative transition properties of Ho:BaF2 crystals were analyzed by the Judd–Ofelt (J–O) theory[33,34]. For J–O intensity parameters Ωt(t=2,4,6), Ω2 represents the symmetry and covalent bond strength of the Ho3+ ion coordination structure. Ω4 and Ω6 represent the whole performance of crystals such as rigidity and viscosity of matrix. Ω4/Ω6 is the spectroscopic quality factor that can indicate the stimulated emission efficiency in the laser gain medium. For the Ho:BaF2 crystal, the effective intensity parameters Ωt(t=2,4,6) were calculated to be Ω2=0.62×1020cm2, Ω4=2.40×1020cm2, and Ω6=1.09×1020cm2. The value of Ω4/Ω6 is 2.20, which is higher than those of Ho3+ ion-doped CYA, YAG, YAP, YLF, LLF, ZnWO4, YVO4, and HoAl3(BO3)4 but smaller than that of Ho:GdScO3[3545]. These results indicate that the spectral quality of the Ho:BaF2 crystal is superior to that of other crystals, except for GdScO3. The radiative lifetimes of I56 and I57 energy levels were calculated to be 10.018 and 19.947 ms, which are larger than those of Ho3+ ion-doped YLF, LLF, GLF, GdScO3, and Lu2O3 crystals[39,40,44,46]. The emission spectrum in the range of 1700–2300 nm and the fluorescence decay curve of the I57 level under 640 nm excitation are shown in Figs. 5(a) and 5(b). The stimulated emission cross-section σem(λ) can be calculated using the Füchtbauer–Landenburg (F–L) formula[36]. As shown in Fig. 5(a), the emission cross-section of I57I58 transition reaches 3.67×1021cm2 at 2047 nm with an FWHM of 134.5nm.

    (a) Room temperature emission cross-section of the Ho:BaF2 crystal and (b) fluorescence decay curve of the 5I7 level in the Ho:BaF2 crystal.

    Figure 5.(a) Room temperature emission cross-section of the Ho:BaF2 crystal and (b) fluorescence decay curve of the 5I7 level in the Ho:BaF2 crystal.

    For laser experiments, the saturation emission intensity Isat of the laser is inversely proportional to the quality factor Q, as described by[47,48]Isat=hv/Q.

    Here, Q=σem(λ)×τ represents the energy loss of the laser gain medium within the laser resonator. A larger Q value indicates a lower energy loss and laser threshold[32]. Therefore, a larger emission cross-section σem(λ) and longer energy level lifetime τ are more favorable for achieving efficient laser operation in the gain medium. From Fig. 5(b), it can be seen that the lifetime of the I57 level for the 2μm emission band in the Ho:BaF2 crystal reaches 17.98 ms, which is longer than those of Lu2O3 (12.4 ms), YAG (7.8 ms), CYA (2.01 ms), PbF2 (13.6 ms), KYF (2.6 ms), YVO4 (4.23 ms), GdScO3 (6.13 ms), LYF (16.1 ms), BYF (17.9 ms), YAP (8.1 ms), La2Be2O5 (1.04 ms), Sc2O3 (5.74 ms), YScO3 (5.89 ms), and CNGG (6.41 ms) crystals[29,36,42,44,46,4953]. The Q value of the Ho:BaF2 crystal is calculated to be 6.60×1020cm2·ms, and the Q values of other gain media are listed in Fig. 6. It can be seen that the Q value of the Ho:BaF2 crystal is higher than those of Ho3+-doped KCaF3, Lu2O3, YAG, CYA, KYF, YVO4, GdScO3, La2Be2O5, Sc2O3, CNGG, and YScO3[29,36,44,46,4953], and it is comparable to those of Ho3+-doped YAP and LaF3[49]. It is only slightly lower than those of Ho3+ doped YVO4 and PbF2[42,50]. This result indicates that the Ho:BaF2 crystal has the potential to achieve laser output in the 2μm band with a relatively low laser threshold.

    Relationship between the quality factor Q and the wavelength for Ho:BaF2 and other Ho3+ ion-doped gain media.

    Figure 6.Relationship between the quality factor Q and the wavelength for Ho:BaF2 and other Ho3+ ion-doped gain media.

    The gain cross-section σgain can be calculated using the absorption and emission cross-sections with the following equation: σgain=βσem(1β)σabs,where β=N2(I57)/NHo represents the inversion ratio or the excited state population fraction, which is equal to the ratio of the electron population densities of the I57 and I58 levels[32,54]. The gain cross-section of the Ho:BaF2 crystal in the range of 1800–2200 nm is shown in Fig. 7. When β=0.3, the Ho:BaF2 crystal achieves a broadband tunable positive gain cross-section in the range of 2037–2200 nm with an FWHM=41nm, which is broader than those of Ho:SrF2, Ho:Lu2O3, and Ho: YAG[27,28,54]. When β0.4, the FWHM of the positive gain cross-section for the Ho:BaF2 increases from 64.49 to 132.98 nm.

    Calculated gain cross-section of the Ho:BaF2 crystal.

    Figure 7.Calculated gain cross-section of the Ho:BaF2 crystal.

    3.3. Continuous-wave and tunable laser performance

    The relationship between the absorbed pump power and the CW laser output power of the 1% (atom fraction) Ho:BaF2 crystal at different transmittances (Toc) of 0.5%, 1%, 1.5%, 3%, 5%, and 10% is shown in Fig. 8(a). During CW laser operation, the laser thresholds of the Ho:BaF2 crystal were 0.399, 0.586, 0.481, 0.438, 0.666, and 0.742 W for Toc values of 0.5%, 1%, 1.5%, 3%, 5%, and 10%, respectively. The lowest laser threshold was 0.399 W at Toc=0.5%. The relationship between the CW output power and absorbed pump power is linear at different Toc values, indicating that thermal effects did not negatively impact laser operation. At Toc=5%, the sample achieved a maximum CW laser output of 1.501 W with an absorbed pump power of 5.88 W, corresponding to a slope efficiency (η) of 29.3%. The CW laser output power at 2μm is higher than those in YAG (1.13 W)[55], SrF2 crystal (0.58 W)[26], SrF2 fiber (1 W)[54], GdScO3 (0.97 W)[24], CaF2 ceramics (0.85 W)[56], crystal (1.11 W)[25], CYLA (0.51 W)[57], CYA (0.95 W)[22], CALGO (1.01 W)[58], CGA (0.52 W)[59], and BSO (0.73 W)[60]. The central wavelengths of the laser output were 2085 and 2073 nm, as shown in Fig. 8(b). The fiber laser produced an output power of 19.5 W, resulting in an optical-to-optical conversion efficiency of 30.1%. However, due to the limitations of the pump source’s maximum output power and the losses caused by the mirror assembly in the laser optical path, the crystal did not reach its saturation absorption power. Consequently, the laser performance has potential for further improvement.

    (a) Relationship between output power and absorbed pump power at different Toc and (b) laser spectra with Toc = 5% of the Ho:BaF2 crystal.

    Figure 8.(a) Relationship between output power and absorbed pump power at different Toc and (b) laser spectra with Toc = 5% of the Ho:BaF2 crystal.

    The results in Fig. 9 demonstrate that the 1% (atom fraction) Ho:BaF2 crystal exhibits a very broad CW laser tuning range, with tuning bandwidths of 166.4 nm (2025.7–2192.1 nm) and 134 nm (2040.2–2174.2 nm) observed at Toc=0.5% and Toc=1%, respectively. It is well known that the broad CW laser tuning range is advantageous for achieving ultrashort-pulse laser operation. The CW tunable ranges of Ho-doped crystals and other gain media are presented in Table 1. It can be seen that the tuning range of the Ho-doped BaF2 crystal is broader than those of other Ho-doped and Tm/Ho-co-doped gain media, such as Ho-doped YAG[55], CYA[57], SrF2 fiber[54], CaF2 crystal[25], silica fiber[61], YLF[62], all-fiber[63], glass fiber[64], SSO[65] and Tm/Ho-co-doped SrF2 crystal[26], fiber[66], CYLA[67], YAP[68], and LLF[69]. Among these gain media, some have already achieved femtosecond ultrafast laser operation, such as SrF2 (246 fs)[26], CYA (87 fs)[22], CALGO (52 fs)[58], CGA (218 fs)[59], and Tm/Ho co-doped glass fiber (150 fs)[66]. For the Ho-doped BaF2 crystal, the higher CW output power and broader tunable range indicate that the Ho:BaF2 crystal has great potential to achieve femtosecond ultrafast laser operation with higher pulse energy and shorter pulse duration at 2μm.

    • Table 1. Pulse Widths, CW Output Powers, and Tuning Ranges of the Ho:BaF2 Crystal and Other Gain Media

      Table 1. Pulse Widths, CW Output Powers, and Tuning Ranges of the Ho:BaF2 Crystal and Other Gain Media

      Gain mediumOutput power(W)Tuning range (nm)Pulse widthRef.
      Ho:BaF21.5134 (Toc = 1%)166.2 (Toc = 0.5%)Next workThis work
      Ho:YAG1.1360[55]
      Ho:SrF21.062779 ns[54]
      Ho:CaF21.1160[25]
      Ho:CYA11.345[57]
      Ho:BSO0.73[60]
      Ho:SSO110[65]
      Ho:YLF0.542.5[62]
      Ho:silica fiber4.3160[61]
      Ho:All-fiber255[63]
      Ho:glass fiber18.3128[64]
      Tm,Ho:SrF20.58144.2246 fs[26]
      Tm,Ho:fiber60150 fs[66]
      Tm,Ho:CYLA0.51123.4560 ns[67]
      Tm,Ho:YAP93.5[68]
      Tm,Ho:LLF1855 ns[69]

    Wavelength tuning curve of the Ho:BaF2 crystal with Toc = 0.5% and Toc = 1%.

    Figure 9.Wavelength tuning curve of the Ho:BaF2 crystal with Toc = 0.5% and Toc = 1%.

    4. Conclusion

    In summary, we report crystal growth, spectral characterization, and the first tunable laser operation of the Ho:BaF2 crystal in the 2μm range. The XRD and ICP-OES results indicated that the sample structure remained intact, and the expected doping effect was achieved. The maximum absorption cross-section for the I58I57 absorption band was 3.51×1021cm2, with an FWHM of 112.3 nm. The emission cross-section reached 3.67×1021cm2 at 2047 nm, with an FWHM of 134.5 nm. The fluorescence lifetime of the I57 level was 17.98 ms, and the quality factor of the crystal was calculated to be Q=6.60×1020cm2·ms. Spectral parameters of the crystal were analyzed by the J–O theory, and Ω2, Ω4, Ω6, and Ω4/Ω6 of the spectral quality factor were calculated to be 0.62×1020cm2, 2.40×1020cm2, 1.09×1020cm2, and 2.20, respectively. The CW laser experiments were conducted by a Tm-doped fiber with the wavelength of 1908 nm as the efficient pumping source, and a maximum CW laser output power of 1.5 W with a slope efficiency of 29.3% was achieved. Additionally, tunable laser operation was achieved with tuning ranges of 166.4 nm (2025.7–2192.1 nm) and 134 nm (2040.2–2174.2 nm) at Toc=0.5% and Toc=1.0%, respectively. These results indicate that Ho:BaF2 crystals are significantly used as ultrafast laser gain media with excellent performance potential in the 2μm wavelength range.

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    [37] J. Šulc, M. Němec, D. Vyhlídal et al. Holmium doping concentration influence on Ho:YAG crystal spectroscopic properties. Solid State Lasers XXX: Technology and Devices, 11664, 105(2021).

    [38] J. Šulc, M. Němec, M. Jelínek et al. Anisotropy of spectroscopic and laser properties of Ho:YAP crystal. Solid State Lasers XXXI: Technology and Devices, 11980, 74(2022).

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    Xinyu Qian, Ning Zhang, Wudi Wang, Qingsong Song, Yanyan Xue, Song Wang, Feng Wu, Chenbo Zhang, Huili Tang, Xiaodong Xu, Yongguang Zhao, Qingguo Wang, Jun Xu, "Spectroscopic characterization and efficient tunable lasers of Ho:BaF2 single crystals," Chin. Opt. Lett. 23, 071403 (2025)

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    Paper Information

    Category: Lasers, Optical Amplifiers, and Laser Optics

    Received: Jan. 2, 2025

    Accepted: Feb. 19, 2025

    Published Online: Jun. 11, 2025

    The Author Email: Yanyan Xue (xueyanyan@mail.sic.ac.cn), Qingguo Wang (qgwang@tongji.edu.cn)

    DOI:10.3788/COL202523.071403

    CSTR:32184.14.COL202523.071403

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