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
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.
【AIGC One Sentence Reading】:Ho:BaF2 crystal grown by TGT exhibits low threshold, broadband tunability, achieving 1.5W output with 29.3% efficiency, widest tunable range in 2µm band.
【AIGC Short Abstract】:A high-quality Ho:BaF2 crystal was grown using TGT, exhibiting low laser threshold and broadband tunability. CW laser operation at 2 µm was demonstrated, with spectral parameters analyzed by J-O theory. Pumped by a 1908 nm fiber laser, it achieved 1.5 W output power and 29.3% slope efficiency, plus a record 166.4 nm tunable bandwidth, suggesting potential for femtosecond laser output.
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In recent years, mid-infrared lasers around the μ 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[1–3]. Currently, μ 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, ions can promote cross relaxation through high-concentration doping, which has enabled high slope efficiency and high-power laser output at μ. As a result, -doped lasers have increasingly moved toward practical applications[7–9]. However, it is noteworthy that the emission wavelength of ions is relatively shorter than that of ions. The transition in 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 ions and resonant pumping with a Tm-doped laser[10–12]. Beyond the 2 µm range, ions can also generate multi-wavelength laser output across the near-infrared to mid- and far-infrared regions, including at 1.2 µm ()[13,14], 2.9 µm ()[15,16], and 3.9 µm ()[17].
A continuous-wave (CW) μ wavelength laser from ions was first reported in 1965[18]. With the growing demand for ultrafast lasers in the μ range for applications such as chirped-pulse amplification systems, higher-harmonic terahertz generation, high-resolution molecular spectroscopy, and synchronously pumped optical parametric oscillators[19–21], Ho-doped and Tm/Ho co-doped laser gain media have become research hotspots. Currently, 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 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 crystal, 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 doping. Both and fluoride crystals had achieved ultrafast laser output[25,26]. In 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 -doped lasers in crystals.
In this paper, a high-quality 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 ions in the μ range in the crystal. Additionally, the wavelength of the CW laser could be tuned using a Lyot filter with a tuning range of 166 nm, which is currently the widest tuning width among -doped fluoride crystals, to the best of our knowledge.
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2. Experiment
A high-quality 1% (atom fraction) crystal was grown by the TGT. High-purity (99.99%) and powders were accurately weighed according to the chemical formula, and 1% (mass fraction) powder was added as an oxygen scavenger. The raw material powders were heated in a porous graphite crucible in a vacuum state (). 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) crystal.
Figure 1.Photographs of (a) the as-grown Ho:BaF2 crystal and (b) processed and polished Ho:BaF2 crystals.
samples with a size of 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 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) 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 and a laser spot diameter of 22 µm. Mirrors , , and are all coated with high-reflectivity coatings. Specifically, the curvature radius of planar concave mirrors and was . A Lyot filter was placed before the plane-wedged output coupler (OC) to enable tunable laser operation. The transmittances of the plane-wedged OC are 0.5%, 1%, 1.5%, 3%, 5%, and 10%, respectively.
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.
The room temperature XRD pattern of powder is shown in Fig. 3. The XRD results indicate that all diffraction peaks match well with the undoped (PDF#85-1341), and no diffraction peaks of other phases or impurities are observed. The sample retains the cubic structure of , and the calculated lattice parameter is Å. This result is slightly smaller than that of undoped (Å), which can be attributed to the smaller ionic radius of ions (Å, ) compared to ions (Å, ). The actual doping concentration of ions in the 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 ().
Figure 3.Room temperature XRD pattern of the powdered Ho:BaF2 crystal.
Figure 4 shows the room temperature absorption spectra of the crystal. In the range of 400–2200 nm, there are six primary absorption bands, corresponding to energy transitions from the ground state level to the , , , , , and levels. For the transition, the absorption cross-section reaches with a full width at half-maximum (FWHM) of 11.77 nm centered at 637 nm. For the transition, the absorption cross-section reaches with an FWHM of 48.19 nm at 1157 nm. For the transition, the absorption cross-section reaches a maximum of at 1951 nm, while the FWHM of this absorption band is 112.30 nm, which is larger than those of Ho:YAG, , and [27–29]. The broadband absorption of the transition in 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 ( for ) enhances compatibility with diverse pump sources, such as LDs and Tm-doped fiber lasers, thereby improving pump efficiency and simplifying optical design[32].
Figure 4.Room temperature absorption cross-section of the Ho:BaF2 crystal.
The radiative transition properties of crystals were analyzed by the Judd–Ofelt (J–O) theory[33,34]. For J–O intensity parameters , represents the symmetry and covalent bond strength of the ion coordination structure. and represent the whole performance of crystals such as rigidity and viscosity of matrix. is the spectroscopic quality factor that can indicate the stimulated emission efficiency in the laser gain medium. For the crystal, the effective intensity parameters were calculated to be , , and . The value of is 2.20, which is higher than those of ion-doped CYA, YAG, YAP, YLF, LLF, , , and but smaller than that of [35–45]. These results indicate that the spectral quality of the crystal is superior to that of other crystals, except for . The radiative lifetimes of and energy levels were calculated to be 10.018 and 19.947 ms, which are larger than those of ion-doped YLF, LLF, GLF, , and crystals[39,40,44,46]. The emission spectrum in the range of 1700–2300 nm and the fluorescence decay curve of the level under 640 nm excitation are shown in Figs. 5(a) and 5(b). The stimulated emission cross-section can be calculated using the Füchtbauer–Landenburg (F–L) formula[36]. As shown in Fig. 5(a), the emission cross-section of transition reaches at 2047 nm with an FWHM of .
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 of the laser is inversely proportional to the quality factor , as described by[47,48]
Here, represents the energy loss of the laser gain medium within the laser resonator. A larger value indicates a lower energy loss and laser threshold[32]. Therefore, a larger emission cross-section and longer energy level lifetime 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 level for the μ emission band in the crystal reaches 17.98 ms, which is longer than those of (12.4 ms), YAG (7.8 ms), CYA (2.01 ms), (13.6 ms), KYF (2.6 ms), (4.23 ms), (6.13 ms), LYF (16.1 ms), BYF (17.9 ms), YAP (8.1 ms), (1.04 ms), (5.74 ms), (5.89 ms), and CNGG (6.41 ms) crystals[29,36,42,44,46,49–53]. The value of the crystal is calculated to be , and the values of other gain media are listed in Fig. 6. It can be seen that the value of the crystal is higher than those of -doped , , YAG, CYA, KYF, , , , , CNGG, and [29,36,44,46,49–53], and it is comparable to those of -doped YAP and [49]. It is only slightly lower than those of doped YVO4 and [42,50]. This result indicates that the Ho:BaF2 crystal has the potential to achieve laser output in the μ band with a relatively low laser threshold.
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 can be calculated using the absorption and emission cross-sections with the following equation: where represents the inversion ratio or the excited state population fraction, which is equal to the ratio of the electron population densities of the and levels[32,54]. The gain cross-section of the crystal in the range of 1800–2200 nm is shown in Fig. 7. When , the crystal achieves a broadband tunable positive gain cross-section in the range of 2037–2200 nm with an , which is broader than those of , , and Ho: YAG[27,28,54]. When , the FWHM of the positive gain cross-section for the increases from 64.49 to 132.98 nm.
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) crystal at different transmittances () 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 crystal were 0.399, 0.586, 0.481, 0.438, 0.666, and 0.742 W for values of 0.5%, 1%, 1.5%, 3%, 5%, and 10%, respectively. The lowest laser threshold was 0.399 W at . The relationship between the CW output power and absorbed pump power is linear at different values, indicating that thermal effects did not negatively impact laser operation. At %, 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 () of 29.3%. The CW laser output power at μ is higher than those in YAG (1.13 W)[55], crystal (0.58 W)[26], fiber (1 W)[54], (0.97 W)[24], 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.
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) 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 and , 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 crystal is broader than those of other Ho-doped and Tm/Ho-co-doped gain media, such as Ho-doped YAG[55], CYA[57], fiber[54], crystal[25], silica fiber[61], YLF[62], all-fiber[63], glass fiber[64], SSO[65] and Tm/Ho-co-doped 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 (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 crystal, the higher CW output power and broader tunable range indicate that the crystal has great potential to achieve femtosecond ultrafast laser operation with higher pulse energy and shorter pulse duration at μ.
Table 1. Pulse Widths, CW Output Powers, and Tuning Ranges of the Ho:BaF2 Crystal and Other Gain Media
In summary, we report crystal growth, spectral characterization, and the first tunable laser operation of the crystal in the μ 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 absorption band was , with an FWHM of 112.3 nm. The emission cross-section reached at 2047 nm, with an FWHM of 134.5 nm. The fluorescence lifetime of the level was 17.98 ms, and the quality factor of the crystal was calculated to be . Spectral parameters of the crystal were analyzed by the J–O theory, and , , , and of the spectral quality factor were calculated to be , , , 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 and , respectively. These results indicate that crystals are significantly used as ultrafast laser gain media with excellent performance potential in the μ wavelength range.
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