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Abstract
The EFG technique was used to successfully grow a crystal of Ho,Pr:Lu2O3. The crystal's spectroscopic features were investigated. Ho,Pr:Lu2O3 has an absorption cross-section of 0.47 × 10−20 cm2 at 647 nm and 0.20 × 10−20 cm2 at 1147 nm. The fluorescence characteristics of the grown crystal at 2.9 µm were examined using the J-O theory. Calculations were made for the intensity parameters Ωt (t = 2, 4, 6), radiative transition rates, branching ratios, and radiative lifetime. Ho,Pr:Lu2O3 demonstrated significant emission at 2104 and 2893 nm, with stimulated emission cross sections of 2.96 × 10−21 cm2 and 4.24 × 10−21 cm2, respectively. Ho3+ 5I6 and 5I7 levels were found to have fluorescence lifetimes of 0.55 ms and 3.23 ms, respectively.
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1. Introduction
Due to the vibrational absorption band of water in this spectral range, mid-IR (MIR) lasers operating at 3 µm have received a lot of interest recently for multiple applications in medical and sensing technology. Optical parametric oscillators (OPO) and efficient, high-quality pump sources for longer-wavelength MIR lasers can both utilize it [1–4]. Ho3+ and Er3+ are the two options for the active ions because of the transitions: Ho3+:5I6→5I7 and Er3+:4I11/2→4I13/2. Ho3+ doped lasers develop more slowly than Er3+ doped lasers because they lack a pumping source [5–8]. Fortunately, Ho3+ ions can now be pumped from the 5I8 energy level to the 5I6 energy level, which is the upper laser level of 3 µm, thanks to the development of the 1150 nm Raman fiber laser. Therefore, there is a need for quick progress in the investigation of direct mid-infrared laser production by pumping Ho3+ ions with 1150 nm Raman fiber lasers [9–13]. But because the upper 5I6 level's fluorescence lifetime is significantly less than that of the lower level 5I7, it is unable to achieve the 2.9 µm emission effectively [14]. We require suitable deactivated ions to depopulate the Ho3+:5I7 manifold for population inversion as well as a suitable host with low phonon energy to inhibit multi-phonon de-excitation to obtain strong Ho3+:5I6→5I7 MIR emission for practical laser operations [15].
Fortunately, investigations of Ho,Pr-codoped fibers and crystals have shown that Pr3+ has the potential to quench the lower Ho3+:5I7 manifold while scarcely depopulating the higher Ho3+:5I6 manifold [16–20]. Numerous oxide host materials have better thermo-mechanical qualities, such as exceptional hardness and good thermal conductivity, in comparison to fluoride crystals. In comparison to YAG (11 Wm-1K-1), Lu2O3 crystal has a greater thermal conductivity of 12.5 Wm-1K-1 and comparably low phonon energy of 618 cm-1. Additionally, the Lu2O3 crystal's cubic structure reduces cracking due to uneven thermal expansion and the thermal lensing effect [21]. According to all the findings, Lu2O3 crystal will make a great laser host crystal. Due to its extremely high melting point of 2450°C, the rare earth ion-doped Lu2O3 laser crystal has received little attention. Lu2O3 single crystals with good optical properties and huge diameters are particularly challenging to grow [22].
For the first time, Ho3+,Pr3+:Lu2O3 single crystal has been successfully grown in this paper using an EFG (Edge-Defined Film-Fed Growth) method. The structure, spectral properties, and Judd-Ofelt (J-O) theory analysis of Ho3+,Pr3+:Lu2O3 crystal were investigated.
2. Experimental procedure
In an induction heated furnace, a Ho3+, Pr3+ co-doped Lu2O3 crystal was grown using the EFG method. The following molar compositions of 99.99% pure Lu2O3, Ho2O3, and Pr6O11 powders have been used, respectively: 1 at.% Ho2O3, 0.1 at.% Pr6O11, and 98.9 at.% Lu2O3. After being compressed into bulks, the combined powders were added to the tungsten crucible. The argon gas was added to the furnace as a protective atmosphere when the air pressure within the furnace dropped to 8 Pa. The crucible was then maintained at 2450°C for one hour. The growth rate is 2–10 mm/h. Figure 1 presents the annealed Ho,Pr co-doped Lu2O3 crystals with yellow color and size of 6.5 × 5.5 × 0.89 mm3.
The agate mortar was used to grind the powder samples for the XRD experiments, and a Bruker SMART APEX II 4 K CCD diffractometer with Cu Kα (50 kV, 40 mA) irradiation (λ=0.71073 Å) was used to record the results. The XRD data was recorded at the 2θ angle from 10° to 80° with a scanning rate of 0.015° and an exposure time of 0.9 s. To determine the cell properties of the grown crystals, the XRD curves were analyzed using the Jade 6.0 program.
The samples’ absorption spectra between 300 nm and 2300 nm were measured using a Spectrometer (Cary 5000). The FLS 1000 (Edinburgh Instruments, England) was used to measure the emission spectra and decay time in the wavelength ranges of 1700–2300 nm and 2700–3200 nm when excited by a 640 nm pump. At room temperature, all measurements were made.
3. Results and discussion
3.1 Crystal structure
The concentrations of Ho3+ and Pr3+ ions in Ho,Pr:Lu2O3 crystals were determined by ICP-AES (inductively coupled plasma-atomic emission spectrometry) and the actual concentrations of Ho3+ and Pr3+ ions were measured to be 0.81 at.% and 0.04 at.%, respectively, and the population density of Ho3+ and Pr3+ is 2.30 × 1020 cm-3 and 1.17 × 1019 cm-3. The Ho3+,Pr3+:Lu2O3 crystal's XRD pattern is depicted in Fig. 2, and it agrees well with the Lu2O3 single crystal standard card, PDF#43-1021. It is verified that following co-doping with Ho3+ and Pr3+, the crystal structure of Ho3+,Pr3+:Lu2O3 does not change appreciably. Jade program computed the cell parameters of Ho3+,Pr3+:Lu2O3 crystal as 1.040 nm, which was near to the cell characteristics of pure Lu2O3 crystal (1.039 nm).
3.2 Absorption spectra
For spectroscopic analysis, a sample with a thickness of 0.96 mm was cut and polished. Figure 3 displays the measured absorption spectra in the 400–2200 nm region. The transitions from the ground state Ho3+ 5I8 level to the excited states: 3G5, 5F1 + 5G6, 5S2 + 5F4, 5F5, 5I6, and 5I7 levels, are represented by six absorption peaks with respective FWHM values of 8.53 nm, 3.18 nm, 6.23 nm, 15.49 nm, 20.96 nm, and 21.20 nm. These absorption peaks are centered at 416 nm, 449 nm, 537 nm, 649 nm, 1147 nm, and 1930nm, respectively. According to the figure, the absorption coefficients were 0.93 cm-1, 18.49 cm-1, 1.83 cm-1, 1.31 cm-1, 0.47 cm-1, and 1.63 cm-1. The related absorption cross sections were 0.40 × 10−20 cm2, 8.04 × 10−20 cm2, 0.80 × 10−20 cm2, 0.57 × 10−20 cm2, 0.20 × 10−20 cm2, and 0.71 × 10−20 cm2, respectively. In addition, Fig. 3 shows a weak absorption peak at 1499 nm, corresponding to the 3H4→3F3,4 transition of Pr3+, with an absorption coefficient of 0.16 cm-1 and absorption cross section of 1.37 × 10−20 cm2. The widened FWHM and large absorption cross section at 649 nm made it easy for the 640 nm LD to pump the system. On the other hand, the absorption at around 1150 nm is also sufficiently strong to match well with diode lasers built on highly strained InGaAs quantum wells that operate in the range beyond 1100 nm or with fiber lasers doped with Yb [16,23,24].
3.3 Analysis of J-O theory
The calculated line strength Scal(J,J′) and absorption line strength Sed(J,J′) of Ho3+ in a Ho,Pr:Lu2O3 crystal were calculated using the J-O theoretical framework and are shown in Table 1 [25–27].
Table 1. The calculated average wavelength $\bar{{\boldsymbol \lambda }}$, absorption line strength ${{\boldsymbol S}_{{\boldsymbol ed}}}$(J,J′) and calculated line strength ${\boldsymbol S}_{{\boldsymbol ed}}^{{\boldsymbol cal}}$(J,J′) of Ho,Pr:Lu2O3 crystal.
The values of Ω2, Ω4, and Ω6 are calculated to be 7.10 × 10−20 cm2, 2.67 × 10−20 cm2, and 0.33 × 10−20 cm2 for Ho,Pr:Lu2O3 crystal, respectively. Table 2 lists the J-O intensity characteristics of some Ho3+ doped and Ho3+/Pr3+ co-doped crystals. As is well known, the Ω4/Ω6 is frequently utilized as a criterion for spectroscopic quality. Ho,Pr:Lu2O3 has a value for Ω4/Ω6 that is 8.09, which is greater than the values for Ho:YVO4, Ho:SrMoO4, Ho:Lu2O3, Ho,Pr:Sc2O3, and Ho:GdScO3 [15,28–31]. The findings show that Ho,Pr:Lu2O3 crystal has a greater stiffness than Ho:YVO4, Ho:SrMoO4, Ho:Lu2O3, Ho,Pr:Sc2O3, and Ho:GdScO3 crystals.
Table 2. The J-O intensity parameters of Ho3+ doped and Ho3+/Pr3+ co-doped crystals.
Table 3 includes the radiative rate A(J-J′), fluorescence branching ratio βJJ′, and radiative lifetime τrad. The laser power of possible transitions and the radiative rad were strongly connected. In a Ho,Pr:Lu2O3 crystal, the radiative lifetime rad of the Ho3+ 5I6 state was determined to be 7.972 ms. According to calculations, the Ho,Pr:Lu2O3 crystal has a fluorescence branching ratio of 27.496% for the 5I6→5I7 transition. In light of the findings, the Ho,Pr:Lu2O3 crystal's 5I6 energy level is a potential one for laser operation.
Table 3. Radiative transition rates A(J-J′), branching ratios βJJ′ and radiative lifetime τrad of Ho,Pr:Lu2O3 crystal.
Figure 4 depicts the emission spectra of a co-doped Ho,Pr:Lu2O3 crystal stimulated at 640 nm at room temperature. Ho3+ 5I7→5I8 and 5I6→5I7 transitions are represented by the emission bands at 2 µm and 3 µm, respectively. The Fuchtbauer-Ladenburg equation is then used to calculate the emission cross sections σem [40]:
3.4 Emission spectra
3.5 Fluorescence lifetime
The time-resolved decays of the 5I7 and 5I6 multiplets of the two crystals were measured and are depicted in Fig. 5 to further explore the energy interaction mechanism. The two experiment curves excited by a 640 nm laser source are amenable to fitting using a single exponential function. Given that the observed lifetime of the 5I6 manifold in the Ho,Pr:Lu2O3 crystal is 0.55 ms rather than the Ho:Lu2O3 crystal's 0.62 ms, it may be concluded that the co-doping of Pr3+ ions does not affect the higher laser level of Ho3+, and both the nonradiative transition and the transfer of energy to unwanted impurities result in a fluorescence lifetime much smaller than the radiative lifetime (7.97 ms) for the 5I6 energy level. However, compared to the date for the Ho:Lu2O3 crystal, the measured lifespan of the 5I7 manifold in the Ho,Pr:Lu2O3 crystal is 3.23 ms, which is shorter than the Ho:Lu2O3 crystal (12.4 ms) [30]. This reduction in observed lifetime supports the hypothesis that Pr3+ ions can cause the population inversion and enhance laser operation by depopulating the Ho3+:5I7 for 2.9 µm emission in Lu2O3 crystal. The characteristics of the Ho,Pr:Lu2O3 crystal's peak wavelength, FWHM, emission cross section σem, and fluorescence lifetime τ are listed in Table 4. The equation below can be used to estimate how effectively Ho3+ and Pr3+ ions transmit energy [20]:
where τHo,Pr and τHo are the lifetimes of Ho3+ in Ho,Pr:Lu2O3 and Ho:Lu2O3 crystals at 2 µm. Since the value was calculated to be 74%, it can be concluded that the Pr3+ ions effectively quench the excited-state population in the 5I7 level of Ho3+ via ET and increase the emission of 2.9 µm.Table 4. The emission peak wavelength, FWHM, Fluorescence lifetime τ, and emission cross section σem of Ho,Pr:Lu2O3 crystal.
4. Conclusions
The EFG technique proved effective in growing the Ho,Pr:Lu2O3 crystal. The XRD pattern revealed that the cell parameters are a = b = c = 1.04 nm. Ho,Pr:Lu2O3 has an absorption cross-section of 0.47 × 10−20 cm2 at 647 nm and 0.20 × 10−20 cm2 at 1147 nm. Under excitation at 640 nm, Ho3+:5I6→5I7 emission at ∼3 µm was increased. At ∼3 µm, the fluorescence branching ratio is 27.496%, the emission cross-section is 4.24 × 10−21 cm2, and the observed fluorescence lifetime is 0.55 ms, respectively. The Ho,Pr:Lu2O3 crystal can be considered a suitable gain medium for the ∼3 µm laser.
Funding
National Key Research and Development Program of China (No.2022YFB3605701); National Natural Science Foundation of China (52032009, 61621001, 62205247).
Acknowledgments
This work is partially supported by the National key Research and Development Program of China (No.2022YFB3605701) and the National Natural Science Foundation of China (No.52032009, No.62205247, and No. 61621001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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