Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG laser crystals with high optical quality were successfully grown by the Czochralski method. Compared with Ho: LuAG and Yb, Ho: LuAG crystals, Yb, Ho, Pr: LuAG crystal not only shows a better absorption characteristic but also exhibits weaker ~2 μm emission, as well as superior mid-infrared (MIR) at ~3 μm emission. The ~3 μm MIR emission intensities of Yb, Ho, Pr: LuAG under excitation of a common 970 nm laser diode (LD) is almost nine times that of the Ho: LuAG crystal and twice as much as that of the Yb, Ho: LuAG crystal. The crystal growth, absorption spectra, J-O parameters, emission spectra, fluorescence lifetimes and the energy transfer mechanism in Yb, Ho, Pr: LuAG crystal were studied in this work. The energy transition efficiency from the lower laser level of Ho3+: 5I7 to Pr3+: 3F2 level is as high as 66.6%, indicating that the Pr3+ ion is an effective deactivation ion for Ho3+ ion in Lu3Al5O12 crystal. All these results show that Yb, Ho, Pr: LuAG crystal may become a promising material for developing solid state lasers at around 3 μm under a conventional 970 nm LD pump.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Over the past several years, mid-infrared (MIR) lasers at ~3 μm have gained attention for a variety of applications including remote sensing and dental surgery due to the vibration absorption band of water in this spectral region [1, 2]. In addition, the ~3 μm lasers can act as the pumping source of optical parametric oscillation (OPO) .
Such lasers can be realized by doping Er3+ or Ho3+ as activator ions. However, the crystals of doping Er3+ ions can only realize a single wavelength of laser output in a specific host material, such as 2.94 μm in Er: YAG crystal  and 2.79 μm in Er: YSGG crystal . The doping concentration of Er3+ ions is also generally required to be higher than 30 at%, which causes difficulties in achieving high-quality laser crystals . In contrast, Ho3+ is a well-known candidate for ~3 μm lasers owing to the 5I6 → 5I7 transition, which can realize lasers of 2.8-3.1 μm in various host materials [7–10], such as oxide YAlO3 (YAP), Y3Al5O12 (YAG), and fluoride LiYF4 (LYF). However, the ~3 μm emission cannot be obtained efficiently from Ho3+-singly-doped hosts due to the two bottle-necks . On the one hand, Ho3+ activator ions lack suitable absorption bands matching well with current laser diode (LD). On the other hand, the fluorescence lifetime of the upper 5I6 level is considerably shorter than that of the lower 5I7 level. Fortunately, there are two main schemes to remove the bottleneck effect. One is choosing Yb3+ ions as a sensitizer to absorb and transfer pumping energy to the Ho3+ ions [12–14]. The other way is co-doping with Pr3+ ions as a deactivator to quench the lower level of Ho3+: 5I7, while hardly depopulate the upper level of Ho3+: 5I6. Good results have been achieved in Ho, Pr: LLF crystal with the lifetime of the upper 5I6 level decreasing from 1.8ms to 1.47ms and the lifetime of the lower 5I7 level decreasing from 16ms to 1.97ms [15, 16].
Host material is another factor that should be considered to get powerful ~3 μm emission from Ho3+. As we know, Y3Al5O12 (YAG) single crystal is one of the most popular laser crystals due to its excellent optical, thermal, mechanical as well as physicochemical properties [17, 18]. In this work, Lu3Al5O12 (LuAG) single crystal, an isomorphic material of YAG, was chosen as the host matrix for reasons: (i) LuAG has a large manifold splitting, leading to a low thermal occupation factor for the lower laser level . (ii) the molar mass difference between Lu3+ and doped rare earth ions for LuAG is slighter, which would make a weaker decease in thermal conductivity of rare ions doped LuAG . (iii) LuAG is harder than YAG and has a higher melting point, which is believed to have a higher damage threshold . Therefore, the Yb, Ho, Pr: LuAG crystal may be a potential laser material for realizing 2.8-3.1 μm lasers.
To our knowledge, there is still no report on the growth of Yb, Ho, Pr: LuAG crystal or the ~3 μm emission in this crystal up to now. In this paper, the high optical quality Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG laser crystals were successfully grown by the Czochralski (Cz) method for the first time. Yb3+ and Pr3+ were demonstrated to enhance the Ho3+: 5I6 → 5I7 ~3 μm emission by efficient energy transfer (ET) from Yb3+: 2F5/2 to Ho3+: 5I6 and Ho3+: 5I7 to Pr3+: 3F2. The absorption spectra, J-O parameters, emission spectra, fluorescence decay curves and transfer mechanism of the Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals were compared and analyzed.
The Ho3+ (2 at. %) single doped, Yb3+ (10 at. %)/ Ho3+ (2 at. %) co-doped and Yb3+ (10 at. %)/ Ho3+ (2 at. %)/ Pr3+ (0.5 at. %) co-doped LuAG crystals were grown by the traditional Czochralski method with intermediate frequency induction heating. Lu2O3 (99.999%), Al2O3 (99.999%), Yb2O3 (99.999%), Ho2O3 (99.999%) and Pr6O11 (99.999%) were used as raw material for crystal growth. Mix all the ingredients and put into the mixer for stir 20 hours. Then, the mixture was pressed into disks and heated in air at 1300°C for 15 h for synthesizing the polycrystalline material. After that, the polycrystalline material was loaded into a 60-mm-diameter iridium crucible for crystal growth. The crystal was grown in nitrogen gas (with high purity of 99.99%) environment. The <001> oriented seed of dimension 4 × 4 × 35 mm3 was used for crystal growth. The rotation rate and pulling speed were kept at 8-12 rpm and 0.6-0.8 mm/h, respectively. After growth, the crystals were cooled to room temperature slowly for 53 hours and annealed at 1300°C in air for 24 h in order to prevent crystal crack as well as oxidize some Yb2+ ions.
Samples with dimension of Φ30 mm × 1 mm were cut from the crystals and then polished on both faces for spectroscopic measurements. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to measure the concentrations of Yb3+, Ho3+, and Pr3+ ions in the as grown crystals. The absorption spectrums of the as-grown crystals in the wavelength of 300-2250 nm were recorded by a UV-vis-NIR spectrophotometer (UV-3150, Shimadzu, Japan). The fluorescence spectra in range of 1800 nm to 2200 nm, 2700 nm to 3050 nm and fluorescence decay profiles of the three crystals samples were acquired by Edinburgh Instruments FLS920 and FSP920 spectrophotometers with a laser diode (LD) as the pump source (Yb, Ho: LuAG and Yb, Ho, Pr: LuAG excited at 970 nm, while Ho: LuAG excited at 637nm), and an optical parametric oscillator pulse laser. All measurements were done at room temperature.
3. Results and discussions
For the distribution coefficient measurement, the sample was cut from the upper part of the as-grown crystals, and then was ground to powder in an agate mortar. The single doped crystal was measured to be (2.01 ± 0.01) at. % ((2.872 ± 0.14) × 1020 ions/cm3) of Ho3+. The double doped crystal was measured to be (10.5 ± 0.01) at. % ((1.501 ± 0.002) × 1021 ions/cm3) of Yb3+, and (2.05 ± 0.01) at. % ((2.929 ± 0.14) × 1020 ions/cm3) of Ho3+. The triply doped crystal was measured to be (10.7 ± 0.01) at. % ((1.530 ± 0.002) × 1021 ions/cm3) of Yb3+, (2.05 ± 0.01) at. % ((2.929 ± 0.14) × 1020 ions/cm3) of Ho3+ and (0.36 ± 0.01) at. % ((0.515 ± 0.14) × 1020 ions/cm3) of Pr3+, respectively. Based on measurement results, the segregation coefficient of Yb3+, Ho3+, and Pr3+ ions in LuAG crystal is about 1.07, 1.02, and 0.72.
The absorption spectra of Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals in the wavelength range of 300-2250 nm is shown in Fig. 1, and the absorption peaks are basically the same among these crystals except in the range of 850-1050 nm as well as 1340-1570 nm. On one side, the absorption peaks centered at wavelength 970 nm with very large intensity correspond to the transitions of Yb3+ ions from the ground state 2F7/2 to the exited states 2F5/2, which makes this crystal propitious to be pumped by commercialized InGaAs LD, indicating that importing of Yb3+ ions is expected to provide an efficient excitation channel to Ho3+ ions. On the other side, the absorption peaks located at wavelength from 1340 to 1570 nm correspond to the transitions of Pr3+ ions from the ground state 3H4 to the excited states 3F3 and 3F4. The efficient ET will occur from the 5I7 excited state of Ho3+ to the 3F2, 3H6 states of Pr3+ due to the fact that a strong overlap between the 5I8 → 5I7 absorption transition of Ho3+ and the 3H4 → 3F2, 3H6 absorption transition of Pr3+ in the wavelength range of 1800-2100 nm . The absorption cross-sections of Yb3+ ions at 936 nm are calculated to be 5.93 × 10−21cm2 for the Yb, Ho, Pr: LuAG.
The Judd-Ofelt (J-O) theory is applied to analyze the room temperature absorption spectrum, and then the branching ratios of the various transitions and the radiative lifetime of the level can be evaluated [22, 23]. The details of J-O analysis are similar to that reported in Refs . The Judd-Ofelt intensity parameters Ω2,4,6, (shown in Table 1) of Ho3+ can be figured out utilizing the reduced matrix elements of the unit tensor operators provided by Brian M. Walsh al . It is well known that Ω2 is affected by the symmetry of rare-earth ions site. The value of Ω2 drops with the improved symmetry . The larger Ω2 of Ho3+ in our Yb, Ho, Pr: LuAG crystal indicates that a lower symmetry surrounding Ho3+ ions is caused by the introduction of Yb3+ and Pr3+ ions . Further calculation about the fluorescence branching ratio β of 5I6 → 5I7 transition in the Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals are about 12%, which is much higher than Ho: LuAG, indicating that the co-doping of Yb3+ and Pr3+ is beneficial to the ~3 μm fluorescence emission efficiency of Ho3+.
The fluorescence spectra of the Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals in the range of 1800-2200 nm and 2700-3050 nm is shown in Fig. 2. The sampling interval of fluorescence spectra is 1 nm in the range of 1800-2200nm and the range of 2700-3050 nm is 5 nm. The observed many emission bands around 2760, 2785, 2810, 2860, 2890, 2950, and 3010 nm are assigned to the transition of 5I6 to 5I7 stark sublevels of Ho3+ ions. By comparing, it is evident that the emission intensities of Yb, Ho, Pr: LuAG is almost nine times that of the Ho: LuAG crystal and twice as much as that of the Yb, Ho: LuAG crystal at 2.95 μm. However, emission in the range of 1800-2200 nm is reverse. This enhanced 2.95 μm emission and reduced ~2 μm emission in the Yb, Ho, Pr: LuAG crystal is mainly attributed to the quenching effect of the Ho3+: 5I7 by energy transition (ET) to Pr3+: 3F2. Thus, output of 2.8-3.1 μm lasers may be realized on the Yb, Ho, Pr: LuAG crystal.
To further study the effect of Yb3+ and Pr3+ co-doping on the ~3 μm photoluminescence of Ho3+, the corresponding emission cross sections are subsequently calculated by Fuchtbauer-Ladenburg equation :Fig. 2(a), the maximum emission cross-section of Yb, Ho, Pr: LuAG crystal is as large as 1.75 × 10−20 cm2 at 2950 nm, which is larger than that of Ho: LuAG (1.55 × 10−20 cm2) and Yb, Ho: LuAG (1.60 × 10−20 cm2) crystals.
The fluorescence decay curves of the Ho: LuAG, Yb, Ho: LuAG and Yb, Ho, Pr: LuAG crystals excited by OPO pulse lasers show a single exponential decay behavior, which are shown in Fig. 3. By single-exponential fitting, the fluorescence lifetime τmeas of the 5I6 and 5I7 manifold in Yb, Ho, Pr: LuAG crystal is 0.13 ms and 3.73 ms, respectively. Despite the lifetime of Ho3+: 5I6 in Yb, Ho, Pr: LuAG crystal decreases a little from 0.15 ms to 0.13 ms due to the energy transfer process Ho3+: 5I6 → Pr3+: 3F4 (ET2), the Yb, Ho, Pr: LuAG crystal exists a quicker attenuation of the lower level lifetime compared with the 5I7 lifetimes of the Ho: LuAG (5I7: 11.15 ms), Yb, Ho: LuAG (5I7: 11.18 ms), which is mainly attributed to the energy transfer from the Ho3+: 5I7 to the Pr3+: 3F2 + 3H6 (ET3), as well as the up-conversion (UC) process and strong excited-state absorption (ESA) from Ho3+: 5I7 → 5F5, as shown is Fig. 4. Meanwhile, energy transfer from Yb3+: 2F5/2 to Ho3+: 5I6 (ET1) under a 970 nm LD excitation is benefit to make the possibility of population inversion for Ho3+: 5I6 → 5I7. These processes not only depopulate the Ho3+: 5I7, but also recycle energy to Ho3+: 5I6 by nonradiative transition (NT), which consequently makes contribution to 2.95 μm emission. The present experimental results show that the lifetime of the upper level (5I6: 0.13 ms) of Ho3+ ions is still much shorter than lower level (5I7: 3.73 ms) for Yb, Ho, Pr: LuAG crystal. However, two improvements can be made in future research work: (i) The concentration ratio of doped ions between Yb3+, Ho3+, Pr3+ ions can be further optimized, which may further enhance the fluorescence intensity of 3 μm, and shorten the lifetime of the lower level (Ho3+: 5I7); (ii) The crystal quality can be further improved by optimizing the crystal growth process, which may further improve the spectral performances of the as-grown crystals.
On the one hand, it is worth noting that the energy transfer from Yb3+ to Ho3+ ions is a phonon-assisted process due to the fact that the small energy mismatch between the 2F5/2 level of Yb3+ and 5I6 level of Ho3+ . The ability of a donor (Yb3+) to transfer its energy to an acceptor (Ho3+) is an important factor to evaluate the donor (Yb3+) as a sensitizer. On the other hand, co-doping Pr3+ deactivation ion can be effective in depopulation of 5I7 level of Ho3+, because its energy level 5I7 is adjacent to 3F2 level of Pr3+. The energy transfer efficiency of ET2 and ET3 processes can be estimated from the measured lifetime of the 2.95 μm and 2 μm emissions by the following equation: η = 1 - τYb/Ho/Pr/τYb/Ho , where τYb/Ho and τYb/Ho/Pr are the Ho3+ lifetimes monitored at 2.95 μm and 2 μm, respectively. It can be calculated that the value of ηET2 = 13.3%, ηET3 = 66.6%. Therefore, under 970 nm pumping scheme, co-doping of both Yb3+ and Pr3+ with Ho3+ can turn on the possibility of excellent ~3 μm lasers.
In conclusion, an efficient emission at ~3 μm was observed in the Yb, Ho, Pr: LuAG crystal under the excitation of a common 970 nm LD. The relevant absorption and emission cross sections as well as the fluorescence lifetimes were calculated and compared with Ho: LuAG and Yb, Ho: LuAG crystals. Studies showed that Yb3+ ions acted as a sensitizer owing to the up-conversion process, excited-state absorption from Ho3+: 5I7 → 5F5, and energy transfer via Yb3+: 2F5/2 → Ho3+: 5I6, which not only increased the absorption cross section but also benefit to make the possibility of population inversion for Ho3+: 5I6 → 5I7. In addition, with the existence of Pr3+ ions, the ~2 μm emission is weak due to the fact that Pr3+ depopulates the Ho3+: 5I7 level and has little influence on the Ho3+: 5I6 level at the same time, which also benefits the possible population inversion for Ho3+: 5I6 → 5I7. The results and analyzed energy transfer indicate co-doping of Yb3+, Pr3+ with Ho3+ in LuAG crystal could be considered as an excellent candidate for mid-infrared lasers.
The National Key Research and Development Program of China (2017YFB1104500); National Natural Science Foundation of China (NSFC) (51702124, 61735005, 11704155, 61475067); Guangdong Project of Science and Technology Grants (2016B090917002, 2016B090926004); Guangzhou Union Project of Science and Technology Grants (201604040006). The Research project of scientific research cultivation and innovation fund of Jinan University (11617329). Guangdong Project of Featured Innovation Grants (2017KTSCX012).
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