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Growth, structure, and spectroscopic properties of a Tm3+, Ho3+ co-doped Lu2O3 crystal for ~2.1 μm lasers
Optical Materials 96 (2019) 109277
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Growth, structure, and spectroscopic properties of a Tm3+, Ho3+ co-doped Lu2O3 crystal for ~2.1 μm lasers
T
Shanming Lia,b, Lianhan Zhanga, Xiaojun Tanc, Wen Dengc, Mingzhu Hea, Guangzhu Chena,b, Min Xua, Yilun Yanga,b, Shulong Zhanga,b, Peixiong Zhangd, Zhenqiang Chend, Yin Hanga,* a
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China c School of Physical Science and Technology, Guangxi University, Guangxi, 530004, China d Department of Optoelectronic Engineering, Jinan University, Guangzhou, Guangdong, 510632, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Crystal growth Lu2O3 Tm3+ ions Ho3+ ions
The Tm,Ho:Lu2O3 crystal was grown by the optical floating zone method for the first time. The XRD pattern shows that it belongs to the cubic crystal system (Ia‾3 space group). The absorption cross-section at 796 nm is 4.21 × 10−21 cm2, the full width at half maximum (FWHM) is 54 nm. The emission cross-section at 2092 nm is approximately 0.20 × 10−20 cm2 . The broad fluorescence band extends to 2200 nm where almost no absorption was found. The fluoresce lifetime at 2092 nm is 3.73 ms. With the help of up-conversion spectra, the energy transfer mechanism between Tm3+ and Ho3+ ions was investigated systematically. The potential utility of the Tm,Ho:Lu2O3 crystal as tunable and ultra-short laser media at ~2.1 μm can be expected.
1. Introduction During the last decades, 2 μm lasers based on Tm3+-Ho3+ co-doped host media have attracted much attention due to the wide application in light detection and ranging (LIDAR), free-space optical communication, atmospheric sensors, and the laser microsurgery [1–4]. Tm3+ is well-known for the 3H6 → 3F4 transition which can be pumped by the commercial AlGaAs laser diodes. The efficient cross-relaxation (CR) process (3H4+3H6→3F4+3F4) results in a quantum efficiency of 200% leading to the low heat load and high slope efficiency [5,6]. Compared with Tm3+ ions, Ho3+ ions have the advantages of longer emission wavelength due to the transition of 5I7 → 5I8, larger emission crosssection, and longer fluorescence lifetime near 2 μm . Therefore, Tm, Ho co-doped laser materials bring the feature of enhanced pump efficiency of AlGaAs laser diodes by Tm3+ ions and possess the advantages of Ho3+ ions by means of energy transfer (Tm3+:3F4 → Ho3+:5I7) [7]. In recent years, many Tm,Ho co-doped laser materials have been explored for the study of 2 μm fluorescence properties [7–12,22]. Rare-earth cubic sesquioxide crystals (Re2O3, where Re= Lu, Y, Sc) are one kind of the most outstanding crystals among the laser hosts. The cubic structure belongs to the Ia(3) space group. Each unit cell includes 32 cations (16 formulas), 24 of which have C2 symmetry and the others have C3i symmetry, corresponding to the green and blue atoms in Fig. 1 [13].
*
Notably, the optical properties of these crystals mainly depend on the C2 sites because the electric-dipole transitions of C3i sites are forbidden as a result of the inversion symmetry [14]. Both sites have a 6-fold coordination surrounded by oxygen ions. Thermal conductivity can be one of the most attractive properties in sesquioxides, for example, 12.8 W/(mK) for Lu2O3, 13 W/(mK) for Y2O3, and 18 W/(mK) for Sc2O3, respectively. Considering the influence of doping on thermal conductivity, Lu2O3 crystal is the best of all because of the similar mass of doping ions [26]. In addition, the sesquioxides exhibit a broad transparency range (0.22~8 μm) and moderate maximum phonon energy (~612 cm−1) [15–17]. Recently, a lot of research based on the various rare earth ions doped sesquioxides has been reported for the application of near and mid-infrared lasers. So far, however, there have been few reports on Tm3+ and Ho3+ co-doped sesquioxides. For the above reasons, Tm3+ and Ho3+ co-doped Lu2O3 crystal was grown for the first time as far as we know. In the paper, the Tm3+ ions (5 at. %) and Ho3+ ions (0.5 at. %) co-doped Lu2O3 crystal was successfully prepared and the detailed growth procedure was introduced. The structural and optical properties were investigated systematically. Up-conversion experiments were carried out in order to obtain detailed information on the excited states dynamics of the investigated material. All the results indicate that Tm,Ho:Lu2O3 crystal can be a perfect candidate for the 2.1 μm laser medium.
Corresponding author. E-mail address: [email protected] (Y. Hang).
https://doi.org/10.1016/j.optmat.2019.109277 Received 7 May 2019; Received in revised form 16 July 2019; Accepted 24 July 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 96 (2019) 109277
S. Li, et al.
Fig. 2. The as-grown Tm,Ho:Lu2O3 crystal.
3. Results and discussion 3.1. Structure characterization The as-grown Tm,Ho:Lu2O3 crystal is shown in Fig. 2. It is a little pale yellow because of the color of Ho3+ ions, which can be found in all of the Ho3+ doped crystals [18–20]. The crystal is transparent without crack indicating that the quality of the Tm,Ho:Lu2O3 crystal is perfect. The concentrations of Tm3+ and Ho3+ ions in the crystal measured by ICP-AES are 9.9 × 1020 cm−3 (3.5 at%) and 1.3 × 1020 cm−3 (0.47 at%), respectively. The corresponding segregation coefficients of Tm3+ and Ho3+ ions are 0.70 and 0.94, respectively. Fig. 3 shows the X-ray diffraction (XRD) pattern of the Tm,Ho:Lu2O3 crystal, as well as the XRD pattern of the Lu2O3 crystal from the database (PDF #65–3172). All of the peaks match well with the Lu2O3 crystal without impurity peaks, which means that the as-grown Tm,Ho:Lu2O3 crystal remains cubic crystal system (Ia‾3 space group). In addition, the peak position of the Tm,Ho:Lu2O3 crystal shifts slightly to the left. It is obvious from the grating equation (Eq. (1)) and the formula for calculating the interplanar spacing of cubic crystal systems (Eq. (2)) that the left shift is due to an increase in the lattice constant. After calculating with the software (JADE), the result is a= 10.675Å , larger than that of the Lu2O3 from PDF #65–3172 (a= 10.390Å ). The enlarged lattice constant comes from the fact that the radii of the doped ions Tm3+ (0.869 Å ) and Ho3+ (0.901 Å ) are larger than that of the Lu3+ (0.848 Å ).
Fig. 1. Schematic of the structure of the cubic sesquioxides (left: three-dimensional view; right: the C3i and C2 sites).
2. Experiments 2.1. Crystal growth The Tm,Ho:Lu2O3 crystal was grown by the optical floating zone method. To grow this crystal, raw materials of Lu2O3, Ho2O3 and Tm2O3 powders with 5 N purity were weighted according to (Tm0.05Ho0.005Lu0.945)2O3. The raw powders were mixed together, shaped into two rods and pressed under a hydrostatic pressure of 210 MPa for 3 min. The obtained rods were then put into an alumina crucible and sintered at 1500 °C for 30 h. Then two poly-crystal rods were obtained. One was used as the seed crystal. The other was used as the feeding rod. The crystal was grown by the optical floating zone furnace (FZ-T-12000-X-VII-VPO-GU-PC, Crystal Systems Corporation, Japan). The heat sources are four Xenon lamps. They are fixed at the focal points of four confocal ellipsoids. The reflector focusing system consists of four gold-plated elliptical mirrors. The melting of rods and the growth of crystals are controlled by power. In this experiment, the growth atmosphere was air and it took 2 h to heat up to melting degree. Then the growth rate was 5 mm/h, and the rotation rate was 5 rpm for both the seed crystal and the feeding rod. After crystal growth, the obtained Tm,Ho:Lu2O3 crystal was cooled down to room temperature in 3 h. After all these processes, a transparent Tm,Ho:Lu2O3 crystal with 4 mm in diameter and 36 mm in length was thus obtained without any bubbles.
2dsinθ = mλ
(1)
d= a/ h2 + k2 + l2
(2)
3.2. Absorption spectra To evaluate the absorption properties of Tm,Ho:Lu2O3 crystal, the absorption spectra were measured at room temperature. Fig. 4 shows
2.2. Experimental methods For systematical analysis, the grown crystal was cut into several samples polished mechanically to optical quality. The samples were annealed at 1500 °C in the air for 20 h in the muffle furnace. The crystal structure identification was undertaken on a D⁄max2550 X-ray Diffraction (XRD) using Cu Kα1 radiation. The concentrations of the Tm3+ and Ho3+ ions were measured by inductively coupled plasma atomic emission spectrometry analysis (ICP-AES). The absorption spectrum of the grown crystals was recorded by a Lambda 1050 UV/ VIS/NIR spectrophotometer (PerkinElmer). The fluorescence spectrum in the range of 300–750 nm and 1600–2200 nm was measured by an Edinburgh Instruments FLSP920 steady-state spectrometer under the 808 nm excitation. The fluorescence decay curve of Tm,Ho:Lu2O3 crystal was measured at 2092 nm under pulse excitation of 808 nm. All the measurements were taken at room temperature.
Fig. 3. XRD patterns of the as-grown Tm,Ho:Lu2O3 crystal and the database (PDF #65–3172) of Lu2O3. 2
Optical Materials 96 (2019) 109277
S. Li, et al.
Fig. 4. The absorption spectrum of the Tm,Ho:Lu2O3 crystal at room temperature; The inset is the enlarged absorption cross-section spectra around 800 nm.
the absorption spectra of Tm,Ho:Lu2O3 crystal in the range of 300–2100 nm. There are five main absorption peaks centered at around 362 nm, 684 nm, 796 nm, 1204 nm, and 1630 nm, corresponding to the transitions from Tm3+: 3H6 to Tm3+: 1D2, 3F3, 3H4, 3H5, and 3F4, respectively. In addition, five main absorption peaks are visible with centers at 448 nm, 536 nm, 658 nm, 1148 nm, and 1866 nm, corresponding to the transitions from Ho3+: 5I8 to Ho3+: 5F1+5G6, 5F4+5S2, 5 F5, 5I6, and 5I7, respectively. The peak at 796 nm is important as it matches well with the emission of the commercial AlGaAs LD. The absorption coefficients (α ) at 796 nm and 808 nm are 4.22 and 1.94 cm−1, respectively. Calculated by Eq. (3), the absorption crosssections are 4.21 × 10−21 and 1.94 × 10−21, where NTm is the lattice concentration of Tm in Tm,Ho:Lu2O3 crystal:
σabs = α/NTm
(3)
Notably, the full width at half maximum (FWHM) is about 54 nm. Such a broad absorption band is advantageous to decrease the crucial control to the temperature stability of the output wavelength of the AlGaAs LD.
Fig. 5. The fluorescence spectrum of the Tm,Ho:Lu2O3 crystal under 808 nm excitation.
3.3. Fluorescence spectra of 2.1 μm emission
peaks at 2092 nm and extend to 2200 nm. It is particularly beneficial to the quasi-three level system. The emission cross-section was calculated with the help of the F-L equation:
The fluorescence spectra in the range of 1700–2200 nm were recorded under the 808 nm excitation at room temperature, corresponding to the combination of the Tm3+:3F4 → 3H6 and Ho3+:5I7 → 5I8 transitions. The two transitions exhibit many peaks shown in Fig. 5. These peaks are due to the splitting of the energy levels caused by the crystal field. The ground states of Ho3+ and Tm3+ are composed of 17 and 13 stark components respectively, the emission states of 15 and 9 components [26]. Owing to the non-negligible population of the Stark levels at the experiment temperature, two or more transitions have similar energy distances, therefore, close peaks are observed. Besides, considering the effect of the temperature, a large number of components were broadened in consequence of accidentally coincident transitions, so it shows a broad fluorescence band with the full width at half maximum of nearly 250 nm. In addition, the large splitting of the ground-state multiplet enables a long laser emission wavelength in the cubic Re2O3 crystals. In Tm,Ho:Lu2O3 crystal, the absorption spectra ends at about 2100 nm. However, the emission spectra show intense
σem (λ) =
λ5I(λ) 8πcτ∫ n2 (λ)λI(λ)dλ
(4)
where I(λ) is the fluorescence intensity, τ is the radiative lifetime of the upper laser level, n is the refractive index, c is the velocity of light, and λ is the emission wavelength. Further, it is necessary to separate the spectra of Ho3+ from the combined one. Firstly, we did peak-fitting with the Origin software in our data processing. Then, the transitions derive from stark energy level of Ho3+ were isolated combined with Ref. 26. Finally, the spectra of Ho3+ ions can be obtained by summing the above separation peaks with the Adj.R–S of 0.995 which shows high accuracy. The emission cross-section at 2092 nm is approximately 0.20 × 10−20 cm2 . Considering the appropriate emission cross section and wide fluorescence bandwidth, the Tm,Ho:Lu2O3 crystal shows perfect property for the generation of ultra-short pulse. 3
Optical Materials 96 (2019) 109277
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by an 808 nm laser pulse and fitted well by the single-exponential function. The measurement was performed with small excited volume to limit the effect of radiation trapping to a large extent. The measured fluorescence lifetime of 2.092 μm emission is 3.73 ms as shown in Fig. 6. In addition, a similar value of the fluorescence lifetime was obtained under the 640 nm excitation. The lifetime at 2.092 μm in Tm3+ and Ho3+ co-doped Lu2O3 crystal is much lower than 10 ms in Ho3+ singlydoped Lu2O3 crystal [21]. To understand this phenomenon, the upconversion spectra from 300 nm to 750 nm were recorded under the excitation of an 808 nm laser diode. As shown in Fig. 7, the spectra exhibit three peaks centered at 551 nm, 661 nm, and 705 nm, corresponding to the transitions of Ho3+: 5F4+5S2→5I8, Ho3+: 5F5→5I8, and Tm3+:3F2,3→3H6, respectively [23,24]. The corresponding energy transfer mechanism is presented in Fig. 8. All of the transitions derive from the excited state absorption (ESA) of the Ho3+: 5I7 level. The excited state (Ho3+: 5I7 level) jumps up to the Ho3+: 5F4+5S2 level after absorbing the pump energy and then populates the Ho3+: 5F5→5I8 and Tm3+:3F2,3→3H6 levels with non-radiation transition. The fluorescence lifetime at 551 nm was measured to be 10 μs under an 808 nm laser diode pulse as shown in the inset of Fig. 7. That's to say, the ESA process indeed depopulates the Ho3+: 5I7 level but just to a small extent. Besides, the energy transfer up-conversion (Tm3+:3F4+ Ho3+: 5I7 →Tm3+:3H6 + Ho3+: 5I5) also has an impact on it, which has been reported in Ref. 7. What's more, the energy transfer process between Tm3+:3F4 and Ho3+: 5I7 energy levels are reversible to a quasi-detailed balance, so the lifetime of Tm3+:3F4 manifold also affects the lifetime of Ho3+: 5I7 energy level in the equilibrium state [25]. As a consequence, the fluorescence lifetime of the 2.092 μm emission drops in Tm,Ho:Lu2O3 crystal. For a longer lifetime, the concentrations of the Tm3+ and Ho3+ ions should be reduced due to the cationic density (3 × 1022 cm−3) [26].
Fig. 6. The fluorescence decay curve of the 2.092 μm emission under 808 nm excitation.
4. Conclusions For the first time, the Tm,Ho:Lu2O3 crystal was grown by the optical floating zone method and its optical properties at around 2.1 μm was investigated at room temperature. The XRD pattern was recorded and the lattice parameter was calculated: a= 10.675Å . The absorption crosssection at 796 nm is 4.21 × 10−21cm2 with the FWHM of 54 nm, which is suitable for the AlGaAs LD. The emission cross-section at 2092 nm was 0.20 × 10−20 cm2 with the FWHM of nearly 250 nm and the fluorescence lifetime at 2092 nm was 3.73 ms. In addition, the energy transfer mechanism between Tm3+ and Ho3+ ions was analyzed. The results proved that Tm,Ho:Lu2O3 crystal can be a potential for the tunable lasers or ultra-short pulse generation at around 2.1 μm .
Fig. 7. The up-conversion fluorescence spectrum under 808 nm excitation; the inset is the decay curve of the 551 nm emission fitted by single-exponential function.
Conflicts of interest The authors declared that there is no conflict of interest. Acknowledgments This work was supported by National Natural Science Foundation of China (51472257, 51502321, 51872307); National Key R&D Program of China [2016YFB0701002, 2016YFB0402105, 2016YFB1102302]; Strategic Priority Research Program (B) [No. XDB16]; Major Project of Shanghai Science and Technology Research Foundation [ 16JC1420600]. Fig. 8. The energy transfer mechanism of between Tm3+ and Ho3+ ions (CR: cross relaxation; ESA: excited state absorption; ET: energy transfer).
References [1] R. Targ, B.C. Steakley, J.G. Hawley, L.L. Ames, P. Forney, D. Swanson, et al., Coherent lidar airborne wind sensor II: flight-test results at 2 and 10 μm, Appl. Optic. 35 (1996) 7117–7127. [2] G.J. Koch, B.W. Barnes, M. Petros, J.Y. Beyon, F. Amzajerdian, et al., Coherent differential absorption lidar measurements of CO2, Appl. Optic. 43 (2004) 5092–5099.
3.4. Fluorescence lifetime and energy transfer processes In order to study the transient fluorescence characteristics, the fluorescence decay curve of Ho3+:5I7 → 5I8 was measured when excited 4
Optical Materials 96 (2019) 109277
S. Li, et al. [3] A. Godard, Infrared (2–12 μm) solid-state laser sources: a review, C. R. Phys. 8 (2007) 1100–1128. [4] B.M. Walsh, H.R. Lee, N.P. Barnes, Mid infrared lasers for remote sensing applications, J. Lumin. 169 (2016) 400–405. [5] I. Razumova, et al., Spectral-luminescent properties of Tm: YLF crystal, J. Alloy, Compd 225 (1995) 129–132. [6] R. Calderón-Villajos, C. Zaldo, C. Cascales, Micro-and nanosized architectures in hydrothermal Tm3+-doped GdVO4: chemical insights towards preservation of the emission efficiency, CrystEngComm 14 (2012) 2756–2768. [7] P. Loiko, J.M. Serres, X. Mateos, K. Yumashev, N. Kuleshov, V. Petrov, et al., Microchip laser operation of Tm, Ho: KLu(WO4)2 crystal, Opt. Express 22 (2014) 27976–27984. [8] Yunyun Liu, et al., Crystal growth and spectroscopic analysis of Tm, Ho singlydoped and Tm/Ho co-doped CaLaGa3O7 NIR laser crystals, Opt. Mater. 75 (2018) 744–750. [9] L. Shen, M. Cai, et al., Preparation and investigation of Tm3+/Ho3+ co-doped germanate-tellurite glass as promising materials for ultrashort pulse laser, Opt. Mater. 67 (2017) 125–131. [10] C. Wang, H. Yin, A. Li, Y. Wu, S. Zhu, et al., Growth, thermal and spectral properties of Tm3+, Ho3+ co-doped NaGd (MoO4) 2 Crystal, J. Alloy, Compd 615 (2014) 482–487. [11] L. Li, X. Yang, L. Zhou, W. Xie, Y. Wang, Shen, X. Duan, Active/passive Q-switching operation of 2 μm Tm, Ho: YAP laser with an acousto-optical Q-switch/MoS2 saturable absorber mirror, Photonics Res. 6 (2018) 614–619. [12] S. Li, L. Zhang, C. Li, et al., Growth, thermal conductivity, spectra, and 2 μm continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal, J. Lumin. 210 (2019) 142–145. [13] C.R. Stanek, K.J. McClellan, B.P. Uberuaga, K.E. Sickafus, M.R. Levy, R.W. Grimes, Phys. Rev. B 75 (2007) 134101. [14] C. Kränkel, Rare-earth-doped sesquioxides for diode-pumped high-power lasers in the 1-, 2-, and 3-μm spectral range, IEEE J. Sel. Top. Quant. 21 (2015) 250–262. [15] M.V. Abrashev, N.D. Todorov, J. Geshev, Raman spectra of R2O3 (R—rare earth)
[16]
[17]
[18] [19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
5
sesquioxides with C-type bixbyite crystal structure: a comparative study, J. Appl. Phys. 116 (2014) 103508. P. Loiko, P. Koopmann, X. Mateos, J.M. Serres, V. Jambunathan, A. Lucianetti, V. Petrov, Highly efficient, compact Tm3+: RE2O3 (RE= Y, Lu, Sc) sesquioxide lasers based on thermal guiding, IEEE J. Sel. Top. Quant. 25 (2018) 1–13. L. Laversenne, Y. Guyot, C. Goutaudier, M.T. Cohen-Adad, G. Boulon, Optimization of spectroscopic properties of Yb3+-doped refractory sesquioxides: cubic Y2O3, Lu2O3 and monoclinic Gd2O3, Opt. Mater. 16 (2001) 475–483. S. Yang, H. Xia, J. Zhang, Y. Jiang, Y. Shi, X. Gu, et al., Growth and spectral properties of Ho3+ doped α-NaYF4 single crystal, Opt. Mater. 45 (2015) 209–214. C. Zhao, Y. Hang, L. Zhang, J. Yin, P. Hu, E. Ma, Polarized spectroscopic properties of Ho3+-doped LuLiF4 single crystal for 2 μm and 2.9 μm lasers, Opt. Mater. 33 (2011) 1610–1615. J. Hong, L. Zhang, P. Zhang, M. Xu, Y. Hang, Ho, LaF3 single crystal as potential material for 2 μm and 2.9 μm lasers, Infrared Phys. Technol. 76 (2016) 636–640. P. Koopmann, S. Lamrini, K. Scholle, M. Schäfer, P. Fuhrberg, G. Huber, Multi-watt laser operation and laser parameters of Ho-doped Lu2O3 at 2.12 μm, Opt. Mater. Express 1 (2011) 1447–1456. V. Sudesh, K. Asai, Spectroscopic and diode-pumped-laser properties of Tm, Ho: YLF; Tm, Ho: LuLF; and Tm, Ho: LuAG crystals: a comparative study, JOSA B 20 (2003) 1829–1837. J.F. Tang, G.N. Li, C. Yang, J. Gou, D.H. Luo, H. He, Power driven tunable white upconversion luminescence from Lu2TeO6 tri-doped with Yb3+, Tm3+ and Ho3+, CrystEngComm 17 (2015) 9048–9054. V. Lojpur, M. Nikolic, L. Mancic, O. Milosevic, M.D. Dramicanin, Y2O3: Yb, Tm and Y2O3: Yb, Ho powders for low-temperature thermometry based on up-conversion fluorescence, Ceram. Int. 39 (2013) 1129–1134. B.M. Walsh, N.P. Barnes, B. Di Bartolo, The temperature dependence of energy transfer between the Tm 3F4 and Ho 5I7 manifolds of Tm-sensitized Ho luminescence in YAG and YLF, J. Lumin. 90 (2000) 39–48. P. Koopmann, Thulium- and Holmium-Doped Sesquioxides for 2 μm Lasers, Ph.D. Dissertation Univ. Hamburg, Hamburg, Germany, 2012.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Growth, structure, and spectroscopic properties of a Tm3+, Ho3+ co-doped Lu2O3 crystal for ~2.1 μm lasers
T
Shanming Lia,b, Lianhan Zhanga, Xiaojun Tanc, Wen Dengc, Mingzhu Hea, Guangzhu Chena,b, Min Xua, Yilun Yanga,b, Shulong Zhanga,b, Peixiong Zhangd, Zhenqiang Chend, Yin Hanga,* a
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China c School of Physical Science and Technology, Guangxi University, Guangxi, 530004, China d Department of Optoelectronic Engineering, Jinan University, Guangzhou, Guangdong, 510632, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Crystal growth Lu2O3 Tm3+ ions Ho3+ ions
The Tm,Ho:Lu2O3 crystal was grown by the optical floating zone method for the first time. The XRD pattern shows that it belongs to the cubic crystal system (Ia‾3 space group). The absorption cross-section at 796 nm is 4.21 × 10−21 cm2, the full width at half maximum (FWHM) is 54 nm. The emission cross-section at 2092 nm is approximately 0.20 × 10−20 cm2 . The broad fluorescence band extends to 2200 nm where almost no absorption was found. The fluoresce lifetime at 2092 nm is 3.73 ms. With the help of up-conversion spectra, the energy transfer mechanism between Tm3+ and Ho3+ ions was investigated systematically. The potential utility of the Tm,Ho:Lu2O3 crystal as tunable and ultra-short laser media at ~2.1 μm can be expected.
1. Introduction During the last decades, 2 μm lasers based on Tm3+-Ho3+ co-doped host media have attracted much attention due to the wide application in light detection and ranging (LIDAR), free-space optical communication, atmospheric sensors, and the laser microsurgery [1–4]. Tm3+ is well-known for the 3H6 → 3F4 transition which can be pumped by the commercial AlGaAs laser diodes. The efficient cross-relaxation (CR) process (3H4+3H6→3F4+3F4) results in a quantum efficiency of 200% leading to the low heat load and high slope efficiency [5,6]. Compared with Tm3+ ions, Ho3+ ions have the advantages of longer emission wavelength due to the transition of 5I7 → 5I8, larger emission crosssection, and longer fluorescence lifetime near 2 μm . Therefore, Tm, Ho co-doped laser materials bring the feature of enhanced pump efficiency of AlGaAs laser diodes by Tm3+ ions and possess the advantages of Ho3+ ions by means of energy transfer (Tm3+:3F4 → Ho3+:5I7) [7]. In recent years, many Tm,Ho co-doped laser materials have been explored for the study of 2 μm fluorescence properties [7–12,22]. Rare-earth cubic sesquioxide crystals (Re2O3, where Re= Lu, Y, Sc) are one kind of the most outstanding crystals among the laser hosts. The cubic structure belongs to the Ia(3) space group. Each unit cell includes 32 cations (16 formulas), 24 of which have C2 symmetry and the others have C3i symmetry, corresponding to the green and blue atoms in Fig. 1 [13].
*
Notably, the optical properties of these crystals mainly depend on the C2 sites because the electric-dipole transitions of C3i sites are forbidden as a result of the inversion symmetry [14]. Both sites have a 6-fold coordination surrounded by oxygen ions. Thermal conductivity can be one of the most attractive properties in sesquioxides, for example, 12.8 W/(mK) for Lu2O3, 13 W/(mK) for Y2O3, and 18 W/(mK) for Sc2O3, respectively. Considering the influence of doping on thermal conductivity, Lu2O3 crystal is the best of all because of the similar mass of doping ions [26]. In addition, the sesquioxides exhibit a broad transparency range (0.22~8 μm) and moderate maximum phonon energy (~612 cm−1) [15–17]. Recently, a lot of research based on the various rare earth ions doped sesquioxides has been reported for the application of near and mid-infrared lasers. So far, however, there have been few reports on Tm3+ and Ho3+ co-doped sesquioxides. For the above reasons, Tm3+ and Ho3+ co-doped Lu2O3 crystal was grown for the first time as far as we know. In the paper, the Tm3+ ions (5 at. %) and Ho3+ ions (0.5 at. %) co-doped Lu2O3 crystal was successfully prepared and the detailed growth procedure was introduced. The structural and optical properties were investigated systematically. Up-conversion experiments were carried out in order to obtain detailed information on the excited states dynamics of the investigated material. All the results indicate that Tm,Ho:Lu2O3 crystal can be a perfect candidate for the 2.1 μm laser medium.
Corresponding author. E-mail address: [email protected] (Y. Hang).
https://doi.org/10.1016/j.optmat.2019.109277 Received 7 May 2019; Received in revised form 16 July 2019; Accepted 24 July 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 96 (2019) 109277
S. Li, et al.
Fig. 2. The as-grown Tm,Ho:Lu2O3 crystal.
3. Results and discussion 3.1. Structure characterization The as-grown Tm,Ho:Lu2O3 crystal is shown in Fig. 2. It is a little pale yellow because of the color of Ho3+ ions, which can be found in all of the Ho3+ doped crystals [18–20]. The crystal is transparent without crack indicating that the quality of the Tm,Ho:Lu2O3 crystal is perfect. The concentrations of Tm3+ and Ho3+ ions in the crystal measured by ICP-AES are 9.9 × 1020 cm−3 (3.5 at%) and 1.3 × 1020 cm−3 (0.47 at%), respectively. The corresponding segregation coefficients of Tm3+ and Ho3+ ions are 0.70 and 0.94, respectively. Fig. 3 shows the X-ray diffraction (XRD) pattern of the Tm,Ho:Lu2O3 crystal, as well as the XRD pattern of the Lu2O3 crystal from the database (PDF #65–3172). All of the peaks match well with the Lu2O3 crystal without impurity peaks, which means that the as-grown Tm,Ho:Lu2O3 crystal remains cubic crystal system (Ia‾3 space group). In addition, the peak position of the Tm,Ho:Lu2O3 crystal shifts slightly to the left. It is obvious from the grating equation (Eq. (1)) and the formula for calculating the interplanar spacing of cubic crystal systems (Eq. (2)) that the left shift is due to an increase in the lattice constant. After calculating with the software (JADE), the result is a= 10.675Å , larger than that of the Lu2O3 from PDF #65–3172 (a= 10.390Å ). The enlarged lattice constant comes from the fact that the radii of the doped ions Tm3+ (0.869 Å ) and Ho3+ (0.901 Å ) are larger than that of the Lu3+ (0.848 Å ).
Fig. 1. Schematic of the structure of the cubic sesquioxides (left: three-dimensional view; right: the C3i and C2 sites).
2. Experiments 2.1. Crystal growth The Tm,Ho:Lu2O3 crystal was grown by the optical floating zone method. To grow this crystal, raw materials of Lu2O3, Ho2O3 and Tm2O3 powders with 5 N purity were weighted according to (Tm0.05Ho0.005Lu0.945)2O3. The raw powders were mixed together, shaped into two rods and pressed under a hydrostatic pressure of 210 MPa for 3 min. The obtained rods were then put into an alumina crucible and sintered at 1500 °C for 30 h. Then two poly-crystal rods were obtained. One was used as the seed crystal. The other was used as the feeding rod. The crystal was grown by the optical floating zone furnace (FZ-T-12000-X-VII-VPO-GU-PC, Crystal Systems Corporation, Japan). The heat sources are four Xenon lamps. They are fixed at the focal points of four confocal ellipsoids. The reflector focusing system consists of four gold-plated elliptical mirrors. The melting of rods and the growth of crystals are controlled by power. In this experiment, the growth atmosphere was air and it took 2 h to heat up to melting degree. Then the growth rate was 5 mm/h, and the rotation rate was 5 rpm for both the seed crystal and the feeding rod. After crystal growth, the obtained Tm,Ho:Lu2O3 crystal was cooled down to room temperature in 3 h. After all these processes, a transparent Tm,Ho:Lu2O3 crystal with 4 mm in diameter and 36 mm in length was thus obtained without any bubbles.
2dsinθ = mλ
(1)
d= a/ h2 + k2 + l2
(2)
3.2. Absorption spectra To evaluate the absorption properties of Tm,Ho:Lu2O3 crystal, the absorption spectra were measured at room temperature. Fig. 4 shows
2.2. Experimental methods For systematical analysis, the grown crystal was cut into several samples polished mechanically to optical quality. The samples were annealed at 1500 °C in the air for 20 h in the muffle furnace. The crystal structure identification was undertaken on a D⁄max2550 X-ray Diffraction (XRD) using Cu Kα1 radiation. The concentrations of the Tm3+ and Ho3+ ions were measured by inductively coupled plasma atomic emission spectrometry analysis (ICP-AES). The absorption spectrum of the grown crystals was recorded by a Lambda 1050 UV/ VIS/NIR spectrophotometer (PerkinElmer). The fluorescence spectrum in the range of 300–750 nm and 1600–2200 nm was measured by an Edinburgh Instruments FLSP920 steady-state spectrometer under the 808 nm excitation. The fluorescence decay curve of Tm,Ho:Lu2O3 crystal was measured at 2092 nm under pulse excitation of 808 nm. All the measurements were taken at room temperature.
Fig. 3. XRD patterns of the as-grown Tm,Ho:Lu2O3 crystal and the database (PDF #65–3172) of Lu2O3. 2
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Fig. 4. The absorption spectrum of the Tm,Ho:Lu2O3 crystal at room temperature; The inset is the enlarged absorption cross-section spectra around 800 nm.
the absorption spectra of Tm,Ho:Lu2O3 crystal in the range of 300–2100 nm. There are five main absorption peaks centered at around 362 nm, 684 nm, 796 nm, 1204 nm, and 1630 nm, corresponding to the transitions from Tm3+: 3H6 to Tm3+: 1D2, 3F3, 3H4, 3H5, and 3F4, respectively. In addition, five main absorption peaks are visible with centers at 448 nm, 536 nm, 658 nm, 1148 nm, and 1866 nm, corresponding to the transitions from Ho3+: 5I8 to Ho3+: 5F1+5G6, 5F4+5S2, 5 F5, 5I6, and 5I7, respectively. The peak at 796 nm is important as it matches well with the emission of the commercial AlGaAs LD. The absorption coefficients (α ) at 796 nm and 808 nm are 4.22 and 1.94 cm−1, respectively. Calculated by Eq. (3), the absorption crosssections are 4.21 × 10−21 and 1.94 × 10−21, where NTm is the lattice concentration of Tm in Tm,Ho:Lu2O3 crystal:
σabs = α/NTm
(3)
Notably, the full width at half maximum (FWHM) is about 54 nm. Such a broad absorption band is advantageous to decrease the crucial control to the temperature stability of the output wavelength of the AlGaAs LD.
Fig. 5. The fluorescence spectrum of the Tm,Ho:Lu2O3 crystal under 808 nm excitation.
3.3. Fluorescence spectra of 2.1 μm emission
peaks at 2092 nm and extend to 2200 nm. It is particularly beneficial to the quasi-three level system. The emission cross-section was calculated with the help of the F-L equation:
The fluorescence spectra in the range of 1700–2200 nm were recorded under the 808 nm excitation at room temperature, corresponding to the combination of the Tm3+:3F4 → 3H6 and Ho3+:5I7 → 5I8 transitions. The two transitions exhibit many peaks shown in Fig. 5. These peaks are due to the splitting of the energy levels caused by the crystal field. The ground states of Ho3+ and Tm3+ are composed of 17 and 13 stark components respectively, the emission states of 15 and 9 components [26]. Owing to the non-negligible population of the Stark levels at the experiment temperature, two or more transitions have similar energy distances, therefore, close peaks are observed. Besides, considering the effect of the temperature, a large number of components were broadened in consequence of accidentally coincident transitions, so it shows a broad fluorescence band with the full width at half maximum of nearly 250 nm. In addition, the large splitting of the ground-state multiplet enables a long laser emission wavelength in the cubic Re2O3 crystals. In Tm,Ho:Lu2O3 crystal, the absorption spectra ends at about 2100 nm. However, the emission spectra show intense
σem (λ) =
λ5I(λ) 8πcτ∫ n2 (λ)λI(λ)dλ
(4)
where I(λ) is the fluorescence intensity, τ is the radiative lifetime of the upper laser level, n is the refractive index, c is the velocity of light, and λ is the emission wavelength. Further, it is necessary to separate the spectra of Ho3+ from the combined one. Firstly, we did peak-fitting with the Origin software in our data processing. Then, the transitions derive from stark energy level of Ho3+ were isolated combined with Ref. 26. Finally, the spectra of Ho3+ ions can be obtained by summing the above separation peaks with the Adj.R–S of 0.995 which shows high accuracy. The emission cross-section at 2092 nm is approximately 0.20 × 10−20 cm2 . Considering the appropriate emission cross section and wide fluorescence bandwidth, the Tm,Ho:Lu2O3 crystal shows perfect property for the generation of ultra-short pulse. 3
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by an 808 nm laser pulse and fitted well by the single-exponential function. The measurement was performed with small excited volume to limit the effect of radiation trapping to a large extent. The measured fluorescence lifetime of 2.092 μm emission is 3.73 ms as shown in Fig. 6. In addition, a similar value of the fluorescence lifetime was obtained under the 640 nm excitation. The lifetime at 2.092 μm in Tm3+ and Ho3+ co-doped Lu2O3 crystal is much lower than 10 ms in Ho3+ singlydoped Lu2O3 crystal [21]. To understand this phenomenon, the upconversion spectra from 300 nm to 750 nm were recorded under the excitation of an 808 nm laser diode. As shown in Fig. 7, the spectra exhibit three peaks centered at 551 nm, 661 nm, and 705 nm, corresponding to the transitions of Ho3+: 5F4+5S2→5I8, Ho3+: 5F5→5I8, and Tm3+:3F2,3→3H6, respectively [23,24]. The corresponding energy transfer mechanism is presented in Fig. 8. All of the transitions derive from the excited state absorption (ESA) of the Ho3+: 5I7 level. The excited state (Ho3+: 5I7 level) jumps up to the Ho3+: 5F4+5S2 level after absorbing the pump energy and then populates the Ho3+: 5F5→5I8 and Tm3+:3F2,3→3H6 levels with non-radiation transition. The fluorescence lifetime at 551 nm was measured to be 10 μs under an 808 nm laser diode pulse as shown in the inset of Fig. 7. That's to say, the ESA process indeed depopulates the Ho3+: 5I7 level but just to a small extent. Besides, the energy transfer up-conversion (Tm3+:3F4+ Ho3+: 5I7 →Tm3+:3H6 + Ho3+: 5I5) also has an impact on it, which has been reported in Ref. 7. What's more, the energy transfer process between Tm3+:3F4 and Ho3+: 5I7 energy levels are reversible to a quasi-detailed balance, so the lifetime of Tm3+:3F4 manifold also affects the lifetime of Ho3+: 5I7 energy level in the equilibrium state [25]. As a consequence, the fluorescence lifetime of the 2.092 μm emission drops in Tm,Ho:Lu2O3 crystal. For a longer lifetime, the concentrations of the Tm3+ and Ho3+ ions should be reduced due to the cationic density (3 × 1022 cm−3) [26].
Fig. 6. The fluorescence decay curve of the 2.092 μm emission under 808 nm excitation.
4. Conclusions For the first time, the Tm,Ho:Lu2O3 crystal was grown by the optical floating zone method and its optical properties at around 2.1 μm was investigated at room temperature. The XRD pattern was recorded and the lattice parameter was calculated: a= 10.675Å . The absorption crosssection at 796 nm is 4.21 × 10−21cm2 with the FWHM of 54 nm, which is suitable for the AlGaAs LD. The emission cross-section at 2092 nm was 0.20 × 10−20 cm2 with the FWHM of nearly 250 nm and the fluorescence lifetime at 2092 nm was 3.73 ms. In addition, the energy transfer mechanism between Tm3+ and Ho3+ ions was analyzed. The results proved that Tm,Ho:Lu2O3 crystal can be a potential for the tunable lasers or ultra-short pulse generation at around 2.1 μm .
Fig. 7. The up-conversion fluorescence spectrum under 808 nm excitation; the inset is the decay curve of the 551 nm emission fitted by single-exponential function.
Conflicts of interest The authors declared that there is no conflict of interest. Acknowledgments This work was supported by National Natural Science Foundation of China (51472257, 51502321, 51872307); National Key R&D Program of China [2016YFB0701002, 2016YFB0402105, 2016YFB1102302]; Strategic Priority Research Program (B) [No. XDB16]; Major Project of Shanghai Science and Technology Research Foundation [ 16JC1420600]. Fig. 8. The energy transfer mechanism of between Tm3+ and Ho3+ ions (CR: cross relaxation; ESA: excited state absorption; ET: energy transfer).
References [1] R. Targ, B.C. Steakley, J.G. Hawley, L.L. Ames, P. Forney, D. Swanson, et al., Coherent lidar airborne wind sensor II: flight-test results at 2 and 10 μm, Appl. Optic. 35 (1996) 7117–7127. [2] G.J. Koch, B.W. Barnes, M. Petros, J.Y. Beyon, F. Amzajerdian, et al., Coherent differential absorption lidar measurements of CO2, Appl. Optic. 43 (2004) 5092–5099.
3.4. Fluorescence lifetime and energy transfer processes In order to study the transient fluorescence characteristics, the fluorescence decay curve of Ho3+:5I7 → 5I8 was measured when excited 4
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S. Li, et al. [3] A. Godard, Infrared (2–12 μm) solid-state laser sources: a review, C. R. Phys. 8 (2007) 1100–1128. [4] B.M. Walsh, H.R. Lee, N.P. Barnes, Mid infrared lasers for remote sensing applications, J. Lumin. 169 (2016) 400–405. [5] I. Razumova, et al., Spectral-luminescent properties of Tm: YLF crystal, J. Alloy, Compd 225 (1995) 129–132. [6] R. Calderón-Villajos, C. Zaldo, C. Cascales, Micro-and nanosized architectures in hydrothermal Tm3+-doped GdVO4: chemical insights towards preservation of the emission efficiency, CrystEngComm 14 (2012) 2756–2768. [7] P. Loiko, J.M. Serres, X. Mateos, K. Yumashev, N. Kuleshov, V. Petrov, et al., Microchip laser operation of Tm, Ho: KLu(WO4)2 crystal, Opt. Express 22 (2014) 27976–27984. [8] Yunyun Liu, et al., Crystal growth and spectroscopic analysis of Tm, Ho singlydoped and Tm/Ho co-doped CaLaGa3O7 NIR laser crystals, Opt. Mater. 75 (2018) 744–750. [9] L. Shen, M. Cai, et al., Preparation and investigation of Tm3+/Ho3+ co-doped germanate-tellurite glass as promising materials for ultrashort pulse laser, Opt. Mater. 67 (2017) 125–131. [10] C. Wang, H. Yin, A. Li, Y. Wu, S. Zhu, et al., Growth, thermal and spectral properties of Tm3+, Ho3+ co-doped NaGd (MoO4) 2 Crystal, J. Alloy, Compd 615 (2014) 482–487. [11] L. Li, X. Yang, L. Zhou, W. Xie, Y. Wang, Shen, X. Duan, Active/passive Q-switching operation of 2 μm Tm, Ho: YAP laser with an acousto-optical Q-switch/MoS2 saturable absorber mirror, Photonics Res. 6 (2018) 614–619. [12] S. Li, L. Zhang, C. Li, et al., Growth, thermal conductivity, spectra, and 2 μm continuous-wave characteristics of Tm3+, Ho3+ co-doped LaF3 crystal, J. Lumin. 210 (2019) 142–145. [13] C.R. Stanek, K.J. McClellan, B.P. Uberuaga, K.E. Sickafus, M.R. Levy, R.W. Grimes, Phys. Rev. B 75 (2007) 134101. [14] C. Kränkel, Rare-earth-doped sesquioxides for diode-pumped high-power lasers in the 1-, 2-, and 3-μm spectral range, IEEE J. Sel. Top. Quant. 21 (2015) 250–262. [15] M.V. Abrashev, N.D. Todorov, J. Geshev, Raman spectra of R2O3 (R—rare earth)
[16]
[17]
[18] [19]
[20] [21]
[22]
[23]
[24]
[25]
[26]
5
sesquioxides with C-type bixbyite crystal structure: a comparative study, J. Appl. Phys. 116 (2014) 103508. P. Loiko, P. Koopmann, X. Mateos, J.M. Serres, V. Jambunathan, A. Lucianetti, V. Petrov, Highly efficient, compact Tm3+: RE2O3 (RE= Y, Lu, Sc) sesquioxide lasers based on thermal guiding, IEEE J. Sel. Top. Quant. 25 (2018) 1–13. L. Laversenne, Y. Guyot, C. Goutaudier, M.T. Cohen-Adad, G. Boulon, Optimization of spectroscopic properties of Yb3+-doped refractory sesquioxides: cubic Y2O3, Lu2O3 and monoclinic Gd2O3, Opt. Mater. 16 (2001) 475–483. S. Yang, H. Xia, J. Zhang, Y. Jiang, Y. Shi, X. Gu, et al., Growth and spectral properties of Ho3+ doped α-NaYF4 single crystal, Opt. Mater. 45 (2015) 209–214. C. Zhao, Y. Hang, L. Zhang, J. Yin, P. Hu, E. Ma, Polarized spectroscopic properties of Ho3+-doped LuLiF4 single crystal for 2 μm and 2.9 μm lasers, Opt. Mater. 33 (2011) 1610–1615. J. Hong, L. Zhang, P. Zhang, M. Xu, Y. Hang, Ho, LaF3 single crystal as potential material for 2 μm and 2.9 μm lasers, Infrared Phys. Technol. 76 (2016) 636–640. P. Koopmann, S. Lamrini, K. Scholle, M. Schäfer, P. Fuhrberg, G. Huber, Multi-watt laser operation and laser parameters of Ho-doped Lu2O3 at 2.12 μm, Opt. Mater. Express 1 (2011) 1447–1456. V. Sudesh, K. Asai, Spectroscopic and diode-pumped-laser properties of Tm, Ho: YLF; Tm, Ho: LuLF; and Tm, Ho: LuAG crystals: a comparative study, JOSA B 20 (2003) 1829–1837. J.F. Tang, G.N. Li, C. Yang, J. Gou, D.H. Luo, H. He, Power driven tunable white upconversion luminescence from Lu2TeO6 tri-doped with Yb3+, Tm3+ and Ho3+, CrystEngComm 17 (2015) 9048–9054. V. Lojpur, M. Nikolic, L. Mancic, O. Milosevic, M.D. Dramicanin, Y2O3: Yb, Tm and Y2O3: Yb, Ho powders for low-temperature thermometry based on up-conversion fluorescence, Ceram. Int. 39 (2013) 1129–1134. B.M. Walsh, N.P. Barnes, B. Di Bartolo, The temperature dependence of energy transfer between the Tm 3F4 and Ho 5I7 manifolds of Tm-sensitized Ho luminescence in YAG and YLF, J. Lumin. 90 (2000) 39–48. P. Koopmann, Thulium- and Holmium-Doped Sesquioxides for 2 μm Lasers, Ph.D. Dissertation Univ. Hamburg, Hamburg, Germany, 2012.