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Effective enhancement of 2.87 µm fluorescence via Yb3+ in Ho3+:LaF3 laser crystal
Journal of Luminescence 203 (2018) 730–734
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Effective enhancement of 2.87 µm fluorescence via Yb3+ in Ho3+:LaF3 laser crystal
T
Shanming Lia,b, Lianhan Zhanga, Mingzhu Hea, Guangzhu Chena, Peixiong Zhangc, Yilun Yanga,b, ⁎ Min Xua, Tao Yand, Ning Yed, 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 University of Chinese Academy of Sciences, Beijing 100039, China c Department of Optoelectronic Engineering, Jinan University, Guangzhou, Guangdong 510632, China d Fujian Institute of Research on the Structure of the Matter, Chinese Academy of Sciences, Fuzhou 350002, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Yb,Ho:LaF3 Laser crystal Spectroscopy
A novel Yb3+, Ho3+:LaF3 crystal was successfully grown and analyzed. Intense emission at 2.87 µm was observed in Yb3+, Ho3+:LaF3 crystal for the first time. Yb3+ ions not only provide sensitization mechanism for conventional LD pump, but also reduce self-termination by comparing with Ho3+:LaF3 crystal. The emission cross-section, quantum efficiency and fluorescence branching ratio at 2.87 µm are calculated to be 1.21 × 10–20 cm2, 82.5%, and 27.1%, respectively. The results indicate Yb3+, Ho3+:LaF3 crystal an attractive host for solid-state lasers around 2.87 µm under a 974 nm LD pump.
1. Introduction In recent years, mid-infrared lasers at around 3 µm have attracted much attention due to wide applications in biomedical treatments, molecular fingerprint identification and optical parametric oscillation pump source [1–3]. Ho3+ ions are ideal active ions operating at around 3 µm attributed to the transition from 5I6 to 5I7 [4–6]. However, none laser diode can meet the absorption of 5I6 energy level; Self- termination is obvious as a result of the difficulty to realize population inversion. As far as we know, Yb3+ ions can be easily pumped under commercial laser diode, as a result of the advanced development of InGaAs diode. In addition, there is energy transfer between Yb3+ and Ho3+ ions, which is popular in relative up-conversion luminescence materials [7–9]. Therefore, codoping Yb3+ ions can open up the pump possibility of Ho3+ ions at 3 µm. Additionally, novel laser material with low phonon energy needs to be discovered. Our research group has been researching on infrared laser materials for a few years. We found that LaF3 crystal is such a candidate with low phonon energy (~350 cm−1), similar ion-substitution-radius and perfect infrared transmittance [10]. Relevant studies of infrared emission in rare earth doped LaF3 crystal have been reported in ref.10 and ref.11. To our knowledge, there have not been reports on the growth or the
~3 µm emission of Yb3+, Ho3+:LaF3 crystal. In this paper, Yb3+, Ho3+:LaF3 crystal was successfully prepared as well as Ho3+:LaF3 and Yb3+:LaF3 crystal as comparison. Theoretical calculation and spectral investigation suggest that Yb3+ ions are effective to sensitize Ho3+:5I6 and beneficial to decrease the self-termination by shortening lifetime of the lower energy level. Intense emission at 2.87 µm has been observed for potential mid- infrared lasers pumped under 974 nm InGaAs laser diode. 2. Experiments 2 at% Yb3+/ 2 at% Ho3+ co-doped LaF3 crystal was grown by the Bridgman method. The raw materials were fluoride powder of the LaF3, HoF3 and YbF3 provided commercially, all with high purity of 99.99%. In comparison, 2 at% Yb3+:LaF3 and 2 at% Ho3+:LaF3 crystals were also grown. All crystals crystallized spontaneously in a graphite crucible under the atmosphere of high-purity Argon (70%) and Carbon tetrafluoride (30%) during the whole procedure, including heating, cooling growth and annealing. The detailed process can be found in ref.10. The insets of Fig. 1 show two types of the grown crystals: Ho3+ and Yb3+, Ho3+ co-doped LaF3 crystals, the size of which are around ∅20 × 25 mm3. The grown crystals were cut into several samples polished mechanically to spectral quality for systematical analysis, the
⁎ Corresponding author at: Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail addresses: [email protected] (S. Li), [email protected] (Y. Hang).
https://doi.org/10.1016/j.jlumin.2018.07.027 Received 22 May 2018; Received in revised form 10 July 2018; Accepted 19 July 2018 Available online 21 July 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
Journal of Luminescence 203 (2018) 730–734
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Fig. 1. Comparison of absorption spectra between Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals in the range of 300–2200 nm; the insets are two types of LaF3 crystals.
sizes of which are 10 × 10 × 1 mm3. The concentrations of Yb3+ and Ho3+ ions were measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The absorption spectra of the grown crystals were recorded by a Lambda 1050 UV/VIS/NIR spectrophotometer (Perkin-Elmer). The fluorescence spectrum in the wavelength of 2700–3100 nm for the Yb3+,Ho3+:LaF3 crystal was measured with Edinburgh Instruments FLS920 under excitation of 974 nm. The fluorescence decay curves of Yb3+,Ho3+:LaF3 crystal were measured at 2870 nm under pulse excitation of 974 nm. The decay curves of the Yb3+:2F5/2 energy level of Yb3+-doped and Yb3+, Ho3+ co-doped LaF3 crystals were measured at 1040 nm under excitation of 974 nm. All the measurements were taken at room temperature. 3. Results and discussion Fig. 2. Mid-infrared transmittance spectrum of Yb3+, Ho3+:LaF3 crystal.
The absorption spectra of Ho3+ doped LaF3 crystal and Yb3+, Ho3+ co-doped LaF3 crystal are shown in Fig. 1. There are 7 main absorption bands with the peaks centered at 360 nm, 414 nm, 448 nm, 536 nm, 636 nm, 882 nm, 1142 nm and 1926 nm in the Ho3+:LaF3 crystal, which correspond to the transitions from Ho3+: 5I8 to 3K7 +5G4, 3G5, 5 G6 +5F1, 5F4 +5S2, 5F5, 5I5, 5I6 and 5I7, respectively [15]. It is a pity that no peaks satisfy the commercial laser diode. As for Yb3+, Ho3+: LaF3 crystal, there is an obvious absorption peak in the range of 851–1002 nm centered at 974 nm, which corresponds to the transition from 2F7/2 to 2F5/2. Lattice-site concentrations of the Ho3+ ions in Ho3+ doped and Yb3+, Ho3+ co-doped LaF3 crystals were measured by ICPAES to be 1.89 × 1020 cm−3 and 1.82 × 1020 cm−3, respectively. And the values of Yb3+ ions (N) in Yb3+ doped and Yb3+, Ho3+ co-doped LaF3 crystals were 1.70 × 1020 cm−3 and 1.76 × 1020 cm−3, respectively. By the following equation: σa = α/N , where α is the absorption coefficient of Yb3+, the absorption cross-section σa can be get to be 0.28 × 10–20 cm2. In addition, the Yb3+:2F5/2 energy level is approximately 1500 cm−1 higher than the Ho3+: 5I6 energy level [11], which makes it possible for the non-radiative energy transfer from the formal to the latter, thus enhancing the upper energy level of the MIR emission around 2.8 µm. Therefore, Yb3+ ion, which can be pumped by commercialized InGaAs LD, is expected to be an effective sensitized ion for mid-infrared emission of the Ho3+:LaF3 crystal. The mid-infrared transmittance spectrum of Yb3+,Ho3+:LaF3 crystal was measured at room temperature shown in Fig. 2. There is a wide transmittance band from 2.5 μm to 9 μm . The transmittance at
2.87 μm is as high as 91%. The 9% loss contains the Fresnel reflections, scattering, and absorption of the crystal. It is well-known that fluoride absorbs water easily and the content of OH−1 groups, which resulting in the absorption band region from 2.5 to 4 µm, can be calculated by α OH − =(lnTmax /T3μm)/L , where Tmax and T3 μm are the baseline and 3 µm transmittance, L is thickness of the sample [19]. The calculated value at 3 µm is 0.02 cm−1, indicating that few free OH− groups were brought into the crystal during the crystal growth. Fig. 3. shows the MIR emission spectra of Yb3+, Ho3+:LaF3 crystal under 974 nm continuous-wavelength (CW) LD pumping recorded by an Edinburgh Instruments FLS920 spectrophotometer. The power density of the excitation source remains stable during wavelength scanning. There is an evident mid-IR fluorescence centered at 2.87 µm with a full width at half maximum (FWHM) of 112 nm. In order to evaluate emission properties of Yb3+, Ho3+:LaF3 crystal systematically, Judd-Ofelt theory was used to calculate the spectral parameters [12–14]. The intensity parameters Ωt(t = 2,4,6) were obtained by the least-square fitting, and the results are Ω2(1.02 ×10–20 cm2), Ω4(1.13 ×10–20 cm2), Ω6(1.52 ×10–20 cm2), respectively. On these parameters, radiative lifetimes (τr) of 5I6 and 5I7 were calculated to be 13.21 ms and 25.76 ms, respectively. The emission cross-section was calculated to be 1.21 × 10–20 cm2. The fluorescence branching ratio at 2.87 µm is 27.1%, which is larger than reported materials such as PbF2
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5 F4 +5S2 → 5I7 in Yb3+, Ho3+:LaF3 crystal, respectively. This means that co-doping Yb3+ ions plays an important part on the access to Ho3+:5F4 +5S2 and 5F5 electronic states, especially 5F4 +5S2, as the fluorescence intensity at 541 nm and 750 nm were enhanced several times via Yb3+ co-doping. Therefore, up-conversion makes the energy transfer from Yb3+ ions to Ho3+ ions, including two cross-relaxations: 2 F5/2 +5I7 → 5F5 and 2F5/2 +5I6 → 5S2 +5F4 [15,16], which is the key point of sensitization mechanism. For further investigation of Yb3+ ion in Ho3+:LaF3 crystal, fluorescence decay curves of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals were measured under excitation of 974 nm and 640 nm, respectively. All the fluorescence lifetimes (τf) were fitted by single exponential function shown in Fig. 6. τf is 10.90 ms at 2.87 µm. The quantum efficiency, which is evaluated by η = τf /τr , where τf and τr are the measured and calculated (by Judd-Ofelt theory) radiative lifetime of 5I6 level for Ho3+ ions in Yb3+, Ho3+:LaF3 crystal. η2.87 = 82.5%. High η means high energy utilization efficiency for fluorescence. By comparing lifetimes of Ho3+:5I6, 5I7 energy levels between Yb3+ doped and un-doped crystals, we can see that Yb3+ ions prolong 2.87 µm fluorescence lifetime slightly from 9.03 ms to 10.90 ms. In addition, lifetime of Ho3+: 5I7 level decreases evidently from 25.12 ms to 19.61 ms. The population inversion ratio, defined as ηi = τup/τdown , increased by 19.7% from 35.9% to 55.6%, where τup and τdown are lifetimes of the upper and lower energy levels, respectively. To explain the decrease in self- termination effect (significant shorten of τf between 5I6 and 5I7), schematic energy-transfer of Yb3+ and Ho3+ ions in LaF3 crystal is shown in Fig. 7a. Actually, researchers have explained similar model in other fluoride and oxide crystals, such as PbF2, Gd3Ga5O12 and GdYTaO4 [15–17]. The up-conversion (UC) is the result of the energy transfer from the Yb3+ (donor) to the Ho3+ (acceptor) promoting electrons from the ground state to different excited states. Yb3+ ions contribute to MIR laser emission in two ways [15]. On one hand, when pumped by 974 nm LD, population on 2F7/2 level can be excited to 2F5/2 level in Yb3+, and then populate easily on Ho3+: 5I6 by energy transfer (ET) with the help of lattice vibration. This is main reason for the lifetime increase of 5I6. On the other hand, crossrelaxation between excited states of Yb3+: 2F5/2 and Ho3+: 5I7 depopulate the 5I7 level by exciting them to 5F5 level, which leads to the lifetime decrease of 5I7. Besides, population on 5I7 can also be excited to 5 F5 via excited state absorption (ESA) under excitation of 974 nm. All processes devote to the stimulated emission from 5I6 to 5I7. Since codoping Yb3+ ions is advantageous to 2.87 µm fluorescence, further experiments are needed to invert the population. There is an optimal concentration of Yb3+ ions as researchers have found in fluoride crystal, such as PbF2 [15]. Therefore, studies focus on influence of the concentration of Yb3+ ions will be made to get optimal 2.87 µm emission. The transition from 2F5/2 to 2F7/2 was analyzed systematically for comprehension of the Yb3+-Ho3+ energy transfer model. Fig. 7b. shows the comparison of Yb3+:2F5/2 fluorescence decay curves at 1040 nm under excitation of 974 nm LD between Yb3+, Ho3+:LaF3 and Yb3+:LaF3 crystals. The measured fluorescence lifetime of Yb3+: 2F5/2 level decreases from 3.12 ms to 1.71 ms when Ho3+ ions involved, indicating energy transfer from Yb3+ ion to Ho3+ ion. To evaluate the efficiency, energy transfer efficiency (ηET) was calculated to be 45.2% by equation: ηET = 1−τYbHo/τYb , which far away from the reported results in the GGG crystal whose ηET is amazing 96.2% [16]. Actually, the lattice-site concentrations of the Yb3+ ions in two samples are not the same. However, lattice-site concentrations of the Yb3+ ions don’t have much influence when low concentration doping, which proved in fluoride crystals [18]. The conjecture for this may be that energy transfer between Yb3+: 2F5/2 and Ho3+:5I6 are off-resonant and depend on phonon-assist, so maximum phonon energy plays a significant role. This value in LaF3 crystal is 350 cm−1 [11]. Compared with oxide laser crystals, weak lattice vibrations (one to two phonons) can’t facilitate so much energy transfer. However, low phonon energy reduces the
Fig. 3. MIR emission spectra of Yb3+, Ho3+:LaF3 crystal.
Fig. 4. Comparison of the emission spectra ranging from 1100 to 1300 nm of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals.
[15], GGG [16], indicating higher lasing possibility at 2.87 µm. The emission spectra ranging from 1100 to 1300 nm of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals were measured under excitation of the same 974 nm CW LD shown in Fig. 4. No peak occurs in the spectrum of Ho3+:LaF3 crystal. Contrastively, there is an obvious peak centered at 1194 nm derived from the transition from Ho3+:5I6 level to Ho3+:5I8 level. This demonstrates that energy transfer exists from Yb3+ ions to Ho3+ ions: the introduction of Yb3+ ions makes it possible to get the electronic state of Ho3+:5I6 level (the upper level of 2.87 µm emission) under 974 nm LD excitation. Fig. 5 shows comparison of visible-to-near-IR up-conversion fluorescence spectra of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals, which were excited by a 974 nm CW LD. By comparison, there are significantly enhanced peaks centered at 541 nm, 646 nm and 750 nm derived from the transitions from Ho3+: 5F4 +5S2 → 5I8, 5F5 → 5I8 and
Fig. 5. Comparison of visible-to-near-IR up-conversion fluorescence spectra of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals. 732
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Fig. 6. Fluorescence decay curves of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals.
Fig. 7. a. Schematic energy-transfer of Yb3+ and Ho3+ ions in LaF3 crystal (CR: cross-relaxation; ESA: excited state absorption; ET: energy transfer); b. Comparison of Yb3+:2F5/2 fluorescence decay curves between Yb3+, Ho3+:LaF3 and Yb3+:LaF3 crystal. 733
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probability of non-radiative transition, which is obviously helpful for laser oscillation.
[3] K.L. Vodopyanov, F. Ganikhanov, J.P. Maffetone, et al., ZnGeP2 optical parametric oscillator with 3.8–12.4 μm tunability, Opt. Lett. 25 (11) (2000) 1841–1843. [4] S.R. Bowman, W.S. Rabinovich, A.P. Bowman, et al., 3 μs laser performance of Ho: YAlO3 and Nd,Ho:YAlO3, IEEE J. Quantum Electron. 26 (3) (1990) 403–406. [5] M. Eichhorn, Quasi-three-level solid-state lasers in the near and mid infrared based on trivalent rare earth ions, Appl. Phys. B 93 (2–3) (2008) 269. [6] S.R. Bowman, W.S. Rabinovich, B.J. Feldman, et al., Tuning the 3 μm Ho: YAlO3 Laser [C]//Advanced Solid State Lasers, Opt. Soc. Am. (1990) (MML4). [7] A.V. Kir’yanov, V. Aboites, A.M. Belovolov, et al., Powerful visible (530–770 nm) luminescence in Yb,Ho: GG G with IR diode pumping, Opt. Express 10 (16) (2002) 832–839. [8] W. Liu, J. Sun, X. Li, et al., Laser induced thermal effect on upconversion luminescence and temperature-dependent upconversion mechanism in Ho3+/Yb3+-codoped Gd2(WO4)3 phosphor, Opt. Mater. 35 (7) (2013) 1487–1492. [9] P.E.A. Möbert, A. Diening, E. Heumann, et al., Room-temperature continuous-wave upconversion-pumped laser emission in Ho,Yb:KYF4 at 756, 1070, and 1390 nm, Laser Phys. 8 (1) (1998) 210–213. [10] S. Li, L. Zhang, P. Zhang, et al., Spectroscopic characterizations of Dy: LaF3 crystal, Infrared Phys. Technol. 87 (2017) 65–71. [11] J. Hong, L. Zhang, P. Zhang, et al., Ho: LaF3 single crystal as potential material for 2 μm and 2.9 μm lasers, Infrared Phys. Technol. 76 (2016) 636–640. [12] W.T. Carnall, P.R. Fields, K. Rajnak, Electronic energy levels in the trivalent lanthanide aquo ions. I. Pr3+, Nd3+, Pm3+, Sm3+, Dy3+, Ho3+, Er3+, and Tm3+, J. Chem. Phys. 49 (10) (1968) 4424–4442. [13] W.T. Carnall, P.R. Fields, K. Rajnak, Spectral intensities of the trivalent lanthanides and actinides in solution. II. Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Ho3+, J. Chem. Phys. 49 (10) (1968) 4412–4423. [14] M. Malinowski, Z. Frukacz, M. Szuflińska, et al., Optical transitions of Ho3+ in YAG, J. Alloy. Compd. 300 (2000) 389–394. [15] P. Zhang, B. Zhang, J. Hong, et al., Enhanced emission of 2.86 μm from diodepumped Ho 3+/Yb 3+-codoped PbF2 crystal, Opt. Express 23 (4) (2015) 3920–3927. [16] Y. Wang, J. Li, Z. Zhu, et al., Activation effect of Ho3+ at 2.84 μm MIR luminescence by Yb 3+ ions in GGG crystal, Opt. Lett. 38 (20) (2013) 3988–3990. [17] R. Dou, Q. Zhang, D. Sun, et al., Growth, thermal, and spectroscopic properties of a 2.911 μm Yb,Ho:GdYTaO4 laser crystal, Cryst. Eng. Comm. 16 (48) (2014) 11007–11012. [18] A. Bensalah, M. Ito, Y. Guyot, et al., Spectroscopic properties and quenching processes of Yb3+ in fluoride single crystals for laser applications, J. Lumin. 122 (2007) 444–446. [19] G. Wang, S. Dai, J. Zhang, et al., Effect of F− ions on emission cross-section and fluorescence lifetime of Yb3+-doped tellurite glasses, J. Non-Cryst. Solids 351 (24–26) (2005) 2147–2151.
4. Conclusion In conclusion, Yb3+, Ho3+:LaF3 crystal was grown by Bridgman method and the enhanced 2.87 µm fluorescence has been observed under excitation of 974 LD for the first time. On the absorption spectra and J-O theory, the fluorescence branching ratio, emission cross-section and quantum efficiency were calculated to be 27.1%, 1.21 × 10–20 cm2 and 82.5%, respectively. Decay curves of the 5I6 and 5I7 levels of the Yb3+, Ho3+:LaF3 crystal and Ho3+:LaF3 crystal were measured and fitted by single exponential function. The population inversion ratio increased by 19.7%. Energy-transfer mechanism and transmittance were analyzed systematically. It can be concluded that Yb3+ ions play a key role on the Yb3+, Ho3+:LaF3 crystal for 2.87 µm laser materials pumped by InGaAs LD. Acknowledgments This work was supported by National Natural Science Foundation of China (NSFC) [51472257, 51502321]; 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]. References [1] J.S. Sanghera, L.B. Shaw, I.D. Aggarwal, Chalcogenide glass-fiber-based mid-IR sources and applications, IEEE J. Sel. Top. Quantum Electron. 15 (1) (2009) 114–119. [2] M. Tacke, New developments and applications of tunable IR lead salt lasers, Infrared Phys. Technol. 36 (1) (1995) 447–463.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Effective enhancement of 2.87 µm fluorescence via Yb3+ in Ho3+:LaF3 laser crystal
T
Shanming Lia,b, Lianhan Zhanga, Mingzhu Hea, Guangzhu Chena, Peixiong Zhangc, Yilun Yanga,b, ⁎ Min Xua, Tao Yand, Ning Yed, 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 University of Chinese Academy of Sciences, Beijing 100039, China c Department of Optoelectronic Engineering, Jinan University, Guangzhou, Guangdong 510632, China d Fujian Institute of Research on the Structure of the Matter, Chinese Academy of Sciences, Fuzhou 350002, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Yb,Ho:LaF3 Laser crystal Spectroscopy
A novel Yb3+, Ho3+:LaF3 crystal was successfully grown and analyzed. Intense emission at 2.87 µm was observed in Yb3+, Ho3+:LaF3 crystal for the first time. Yb3+ ions not only provide sensitization mechanism for conventional LD pump, but also reduce self-termination by comparing with Ho3+:LaF3 crystal. The emission cross-section, quantum efficiency and fluorescence branching ratio at 2.87 µm are calculated to be 1.21 × 10–20 cm2, 82.5%, and 27.1%, respectively. The results indicate Yb3+, Ho3+:LaF3 crystal an attractive host for solid-state lasers around 2.87 µm under a 974 nm LD pump.
1. Introduction In recent years, mid-infrared lasers at around 3 µm have attracted much attention due to wide applications in biomedical treatments, molecular fingerprint identification and optical parametric oscillation pump source [1–3]. Ho3+ ions are ideal active ions operating at around 3 µm attributed to the transition from 5I6 to 5I7 [4–6]. However, none laser diode can meet the absorption of 5I6 energy level; Self- termination is obvious as a result of the difficulty to realize population inversion. As far as we know, Yb3+ ions can be easily pumped under commercial laser diode, as a result of the advanced development of InGaAs diode. In addition, there is energy transfer between Yb3+ and Ho3+ ions, which is popular in relative up-conversion luminescence materials [7–9]. Therefore, codoping Yb3+ ions can open up the pump possibility of Ho3+ ions at 3 µm. Additionally, novel laser material with low phonon energy needs to be discovered. Our research group has been researching on infrared laser materials for a few years. We found that LaF3 crystal is such a candidate with low phonon energy (~350 cm−1), similar ion-substitution-radius and perfect infrared transmittance [10]. Relevant studies of infrared emission in rare earth doped LaF3 crystal have been reported in ref.10 and ref.11. To our knowledge, there have not been reports on the growth or the
~3 µm emission of Yb3+, Ho3+:LaF3 crystal. In this paper, Yb3+, Ho3+:LaF3 crystal was successfully prepared as well as Ho3+:LaF3 and Yb3+:LaF3 crystal as comparison. Theoretical calculation and spectral investigation suggest that Yb3+ ions are effective to sensitize Ho3+:5I6 and beneficial to decrease the self-termination by shortening lifetime of the lower energy level. Intense emission at 2.87 µm has been observed for potential mid- infrared lasers pumped under 974 nm InGaAs laser diode. 2. Experiments 2 at% Yb3+/ 2 at% Ho3+ co-doped LaF3 crystal was grown by the Bridgman method. The raw materials were fluoride powder of the LaF3, HoF3 and YbF3 provided commercially, all with high purity of 99.99%. In comparison, 2 at% Yb3+:LaF3 and 2 at% Ho3+:LaF3 crystals were also grown. All crystals crystallized spontaneously in a graphite crucible under the atmosphere of high-purity Argon (70%) and Carbon tetrafluoride (30%) during the whole procedure, including heating, cooling growth and annealing. The detailed process can be found in ref.10. The insets of Fig. 1 show two types of the grown crystals: Ho3+ and Yb3+, Ho3+ co-doped LaF3 crystals, the size of which are around ∅20 × 25 mm3. The grown crystals were cut into several samples polished mechanically to spectral quality for systematical analysis, the
⁎ Corresponding author at: Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China. E-mail addresses: [email protected] (S. Li), [email protected] (Y. Hang).
https://doi.org/10.1016/j.jlumin.2018.07.027 Received 22 May 2018; Received in revised form 10 July 2018; Accepted 19 July 2018 Available online 21 July 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.
Journal of Luminescence 203 (2018) 730–734
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Fig. 1. Comparison of absorption spectra between Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals in the range of 300–2200 nm; the insets are two types of LaF3 crystals.
sizes of which are 10 × 10 × 1 mm3. The concentrations of Yb3+ and Ho3+ ions were measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The absorption spectra of the grown crystals were recorded by a Lambda 1050 UV/VIS/NIR spectrophotometer (Perkin-Elmer). The fluorescence spectrum in the wavelength of 2700–3100 nm for the Yb3+,Ho3+:LaF3 crystal was measured with Edinburgh Instruments FLS920 under excitation of 974 nm. The fluorescence decay curves of Yb3+,Ho3+:LaF3 crystal were measured at 2870 nm under pulse excitation of 974 nm. The decay curves of the Yb3+:2F5/2 energy level of Yb3+-doped and Yb3+, Ho3+ co-doped LaF3 crystals were measured at 1040 nm under excitation of 974 nm. All the measurements were taken at room temperature. 3. Results and discussion Fig. 2. Mid-infrared transmittance spectrum of Yb3+, Ho3+:LaF3 crystal.
The absorption spectra of Ho3+ doped LaF3 crystal and Yb3+, Ho3+ co-doped LaF3 crystal are shown in Fig. 1. There are 7 main absorption bands with the peaks centered at 360 nm, 414 nm, 448 nm, 536 nm, 636 nm, 882 nm, 1142 nm and 1926 nm in the Ho3+:LaF3 crystal, which correspond to the transitions from Ho3+: 5I8 to 3K7 +5G4, 3G5, 5 G6 +5F1, 5F4 +5S2, 5F5, 5I5, 5I6 and 5I7, respectively [15]. It is a pity that no peaks satisfy the commercial laser diode. As for Yb3+, Ho3+: LaF3 crystal, there is an obvious absorption peak in the range of 851–1002 nm centered at 974 nm, which corresponds to the transition from 2F7/2 to 2F5/2. Lattice-site concentrations of the Ho3+ ions in Ho3+ doped and Yb3+, Ho3+ co-doped LaF3 crystals were measured by ICPAES to be 1.89 × 1020 cm−3 and 1.82 × 1020 cm−3, respectively. And the values of Yb3+ ions (N) in Yb3+ doped and Yb3+, Ho3+ co-doped LaF3 crystals were 1.70 × 1020 cm−3 and 1.76 × 1020 cm−3, respectively. By the following equation: σa = α/N , where α is the absorption coefficient of Yb3+, the absorption cross-section σa can be get to be 0.28 × 10–20 cm2. In addition, the Yb3+:2F5/2 energy level is approximately 1500 cm−1 higher than the Ho3+: 5I6 energy level [11], which makes it possible for the non-radiative energy transfer from the formal to the latter, thus enhancing the upper energy level of the MIR emission around 2.8 µm. Therefore, Yb3+ ion, which can be pumped by commercialized InGaAs LD, is expected to be an effective sensitized ion for mid-infrared emission of the Ho3+:LaF3 crystal. The mid-infrared transmittance spectrum of Yb3+,Ho3+:LaF3 crystal was measured at room temperature shown in Fig. 2. There is a wide transmittance band from 2.5 μm to 9 μm . The transmittance at
2.87 μm is as high as 91%. The 9% loss contains the Fresnel reflections, scattering, and absorption of the crystal. It is well-known that fluoride absorbs water easily and the content of OH−1 groups, which resulting in the absorption band region from 2.5 to 4 µm, can be calculated by α OH − =(lnTmax /T3μm)/L , where Tmax and T3 μm are the baseline and 3 µm transmittance, L is thickness of the sample [19]. The calculated value at 3 µm is 0.02 cm−1, indicating that few free OH− groups were brought into the crystal during the crystal growth. Fig. 3. shows the MIR emission spectra of Yb3+, Ho3+:LaF3 crystal under 974 nm continuous-wavelength (CW) LD pumping recorded by an Edinburgh Instruments FLS920 spectrophotometer. The power density of the excitation source remains stable during wavelength scanning. There is an evident mid-IR fluorescence centered at 2.87 µm with a full width at half maximum (FWHM) of 112 nm. In order to evaluate emission properties of Yb3+, Ho3+:LaF3 crystal systematically, Judd-Ofelt theory was used to calculate the spectral parameters [12–14]. The intensity parameters Ωt(t = 2,4,6) were obtained by the least-square fitting, and the results are Ω2(1.02 ×10–20 cm2), Ω4(1.13 ×10–20 cm2), Ω6(1.52 ×10–20 cm2), respectively. On these parameters, radiative lifetimes (τr) of 5I6 and 5I7 were calculated to be 13.21 ms and 25.76 ms, respectively. The emission cross-section was calculated to be 1.21 × 10–20 cm2. The fluorescence branching ratio at 2.87 µm is 27.1%, which is larger than reported materials such as PbF2
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5 F4 +5S2 → 5I7 in Yb3+, Ho3+:LaF3 crystal, respectively. This means that co-doping Yb3+ ions plays an important part on the access to Ho3+:5F4 +5S2 and 5F5 electronic states, especially 5F4 +5S2, as the fluorescence intensity at 541 nm and 750 nm were enhanced several times via Yb3+ co-doping. Therefore, up-conversion makes the energy transfer from Yb3+ ions to Ho3+ ions, including two cross-relaxations: 2 F5/2 +5I7 → 5F5 and 2F5/2 +5I6 → 5S2 +5F4 [15,16], which is the key point of sensitization mechanism. For further investigation of Yb3+ ion in Ho3+:LaF3 crystal, fluorescence decay curves of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals were measured under excitation of 974 nm and 640 nm, respectively. All the fluorescence lifetimes (τf) were fitted by single exponential function shown in Fig. 6. τf is 10.90 ms at 2.87 µm. The quantum efficiency, which is evaluated by η = τf /τr , where τf and τr are the measured and calculated (by Judd-Ofelt theory) radiative lifetime of 5I6 level for Ho3+ ions in Yb3+, Ho3+:LaF3 crystal. η2.87 = 82.5%. High η means high energy utilization efficiency for fluorescence. By comparing lifetimes of Ho3+:5I6, 5I7 energy levels between Yb3+ doped and un-doped crystals, we can see that Yb3+ ions prolong 2.87 µm fluorescence lifetime slightly from 9.03 ms to 10.90 ms. In addition, lifetime of Ho3+: 5I7 level decreases evidently from 25.12 ms to 19.61 ms. The population inversion ratio, defined as ηi = τup/τdown , increased by 19.7% from 35.9% to 55.6%, where τup and τdown are lifetimes of the upper and lower energy levels, respectively. To explain the decrease in self- termination effect (significant shorten of τf between 5I6 and 5I7), schematic energy-transfer of Yb3+ and Ho3+ ions in LaF3 crystal is shown in Fig. 7a. Actually, researchers have explained similar model in other fluoride and oxide crystals, such as PbF2, Gd3Ga5O12 and GdYTaO4 [15–17]. The up-conversion (UC) is the result of the energy transfer from the Yb3+ (donor) to the Ho3+ (acceptor) promoting electrons from the ground state to different excited states. Yb3+ ions contribute to MIR laser emission in two ways [15]. On one hand, when pumped by 974 nm LD, population on 2F7/2 level can be excited to 2F5/2 level in Yb3+, and then populate easily on Ho3+: 5I6 by energy transfer (ET) with the help of lattice vibration. This is main reason for the lifetime increase of 5I6. On the other hand, crossrelaxation between excited states of Yb3+: 2F5/2 and Ho3+: 5I7 depopulate the 5I7 level by exciting them to 5F5 level, which leads to the lifetime decrease of 5I7. Besides, population on 5I7 can also be excited to 5 F5 via excited state absorption (ESA) under excitation of 974 nm. All processes devote to the stimulated emission from 5I6 to 5I7. Since codoping Yb3+ ions is advantageous to 2.87 µm fluorescence, further experiments are needed to invert the population. There is an optimal concentration of Yb3+ ions as researchers have found in fluoride crystal, such as PbF2 [15]. Therefore, studies focus on influence of the concentration of Yb3+ ions will be made to get optimal 2.87 µm emission. The transition from 2F5/2 to 2F7/2 was analyzed systematically for comprehension of the Yb3+-Ho3+ energy transfer model. Fig. 7b. shows the comparison of Yb3+:2F5/2 fluorescence decay curves at 1040 nm under excitation of 974 nm LD between Yb3+, Ho3+:LaF3 and Yb3+:LaF3 crystals. The measured fluorescence lifetime of Yb3+: 2F5/2 level decreases from 3.12 ms to 1.71 ms when Ho3+ ions involved, indicating energy transfer from Yb3+ ion to Ho3+ ion. To evaluate the efficiency, energy transfer efficiency (ηET) was calculated to be 45.2% by equation: ηET = 1−τYbHo/τYb , which far away from the reported results in the GGG crystal whose ηET is amazing 96.2% [16]. Actually, the lattice-site concentrations of the Yb3+ ions in two samples are not the same. However, lattice-site concentrations of the Yb3+ ions don’t have much influence when low concentration doping, which proved in fluoride crystals [18]. The conjecture for this may be that energy transfer between Yb3+: 2F5/2 and Ho3+:5I6 are off-resonant and depend on phonon-assist, so maximum phonon energy plays a significant role. This value in LaF3 crystal is 350 cm−1 [11]. Compared with oxide laser crystals, weak lattice vibrations (one to two phonons) can’t facilitate so much energy transfer. However, low phonon energy reduces the
Fig. 3. MIR emission spectra of Yb3+, Ho3+:LaF3 crystal.
Fig. 4. Comparison of the emission spectra ranging from 1100 to 1300 nm of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals.
[15], GGG [16], indicating higher lasing possibility at 2.87 µm. The emission spectra ranging from 1100 to 1300 nm of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals were measured under excitation of the same 974 nm CW LD shown in Fig. 4. No peak occurs in the spectrum of Ho3+:LaF3 crystal. Contrastively, there is an obvious peak centered at 1194 nm derived from the transition from Ho3+:5I6 level to Ho3+:5I8 level. This demonstrates that energy transfer exists from Yb3+ ions to Ho3+ ions: the introduction of Yb3+ ions makes it possible to get the electronic state of Ho3+:5I6 level (the upper level of 2.87 µm emission) under 974 nm LD excitation. Fig. 5 shows comparison of visible-to-near-IR up-conversion fluorescence spectra of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals, which were excited by a 974 nm CW LD. By comparison, there are significantly enhanced peaks centered at 541 nm, 646 nm and 750 nm derived from the transitions from Ho3+: 5F4 +5S2 → 5I8, 5F5 → 5I8 and
Fig. 5. Comparison of visible-to-near-IR up-conversion fluorescence spectra of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals. 732
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Fig. 6. Fluorescence decay curves of Yb3+, Ho3+:LaF3 and Ho3+:LaF3 crystals.
Fig. 7. a. Schematic energy-transfer of Yb3+ and Ho3+ ions in LaF3 crystal (CR: cross-relaxation; ESA: excited state absorption; ET: energy transfer); b. Comparison of Yb3+:2F5/2 fluorescence decay curves between Yb3+, Ho3+:LaF3 and Yb3+:LaF3 crystal. 733
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probability of non-radiative transition, which is obviously helpful for laser oscillation.
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4. Conclusion In conclusion, Yb3+, Ho3+:LaF3 crystal was grown by Bridgman method and the enhanced 2.87 µm fluorescence has been observed under excitation of 974 LD for the first time. On the absorption spectra and J-O theory, the fluorescence branching ratio, emission cross-section and quantum efficiency were calculated to be 27.1%, 1.21 × 10–20 cm2 and 82.5%, respectively. Decay curves of the 5I6 and 5I7 levels of the Yb3+, Ho3+:LaF3 crystal and Ho3+:LaF3 crystal were measured and fitted by single exponential function. The population inversion ratio increased by 19.7%. Energy-transfer mechanism and transmittance were analyzed systematically. It can be concluded that Yb3+ ions play a key role on the Yb3+, Ho3+:LaF3 crystal for 2.87 µm laser materials pumped by InGaAs LD. Acknowledgments This work was supported by National Natural Science Foundation of China (NSFC) [51472257, 51502321]; 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]. References [1] J.S. Sanghera, L.B. Shaw, I.D. Aggarwal, Chalcogenide glass-fiber-based mid-IR sources and applications, IEEE J. Sel. Top. Quantum Electron. 15 (1) (2009) 114–119. [2] M. Tacke, New developments and applications of tunable IR lead salt lasers, Infrared Phys. Technol. 36 (1) (1995) 447–463.
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