A novel Ho3+/Yb3+-codoped PbF2 mid-IR laser crystal was successfully grown and analyzed. Enhanced emission at 2.86 μm was observed from the crystal under excitation of a common 970 nm laser diode for the first time. The effect of Yb3+ codoping on the 2.86 μm photoluminescence of Ho3+ was investigated. In comparison to Ho3+-singly doped PbF2 crystal, the Ho3+/Yb3+-codoped PbF2 crystal possessed comparable quantum efficiency (88.8%), and fluorescence branching ratio (20.52%) along with a larger calculated emission cross section (1.90×10−20 cm2) corresponding to the laser transition 5I6→5I7 of Ho3+. It was found that the introduced Yb3+ enhanced the 2.86 μm emission by depopulating the Ho3+:5I7 level. The energy transfer (ET) efficiency from Yb3+:2F5/2 to Ho3+:5I6 is as high as 96.7%, indicating that Yb3+ ion is an effective sensitizer for Ho3+ ion in PbF2 crystal. These results suggest that Ho3+/Yb3+-codoped PbF2 crystal may become an attractive host for developing solid state lasers at around 2.86 μm under a conventional 970 nm LD pump.
© 2015 Optical Society of America
Mid-IR (MIR) lasers operating around 3 μm region have attracted much attention in recent years due to their wide applications in atmospheric monitoring, medical surgery, remote sensing, and scientific research [1–5]. It is well know that Ho3+ is a natural candidate for ∼3 μm lasers owing to the 5I6→5I7 transition [6, 7]. However, the ∼3 μm laser operation cannot be obtained efficiently due to (i) the lack of commercialized laser diodes (LD) corresponding to the intrinsic absorption of Ho3+ ions, and (ii) the population bottleneck effect that occurs with the 5I6→5I7 transition which is a self-terminated transition. In order to conquer these problems to turn on the probability to acquire ∼3 μm lasing from Ho3+, we need (i) a proper sensitizer ion with large absorption cross section for Ho3+ ion, and (ii) an appropriate deactivated ion with efficient depopulation of Ho3+:5I7 for population inversion.
Fortunately, Yb3+ ions, which have a sufficient absorption cross section in the NIR region, are often codoped as sensitizer ions to transfer the accumulated excitation energy to the activator ions. As the energy level scheme illustrated in Fig. 1 shows, under the excitation of a common 970 nm LD pump, the Yb3+ ion is excited to the excited state (2F5/2), and then transfer the excitation energy to Ho3+:5I6. Up to now, effective energy transformation from Yb3+:2F5/2 to Ho3+:5I6 have been achieved in luminescent materials [8, 9].
Figure 1 also shows the energy level scheme of Yb3+ and Ho3+ in comparison to Er3+. In the Er3+ system, the ∼3 μm laser operation on the self-terminating transition (4I11/2→4I13/2) is possible due to an up-conversion process from the lower laser level (4I13/2) under laser diode excitation, which leads to a strong population of the excited state of erbium ion. However, up-conversion is less efficient in a single Ho3+ doped crystal and laser oscillation under laser diode excitation of the 5I6 upper laser level is less common. Fortunately, these problems can be partly solved by Yb3+ codoping. After the energy transfers from Yb3+:2F5/2 to Ho3+:5I6 under a 970 nm LD excitation, an up-conversion (UC) process from 5I7 multiplet to 5F5 multiplet can be achieved by the energy exchange from a second excited Yb3+ ion. At the same time, population on the 5I7 level is also excited to 5F5 level by strong excited-state absorption (ESA) at the pumping of 970 nm LD. These processes not only depopulate the lower laser level (5I7), but also recycle energy to the upper laser level (5I6) by nonradiative transition, which makes the possibility of population inversion for Ho3+:5I6→5I7.
To date, lots of previous works have mainly paid attention to the ∼2 μm emissions owing to the 5I7 → 5I8 transition in Ho3+/Yb3+ co-doped crystals [10, 11]. However, to our knowledge, there are few results focusing on the ∼3 μm emissions, and limited to oxide crystals [12, 13]. It is well known that one important factor (a proper host) should be considered in achieving intense Ho3+:5I6 → 5I7 MIR emission for practical laser operation. The ideal host material is expected to possess low phonon energy, which is beneficial for suppressing multiphonon de-excitation processes. Among many alternatives, fluoride crystals are more favorable in several respects: (i) lower phonon energy suppressing nonradiative relaxation between adjacent energy levels, (ii) longer fluorescence lifetime improving energy storage, and (iii) lower refractive index limiting nonlinear effects under intense laser pump [14–16]. In our previous work , we have reported a new MIR laser crystal Ho3+:PbF2, which has low phonon energy of 257 cm−1, good 2.8 μm MIR emission performances, and low absorption coefficient in typical H2O absorption band at 3 μm. PbF2 crystallizes in the cubic space group (Fm3̄m) with fluorite structure, which makes it possible to grow into the form of large-size transparent single crystals . Furthermore, the PbF2 crystal has high thermal conductivity (28 W/m/K), moderate mechanical properties for handling, and is not moisture sensitive . These characteristics make the PbF2 crystal can be used as an extremely suitable host to MIR solid-state lasers.
In this Letter, we reported the efficient 2.86 μm emission in a Ho3+/Yb3+ co-doped PbF2 crystal under a 970 nm LD pump for the first time. Yb3+ ion was demonstrated to be an effective sensitizer for Ho3+ ion in PbF2 crystal, and to greatly facilitate the Ho3+:5I6→5I7 emission by UC and ESA from Ho3+:5I7 to Ho3+:5F5. Besides, the energy transfer efficiency from Yb3+:2F5/2 to Ho3+:5I6, and the spectroscopic investigation of Ho3+/Yb3+:PbF2 crystal were also investigated to demonstrate their feasibility for MIR solid state lasers under a common 970 nm LD pump.
2. Experimental section
In the Yb3+, Ho3+ codoped system, if the concentration of Yb3+ is too small, the absorption coefficient at around 970 nm would be greatly reduced, resulting in a lower pumping absorption efficiency. On the contrary, if the concentration of Yb3+ is too large, the Yb3+ ions would form cluster structures involving at least Yb3+-Yb3+ ions pairs in PbF2 crystal when the Yb3+ doped concentration was larger than 2 mol.%, which would result in the fluorescence quenching . A middle-ground approach was taken in our experiment, the concentration ratio between Ho and Yb was chose as about 1, they were all 2 mol.%, respectively.
The Ho3+ single doped, Yb3+ single doped, and Ho3+/Yb3+ co-doped PbF2 crystals were grown by the Bridgman method. The crystals growth process was similar to our previous work . The concentration of Ho3+, and Yb3+ ions in the crystals were measured by the inductively coupled plasma-atomic emission spectrometry (ICP-AES) method. The single-doped crystals were measured to be 2.20×1020 ions/cm3 of Ho3+, and 2.56×1020 ions/cm3 of Yb3+, respectively. The concentrations of Ho3+ and Yb3+ in double-doped crystal were 2.29×1020 ions/cm3 and 2.59×1020 ions/cm3, respectively. The corresponding segregator coefficients of Ho3+ and Yb3+ were 1.15 and 1.35, respectively. The effective segregation has a close relationship with the ionic radius of doping and host ions. In PbF2 crystal, trivalent rare earth ions usually occupy the Pb2+ sites, and the charge compensation is attained by the presence of interstitial fluorine ion ( ) . The ionic radius of Yb3+ (0.98 Å), and Ho3+ (1.02 Å) are far smaller than that of Pb2+ (1.29 Å), resulting in the segregator coefficients of Yb3+ and Ho3+ are large than 1, indicating that Pb2+ can be easily replaced by doping ions Yb3+, and Ho3+ ions and the doping concentration can be high.
The absorption spectrum of Ho3+/Yb3+:PbF2 crystal was record with a JASCO V-570 UV/VIS spectrophotometer in the range of 300–2300 nm. The fluorescence spectrum in the wavelength of 2700–3100 nm for the Ho3+/Yb3+:PbF2 crystal was measured with Edinburgh Instruments FLS920 under excitation of 970 nm. The fluorescence decay curves of Ho3+/Yb3+ codoped PbF2 crystal was measured at 2860 nm under pulse excitation of 970 nm. The decay curves of the Yb3+:2F5/2 energy level of Yb3+-doped and Yb3+/Ho3+-codoped PbF2 crystals were measured at 1030 nm under excitation of 970 nm. All the measurements were taken at room temperature.
To reduce an artificial lifetime lengthening due to radiation trapping, (i) the thickness of crystals was very thin with a value of 0.5 mm; (ii) the excited laser beam was focused near the edge of crystal; (iii) the measurements were performed by the pin-hole method , the crystals were placed behind a pin-hold with a variable aperture from 0.5 mm to 1.5 mm at an angle of 45° with respect to the excited laser beam and the axis of the collection optics.
3. Experimental results and discussion
The absorption spectrum of Ho3+/Yb3+:PbF2 crystal is shown in Fig. 2. It is obvious to see that there are many typical absorption spectrum of Ho3+ ion, such as the absorption bands centered at around 416, 450, 536, 643, 1150, and 1914 nm, which correspond to the transitions from Ho3+: 5I8 to 5G5, 5G6+5F1, 5S2+5F4, 5F5, 5I6, and 5I7, respectively. These peaks are associated with the 4f10→4f10 transitions of trivalent holmium. However, there is no readily available commercial LD corresponding to these absorption bands. Nevertheless, the absorption band corresponds to the Yb3+ optical transition 2F7/2→2F5/2 centered at 970 nm with very large intensity can be obtained in Ho3+/Yb3+:PbF2 crystal. By reason of the small energy gap between Yb3+:2F5/2 level and Ho3+:5I6 level, Yb3+ ion can act as a sensitizer to Ho3+ ion by nonradiative energy transfer, 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.
According to the Judd-Ofelt theory [22, 23], the intensity parameters Ω2,4,6 of Ho3+ (shown in Table 1) was calculated from the room-temperature absorption spectrum. It can be seen that the Ω2,4,6 of Ho3+ in our Ho3+/Yb3+ co-doped PbF2 crystal are all higher than those of Ho3+ single doped PbF2 crystal . In particular, 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 Ho3+/Yb3+ co-doped PbF2 crystal indicates that a lower symmetry surrounding Ho3+ ions is caused by the introduction of Yb3+ ions . Further calculation about the fluorescence branching ratio β of 5I6→5I7 transition in the Ho3+/Yb3+:PbF2 crystal is as high as 20.52%, which is comparable with the result of Ho3+ single doped PbF2 crystal (20.99%), and still much higher than that of other crystals, indicating that the codoping of Yb3+ ions has little influence on the ∼3 μm fluorescence emission efficiency of Ho3+.
To further study the effect of Yb3+ codoping on the ∼3 μm photoluminescence of Ho3+, the corresponding emission cross sections are subsequently calculated by the Fuchtbauer-Ladenburg equation :Fig. 3. An intense band assigned to 5I6→5I7 transition of Ho3+ centered at 2.86 μm is observed. The maximum emission cross section is 1.90×10−20 cm2 at 2860 nm, which is larger than that of Ho3+ single doped PbF2 crystal (1.44×10−20 cm2). This enhanced fluorescence emission cross section is supposed to be closely associated with the shorter radiative lifetime (from 6.11 ms to 5.79 ms) and comparable fluorescence branching ratio (shown in Table 1).
The inset of Fig. 3 shows the fluorescence decay curves of Ho3+/Yb3+ codoped PbF2 crystals performances monitored at 2860 nm under pulse excitation of 970 nm. The lifetime value is determined by the single exponential fitting of the decay curve, and is measured to be 5.14 ms, which is only 4.8% shorter when compared with the date for the Ho3+ single doped PbF2 crystal (5.4 ms) , indicating that the codoping of Yb3+ ions has litter influence on the 5I6 level of Ho3+. The quantum efficiency, which is evaluated by η =τmeas/τR, where τmeas and τR are the measured and calculated (by Judd-Ofelt theory) radiative lifetime of 5I6 level for Ho3+ ions in Ho3+/Yb3+:PbF2 crystal. Therefore, the value of η was calculated to be 88.8%, which is comparable to that of Ho3+ single doped PbF2 crystal (88.4%) , confirming Yb3+ as an effective sensitizer.
To further understand this phenomenon, the energy transfer between Yb3+ and Ho3+ ions was studied in details. It is worth noting that the energy transfer from Yb3+ to Ho3+ ions is a phonon-assisted process because of 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. And the energy transfer efficiency ηET from Yb3+ to Ho3+ can be described as ηET =1−τYb−Ho/τYb, where τYb−Ho and τYb are lifetime of Yb3+:2F5/2 energy level of Yb3+:PbF2 crystals with and without Ho3+. The decay curves of the Yb3+:2F5/2 energy level of Yb3+-doped and Yb3+/Ho3+-codoped PbF2 crystals are shown in Fig. 4 (a) and 4(b). The inset of Fig. 4 (a) and 4(b) show the Ln of fluorescence intensity I versus time t, which can be well linear fitting, indicating that the these two decay curves of Fig. 4 (a) and 4(b) can be well fitted to single exponential functions. The measured lifetime of the Yb3+:2F5/2 manifold in the Ho3+/Yb3+:PbF2 crystal is 77 μs, which is much shorter than that of Yb3+:PbF2 crystal (2.3 ms), resulting in the calculated energy transfer efficiency ηET from Yb3+ to Ho3+ being as high as 96.7%. This confirms that Yb3+ ion is a suitable sensitizer in Ho3+/Yb3+:PbF2 crystal and can efficiently transfer energy to Ho3+ ion, thus makes this crystal propitious to be pumped by commercialized InGaAs LD.
According to the energy level diagram of Ho3+ and Yb3+ ions (shown in Fig. 1), firstly ions of Yb3+:2F7/2 state are excited to Yb3+:2F5/2 by a 970 nm LD. Then the energy easily transfers to the Ho3+ ions on the ground 5I8 level which will be excited to the 5I6 level. One part of the Ho3+ ions on the 5I6 level will decay radiatively to 5I7 with ∼2.8 μm emission. Other ions on the 5I6 level will decay radiatively to 5I8 with ∼1.2 μm emission, or decay nonradiatively to the 5I7 level by the multiphonon relaxation. Under the excitation of 970 nm LD, the ions on the 5I7 level will undergo the UC and ESA processes to 5F5 level, which depopulates the Ho3+:5I7 level. Then, the ions on 5F5 level decays mainly nonradiatively to the upper laser level 5I6, makes the possibility of population inversion for Ho3+:5I6→5I7, and enhances the 2.86 μm emission. To prove this assumption, the time-resolved decays of the 5I7 multiplet of the Ho3+ single doped and Ho3+/Yb3+ codoped PbF2 crystals were measured, and shown in Fig. 4 (c) and 4(d). The inset of Fig. 4 (c) and 4(d) show the Ln of fluorescence intensity I versus time t, which are unable to be linear fitting, indicating that the these two decay curves of Fig. 4 (c) and 4(d) can not be fitted to single exponential functions. They can be well fitted to dual exponential decaying. The measured lifetime of the 5I7 manifold in the Ho/Yb:PbF2 crystal is 10.8 ms, which is 20.1% shorter when compared with the date for the Ho:PbF2 crystal (13.8 ms). This shortening of the measured lifetime confirms that Yb3+ ions are able to depopulate the Ho3+:5I7 for 2.86 μm emission in PbF2 crystal, which may induce the population inversion and facilitate laser operation.
In conclusion, an efficient emission at 2.86 μm was observed in the Ho3+/Yb3+-codoped PbF2 crystal under the excitation of a common 970 nm LD. Compared with the Ho3+ single doped PbF2 crystal, the Ho3+/Yb3+ codoped PbF2 crystal has comparable fluorescence branching ratio (20.52%), quantum efficiency (88.8%), and higher emission cross section (1.90×10−20 cm2) corresponding to the stimulated emission of Ho3+:5I6→5I7 transition. It was demonstrated that the energy transfer efficiency from Yb3+:2F5/2 to Ho3+:5I6 is as high as 96.7%, indicates that Yb3+ ion can be used as an effective sensitizer for Ho3+ ion. Moreover, it was also shown that the introduced Yb3+ depopulates the Ho3+:5I7 level, which is benefited the possible population inversion for Ho3+:5I6→5I7. The results and analyzed energy transfer indicate codoping of Yb3+ with Ho3+ in PbF2 crystal plays an important role in 2.86 μm emission when the crystal was pumped by a conventional 970 nm LD.
We thank the National Natural Science Foundation of China (Grant No. 51302283), the National Natural Science Foundation of China (Grant No. 51472257), the Shanghai Natural Science Fundation under Projects (Grant No. 13ZR1463400), and the National Natural Science Foundation of China ( 61308042, 61275142) for their financial support.
References and links
1. H. Guo, L. Liu, Y. Wang, C. Hou, W. Li, M. Lu, K. Zou, and B. Peng, “Host dependence of spectroscopic properties of Dy3+- doped and Dy3+, Tm3+-codped Ge-Ga-S-CdI2 chalcohalide glasses,” Opt. Express. 17(17), 15350–15358 (2009). [CrossRef] [PubMed]
2. E. Kasper, M. Kittler, M. Oehme, and T. Arguirov, “Germanium tin: silicon photonics toward the mid-infrared,” Photon. Res. 1(2), 69–76 (2013). [CrossRef]
3. K. Liu, J. Liu, H. Shi, F. Tan, and P. Wang, “High power mid-infrared supercontinuum generation in a single-mode ZBLAN fiber with up to 21.8 W average output power,” Opt. Express. 22(20), 24384–24391 (2014). [CrossRef] [PubMed]
4. J. Zhang, E. Cassan, and X. Zhang, “Enhanced mid-to-near-infrared second harmonic generation in silicon plasmonic microring resonators with low pump power,” Photon. Res. 2(5), 143–149 (2014). [CrossRef]
5. X. Li, X. Liu, L. Zhang, L. Hu, and J. Zhang, “Emission enhancement in Er3+/Pr3+-codoped germanate glasses and their use as a 2.7 μm laser material,” Chin. Opt. Lett. 11(12), 121601 (2013). [CrossRef]
6. P. X. Zhang, Y. Hang, and L. H. Zhang, “Deactivation effects of the lowest excited state of Ho3+ at 2.9 μm emission introduced by Pr3+ ions in LiLuF4 crystal,” Opt. Lett. 37(24), 5241–5243 (2012). [CrossRef] [PubMed]
7. P. X. Zhang, J. G. Yin, B. T. Zhang, L. H. Zhang, J. Q. Hong, J. L. He, and Y. Hang, “Intense 2.8 μm emission of Ho3+ doped PbF2 single crystal,” Opt. Lett. 39(13), 3942–3945 (2014). [CrossRef] [PubMed]
8. J. Pan, R. Xu, Y. Tian, K. Li, L. Hu, and J. Zhang, “2.0 μm emission properties of transparent oxyfuoride glass ceramics doped with Yb3+-Ho3+ ions,” Opt. Mater. 32(11), 1451–1455 (2010). [CrossRef]
9. Q. Zhang, G. Chen, G. Zhang, J. Qiu, and D. Chen, “Spectroscopic properties of Ho3+/Yb3+ codoped lanthanum aluminum germinate glasses with efficient energy transfer,” J. Appl. Phys. 106, 113102 (2009). [CrossRef]
10. R. Lisiecki, W. R. Romanowski, L. Macalik, J. Komar, and M. Berkowski, “Optical study of La3Ga5.5Ta0.5O14 single crystal co-doped with Ho3+ and Yb3+,” Appl. Phys. B. 116, 183–194 (2014). [CrossRef]
11. H. Zhang, D. Sun, J. Luo, J. Chen, H. Yang, J. Xiao, Q. Zhang, and S. Yin, “Growth, thermal, and spectroscopic properties of a Cr, Yb, Ho, Eu:YAP laser crystal,” Opt. Mater. 36(8), 1361–1365 (2014). [CrossRef]
13. A. Diening, S. Kuck, and G. Huber, “Quasi-cw laser oscillation of Yb,Ho:YSGG at 3 μm under laser-diode excitation,” OSA TOPS Advanced Solid State Lasers 19, 221 (1998).
14. F. Cornacchia, A. Toncelli, and M. Tonelli, “2 μm lasers with fluoride crystals: research and development,” Progress in Quantum Electronics 33, 61–109 (2009). [CrossRef]
15. F. Huang, X. Liu, W. Li, L. Hu, and D. Chen, “Energy transfer mechanism in Er3+ doped fuoride glass sensitized by Tm3+ or Ho3+ for 2.7 μm emission,” Chin. Opt. Lett. 12(05), 051601 (2014). [CrossRef]
17. S. E. Hatch, W. F. Parsons, and R. J. Weagley, “Hot-pressed polycrystalline CaF2:Dy2+ laser,” Appl. Phys. Lett. 5, 153 (1964). [CrossRef]
18. W. J. Tropf, M. F. Thomas, and T. J. Harris, Handbook of Optics (McGraw-Hill, 1995).
19. G. Dantelle, M. Mortier, Ph. Goldner, and D. Vivien, “EPR and optical study of Yb3+-doped β-PbF2 single crystals and nanocrystals of glass-ceramics,” J. Phys. Condens. Matter 18, 7905–7922 (2006). [CrossRef]
20. V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett. 81, 1937 (2002). [CrossRef]
21. H. Kuhn, S. T. Fredrich-Thornton, C. Krankel, R. Peters, and K. Petermann, “Model for the calculation of radiation trapping and description of the pinhole method,” Opt. Lett. 32(13), 1908–1910 (2007). [CrossRef] [PubMed]
22. G. S. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys. 37, 511–520 (1962). [CrossRef]
23. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127, 750–761 (1962). [CrossRef]
24. C. K. Jorgensen and R. Reisfeld, “Judd-Ofelt parameters and chemical bonding,” J. Less-Common Met. 93(1), 107–112 (1983). [CrossRef]
25. M. Wang, L. X. Yi, Y. K. Chen, C. L. Yu, G. N. Wang, L. L. Hu, and J. J. Zhang, “Effect of Al(PO3)3 content on physical, chemical and optical properties of fluorophosphate glasses for 2 μm application,” Mater. Chem. Phys. 114, 295–299 (2009). [CrossRef]
26. B. F. Aull and H. P. Jenssen, “Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections,” IEEE J. Quantum Electron. 18(5), 925–930 (1982). [CrossRef]
27. S. A. Payne, L. K. Smith, and W. F. Krupke, “Cross sections and quantum yields of the 3 μm emission for Er3+ and Ho3+ dopants in crystals,” J. Appl. Phys. 77(9), 4274–4279 (1995). [CrossRef]
28. Z. Chengchun, H. Yin, Z. Lianhan, Y. Jigang, H. Pengchao, and M. En, “Polarized spectroscopic properties of Ho3+-doped LuLiF4 single crystal for 2 μm and 2.9 μm lasers,” Opt. Mater. 33(11), 1610–1615 (2011). [CrossRef]
29. A. Dieninga and S. Kuck, “Spectroscopy and diode-pumped laser oscillation of Yb3+, Ho3+-doped yttrium scandium gallium garnet,” J. Appl. Phys. 87(9), 4063–4068 (2000). [CrossRef]
30. L. Feng, J. Wang, Q. Tang, L. Liang, H. Liang, and Q. Su, “Optical properties of Ho3+-doped novel oxyfuoride glasses,” J. Lumin. 124(2), 187–194 (2007). [CrossRef]