A novel Dy3+/Yb3+ co-doped PbF2 mid-IR laser crystal was successfully grown using the vertical Bridgman method. Efficient emission at around 3 μm from the crystal was observed under excitation of a conventional 970 nm laser diode (LD). The energy transfer efficiency from Yb3+ to Dy3+ in Dy3+/Yb3+:PbF2 crystal is as high as (97.7±0.3)%. It is also found that the Dy3+/Yb3+:PbF2 crystal possesses long fluorescence lifetime (15.4±0.2) ms, high quantum efficiency (95.0±0.3)%, and large emission cross section (1.37±0.11)×10−20 cm2 corresponding to the stimulated emission of Dy3+:6H13/2→6H15/2 transition. Additionally, the phonon energy of the crystal was analyzed by the Raman spectrum. These results indicate that Dy3+/Yb3+:PbF2 crystal may become a promising material for 3 μm solid state lasers under a conventional 970 nm LD pump.
© 2015 Optical Society of America
Owing to the strong absorption of radiation by water and hydroxyapatite at near 3 μm region, mid-IR (MIR) lasers operating around 3 μm have attracted much attention in view of the possible applications in medical and sensing technologies [1–6]. It can also be served as efficient and high quality laser pump sources for longer-wavelength mid-IR oscillators, such as mid-IR optical parametric oscillators (OPO) and THz local oscillators [7–12]. Among various rare earth ions, Dy3+ ion is well known to be the important candidate that can provide 3 μm emission due to its 6H13/2→6H15/2 transition. The Dy3+ ion possesses a number of pump bands in the near infrared, such as 1.3 μm (6H15/2→6H9/2+6F11/2), and 1.7 μm (6H15/2→6H11/2). However, it is lack of commercially available high-power laser diodes (LD) corresponding to these pump bands of Dy3+ ion. Fortunately, codoping of Yb3+ as a sensitizer to Dy3+ has been used as a feasible alternative to obtain the 3 μm emission. In the Dy3+, Yb3+ codoped system, the Yb3+ ion is excited to the excited state (2F5/2) under the excitation of a common 970 nm LD pump, then transfer the accumulated excitation energy to Dy3+:6H5/2. Up to now, effective energy transformation from Yb3+:2F5/2 to Dy3+:6H5/2 has been studied in many luminescent hosts [13–15].
In order to get powerful 3 μm MIR emission from Dy3+ ions, the ideal host material is expected to possess low phonon energy suppressing nonradiative relaxation between adjacent energy levels, minimal absorption coefficient in the H2O absorption band at around 3 μm with no effect to the 3 μm emission, and high radiative emission rates improving laser emission efficiency. Among many alternatives, fluoride crystals have been regarded as natural candidates for such rare-earth-doped optical materials. In our previous work [16–18], we have focused our attentions on a new potential gain medium β–PbF2 crystal, due to its combination of good thermal properties, moderate mechanical properties, excellent solubility for rare-earth ions, and high transparency in a wide wavelength range. Moreover, PbF2 crystal has the advantage of being characterized by significantly lower phonon energy, which is beneficial for suppressing multiphonon de-excitation processes. These characteristics render PbF2 crystal as extremely suitable to be used as a host to mid-IR solid–state lasers.
The spectroscopic properties around 3 μm emission of a Dy3+/Yb3+ co-doped PbF2 crystal under a 970 nm LD pump are investigated for the first time in the present Letter. Yb3+ ion was demonstrated to be an effective sensitizer for Dy3+ ion in PbF2 crystal. Additionally, the established structure features from the Raman spectra has also been made to demonstrate the crystal’s feasibility for future applications in MIR lasers under a common 970 nm LD pump.
2. Experimental section
The Dy3+ (2.0 at. %) single doped, Yb3+ (2.0 at. %) single doped, and Dy3+(2.0 at. %)/Yb3+ (2.0 at. %) co-doped PbF2 crystals were grown by the Bridgman technique. The PbF2 (99.99%), DyF3 (99.99%), and YbF3 (99.99%) powers have been used as starting material. The constituent fluorides were weighted and thoroughly mixed. Then, it was heated for 5 h at 200 °C to make dehydration treatment. After that, the polycrystalline material was loaded into a 30 mm–diameter platinum crucible for crystal growth. In order to avoid volatilization of the melt, the assembled crucible was sealed immediately. During the growth of crystal, firstly, the melt in the crucible was melted for 10 h in the high-temperature zone with a temperature of 980 °C. Then the growth process was driven by lowering the crucible at a rate of 0.4 mm/h in the gradient zone with a temperature gradient around 35 °C/cm–40 °C/cm. At last, the as grown crystal was cooled to room temperature at the rate of 30 °C/cm–40 °C/h. Samples were then cut and polished with dimension of 10 mm×10 mm×5 mm for further optical measurements. The thickness of the sample was 5 mm.
The inductively coupled plasma-atomic emission spectrometry (ICP-AES) method was used to measure the concentration of Dy3+, and Yb3+ ions in the crystals. The doping concentrations of Dy and Yb in the co-doped crystal were (2.29±0.01) at. % ((6.59±0.09)×1020 ions/cm3) and (2.02±0.01) at. % ((5.46±0.10)×1020 ions/cm3), respectively. The doping concentrations of the single-doped crystal was (2.05±0.01) at. % ((5.90±0.09)×1020 ions/cm3) of Dy ions, and (2.01±0.01) at. % ((5.43±0.10)×1020 ions/cm3) of Yb ions.
Crystal structure identification was under taken on a D/max2550 X-ray diffraction (XRD) using Cu Kα radiation. The IR transmittance and absorption spectrums of Dy3+/Yb3+:PbF2 crystal were measured by Nicolet 6700 FTIR spectrometer. The Raman spectrum of Dy3+/Yb3+:PbF2 crystal was measured by inVia microscopes Raman spectrometer. The absorption spectrum of Dy3+/Yb3+:PbF2 crystal in the wavelength of 400–2400 nm was measured by JASCO V-570 UV/VIS spectrophotometer. The fluorescence spectrum of Dy3+/Yb3+:PbF2 crystal in the range of 2600–3400 nm was recorded by Edinburgh Instruments FLS920 under excitation of 970 nm. The fluorescence decay curves of Dy3+/Yb3+:PbF2 crystal at the fluorescence peak of 2890 nm was recorded under pulse excitation of 970 nm. The decay curves of the Yb3+–doped and Dy3+/Yb3+-codoped PbF2 crystals at the fluorescence peak of 1030 nm were measured under excitation of 970 nm. All the measurements were taken at room temperature.
3. Experimental results and discussion
The XRD patterns of the Dy3+/Yb3+:PbF2 crystal and the standard pattern of PbF2 (JCPDS 06-0251), are shown in Fig. 1. The diffraction peaks are strong and no second phase diffraction peak is found, indicating that the impurities do not change the essential structure of β–PbF2.
In order to study the phonon energy of the Dy3+/Yb3+:PbF2 crystal, the Raman spectrum was measured and analyzed. The result is shown in Fig. 2(a). As we can see, there are two strong peaks centered at 130 cm−1 and 251 cm−1, which are attributed to the triply degenerate Raman active that the two fluorine lattices against each other while the lead ion lattice is at rest . Therefore, the maximum phonon energy of the Dy3+/Yb3+:PbF2 crystal can be speculated as low as 251 cm−1. This low phonon energy of the Dy3+/Yb3+:PbF2 crystal is beneficial for suppressing multiphonon de-excitation processes, and improving the ∼3 μm emission efficiency.
The IR absorption spectrum of Dy3+/Yb3+:PbF2 crystal between 3–15 μm is shown in Fig. 2(b). As it can be seen, there are no absorption bands at the wavelength of 3–9 μm, indicating that the Dy3+/Yb3+:PbF2 crystal may be a promising material for IR laser applications. The IR transmittance spectrum of Dy3+/Yb3+:PbF2 crystal is also shown in the inset of Fig. 2(b). It is clear to see that the Dy3+/Yb3+:PbF2 crystal shows good IR transmittance around 3 μm and extends up to 9 μm (about (85±2)%). The (15±2)% loss includes to the Fresnel reflections, dispersion, and absorption of the crystal. There is a dip in transmission near at 3 μm, which correspond to the transitions of Dy3+ ions starting from the 6H15/2 ground state to higher level 6H13/2. As we know, the absorption band region from 3 to 4 μm originated from the stretching vibration of free OH−1 groups will participate in the energy transfer of rare–earth ions and reduce the intensity of emission . The content of OH−1 groups in Dy3+/Yb3+:PbF2 crystal can be expressed by the absorption coefficient of OH−1 vibration band at around 3 μm, which can be giving by αOH− =−ln(T/T0)/l, Where T and T0 are the transmitted and incident intensities, respectively, l is the thickness of the measured sample. The absorption coefficient at 3 μm is as low as (0.024±0.008) cm−1, which might have no effect on the 3 μm emission. In other words, the good IR transmission property proves that the Dy3+/Yb3+:PbF2 crystal is a promising candidate for IR laser materials.
The absorption spectrum of Dy3+/Yb3+:PbF2 crystal in the range of 400–2400 nm was shown in Fig. 3. There are six absorption bands centered at around 453, 805, 905, 1094, 1275, and 1697 nm, which correspond to the transitions starting from the 6H15/2 ground state to higher levels 4F9/2, 6F5/2, 6F7/2, 6H7/2+6F9/2, 6F11/2+6H9/2, and 6H11/2, respectively. In the range of 860 nm to 1030 nm, the highest absorption of (2.3±0.1) cm−1 has been observed centered at 974 nm with a full band width at half-maximum (FWHM) of (16±1) nm corresponding to the Yb3+ optical transition 2F7/2→2F5/2, and turns out to be particularly suitable for commercialized 970 nm LD pumping. The strong absorption at around 970 nm indicates that importing of Yb3+ ions is expected to provide an efficient excitation channel. The inset of Fig. 3 shows the simplified energy level diagram of the Dy3+/Yb3+ codoped system. When the crystal is pumped by a 970 nm laser diode, ions of Yb3+:2F7/2 state is excited to Yb3+:2F5/2. Then transferring the energy to the 6H5/2 level of Dy3+ ions, and mainly nonradiatively to the upper laser level 6H13/2, makes the possibility of Dy3+:6H13/2→6H15/2 transition with ∼ 3 μm emission.
According to the absorption spectrum (Fig. 3), the J-O intensity parameters Ω2,4,6 of Dy3+ from the Judd-Ofelt theory [21,22] were calculated (shown in Table 1), from which spontaneous emission probabilities (A), and radiative lifetime (τR) of the transition of Dy3+: 6H13/2→6H15/2 can be also calculated, and also shown in Table 2. The parameter Ω2 reflects the symmetry of the local environment at the Dy3+ site, and dropping with the improved symmetry . As shown in Table 1, the Ω2 of the Dy3+/Yb3+:PbF2 crystal is higher than that of Dy3+:PbF2 crystal, indicating that the codoping of Yb3+ ions would bring about a lower symmetry surrounding Dy3+ ions in PbF2 crystal. The spontaneous emission probabilities (A) and the radiative lifetime (τR) of the transition of Dy3+:6H13/2→6H15/2 in Dy3+/Yb3+:PbF2 crystal is as high as (61.9±0.2) s−1, and (16.2±0.1) ms, respectively, which are comparable with those of Dy3+:PbF2 crystal: (61.1±0.2) s−1 and (16.4±0.1) ms, while much higher than those of other crystals. It indicates that the introduction of Yb3+ ions has little impact on the ∼3 μm fluorescence emission efficiency of Dy3+.
Figure 4 shows the emission spectra of Dy3+/Yb3+:PbF2 crystal at around 3 μm originated from the Dy3+:6H13/2→6H15/2 transition. It is clear to see that the Dy3+/Yb3+:PbF2 crystal shows a broad and smooth emission spectrum from 2600 nm to 3400 nm with FWHM of (196±2) nm, and strong emission at around 3 μm with a peak intensity of 2890 nm. The corresponding stimulated emission cross-section can be calculated according to the Fuchtbauer-Ladenburg theory 
The fluorescence decay curve for the 6H13/2 multiplet of Dy3+ ions in Dy3+/Yb3+:PbF2 crystal under pulse excitation of 970 nm is shown in the inset of Fig. 4. The fluorescence decay curve exhibit single-exponential nature. The measured lifetime of the 6H13/2 multiplet is as long as (15.4±0.2) ms. A long upper laser level lifetime is beneficial in achieving population inversion and increasing energy storage, which will make Dy3+/Yb3+:PbF2 crystal desirable for laser operation around 3.0 μm. The quantum efficiency η of the fluorescent level can be calculated by : η =τmeas/τR, where τmeas and τR refer to the measured and calculated radiative lifetime, respectively. Therefore, the value of η was calculated to be (95.0±0.3)%. This high quantum efficiency is a result of the low phonon energy of the PbF2 crystal host.
The ability of a donor (Yb3+) to transfer its energy to an acceptor (Dy3+) is an important factor to evaluate the donor (Yb3+) as a sensitizer. To further investigate the energy interaction mechanism, the time-resolved decays of the Yb3+:2F5/2 level for the Dy3+/Yb3+:PbF2 crystal and Yb3+:PbF2 crystal were measured, and shown in Fig. 5. The measured lifetime is (52±4) μs for the Dy3+/Yb3+:PbF2 crystal, which is much shorter than that of Yb3+:PbF2 crystal ((2.3±0.2) ms). This shortening of the measured lifetime confirms that Yb3+ ions are able to transfer energy to the Dy3+ ions for 3 μm emission in PbF2 crystal. The energy transfer (ET) efficiency can be estimated by the following equation: ηET =1−τYb−Dy/τYb, where τYb−Dy and τYb are the Yb3+ lifetimes monitored at 1030 nm, with and without Yb3+ ions, respectively. Therefore, the value of ηET was calculated to be (97.7±0.3)%, indicating that the Yb3+ ion can efficiently transfer energy to Dy3+ ion, and confirming that the Dy3+/Yb3+:PbF2 crystal would be propitious to be pumped by commercialized InGaAs LD.
The effective gain cross section G(λ) is given by the following equation: G(λ)=Pσem−(1−P)σabs, where P is the population inversion, corresponding to the concentration ratio of Dy3+ in the 6H13/2 and 6H15/2 levels, σabs and σem are the absorption cross-section, and emission cross-section, respectively. The calculated gain cross-sections as a function of wavelength with different P (P=0.3, 0.4, 0.5, 0.6, 0.7, and 0.9, respectively.) is shown in Fig. 6. It is obvious to see that the gain cross-section becomes positive from 2900 nm once the population inversion level reaches value of 40%, indicating that a low pumping threshold is achieved for the Dy3+:6H13/2→6H15/2 laser operation.
In conclusion, Dy3+/Yb3+ co-doped PbF2 crystal was successfully prepared, and an intense 3 μm emission was obtained under the excitation of a commercialized 970 nm LD. It is found that the Dy3+/Yb3+:PbF2 crystal possesses long fluorescence lifetime (15.4±0.2) ms, high quantum efficiency (95.0±0.3)%, and large emission cross section (1.37±0.11)×10−20 cm2 corresponding to the stimulated emission of Dy3+:6H13/2→6H15/2 transition. It was demonstrated that the energy transfer efficiency from Yb3+ to Dy3+ is as high as (97.7±0.3)%, confirms that the Yb3+ ion can be used as an effective sensitizer for Dy3+ ion. It was also demonstrated that the Dy3+/Yb3+:PbF2 crystal possesses low phonon energy (251 cm−1), and good IR transmission, which is one of the main reasons for the intense 3 μm emission. It suggests that the Dy3+/Yb3+:PbF2 crystal is a promising material for 3 μm laser applications under being pumped by a conventional 970 nm LD.
We thank the National Natural Science Foundation of China, Grant Nos. 51302283, 51472257, 13ZR1463400, 61308042, and 61275142 for their financial support.
References and links
1. 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]
2. 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, 15350–15358 (2009). [CrossRef] [PubMed]
3. E. Kasper, M. Kittler, M. Oehme, and T. Arguirov, “Germanium tin: silicon photonics toward the mid-infrared,” Photonics Res. 1(2), 69–76 (2013). [CrossRef]
4. I. D. Aggarwal, L. B. Shaw, and J. S. Sanghera, “Chalcogenide glass fiber-based Mid-IR sources and applications,” Proc. SPIE 6453, 645312 (2007). [CrossRef]
5. F. Huang, X. Liu, W. Li, L. Hu, and D. Chen, “Energy transfer mechanism in Er3+ doped fluoride glass sensitized by Tm3+ or Ho3+ for 2.7-μm emission,” Chin. Opt. Lett. 12(5), 051601 (2014). [CrossRef]
6. J. S. Sanghera, L. B. Shaw, and I. D. Aggarwal, “Chalcogenide glass-fiber-based mid-IR sources and applications,” IEEE J. Sel. Top. Quantum Electron. 15, 114–119 (2009). [CrossRef]
7. J. Zhang, E. Cassan, and X. Zhang, “Enhanced mid-to-near-infrared second harmonic generation in silicon plasmonic microring resonators with low pump power,” Photonics Res. 2(5), 143–149 (2014). [CrossRef]
8. B. C. Dickinson, P. S. Golding, M. Pollnau, T. A. King, and S. D. Jackson, “Investigation of a 791–nm pulsed–pumped 2.7–μm Er-doped ZBLAN fibre laser,” Opt. Commun. 191, 315–321 (2001). [CrossRef]
9. M. E. Doroshenko, H. Jelinkova, P. Koranda, J. Sulc, T. T. Basiev, V. V. Osiko, V. K. Komar, A. S. Gerasimenko, V. M. Puzikov, V. V. Badikov, and D. V. Badikov, “Tunable mid-infrared laser properties of Cr2+:ZnMgSe and Fe2+:ZnSe crystals,” Laser Phys. Lett. 7, 38–45 (2010). [CrossRef]
10. X. Wang, P. Zhou, X. Wang, H. Xiao, and L. Si, “51.5 W monolithic single frequency 1.97 μm Tm-doped fiber amplifier,” High Power Laser Sci. Eng. 1, 123–125 (2013). [CrossRef]
12. X. S. Zhu and R. Jain, “10–W–level diode–pumped compact 2.78 μm ZBLAN fiber laser,” Opt. Lett. 32, 26–28 (2007). [CrossRef]
13. P. Y. Tigreat, J. L. Doualan, C. Budasca, and R. Moncorge, “Energy transfer processes in (Yb3+,Dy3+) and (Tm3+,Dy3+) codoped LiYF4 and KY3F10 single crystals,” J. Lumin. 94–95, 23–27 (2001). [CrossRef]
14. Y. Dwivedi and S. B. Rai, “Spectroscopic study of Dy3+ and Dy3+/Yb3+ ions co-doped in barium fluoroborate glass,” Opt. Mater. 31, 1472–1477 (2009). [CrossRef]
15. Y. Yang, L. Liu, S. Cai, F. Jiao, C. Mi, X. Su, J. Zhang, F. Yu, X. Li, and Z. Li, “Up–conversion luminescence and near-infrared quantum cutting in Dy3+,Yb3+ co–doped BaGd2ZnO5 nanocrystal,” J. Lumin. 146, 284–287 (2014). [CrossRef]
16. 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]
18. P. Zhang, B. Zhang, J. Hong, L. Zhang, J. He, and Y. Hang, “Enhanced emission of 2.86 μm from diode-pumped Ho3+/Yb3+-codoped PbF2 crystal,” Opt. Express. 23(4), 3920–3927 (2015). [CrossRef] [PubMed]
19. W. Hayes, A. J. Rushworth, J. F. Ryan, R. J. Elliott, and W. G. Kleppmannl, “A Raman-scattering study of disorder in PbF2 at high temperatures,” J. Phys. C. 10, L111 (1977). [CrossRef]
20. G. Wang, S. Dai, J. Zhang, S. Xu, L. Hu, and Z. Jiang, “Effect of F− ions on emission cross-section and fluorescence lifetime of Yb3+-doped tellurite glasses,” J. Non-Cryst. Solids 351, 2147–2151 (2005). [CrossRef]
21. G. S. Ofelt, “Intensities of crystal spectra of rare-earth ions,” J. Chem. Phys. 37, 511–520 (1962). [CrossRef]
22. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127, 750–761 (1962). [CrossRef]
23. 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]
24. M. G. Brik, T. Ishii, A. M. Tkachuk, S. E. Ivanova, and I. K. Razumova, “Calculations of the transitions intensities in the optical spectra of Dy3+:LiYF4,” J. Alloys Compounds 374, 63–68 (2004). [CrossRef]
25. D. G. Dominiak, R. W. Ryba, M. N. Palatnikov, N. V. Sidorov, and V. T. Kalinnikov, “Dysprosium-doped LiNbO3 crystal optical properties and effect of temperature on fluorescence dynamics,” J. Mol. Struct. 704, 139–144 (2004). [CrossRef]
26. H. Wang, J. Li, G. Jia, Z. You, F. Yang, Y. Wei, Y. Wang, Z. Zhu, X. Lu, and Tu, “Optical properties of Dy3+ ions in sodium gadolinium tungstates crystal,” J. Lumin. 126, 452–458 (2007). [CrossRef]
27. 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, 925–930 (1982). [CrossRef]
28. S. Singh, R. G. Smith, and L. G. Van Uitert, “Stimulated-emission cross section and fluorescent quantum efficiency of Nd3+ in yttrium aluminum garnet at room temperature,” Phys. Rev. B. 10, 2566 (1974). [CrossRef]