We report results of the optical properties of Dy-doped CsPbCl3 and KPb2Cl5 bulk crystals for potential applications in yellow solid-state laser development. The crystals were synthesized from purified starting materials and melt-grown by vertical Bridgman technique. Optical transmission measurements revealed characteristic absorption bands from intra-4f transitions of Dy3+ ions. Direct optical excitation at 455 nm (6H15/2 → 4I15/2) resulted in dominant yellow emission bands at ∼575 nm from the 4F9/2 excited state of Dy3+ ions. In addition, both crystals exhibited weaker emission lines in the blue (∼483 nm) and red (∼670 nm) regions. The peak emission-cross sections for the yellow transition (4F9/2 → 6H13/2) were determined to be ∼0.22 × 10−20 cm2 for Dy: CsPbCl3 (λpeak = 576.5 nm) and ∼0.59 × 10−20 cm2 for Dy: KPb2Cl5 (λpeak = 574.5 nm). The spectral properties and decay dynamics of the 4F9/2 excited state were evaluated within the Judd-Ofelt theory to predict total radiative decay rates, branching ratios, and emission quantum efficiencies.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The visible emission properties of Dy3+ doped solids continue to be of current interest for applications in white light source development and yellow solid-state lasers [1–7]. Laser action based on the yellow Dy3+ transition 4F9/2 → 6H13/2 was first reported at cryogenic temperature for Dy: KY(WO4)2 and Dy: KGd(WO4)2 under flashlamp pumping . More recently, blue laser diode pumped Dy3+ solid-state lasers were reported for Dy: YAG  and Dy,Tb: LiLuF4  operating at room temperature. Ternary lead halides including halide perovskites with composition CsPbX3 (X = Br, Cl) have emerged as promising photonic materials for applications in photovoltaics, optoelectronics, and nuclear radiation detection [8–11]. The low-phonon energy of lead halide based materials has also been of interest for rare earth (RE) doping in solid-state laser applications at IR wavelengths [12–16]. For example, Nd doped KPb2Br5 and RbPb2Cl5 crystals have shown lasing at several near-IR wavelengths , whereas Dy doped RbPb2Cl5 has shown mid-IR lasing at 5.5 µm . The low-phonon energies of lead halide based crystals promises also efficient visible emission from RE ions with only weak non-radiative decay. Furthermore, the relatively broad spectral features observed from RE doped ternary lead halides reduces requirements for wavelength stabilization in diode laser pumping and suggests possible yellow laser tuning. In this paper, we present results of the material preparation and visible emission properties of Dy doped CsPbCl3 and KPb2Cl5 crystals melt-grown by vertical Bridgman technique. The visible emission properties were studied and important spectroscopic parameters were derived for applications in yellow laser development.
2. Materials and experimental details
CsPbCl3 (CPC) is a direct semiconductor with a bandgap of ∼ 3.0 eV. It has a density of 4.21 g/cm3 and a melting point of ∼ 600 °C . CPC crystallizes in a close to cubic perovskite structure. CPC has a low moisture sensitivity and can be handled under ambient conditions. The refractive index was derived from published data to be n∼1.75 at visible to near-IR wavelengths . KPb2Cl5 (KPC) is a monoclinic crystal with a band gap of ∼3.77 eV . KPC has a density of 4.629 g/cm3 and a melting point of 434 °C . KPC is considered non-hygroscopic and has a refractive index of n∼2 . Dy: CPC and Dy: KPC crystals were grown by a vertical Bridgman technique employing a two-zone furnace . Beads of PbCl2 (99.999%) were mixed in a stoichiometric ratio with CsCl (99.999%) or KCl (99.999%) to synthesize CPC or KPC, respectively. DyCl3 was added as dopant to the mix at 1-2 wt%. The combined material was loaded into a pre-cleaned quartz ampoule inside an argon-filled glovebox and heated at ∼110 °C under a dynamic vacuum for 2 days. After sealing under vacuum (∼10−6 torr) the growth ampoule was positioned inside the hot zone of a two-zone crystal growth furnace. The temperature of the hot zone was kept at ∼30 °C above the melting point. The ampoule was then lowered at a growth speed of 1-3 mm/h. After the crystal growth was completed, the furnace temperature was slowly reduced to room-temperature over a time period of four days. Single crystalline grains in the size range of 5 to 10 mm could be mined out from each boule. It was observed that Dy: KPC was colorless and stable under ambient conditions without any change in overall transmission. In contrast, the Dy: CPC crystal exhibited a slight yellowish coloration and became milky with reduced transmission after a few hours of exposure to air. The crystal structure of CPC and KPC was confirmed by XRD measurements. The dopant concentration for Dy: CPC was measured to be 1.1 × 1020 cm−3 using inductively coupled plasma optical emission spectroscopy (ICP-OES). The Dy concentration for Dy: KPC was determined to be 5 × 1019 cm−3 by comparison to published absorption cross-section data . Samples of good optical quality with dimensions of ∼5 × 5×3 mm3 were selected and polished for spectroscopic studies. Transmission spectra were measured using a Shimadzu UV-3600 spectrophotometer. Emission spectra, excitation spectra, and lifetimes were recorded employing an Edinburgh Instruments FLS 980 fluorescence spectrometer.
3. Results and discussion
3.1 Transmission, absorption, and Judd-Ofelt analysis
The optical transmission and absorption spectra of Dy: CPC and Dy: KPC crystals are shown in Figs. 1 and 2. For both crystals the transmission in the IR region was over 50%, but decreased significantly at visible wavelengths due to background absorption losses.
Characteristic intra-4f absorption bands of Dy3+ ions were identified in the IR region peaking at ∼0.76, ∼0.81, ∼0.91, ∼1.10, ∼1.29, ∼1.67, and ∼2.9 µm corresponding to transitions from the 6H15/2 ground state to the 6F3/2, 6F5/2, 6F7/2, 6H7/2+6F9/2, 6H9/2+6F11/2, 6H11/2, and 6H13/2 states, respectively. In addition, weaker UV-visible absorption bands were observed for Dy: KPC at 354 nm (6P7/2), 366 nm (6P5/2), 388 nm (4I13/2), 427 nm (4G11/2), 454 nm (4I15/2), and 476 nm (4F9/2), respectively [19,20]. Dy: CPC did not reveal any visible Dy3+ absorption bands most likely due to overlapping defects and the onset of near bandedge absorption . Using five IR Dy3+ absorption bands a Judd-Ofelt (JO) analysis was performed for Dy: CPC. The 2.9 µm absorption band was not included because of possible overlap with OH absorption . The reduced matrix elements were taken from Kaminskii  and yielded the following intensity parameters: Ω2 = 4.05 × 10−20 cm2, Ω4 = 0.06 × 10−20 cm2, and Ω6 = 0.39 × 10−20 cm2. Table 1 summarizes the relevant data and calculated oscillator strengths for Dy: CPC. For Dy: KPC a JO analysis was reported by Nostrand et al.  yielding the intensity parameters: Ω2 = 5.41 × 10−20 cm2, Ω4 = 0.99 × 10−20, Ω6 = 2.96 × 10−20 cm2.
3.2 Visible emission studies and emission cross-sections
Figure 3 depicts the room temperature visible emission spectra of Dy: CPC and Dy: KPC under 455 nm optical pumping into the 6H15/2 → 4I15/2 absorption transition. Both crystals exhibited dominant yellow emission bands from the 4F9/2 → 6H13/2 transition at 576.5 nm (FWHM: 9.5 nm) for Dy: CPC and 574.5 nm (FWHM: 9.1 nm) for Dy: KPC, respectively. Other observed Dy3+ emission lines originating from the 4F9/2 level showed peak wavelengths at 482.6 (4F9/2 → 6H15/2), 668.6 (4F9/2 → 6H11/2), and 755.0 nm (4F9/2 → 6H9/2+6F11/2) for Dy: CPC. The corresponding emission lines for Dy: KPC peaked at the wavelengths of 482.9, 663.7, and 752.5 nm.
The excitation spectra for Dy: CPC and Dy: KPC for a monitor wavelength of ∼575 nm are shown in Fig. 4. It can be seen that intra-4f excitation at ∼455 nm (6H15/2 → 4I15/2) is the most efficient pump wavelength for achieving yellow emission from both crystals. The excitation spectra also reveal that carrier-mediated excitation using above gap energies does not efficiently excite Dy3+ ions in both hosts. The room temperature 4F9/2 emission decay transients monitored at ∼575 nm for Dy: CPC and Dy: KPC are shown in Fig. 5. For lifetime studies low-concentration samples (∼0.5 wt%) were employed to avoid possible cross-relaxation processes . Following an initial fast decaying component, the emission lifetime of Dy: CPC was single exponential with a value of ∼1.0 ms. The initial fast decaying component is attributed to residual broad background emission from the CPC host. Since the decay time for Dy: KPC was slightly non-exponential, the average lifetime was determined according to the following equation:
Based on the existing emission data, the peak emission cross-sections were calculated using the Fuchtbauer-Ladenburg expression :
The main laser relevant spectroscopic parameters for the investigated Dy: CPC and Dy: KPC in comparison to other Dy doped crystals are shown in Table 3. It can be noted that Dy: KPC compares well with Dy: YAG and Dy: LiLuF4 laser materials and exhibits higher emission cross-section and quantum efficiency. Furthermore, the broad absorption and emission features with bandwidth of several nanometers offer advantages for diode laser pumping and yellow laser tunability. Careful studies on possible quenching effects for higher Dy3+ concentrations, however, are still needed to fully explore Dy: KPC as a novel yellow laser material. For Dy: CPC the strong background absorption losses due to intrinsic defects severely limits its potential for laser applications.
Results of the material preparation and optical spectroscopy for Dy: CsPbCl3 and Dy: KPb2Cl5 were presented. High purity starting materials were employed for the synthesis of both compounds and crystal growth experiments were carried out using vertical Bridgman technique. Under resonant excitation into an intra-4f absorption band, Dy: CPC and Dy: KPC exhibited bright yellow emission peaking in the ∼575-577 nm range from the 4F9/2 → 6H13/2 transition of Dy3+ ions. Based on a Judd-Ofelt analysis, it was determined that the yellow emission exhibited high branching ratios (65-78%) with an emission quantum efficiency of ∼82% for Dy: KPC. In addition, Dy: KPC also exhibited a slightly higher peak emission cross-section for the 4F9/2 → 6H13/2 transition compared to other yellow laser crystals [6,7], which indicates the potential of Dy: KPC for solid-state gain media applications. On the contrary, Dy: CPC exhibited non-radiative losses due to unknown defects and possible OH multi-phonon relaxation. Dy: CPC also suffered from intrinsic transmission losses which make this material less attractive for visible laser applications. Future studies on these crystals will focus on additional purification experiments in an effort to reduce background absorption losses and to improve overall crystal quality.
National Science Foundation (DMR-PREM-1827820); Army Research Office (W911NF1810447).
The authors declare no conflicts of interest.
1. Z. Zhang, J. Li, W. Wang, G. Duan, W. Zhao, and Z. Liu, “Luminescence Properties of Dy3+ doped Gd2(WO4)3 Phosphor Prepared by Hydrothermal Method,” IOP Conf. Ser.: Mater. Sci. Eng. 389, 012019 (2018). [CrossRef]
2. E. Cavalli, “Optical spectroscopy of Dy3+ in crystalline hosts: General aspects, personal considerations and some news,” Opt. Mater. X 1, 100014 (2019). [CrossRef]
3. G. Dominiak-Dzik, P. Solarz, W. Ryba-Romanowski, E. Beregi, and L. Kovacs, “Dysprosium-doped YAl3(BO3)4 (YAB) crystals: an investigation of radiative and non-radiative processes,” J. Alloys Compd. 359(1-2), 51–58 (2003). [CrossRef]
4. A. A. Kaminskii, U. Hömmerich, D. Temple, J. T. Seo, K. Ueda, S. Bagayev, and A. A. Pavlyulk, “Visible Laser Action of Dy3+ in Monoclinic KY(WO4)2 and KGd(WO4)2 crystal under Xe-flashlamp pumping,” Jap. J. Appl. Phys. 39(Part 2, No. 3A/B), L208–L211 (2000). [CrossRef]
5. A. Kaminskii A, B. Gruber J, N. Bagaev S, K. Ueda, U. Hommerich, T. Seo J, D. Temple, B. Zandi, A. Kornienko A, B. Dunina E, A. Pavlyuk A, F. Klevtsova R, and A. Kuznetsov F, “Optical spectroscopy and visible stimulated emission of Dy3+ ions in monoclinic KY(WO4)2 and KGd(WO4)2 crystals,” Phys. Rev. B 65(12), 125108 (2002). [CrossRef]
6. S. R. Bowman, S. O’Connor, and N. J. Condon, “Diode pumped yellow dysprosium lasers,” Opt. Express 20(12), 12906–12911 (2012). [CrossRef]
7. G. Bolognesi, D. Parisi, D. Calonico, G. A. Costanzo, F. Levi, P. W. Metz, C. Krankel, G. Huber, and M. Tonelli, “Yellow laser performance of Dy3+ in co-doped Dy, Tb: LiLuF4,” Opt. Lett. 39(23), 6628–6631 (2014). [CrossRef]
8. R. Babu, L. Giribabu, and S. P. Singh, “Recent Advances in Halide-Based Perovskite Crystals and Their Optoelectronic Applications,” Cryst. Growth Des. 18(4), 2645–2664 (2018). [CrossRef]
9. H. H. Ma, M. Imran, Z. Dang, and Z. Hu, “Growth of Metal Halide Perovskite, from Nanocrystal to Micron-Scale Crystal: A Review,” Crystals 8(5), 182 (2018). [CrossRef]
10. C. C. Stoumpos, C. D. Malliakas, J. A. Peters, Z. Lui, M. Sebastian, J. Im, T. C. Chasapis, A. C. Wibowo, D. Y. Chung, A. J. Feeman, B. C. Wessels, and M. G. Kanatzidis, “Crystal Growth of the Perovskite Semiconductor CsPbBr3: A new Material for High-Energy Radiation Detection,” Cryst. Growth Des. 13(7), 2722–2727 (2013). [CrossRef]
11. D. N. Dirin, I. Cheniukh, S. Yakunin, Y. Shynkarenko, and M. V. Kovalenko, “Solution-Grown CsPbBr3 Perovskite Single Crystals for Photon Detection,” Chem. Mater. 28(23), 8470–8474 (2016). [CrossRef]
12. M. C. Nostrand, R. H. Page, S. A. Payne, L. I. Isaenko, and A. P. Yelisseyev, “Optical Properties of Dy3+ and Nd3+ doped KPb2Cl5,” J. Opt. Soc. Am. B 18(3), 264–275 (2001). [CrossRef]
13. K. Rademaker, E. Heuman, G. Huber, S. A. Payne, W. F. Krupke, L. I. Isaenko, and A. Burger, “Laser Activity at 1.18,1.07, and 0.97 (m in the low-phonon-energy hosts KPb2Br5 and RbPb2Br5 doped with Nd3+,” Opt. Lett. 30(7), 729–731 (2005). [CrossRef]
14. U. Hommerich, E. Brown, A. Kabir, D. Hart, S. B. Trivedi, F. Jin, and H. Chen, “Crystal growth and characterization of undoped and Dy-doped TlPb2Br5 for infrared lasers and nuclear radiation detection,” J. Cryst. Growth 479, 89–92 (2017). [CrossRef]
15. A. G. Okhrimchuk, L. N. Butvina, E. M. Dianov, I. A. Shestakova, N. V. Lichkova, V. N. Zagorodney, and A. Shestakov, “Optical spectroscopy of the RbPb2Cl5: Dy3+ laser crystal and oscillation at 5.5 (m at room temperature,” J. Opt. Soc. Am. B 24(10), 2690–2695 (2007). [CrossRef]
16. M. Kobayashi, K. Omata, S. Sugimoto, Y. Tamagawa, T. Kuroiwa, H. Asada, H. Takeuchi, and S. Kondo, “Scintillation characteristics of CsPbCl3 single crystals,” Nucl. Instrum. Methods Phys. Res., Sect. A 592(3), 369–373 (2008). [CrossRef]
17. N. Pandey, A. Kumar, and S. Chakrabarti, “Investigation of the structural, electronic, and optical properties of Mn-doped CsPbCl3: theory and experiment,” RSC Adv. 9(51), 29556–29565 (2019). [CrossRef]
18. L. Isaenko, A. Yelisseyev, A. Tkachuk, and S. Ivanova, (2008) New Monocrystals with Low Phonon Energy for Mid-IR Lasers, In: M. Ebrahim-Zadeh and I. T. Sorokina, eds. Mid-Infrared Coherent Sources and Applications. NATO Science for Peace and Security Series B: Physics and Biophysics, Springer, Dordrecht.
19. A. A. Kaminskii, Crystalline lasers: Physical processes and operating schemes, CRC press, New York (1996).
20. L. Fang, X. Zhou, J. Zhang, H. Xia, B. Chen, and H. Song, “Control of white light emission via co-doping of Dy3+ and Tb3+ ions in LiLuF4 single crystals under UV excitation,” J. Mater. Sci.: Mater. Electron. 31(4), 3405–3414 (2020). [CrossRef]
21. K. Watanabe, M. Koshimizu, T. Yanagida, Y. Fujimoti, and K. Asai, “Luminescence and scintillation properties of La-and La,Ag-doped CsPbCl3 single crystals,” Jpn. J. Appl. Phys. 55(2S), 02BC20 (2016). [CrossRef]
22. C. K. Jayasankar and E. Rukmini, “Spectroscopic investigation of Dy3+ ions in borosulphate glasses,” Phys. B 240(3), 273–288 (1997). [CrossRef]
23. S. Bigotta, M. Tonelli, E. Cavalli, and A. Belletti, “Optical spectra of Dy3+ in KY3F10 and LiLuF4 crystalline fibers,” J. Lumin. 130(1), 13–17 (2010). [CrossRef]
24. Y. Yang, L. Zhang, S. Li, S. Zhang, M. He, M. Xu, and Y. Hing, “Crystal growth and 570 nm emission of Dy3+ doped CeF3 single crystal,” J. Lumin. 215, 116707 (2019). [CrossRef]