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Generation of 1.5 μm emission through an upconversion-mediated looping mechanism in Er3 ∕Sc3-codoped LiNbO3 single crystal Guanying Chen,1,2,* Liming Yang,3,4 Chao Xu,1 Shuwei Hao,1 Hailong Qiu,1 Liang Sun,1 Yuheng Xu,1 and Chunhui Yang1,5 1 2
School of Chemical Engineering and Technology, Harbin Institute of Technology, 150001 Harbin, China
Institute for Lasers, Photonics and Biophotonics, State University of New York at Buffalo, Buffalo, New York 14260, USA 3
Department of Pathophysiology, Harbin Medical University, 150081 Harbin, China 4 e-mail:
[email protected] 5 e-mail:
[email protected] *Corresponding author:
[email protected]
Received November 28, 2011; revised February 11, 2012; accepted February 11, 2012; posted February 13, 2012 (Doc. ID 159056); published March 30, 2012 Important for telecommunications, luminescence of trivalent erbium (Er) at 1.5 μm generally arises from a Stokesshifted downconversion mechanism. We show that this luminescence following direct excitation of the 4 I11∕2 state is generated by upconversion-mediated looping process in Er3 ∕Sc3 -codoped LiNbO3 single crystal. Emissions at 1.0 and 1.5 μm from the 4 I11∕2 and 4 I13∕2 states display linear and quadratic dependences on the excitation density in two separated ranges with a threshold of 20 W∕cm2 . This observation correlates with two- and four-photon processes in green and red upconversion emissions. The mechanism described has implications in the improvement of the output of 1.5 μm luminescence. © 2012 Optical Society of America OCIS codes: 130.3730, 130.4310, 300.2530.
Stokes or anti-Stokes luminescence of trivalent Er3 ions activated in inorganic host lattice is promising for applications such as lasers, biological imaging, infrared detection, and solar cells [1–4]. In particular, the near-infrared (NIR) downconversion emission at 1.5 μm investigated in glasses, semiconductors, and single crystals is of significant interest for optical telecommunications [5–9]. Anti-Stokes upconversion processes that convert the long-wavelength pump sources into short-wavelength emissions are generally believed to be losses for energy conversion into 1.5 μm emission [10]. However, nonradiative cross-relaxations were found to populate the emitting level of 1.5 μm allowing an enhancement of the luminescence intensity [11–13]. An incorporation of cross-relaxation into upconversion process that constitutes a looping, academically known as photo avalanche upconversion, is able to create substantial populations in the intermediary and upper-emitting state [14]. Although the looping mechanism is demonstrated in visible upconversion emissions of Pr3 , Ho3 , Tm3 , and Er3 ions [15], its direct evidence in generation of 1.5 μm downconversion luminescence has not yet been established. Single LiNbO3 crystal doped with Er3 ions has been successfully applied to miniaturized waveguide amplifiers and lasers operating at the important carrier wavelength of 1.5 μm [16,17]. These applications create interests in improving the performance of Er3 :LiNbO3 devices. It is known that the performance of pure LiNbO3 and Er3 :LiNbO3 crystal is severely limited by photo-damage effect when exposed to high laser intensity. A set of photo damage-resistant dopants including divalent Mg2 and Zn2 as well as trivalent In3 and Sc3 were selected to incorporate into the crystal to resist such effect [18,19]. The incorporation varies the transition properties, the occupying sites, and the clusters of Er3 ions in the single crystal [12]. Despite Stokes-shifted mechanism in Er3 :LiNbO3 0146-9592/12/071268-03$15.00/0
[16], the generation mechanism for 1.5 μm luminescence in Er3 :LiNbO3 codoped with damage-resistant dopants remains unexplored. In this letter, we show that this NIR luminescence is generated by upconversion-mediated looping process in Er3 ∕Sc3 -codoped LiNbO3 . Congruent single LiNbO3 crystals doped with 1 mol% Er3 or 1 mol% Er3 and 2 mol% Sc3 were grown by the Czochralski technique [20]. Cuboids of 10 × 8 × 2 mm3 with Y -cut plates were employed for optical characterization. The Stokes NIR emission or anti-Stokes visible upconversion were measured by irradiating Er3 -doped and Er3 ∕Sc3 -codoped single LiNbO3 crystals with a focused 500 mW power-controllable 974 nm diode laser (HiTech Optoelectronics Co. Ltd, Beijing). The spot size of focused laser on the crystal was measured to be about 1 mm in diameter through direct evaluation of the emitting area, which is visible to the naked eye. The emitted visible upconversion radiations were collected by a lens-coupled monochromator (Zolix Instruments Co. Ltd, Beijing) with an attached photomultiplier tube (Hamamatsu CR131). When measuring NIR Stokes emissions, the photomultiplier tube was replaced by a NIR-sensitive InGaAs photo-diode (Thorlabs, DET 410/M). All the spectra were measured using the same geometry and at room temperature. Figure 1 shows the emission spectra of single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 following direct excitation of the 4 I11∕2 state of Er3 with a laser diode emitting at 974 nm. As one can see, the NIR emissions have two peaks with maxima at 1530 and 1000 nm, corresponding to the 4 I13∕2 → 4 I15∕2 and 4 I11∕2 → 4 I15∕2 transitions of Er3 , respectively [21]. The NIR emission of 1530 nm fall into the telecom windows of 1260–1675 nm, which is important for optical communications. The sharp line arises from the scattered laser photons, indicating the lasing wavelength of 974 nm. © 2012 Optical Society of America
April 1, 2012 / Vol. 37, No. 7 / OPTICS LETTERS
Fig. 1. The NIR emission spectra of single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 under laser diode excitation of 974 nm.
To investigate the possible generation mechanisms, we measured the dependence of the intensities of NIR emission bands on the power of excitation at 974 nm and displayed in Fig. 2 for (a) single LiNbO3 crystal doped with 1 mol% Er3 and (b) single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 . The number of photons required to populate the emitting state can be obtained from the equation, I E ∝ P n , where I E is the emission intensity, P is the pump laser power, and n is the number of the laser photons involved [3]. As shown in Fig. 2(a), the NIR emissions at 1000 and 1530 nm in single LiNbO3 crystal doped with 1 mol% Er3 both have a linear dependence on the excitation density in the whole range, suggesting their one-photon generation mechanism. This result is in good agreement with well-established Stokesshift downconversion mechanism where the 4 I13∕2 state was populated by multi-phonon relaxations or radiations from the 4 I11∕2 state directly excited by lasers (consult Fig. 4) [16]. Interestingly, as illustrated in Fig. 2(b), the NIR emissions at 1000 and 1530 nm in single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 have a linear dependence on the laser power density below 20 W∕cm2 , but become quadratic dependence when the excitation density is the range of 20‐40 W∕cm2 . This quadratic dependence suggests that multi-photon processes involving higher energy states are utilized in population of the 4 I11∕2 and the 4 I13∕2 state. Additionally, the same dependence behaviors of 1530 and 1000 nm NIR emissions on the excitation density suggest that they are generated via the same dynamics. To gain an insight into the quadratic dependences of NIR emission at 1530 nm, we further measured visible upconversion spectra from higher energy levels of Er3
Fig. 2. The dependence of the intensities of all NIR emission bands on the excitation density at 974 nm in (a) single LiNbO3 crystal doped with 1 mol% Er 3 and (b) single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 .
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in single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 and displayed in Fig. 3. As illustrated in the figure, the green upconversion with maxima at 530∕550 nm and the red one with maxima at 660 nm are clearly resolved, corresponding to transitions from the 2 H11∕2 ∕4 S3∕2 and the 4 F9∕2 state to the ground 4 I15∕2 state of Er3 ion, respectively [22,23]. It is worth to mention that the green and red upconversion emissions generally arise from two-photon processes in Er3 -doped single LiNbO3 [23]. We also measured the dependence of the intensities of green and red upconversion on the power of excitation in single LiNbO3 crystal doped with 1 mol% Er3 and confirmed this conclusion (data not shown). The inset of Fig. 3 displays the dependence of the intensities of green and red upconversion emissions on the power of excitation at 974 nm in single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 . As one can see, the green upconversion emission has a quadratic dependence on laser power with an excitation density lower than 20 W∕cm2 ; the red upconversion emission is not displayed in this range due to the low signalto-noise ratio resulting from its much lower intensity than the green one. The green and the red upconversion emissions both have a quartic dependence on the laser power when the excitation density exceeds 20 W∕cm2 , correlating well with observations of quadratic dependence of NIR emission intensities on laser excitation power in Fig. 2(b). The much larger n value of n 4 than n 2 illustrates that a positive closed looping mechanism is occurring, since it is hard to envision a four-photon process for the green and the red radiation and a twophoton process for NIR emissions at 1000 and 1530 nm without a positive feedback loop according to Er3 ions’ energy level diagram. Figure 4 depicts energy levels of Er3 ions as well as the proposed upconversion-mediated looping mechanism for the NIR emission at 1530 nm in single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 . It has been established that the intermediate 4 I11∕2 state is resonantly excited from the ground 4 I15∕2 state of Er3 ion by the photon of laser at 974 nm [16]. The radiative decay to the ground state generates NIR emission at 1000 nm. Multiphonon-assisted nonradiative decays and radiations at 2.7 μm can efficiently populate the 4 I13∕2 state from which the important carrier wavelength of 1530 nm is radiated [3]. Alternatively, the Er3 ion in the 4 I11∕2 state can be
Fig. 3. Visible upconversion emission spectra of single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 . The inset displays the dependence of the intensities of two upconversion emission bands on the power of excitation at 974 nm.
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Fig. 4. Energy level diagram of the Er3 ions as well as the proposed mechanisms to produce Stokes NIR emissions and anti-Stokes upconversion emissions.
promoted to the 4 F7∕2 state via excited state absorption 1 (ESA 1) or energy transfer upconversion 1(ETU1) process [22]. These processes are previously considered as upconversion losses in regard to the NIR emission at 1530 nm [11]. The weak red upconversion emission is generated at the 4 F9∕2 state populated by inefficient nonradiative relaxations from the 2 H11∕2 ∕4 S3∕2 state, while the green one is generated at the 2 H11∕2 ∕4 S3∕2 state populated by nonradiative relaxations from the 4 F7∕2 state. However, these mechanisms cannot interpret the quadratic dependence in Fig. 2(b) and quartic dependence in the inset of Fig. 3. It was reported that an incorporation of photo damageresistant dopants into Er3 :LiNbO3 elevate the clustered site concentration of Er3 ions, resulting in efficient cross-relaxations involving the 2 H11∕2 ∕4 S3∕2 state and the ground 4 I15∕2 state [11–13]. Recently, we have demonstrated a new cross-relaxation process of 2 H11∕2 ∕4 S3∕2 4 I15∕2 → 24 I11∕2 in Gd2 O3 :Er3 nanocrystals [24], which introduces a upconversion-mediated feedback to the 4I 11∕2 state, which in combination with ESA 1 constitutes a constructive looping mechanism. In the looping, one Er3 in the 4 I11∕2 state can firstly be excited to the 4 F7∕2 state from which nonradiative relaxations populate the 2 H11∕2 ∕4 S3∕2 state. The 2 H11∕2 ∕4 S3∕2 4 I15∕2 → 24 I11∕2 cross-relaxation process then occur yielding two Er3 ions in the 4 I11∕2 state. When the looping ensues, two can produce four; four produce eight, and so on, leading to photon avalanche-like population of the 4 I11∕2 state and thereby, the 2 H11∕2 ∕4 S3∕2 and 4 F9∕2 state. It is well known that a linear process generally become nonlinear when a positive feedback is involved [14,15]; this also validates here as illustrated by the quadratic dependence in Fig. 2(b). Since the 4 I13∕2 state is populated by radiation or multi-phonon assisted relaxations from the 4 I11∕2 state, the intensity of 1530 nm luminescence, therefore, shares the same behavior as the NIR emission at 1000 nm on the laser excitation power. We calculated the lifetime of the 4 S3∕2 state through use of Judd–Ofelt theory, and we found that the lifetime was significantly lengthened when introducing Sc3 ions into single LiNbO3 crystal doped with 1 mol% Er3 [18]. Along with an increase in the clustered site concentration of Er3 ions, this lengthened lifetime elevates the probability of the 2 H11∕2 ∕4 S3∕2
4I
→ 24 I11∕2 cross-relaxation, favoring the upconversion-mediated looping process in single LiNbO3 crystal doped with 1 mol% Er3 and 2 mol% Sc3 . To conclude, we have investigated the 1.5 μm emission in Er3 ∕Sc3 -codoped single LiNbO3 crystal following direct excitation of the 4 I11∕2 state. This emission and the radiation at 1000 nm both displayed linear and quadratic dependences on the excitation density in two separated ranges with a threshold of 20 W∕cm2 , in coherence to two- and four-photon processes of green and red upconversion emissions. An upconversion-mediated looping mechanism explains the experimental observations, which has implications to improve the output of the 1.5 μm emission. 15∕2
This work was supported by National Natural Science Foundation of China (51102066) and China Postdoctoral Science foundation (20090460911). References 1. J. Amin, J. A. Aust, and N. A. Sanford, Appl. Phys. Lett. 69, 3785 (1996). 2. G. Y. Chen, Y. G. Zhang, G. Somesfalean, Z. G. Zhang, Q. Sun, and F. P. Wang, Appl. Phys. Lett. 89, 163105 (2006). 3. G. Y. Chen, T. Y. Ohulchanskyy, A. Kachynski, H. Ågren, and P. N. Prasad, ACS Nano 5, 4981 (2011). 4. S. Ivanova and F. Pellé, J. Opt. Soc. Am. B 26, 1930 (2009). 5. E. Lallier, Appl. Opt. 31, 5276 (1992). 6. Q. Wang, R. Hui, R. Dahal, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 97, 241105 (2010). 7. S. Yerci, R. Li, S. O. Kucheyev, T. van Buuren, S. N. Basu, and L. Dal Negro, Appl. Phys. Lett. 95, 031107 (2009). 8. L. Tsonev, Opt. Mater. 30, 892 (2008). 9. D. L. Zhang, P. Zhang, W. H. Wong, and E. Y. B. Pun, Cryst. Growth. Des. 8, 2121 (2008). 10. D. L. Zhang and E. Y. B. Pun, J. Appl. Phys. 93, 3141 (2003). 11. D. L. Zhang, C. Wu, Q. Z. Wang, L. Sun, Y. H. Xu, and E. Y. B Pun, Appl. Phys. B 95, 335 (2009). 12. L. Sun, A. H. Li, F. Y. Guo, Qiang Lü, Y. H. Xu, and L. C. Zhao, Appl. Phys. Lett. 91, 071914 (2007). 13. L. Sun, C. H. Yang, A. H. Li, Y. H. Xu, and L. C. Zhao, J. Appl. Phys. 105, 043512 (2009). 14. F. Auzel, Chem. Rev. 104, 139 (2004). 15. M. F. Joubert, Opt. Mater. 11, 181 (1999). 16. S. M. Sher, P. Pintus, F. Di. Pasquale, M. Bianconi, G. B. Montanari, P. D. Nicola, S. Sugliani, and G. Prati, IEEE J. Quant. Electron. 47, 526 (2011). 17. C. Huang and L. McCaughan, IEEE J. Sel. Top. Quantum Electron. 2, 367 (1996). 18. A. H. Li, L. Sun, Z. R. Zheng, W. L. Liu, W. Z. Wu, Y. Q. Yang, and T. Lv, J. Appl. Phys. A 89, 1005 (2007). 19. J. K. Yamamoto, T. Yamazaki, and K. Yamagishi, Appl. Phys. Lett. 64, 3228 (1994). 20. X. H. Zhen, Q. Li, H. T. Li, Y. H. Xu, and L. C. Zhao, Cryst. Res. Technol. 40, 649 (2005). 21. D. M. Gill, L. McCaughan, and J. C. Wright, Phys. Rev. B 53, 2334 (1996). 22. J. J. Ju, M. H. Lee, M. Cha, and H. J. Seo, J. Opt. Soc. Am. B 20, 1990 (2003). 23. S. M. Kostritskii, D. B. Maring, R. F. Tavlykaev, and R. V. Ramaswamy, Appl. Phys. Lett. 76, 2161 (2000). 24. G. Y. Chen, H. J. Liang, H. C. Liu, G. Somesfalean, and Z. G. Zhang, J. Appl. Phys. 105, 114315 (2009).
