Red upconversion emission in LiNbO3 codoped with ... - OSA Publishing

Red upconversion emission in LiNbO3 codoped with Er3+ and Eu3+ Ai-Hua Li,1 Zhi-Ren Zheng,1,∗ Tian-Quan Lu, ¨ 1 Qiang Lu, ¨ 2 and Wei-Long Liu,1 1

Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, P.R.China

2 Center

of Electron Microscope Technology, Mudanjiang Medical College, Mudanjiang 157011, P.R.China ∗ [email protected]

Abstract: Red upconversion (UC) emission at 626 nm is obtained from a LiNbO3 crystal codoped with Er3+ and Eu3+ under 800 nm femtosecond laser excitation. Energy transfer from (2 H11/2 ,4 S3/2 ) levels of Er3+ , which are excited by excited state absorptions, to 5 D1 of Eu3+ followed by cascade to 5 D0 nonradiatively leads to this red UC emission. The energy transfer efficiency of ∼30% is obtained in LiNbO3 :Er3+ (1.0 mol%),Eu3+(0.1 mol%). These initial experimental results indicate that the red UC emission can be obtained from Er3+ /Eu3+ codoped system under diode laser excitation. © 2009 Optical Society of America OCIS codes: 190.7220(Upconversion), 300.6500(Time-resolved spectroscopy).

160.5690(Rare-earth-doped

materials),

References and links 1. A. J. Silversmith, W. Lenth,and R. M. Macfarlane, “Green infrared-pumped erbium upconversion laser,” Appl. Phys. Lett. 51, 1977-1979 (1987). 2. T. Hebert, R. Wannemacher, W. Lenth, and R. M. Macfarlane, “Blue and green cw upconversion lasing in Er:YLiF4 ,” Appl. Phys. Lett. 57, 1727-1729 (1990). 3. E. Downing, L. Hesselink, J. Ralston, and R. Macfarlane, “A Three-Color, Solid-State, Three-Dimensionla Display,” Science 273, 1185-1189 (1996). 4. O. Graydon, “Jets of molten metal make industrial parts,” Opto & Laser Europe 47, 15-20 (1998). 5. S. A. Payne, L. L. Chase, H. W. Newkirk, L. L. Smith, and W. F. Krupke, “LiCaAlF6 :Cr3+ : A Promising New Solid-State Laser Material,” IEEE J. Quantum Electron. 24, 2243-2252 (1988). 6. D. Jaque, J. Capmany, and J. G. Solé, “Continuous wave laser radiation at 669 nm from a self-frequency-doubled laser of YAl3 (BO3 )4 :Nd3+ ,” Appl. Phys. Lett. 74, 1788-1790 (1999). 7. G. Y. Chen, Y. G. Zhang, G. Somesfalean, Z. G. Zhang, Q. Sun, and F. P. Wang, “Two-color upconversion in rare-earth-ion-doped ZrO2 nanocrystals,” Appl. Phys. Lett. 89, 163105-3 (2006). 8. A. S. Gouveia-Neto, L. A. Bueno, R. F. do Nascimento, E. A. da Silva, Jr., E. B. da Costa, and V. B. do Nascimento, “White light generation by frequency upconversion in Tm3+ /Ho3+ /Yb3+ -codoped fluorolead germanate glass,” Appl. Phys. Lett. 91, 091114-3 (2007). 9. H. P. You and M. Nogami, “Three-photon-excited fluorescence of Al2 O3 -SiO2 glass containing Eu3+ ions by femtosecond laser irradiation,” Appl. Phys. Lett. 84, 2076-2078 (2004). 10. D. Hreniak, W. Strek, A. Speghini, M. Bettinelli, G. Boulon, and Y. Guyot, “Infrared induced red luminescence of Eu3+ -doped polycrystalline LiNbO3 ,” Appl. Phys. Lett. 88, 161118-3 (2006). 11. Y. Dwivedi, S. N. Thakud, and S. B. Rai, “Study of frequency upconversion in Yb3+ /Eu3+ by cooperative energy transfer in oxyfluoroborate glass matrix,” Appl. Phys. B 89, 45-51 (2007). 12. F. Pandozzi, F. Vetrone, J.-C. Boyer, R. Naccache, J. A. Capobianco, A. Speghini, and M. Bettinelli, “A spectroscopic analysis of blue and ultraviolet upconverted emissions from Gd3 Ga5 O12 :Tm3+ ,Yb3+ nanocrystals,” J. Phys. Chem. B 109, 17400-17405 (2005).

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Received 22 Aug 2008; revised 12 Jan 2009; accepted 17 Feb 2009; published 27 Feb 2009

2 March 2009 / Vol. 17, No. 5 / OPTICS EXPRESS 3878

13. A. J. Garcia-Adeva, R. Balda, J. Fernández, E. E. Nyein, and U. Hömmerich, “Dynamics of the infrared-to-visible upconversion in an Er3+ doped KPb2 Br5 crystal,” Phys. Rev. B 72, 165116-11 (2005). ´ Péter, and A. F. Muñoz, “Co-emission of Tb3+ and Eu3+ ions 14. E. R. Alvarez, R. F. Sosa, I. Földvári, K. Polgár, A. 3+ 3+ in LiNbO3 :,Tb , Eu single crystals, ” Phys. Stat. Sol. (c) 4, 826-829 (2007). ¨ Andresen, A. N. Bahar, D. Conradi, I. I. Oprea, R. Pankrath, U. Voelker, K. Betzler, M. Wöhlecke, U. Caldiño, 15. A. E. Mart´ın, D. Jaque, and J. G. Solé, “Spectroscopy of Eu3+ ions in congruent strontium barium niobate crystals,” Phys. Rew. B 77, 214102-10 (2008). 16. W. Ryba-Romanowski, S. Golab, G. Dominiak-Dzik, P. Solarz, and T. Lukasiewicz, “Conversion of infrared radiation into red emission in YVO4 :Yb,Ho,” Appl. Phys. Lett. 79, 3026-3028 (2001). 17. J. J. Ju, M.-H. Lee, M. Cha, and H. J. Seo, “Energy transfer in clustered sites of Er3+ ions in LiNbO3 crystals,” J. Opt. Soc. Am. B 20, 1990-1995 (2003). 18. M. Inokuti and F. Hirayama, “Influence of Energy Transfer by the Exchange Mechanism on Donor Luminescence,” J. Chem. Phys. 43, 1978-1989 (1965). 19. L. D. da Vila, L. Gomes, V. G. Tarelho, S. J. L. Ribeiro, and Y. Messadeq, “Mechanism of the Yb-Er energy transfer in fluorozirconate glass,” J. Appl. Phys. 93, 3873-3880 (2003). 20. T. Hayakawa, M. Hayakawa, and M. Nogami, “Excitation-emission properties of Er3+ ions doped in nonlinear optical TeO2 -Nb2 O5 -ZnO glass by 800 nm femtosecond laser excitation,” J. Ceram. Soc. Jpn. 116, 1092-1095 (2008). 21. D. Jaque, M. O. Ramirez, L. E. Bausá, J. G. Solé, E. Cavalli, A. Speghini, and M. Bettinelli, “Nd3+ → Yb3+ energy transfer in the YAl3 (BO3 )4 nonlinear laser crystal,” Phys. Rev. B 68, 035118-9 (2003). 22. H. Z. Chen, Z. R. Zheng, L. Sun, A. H. Li, and W. L. Liu, “Spectroscopic analysis of Eu3+ in In3+ -doped LiNbO3 crystals,” J. Appl. Phys. (In revision). 23. J. Amin, B. Dussardier, T. Schweizer, and Hempstead, “Spectroscopic analysis of Er3+ transition in lithium niobate,” J. Lumin. 69, 17-26 (1996).

1.

Introduction

Besides the potential application in high brightness display of a crystal generating the three fundamental colors [1, 2, 3], red emission is also useful in excitation of photosensitive drugs in medicine [4] and in pumping solid state laser based on Cr3+ , like LiCAF and LiSF [5, 6]. Under one-color diode lasers excitation, red upconversion (UC) emission is usually obtained from Er3+ /Yb3+ or Ho3+ /Yb3+ codoped systems [7, 8]. However, these codoped systems are quite prone to green UC emission, so red UC emission is not efficient nor pure. Eu3+ is an unsurpassable red emitting ion in fluorescence lamps, cathode-ray tubes, and projection television tubes under ultraviolet or vacuum ultraviolet excitation due to the high luminescent quantum yield, high resolution, and adjustable wavelength. But its scarce UC property excludes it as a red UC emitting ions in the past for a long time. Single Eu3+ UC emissions, which originate from three-photon simultaneous absorption of the Eu3+ charge transfer state or from second harmonic generation (SHG) of host under pulse laser excitation, are reported [9, 10]. These processes hardly occur under cw diode laser excitation. On the other hand, a red UC emission in Yb3+ /Eu3+ codoped oxyfluororate glass under cw diode laser excitation is reported [11]. In this work, a red UC emission in Er3+ /Eu3+ codoped LiNbO3 crystal is investigated when excited with 800 nm femtosecond laser. 2.

Experiment

Er3+ /Eu3+ codoped LiNbO3 crystal was grown along the ferroelectric c axis in air by Czochralski technique. 1.0 mol% of Er3+ and 0.1 mol% of Eu3+ were added to a congruent ([Li]/[Nb]=0.946) melt. The sample was cut from the middle of the boules without being polarized to y-cut slice, and then polished to optical grade. A regeneratively amplified 800 nm Ti:sapphire mode-locked laser (Spectra-Physics, Spitfire) delivering 1 kHz and ∼130 fs pulses in duration was loosely focused onto the sample. The emission was collected by a fiber probe, whose other end was connected to the entrance port of a grating-spectrometer (Bruker Optics 250IS/SM), in a direction normal to the incident excitation beam. The spectrometer output was detected by a thermoelectrically cooled intensified CCD (ICCD) detector (Andor, iStar #100086 - $15.00 USD

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DH720). Power dependencies were measured by using neutral density filters. The displayed emission spectrum was corrected for the wavelength response of the ICCD detector according to its quantum efficiency curve. 3.

Results and discussion

The UC emission spectrum of LiNbO3 crystal codoped with Er3+ and Eu3+ is shown in Fig. 1. The emission lines are labeled each with the respective electronic transition involved. The emission is mainly consisted of two emission bands around 550 and 626 nm. It is worth noting that the luminescence intensity of 0.1 mol% Eu3+ is comparable to that of 1.0 mol% Er3+ .

20000 3+

Eu

16000

Lumin. Intensity (a.u.)

:

5

7

D -- F 0

J

J=2

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Er 4

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15/2

J=1

J=4 J=3

0 500

550

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650

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Wavelength (nm)

Fig. 1. Red and green UC emission spectrum of LiNbO3 :Er3+ ,Eu3+ crystal under 800 nm femtosecond laser excitation at room temperature.

To understand the UC mechanism the pump energy dependence of the UC fluorescent intensities are investigated. For an unsaturated UC process, the number of photons that are necessary to populate the upper emitting state can be obtained by [12] n IUC ∝ Ppump ,

(1)

where IUC is the integrated fluorescence intensity, Ppump is the pump energy density, and n is the number of laser photons required to produce one UC photon. The pump energy dependence of UC emission intensities are plotted in Fig. 2. n are obtained from the slope of the graph of ln(IUC ) versus ln(Ppump). In the low pump energy side, n close to 2 indicate two-photon process dominating red and green UC emissions. In the high pump energy side, the slope of 1.06 means the occurrence of saturation for Eu3+ . To further clarify the UC mechanism, the decay kinetics of Eu3+ and Er3+ are measured, and the decay curve of Eu3+ around 626 nm are shown in Fig. 3. The decay curve shows a rise at initial stage followed by an exponential decay. The rise can be ascribed to ET [13] or to population from higher energy level. Whereas the energy gap between 5 D1 and 5 D0 is just 2 quantum of cut-off phonon energy (880 cm−1 ) in LiNbO3 crystal, which leads to very effective multiphonon relaxation from 5 D1 to 5 D0 . The effective multiphonon relaxation from 5 D1 to 5 D is demonstrated by the absence of obvious emission band between 500 and 580 nm in the 0 Eu3+ :LiNbO3 emission spectrum under 393 nm excitation (Fig. 2 of Ref. [14]) and the direct exponential decay in Eu3+ :Sr0.61 Ba0.39 Nb2 O6 under 532 nm excitation into 5 D1 level of Eu3+ [15]. So the assignation of the rise to population from higher energy level is ruled out. Based on #100086 - $15.00 USD

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4

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Lumin. Intensity (a.u.)

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1.76

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Fig. 2. (Color online) Double logarithmic plot of the pump energy dependence of the UC emission intensities of LiNbO3 :Er3+ ,Eu3+ under 800 nm laser excitation at room temperature.

instant decay under excitation in the 5 D1 or higher, the UC mechanisms of two-photon simultaneous absorption or SHG can be ruled out. If the UC mechanisms of two-photon simultaneous absorption or SHG is operative, Eu3+ will be excited to a higher excited state directly, and then cascade to 5 D0 level without delay, which cannot explain the delay of emission signal in experiment. The UC mechanism of SHG can also be excluded by the different temperature dependence between SHG and red UC emission, as shown in Fig. 4. The spectra obtained at 45 and 160 ◦ C are shown in the insert of Fig. 4.

0

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= 414

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Intensity (a.u.)

Intensity (a.u.)

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120

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)

Fig. 3. (Color online) Time evolution of red UC emission after 800 nm femtosecond laser excitation with pump energy of 50µ J/pulse at room temperature. The open dots are the experimental points and full line corresponds to the best fitting to Eq. (2). The inset shows the Er3+ decay curve at 550 nm.

So the delay in the emission signal is ascribed to ET. ET from Er3+ to Eu3+ must oc#100086 - $15.00 USD

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0.16

Intensity of SHG

1.0

Intensity of red emission

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Temperature ( C)

Fig. 4. (Color online) Temperature dependence of the integrated red UC emission and the SHG intensities and the intensity ratios of red emission to SHG under 800 nm laser excitation. The insets show the emission spectra obtained at 45 and 160 ◦ C.

cur because the energy level structure of Eu3+ excludes the possibility of ET UC emission within Eu3+ ions. In view of energy levels matching condition, two channels of ET from Er3+ to Eu3+ can lead to Eu3+ red UC emission under 800 nm excitation: cooperative sensitization (Er3+ :4 I11/2 + Er3+ :4 I11/2 → Eu3+ :5 D2 ), as occurred in Yb3+ /Eu3+ system [11], and ET [Er3+ :(2 H11/2,4 S3/2 ) → Eu3+ :5 D1 ]. The cooperative sensitization from 4 I11/2 level under 4 I9/2 excitation is possible because the population from 4 I9/2 to 4 I13/2 will stay at 4 I11/2 level for 210 µ s. If the cooperative sensitization is operative, the decay curve of Eu3+ can be expressed as I(t) = k[exp(−2t/τEr (4 I11/2 )) − exp(−t/τ )] [16]. But using this expression to fit experimental points, the obtained τEr (4 I11/2 ) (50 µ s) is much shorter than the 4 I11/2 excited state lifetime of Er3+ (210 µ s [17]). This antinomy result excludes the possibility of cooperative sensitization, and the only remained possibility is ET. The Inokuti-Hirayama approach [18] is generally used in ET process, the time-dependent acceptor luminescence has been given by [19] √ t t I(t) = k exp(− ) − exp(−γ6 t − ) , (2) τA τD where k is a coefficient related to the efficiency of the emission process, γ6 is the ET parameter, and τA and τD are the luminescent lifetimes of acceptor and donor ions, respectively. The obtained lifetimes from the fitted line in Fig. 3 are τD = 25 ± 1 µ s and τA = 420 ± 20 µ s. τD is close to the thermally coupled (2 H11/2 ,4 S3/2 ) excited states lifetime (24.5 µ s) (see the inset of Fig. 3). This coincides with ET of [Er3+ :(2 H11/2 ,4 S3/2 ) → Eu3+ :5 D1 ]. τA is ascribed to the 5 D0 excited state lifetime of Eu3+ . Based on the above discussions, our UC emission mechanism is shown in Fig. 5. The Er3+ UC emission mechanisms are extensively investigated, and excited-state absorption (ESA) is adopted here due to the absence of delay in the Er3+ UC emission decay curve at 550 nm (see the inset of Fig. 3). The photon required for the ESA from 4 I13/2 is from the posterior pulse after n (n=1, 2, 3...) ms [20]. The ET from Er3+ :(2 H11/2 ,4 S3/2 ) to Eu3+:5 D1 excites Eu3+ . Populations of Eu3+ at 5 D1 level nonradiatively relax to 5 D0 , from where red UC emission occurs. According to Ref. [21], we calculate the ET efficiency under Er3+ excitation based on emis#100086 - $15.00 USD

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25000

2

H

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, F

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)

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Fig. 5. Energy levels scheme of Er3+ and Eu3+ and proposed UC emission processes. The solid, jagged, and dashed arrows denote radiative transitions, multiphonon relaxations, and ET, respectively. Ground state absorption is abbreviated to GSA.

sion spectrum from the expression

ηt =

α2 ηEr βEr

α 1 R 720nm Eu ηEu 583nm Ilum (λ )d λ R 570nm Er α 1 R 720nm Eu 510nm Ilum (λ )d λ + ηEu 583nm Ilum (λ )d λ

,

(3)

According to available literature [22, 23], the radiative quantum efficiency of the 5 D0 state of Eu3+ (ηEu ) is 0.43, the effective fluorescence quantum efficiency of the (2 H11/2 ,4 S3/2 ) states of Er3+ (ηEr ) is 0.25, the effective fluorescence branching ratio of (2 H11/2 ,4 S3/2 ) → 4 I15/2 (βEr ) is 72%. α is the coupling constant, which includes the mean emission wavelength and the absolute spectral response of the experimental setup. α 1 and α 2 are assumed to be the same since the red emission wavelength is in the proximity of green. The estimated transfer efficiency is about 30%. This value can be enhanced via increasing the concentration of acceptor ion (Eu3+) since the ET efficiency increases with the concentration of acceptor ion [21]. 4.

Conclusion

In summary, red UC emission is observed in a LiNbO3 crystal codoped with Er3+ and Eu3+ under 800 nm femtosecond laser excitation. This red UC emission is a two-photon process and can be ascribed to ET of Er3+ :(2 H11/2 ,4 S3/2 ) → Eu3+ :5 D1 . The ET efficiency of about 30% is obtained in the LiNbO3 crystal codoped with 1.0 mol % of Er3+ and 0.1 mol % of Eu3+ . These results imply that a red UC emission from Eu3+ can occur in Eu3+ /Er3+ codoped or Eu3+ /Er3+ /Yb3+ triply doped systems after Er3+ ions are excited to 4 S3/2 level by cw diode laser. Acknowledgments This work was partially supported by National Natural Science Foundation of China (Grant No. 10774034).

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