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Optics Express

  • Vol. 16, Iss. 16 — Aug. 4, 2008
  • pp: 11907–11914
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Intense ultraviolet upconversion luminescence from hexagonal NaYF4:Yb3+/Tm3+ microcrystals

Guofeng Wang, Weiping Qin, Lili Wang, Guodong Wei, Peifen Zhu, and Ryongjin Kim  »View Author Affiliations


Optics Express, Vol. 16, Issue 16, pp. 11907-11914 (2008)
http://dx.doi.org/10.1364/OE.16.011907


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Abstract

Under 980 nm excitation, unusual 3P23H6(~264 nm) and 3P23F4(~309 nm) emissions from Tm3+ ions were observed in hexagonal NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals. In comparison with the strong emissions from 1D2 and 1I6, the emissions from 1G4 and 3H4 almost vanished due to the efficient cross-relaxation of 1G4+3H43F4+1D2(Tm3+). Double logarithmic plots of the upconversion emission intensity versus the excitation power are neither straight lines nor typical saturation curves. Theoretical analysis indicated that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation.

© 2008 Optical Society of America

1. Introduction

Frequency upconversion (UC) from infrared (IR) to visible/ultraviolet (UV) by materials doped with trivalent rare-earth (RE) ions, such as Tm3+, Er3+, Ho3+, Nd3+, and Pr3+, has attracted intensive attention for more than 30 years [1

1. L. F. Johnson, J. E. Geusic, H. J. Guggenheim, T. Kushida, S. Singh, and L. G. Van Uitert, “Comments on Materials for Efficient Infrared Conversion,” Appl. Phys. Lett. 15, 48–50 (1969). [CrossRef]

,2

2. G. S. He, P. P. Markowicz, T. C. Lin, and P. N. Prasad, “Observation of stimulated emission by direct three-photon excitation,” Nature 415, 767–770 (2002). [CrossRef] [PubMed]

]. Its potential applications include detection of infrared radiation, optical storage, color display, UC lasers, biosensor, and so on [3-7

3. E. Downing, L. Hesselink, J. Ralston, and R. Macfarlane, “A Three-Color, Solid-State, Three-Dimensional Display,” Science 273, 1185–89 (1996). [CrossRef]

]. Most of these applications have been achieved with the help of fluorides due to their low phonon energy and high quantum efficiencies as luminescent hosts [8

8. C. Y. Cao, W. P. Qin, J. S. Zhang, Y. Wang, P. F. Zhu, G. D. Wei, G. F. Wang, R. J. Kin, and L. L. Wang, “Ultraviolet upconversion emissions of Gd3+,” Opt. Lett. 33, 857–859 (2008). [CrossRef] [PubMed]

]. Among RE ions, Tm3+ has attracted increasing attention because it has metastable levels (Fig. 1) suitable for emitting blue and UV UC luminescence [9-11

9. J. J. Owen, A. K. Cheetham, and R. A. Mcfarlane, “Orientation-dependent fluorescence studies and spectroscopic analysis of doped barium yttrium fluoride upconversion laser crystals (BaY2-x-yYbxTmyF8),” J. Opt. Soc. Am. B 15, 684–693 (1998). [CrossRef]

]. Especially, UV UC luminescence from Yb3+-sensitized Tm3+ in fluorides has become one hot topic of optical functional materials in recent years [12

12. X. B. Chen and Z. F. Song, “Study on six-photon and five-photon ultraviolet upconversion luminescence,” J. Opt. Soc. Am. B 24, 965–971 (2006) [CrossRef]

]. Our group has reported strong UV UC emissions from Yb3+/Tm3+-codoped fluoride film and attributed the UV enhancement to the decrease of Judd-Ofelt parameter Ω2, which reflects the symmetry of the crystal field [13

13. G. S. Qin, W. P, Qin, C. F. Wu, S. H. Huang, J. S. Zhang, S. Z. Lu, D. Zhao, and H. Q. Liu, “Enhancement of ultraviolet upconversion in Yb3+ and Tm3+ codoped amorphous fluoride film prepared by pulsed laser deposition,” J. Appl. Phys. 99, 1–3 (2003).

]. Wang et al. have reported intense UV UC luminescence from Yb3+/Tm3+-codoped YF3 nanocrystals embedded glass ceramic [14

14. D. Q. Chen, Y. S. Wang, Y. L. Yu, and P. Huang, “Intense ultraviolet upconversion luminescence from Tm3+/Yb3+:β-YF3, nanocrystals embedded glass ceramic,” Appl. Phys. Lett. 91, 051920–051922 (2007). [CrossRef]

]. Chen et al. demonstrated six-photon and five-photon UV UC luminescence in Yb3+/Tm3+-codoped ZBLAN fluoride glass [12

12. X. B. Chen and Z. F. Song, “Study on six-photon and five-photon ultraviolet upconversion luminescence,” J. Opt. Soc. Am. B 24, 965–971 (2006) [CrossRef]

]. Despite the fact that UV UC emissions have been widely investigated and some novel spectral phenomena have been reported, the study of new approaches to obtain efficient UV luminescence as well as the mechanism of the enhancement is still a challenge for the development of short-wavelength solid-state lasers [12

12. X. B. Chen and Z. F. Song, “Study on six-photon and five-photon ultraviolet upconversion luminescence,” J. Opt. Soc. Am. B 24, 965–971 (2006) [CrossRef]

].

Fig. 1. Energy level diagrams of Yb3+ ions and Tm3+ ions and UC emission mechanism in NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals.

In this study, intense UV UC luminescence was presented in hexagonal NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals under 980 nm excitation. The 3P23H6 and 3P23F4 emissions were observed for the first time. In comparison with the strong emissions from 1D2 and 1I6, the emissions from 1G4 and 3H4 almost vanished.

2. Experimental

Y2O3, Yb2O3, and Tm2O3 (purity ≥ 99.999 %) were supplied by Shanghai Chemical Reagent Company. EDTA, NaF, and HNO3 were supplied by Beijing Chemical Reagent Company, and were of analytical grade. Deionized water was used to prepare solutions. Ln2O3 (Ln=Y,Yb, and Tm) were dissolved in dilute HNO3 by heating to prepare the stock solution of Ln(NO3)3.

In a typical synthesis, 1 mL of 0.5 M Ln(NO3)3 aqueous solution and 0.5 mmol of EDTA were dispensed into 20 mL of deionized water and magnetically stirred for 1 h, forming a chelated Ln-EDTA complex. Then 16 mL of 0.5 M NaF aqueous solution was added to the solution. After vigorous stirring for 1 h, the mixture was transferred into a 50-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained in an oven at 160 °C for 18 h, and then cooled down slowly to room temperature. Subsequently, the suspension was centrifuged at 8000 rpm for 10 min. The resultant product was then washed thoroughly and dried in vacuum at 80 °C. To improve the crystallinity of the nanocrystalline powder, the product was annealed at 400 °C for 2 h.

The crystal structure was analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using a nickel-filtered Cu-Ka radiation (λ=1.4518Å). The size and morphology were investigated by scanning electron microscope (SEM, KYKY 1000B). UC luminescence spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer (2.5 nm for spectral resolution (FWHM) of the spectrophotometer and 400 V for PMT voltage) at room temperature.

Figure 2(a) shows the XRD pattern of the microcrystals. All the diffraction peaks can be indexed to the pure hexagonal NaYF4 (JCPDS 16-0334). No other impurity peaks were detected. The corresponding SEM image (Fig. 2(b)) shows that the NaYF4:Yb3+/Tm3+ microcrystals are hexagonal pillars.

Fig. 2. XRD pattern (a) and SEM image (b) of the NaYF4:Yb3+/Tm3+ microcrystals.

3. Results

3.1. Intense UV and weak visible emissions

Under 980 nm excitation (~320 W/cm2), unusually intense UV and weak visible emissions were observed in NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals, as shown in Fig. 3. These emissions come from the following transitions of Tm3+ ions: 3P23H6 (~264 nm) [11

11. W. T. Carnall, R. Fields, and K. T. Rajnank, “Electronic energy levels in the trivalent lanthanide aquo ions,” J. Chem. Phys. 49, 4424–4442 (1968). [CrossRef]

], 1I63H6 (~291 nm), 3P23F4 (~309 nm), 1I63F4 (~347 nm), 1D23H6 (~363 nm), 1D23F4 (~454 nm), 1G4 3H6 (~475 nm), 1D23H5 (~508 nm), 1D23H4 (~574 nm), 1G43F4 (~642 nm), 3F33H6 (~689 nm), and 3H43H6 (~795 nm). In comparison with usual UC emission spectra of Tm3+ [13

13. G. S. Qin, W. P, Qin, C. F. Wu, S. H. Huang, J. S. Zhang, S. Z. Lu, D. Zhao, and H. Q. Liu, “Enhancement of ultraviolet upconversion in Yb3+ and Tm3+ codoped amorphous fluoride film prepared by pulsed laser deposition,” J. Appl. Phys. 99, 1–3 (2003).

,14

14. D. Q. Chen, Y. S. Wang, Y. L. Yu, and P. Huang, “Intense ultraviolet upconversion luminescence from Tm3+/Yb3+:β-YF3, nanocrystals embedded glass ceramic,” Appl. Phys. Lett. 91, 051920–051922 (2007). [CrossRef]

], the 1G43H6 and 3H43H6 emissions almost vanished. Although Noginov et al. have predicted that the back energy transfer of 1G4 (Tm3+)+2F7/2 (Yb3+)→3H4 (Tm3+)+2F5/2 (Yb3+) could lead the 1G4 level to be quenched [15

15. M. A. Noginov, M. Curley, P. Venkateswarlu, A. Williams, and H. P. Jenssen, “Excitation scheme for the upper energy levels in a Tm:Yb:BaY2F8 laser crystal,” J. Opt. Soc. Am. B 14, 2126–2136 (1997). [CrossRef]

], this phenomenon has never been observed in experiments so far.

Fig. 3. UC luminescence spectrum of NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals under 980 nm excitation (320 W/cm2). Inset: magnification of the spectrum in the range of 430–850 nm.

3.2. Unusual 3P23H6 (~264 nm) and 3P23F4 (~309 nm) emissions

3.3. Power dependence of UC luminescence.

For nearly any UC mechanism, the emission intensity is proportional to the n-th power of the excitation intensity, and the integer n is the number of photons absorbed per upconverted photon emitted [17

17. M. Pollnau, D. R. Gamelin, S. R. Lüthi, and H. U. Güdel, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61, 3337–3346 (2000). [CrossRef]

]. In the present case, the results are different. Figure 4 shows the double logarithmic plots of the emission intensity as a function of excitation power for the 3H43H6, 1G43H6, 1D23H6, 1I63H6, and 3P23H6 emissions. The corresponding slopes (n) of the dependencies are listed in Table 1.

In the initial stage (0.09-0.14 W), n=1.66, 2.80, 3.74, and 4.57 for the 3H43H6, 1G43H6, 1D23H6, and 1I63H6 emissions, respectively. This means that the populations of the states 1I6, 1D2, 1G4, and 3H4 come from five-photon, four-photon, three-photon, and two-photon UC processes, respectively. However, with the increase of excitation power, the values of n changed significantly. In the range from 0.15 to 0.26 W, n=1.23, 1.86, 2.87, 3.14, and 3.17 for the 3H43H6, 1G43H6, 1D23H6, 1I63H6, and 3P23H6 emissions, respectively. In the range from 0.26 to 0.31 W, the UC luminescence deviates from a linear relationship. In the final stage (0.31-0.39 W), n=0.78, 1.78, 3.16, 3.69, and 3.87 for the 3H43H6, 1G43H6, 1D23H6, 1I63H6, and 3P23H6 emissions, respectively. Obviously, when the excitation power is higher than 0.15 W, the slopes (n) of the 3H43H6 and 1G43H6 emission at low laser power are larger than those at high laser power, while the slopes (n) of the 1D23H6, 1I63H6, and 3P23H6 emissions at low laser power are smaller than those at high laser power.

Fig. 4. Plots (log-log) of emission intensity versus excitation power.

Table 1. Experimental slopes (n) of the dependencies log(emission intensity) versus log(excitation power) for different emissions in NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals under 980 nm excitation.

table-icon
View This Table

4. Mechanism analysis

4.1. Analysis on the nearly vanished radiative transitions from 1G4 and 3H4

The populated 1D2 level radiatively relaxes to the ground-state and inter-states, which cause 363, 454, 508, and 574 nm emissions, as shown in the inset of Fig. 3. On the other hand, the 1D2 state may be excited to the 3P2 state via another energy transfer from excited Yb3+, producing 264 and 309 nm emissions, simultaneously. Before this work, there has not been being any experimental evidence to verify which of 3P0,1,2 and 1I6 levels is the terminal level from the 1D2 level for the five-photon UC process of Tm3+ ions. The UC emissions of 3P23H6 (~264 nm) and 3P23F4 (~309 nm) provide evidence for the energy transfer from Yb3+ to Tm3+ populating 3P2. Of course, the UC emissions from the 1I6 are still dominating in the five-photon UC processes due to the rapidly nonradiative relaxation of 3P23P13P01I6. For a relative high doping concentration, the microcrystals therefore emitted intense 347 and 291 nm fluorescence under the NIR excitation. Furthermore, since the photon (980 nm) exactly matches the energy separation between 1D2 and 3P2, the single-photon absorption of 1D23P2 might be exist.

Fig. 5. Dependence of UC luminescence spectra (normalized to 1D23F4 transition) on Tm3+ concentration (20% Yb3+): (a) 0.01%, (b) 0.1%, (c) 0.5%, (d) 1%, and (e) 1.5%.

4.2. Competition between linear decay and UC processes

For any UC mechanism, although the n-photon process corresponds to a slope approximately equal to n, the dependence of UC luminescence intensity on pump power is also expected to decrease in slope. Pollnau et al. attributed the decrease of n to the competition between linear decay and UC processes for the depletion of the intermediate excited states [17

17. M. Pollnau, D. R. Gamelin, S. R. Lüthi, and H. U. Güdel, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61, 3337–3346 (2000). [CrossRef]

]. They reported that the UC luminescence intensity for an n-photon energy transfer process was proportional to the n-th power of the pump power (Pn) in the limit of infinitely small UC rates, while the intensity was proportional to the pump power (P 1) in the limit of infinitely large UC rates. Therefore, the intensity of an UC luminescence excited by the sequential absorption of n photons has a dependence of Pβ on absorbed pump power P, with 1<β<n [17

17. M. Pollnau, D. R. Gamelin, S. R. Lüthi, and H. U. Güdel, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61, 3337–3346 (2000). [CrossRef]

].

Although the population of the state 3H4 comes from a two-photon process, the slope of the 3H43H6 emission was estimated to be 1.23 of pump power in the range from 0.15 to 0.26 W. For the 3H43H6 emission, the intermediate excited state is 3F4. Therefore, The UC becomes the dominant depletion mechanism for 3F4 in NaYF4:Yb3+/Tm3+ microcrystals [18

18. Y. G. Chen, Y. Liu, Z. G. Zhang, B. Aghahadi, G. Somesfalean, Q. Sun, and F. P. Wang, “Four-photon upconversion induced by infrared diode laser excitation in rare-earth-ion-doped Y2O3 nanocrystals,” Chem. Phys. Lett. 448, 127–131 (2007). [CrossRef]

]. For the 1G43H6 emission (a three-photon process), n=1.86 with a large discrepancy from n=3 is observed. It is reasonable to ascribe this to the increase of the nonradiative relaxation from the 3H4 state together with the reduction of n for the 3H4 state [18

18. Y. G. Chen, Y. Liu, Z. G. Zhang, B. Aghahadi, G. Somesfalean, Q. Sun, and F. P. Wang, “Four-photon upconversion induced by infrared diode laser excitation in rare-earth-ion-doped Y2O3 nanocrystals,” Chem. Phys. Lett. 448, 127–131 (2007). [CrossRef]

]. Similarly, n=2.87 with a large discrepancy from n=4 for the 1D23H6 emission, n=3.14 with a large discrepancy from n=5 for the 1I63H6 (~291 nm) emission, and n=3.17 with a large discrepancy from n=5 for the 3P23H6 (~264 nm) emission are also observed.

4.3. Cross-relaxation processes and thermal effect at high laser power

In most cases, the values of n at lower excitation power are higher than those at higher excitation power due to typical saturation phenomenon and thermal quenching. (1) The typical saturation phenomenon is caused by the population exhaustion at the ground state; (2) The high non-radiative rates at high excitation densities lead to an increase of the temperature in the internal sample, inducing a thermal effect and which will lead to the quenching of UC luminescence [19

19. K. Krämer, D. Biner, G. Frei, H. Güdel, M. Hehlen, and S. Lüthi, “Hexagonal Sodium Yttrium Fluoride Based Green and Blue Emitting Upconversion Phosphors,” Chem. Mater. 16, 1244–1251 (2004). [CrossRef]

,20

20. X. Bai, H. W. Song, G. H. Pan, Y. Q. Lei, T. Wang, X. G. Ren, S. Z. Lu, B. Dong, Q. L. Dai, and L. B. Fan, “Size-Dependent Upconversion Luminescence in Er3+/Yb3+-Codoped Nanocrystalline Yttria: Saturation and Thermal Effects,” J. Phys. Chem. C 11113611–13617 (2007). [CrossRef]

]. However, for the 1D23H6, 1I63H6, and 3P23H6 emissions, the values of n in the range from 0.15 to 0.26 W are smaller than those in the range from 0.31 to 0.39 W. For the 3H43H6 and 1G43H6 emissions, although the values of n in the range from 0.15 to 0.26 W are higher than those in the range from 0.31 to 0.39 W, the log-log plots of UC luminescence versus excitation power are not typical saturation curves at all. The typical saturation curves should gradually bend downward.

We suggested that two mechanisms might cause this anomalous phenomenon. First, under different excitation power, different UC mechanisms are dominant [21

21. S. Sivakumar, J. M. van Veggel, and S. May, “Near-Infrared (NIR) to Red and Green Up-Conversion Emission from Silica Sol-Gel Thin Films Made with La0.45Yb0.50Er0.05F3 Nanoparticles, Hetero-Looping-Enhanced Energy Transfer (Hetero-LEET): A New Up-Conversion Process,” J. Am. Chem. Soc. 129620–625 (2007). [CrossRef] [PubMed]

]. When the excitation power is high enough, the 1G4 and 1D2 are populated effectively, which are benefit for the cross-relaxation of 1G4+3H43F4+1D2 between the two neighboring Tm3+ ions [20

20. X. Bai, H. W. Song, G. H. Pan, Y. Q. Lei, T. Wang, X. G. Ren, S. Z. Lu, B. Dong, Q. L. Dai, and L. B. Fan, “Size-Dependent Upconversion Luminescence in Er3+/Yb3+-Codoped Nanocrystalline Yttria: Saturation and Thermal Effects,” J. Phys. Chem. C 11113611–13617 (2007). [CrossRef]

]. This cross-relaxation process can result in the increase of n for the emissions from 1D2, 1I6, and 3P2. Figure 6 shows the UC luminescence spectra under different excitation power density. When the excitation power increase, the effective populations of the 1G4 and the 3H4 intensively enhance the cross-relaxation mentioned above.

Fig. 6. UC luminescence spectra (normalized to 1D23F4 transition) of NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals under different excitation power density.

WNR=WNR(0)(1+<n>)ΔEћω
(1)

where, WNR(0) is the nonradiative relaxation rate at 0 K,ħω is the phonon energy, and <n> is the phonon density of states, which strongly depends on phonon energy and temperature [20

20. X. Bai, H. W. Song, G. H. Pan, Y. Q. Lei, T. Wang, X. G. Ren, S. Z. Lu, B. Dong, Q. L. Dai, and L. B. Fan, “Size-Dependent Upconversion Luminescence in Er3+/Yb3+-Codoped Nanocrystalline Yttria: Saturation and Thermal Effects,” J. Phys. Chem. C 11113611–13617 (2007). [CrossRef]

]

<n>=1exp(ћωkT)1
(2)

where, k is Boltzmann’s constant and T temperature. The phonon density of states increases with increasing temperature. The increased phonon density of states not only results in the increased nonradiative relaxation rate and quenched UC luminescence, but also benefits the phonon-assisted energy transfers from Yb3+ to Tm3+. Therefore, we suggest that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation. At lower excitation power (0.15 – 0.26 W), the thermal effect can be negligible, and the log-log plots of UC luminescence versus excitation power are straight lines. With the increase of excitation power (0.26 – 0.31 W), the plots gradually bend downward, indicating that the nonradiative relaxations increase faster than the phononassisted energy transfers. When the excitation power is high enough (0.31 – 0.39 W), the slopes (n) of the 1D23H6, 1I63H6, and 3P23H6 emissions again increase, indicating that the energy transfers increase faster than the nonradiative relaxations. Of course, the underlying upconversion dynamics are complex and require further study.

5. Conclusions

Under 980 nm excitation, novel UC luminescent properties were presented in hexagonal NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals. The 3P23H6 (~264 nm) and 3P23F4 (~309 nm) emissions were observed for the first time. In comparison with usual UC emission spectrum of Tm3+, the 1G43H6 and 3H43H6 emissions almost vanished. The dependence of the Tm3+ concentration on the UC luminescence indicated that the unusual phenomenon was caused by the efficient cross-relaxation of 1G4+3H43F4+1D2 (Tm3+) under high Tm3+ concentration.

In addition, the number of laser photons absorbed in one UC excitation process, n, changed in different excitation power range, which was theoretically explained considering different UC excitation mechanisms together with thermal effect under higher excitation power.

Acknowledgment

This research was supported by Natural Science Foundation of China (Grant Nos. 10474096 and 50672030).

References and links

1.

L. F. Johnson, J. E. Geusic, H. J. Guggenheim, T. Kushida, S. Singh, and L. G. Van Uitert, “Comments on Materials for Efficient Infrared Conversion,” Appl. Phys. Lett. 15, 48–50 (1969). [CrossRef]

2.

G. S. He, P. P. Markowicz, T. C. Lin, and P. N. Prasad, “Observation of stimulated emission by direct three-photon excitation,” Nature 415, 767–770 (2002). [CrossRef] [PubMed]

3.

E. Downing, L. Hesselink, J. Ralston, and R. Macfarlane, “A Three-Color, Solid-State, Three-Dimensional Display,” Science 273, 1185–89 (1996). [CrossRef]

4.

D. A. Parthenopoulos and P. M. Rentzepis, “Three-Dimensional Optical Storage Memory,” Science 245, 843–845 (1989). [CrossRef] [PubMed]

5.

G. S. He, J. M. Dai, T.C. Lin, P. P. Markowicz, and P. N. Prasad, “Ultrashort 1.5-mu laser excited upconverted stimulated emission based on simultaneous three-photon absorption,” Opt. Lett. 28, 719–721(2003). [CrossRef] [PubMed]

6.

S. Sanders, R. G. Waarts, D. G. Mehuys, and D. F. Wetch, “Laser diode pumped 106mW blue upconversion fiber laser,” Appl. Phys. Lett. 67, 1815–1817 (1995). [CrossRef]

7.

S. Heer, K. Kömpe, H. U. Güdel, and M. Haase, “Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF4 Nanocrystals,” Adv. Mater. 16, 2102–2105 (2004). [CrossRef]

8.

C. Y. Cao, W. P. Qin, J. S. Zhang, Y. Wang, P. F. Zhu, G. D. Wei, G. F. Wang, R. J. Kin, and L. L. Wang, “Ultraviolet upconversion emissions of Gd3+,” Opt. Lett. 33, 857–859 (2008). [CrossRef] [PubMed]

9.

J. J. Owen, A. K. Cheetham, and R. A. Mcfarlane, “Orientation-dependent fluorescence studies and spectroscopic analysis of doped barium yttrium fluoride upconversion laser crystals (BaY2-x-yYbxTmyF8),” J. Opt. Soc. Am. B 15, 684–693 (1998). [CrossRef]

10.

M. P. Hehlen, A. Kuditcher, A. L. Lenef, H. Ni, Q. Shu, S. C. Rand, J. Rai, and S. Rai, “Nonradiative dynamics of avalanche upconversion in Tm:LiYF4,” Phys. Rev. B 61, 1116–1128 (2000). [CrossRef]

11.

W. T. Carnall, R. Fields, and K. T. Rajnank, “Electronic energy levels in the trivalent lanthanide aquo ions,” J. Chem. Phys. 49, 4424–4442 (1968). [CrossRef]

12.

X. B. Chen and Z. F. Song, “Study on six-photon and five-photon ultraviolet upconversion luminescence,” J. Opt. Soc. Am. B 24, 965–971 (2006) [CrossRef]

13.

G. S. Qin, W. P, Qin, C. F. Wu, S. H. Huang, J. S. Zhang, S. Z. Lu, D. Zhao, and H. Q. Liu, “Enhancement of ultraviolet upconversion in Yb3+ and Tm3+ codoped amorphous fluoride film prepared by pulsed laser deposition,” J. Appl. Phys. 99, 1–3 (2003).

14.

D. Q. Chen, Y. S. Wang, Y. L. Yu, and P. Huang, “Intense ultraviolet upconversion luminescence from Tm3+/Yb3+:β-YF3, nanocrystals embedded glass ceramic,” Appl. Phys. Lett. 91, 051920–051922 (2007). [CrossRef]

15.

M. A. Noginov, M. Curley, P. Venkateswarlu, A. Williams, and H. P. Jenssen, “Excitation scheme for the upper energy levels in a Tm:Yb:BaY2F8 laser crystal,” J. Opt. Soc. Am. B 14, 2126–2136 (1997). [CrossRef]

16.

T. Riedener, H. U. Güdel, G. C. Valley, and R. A. McFarlane, “Infrared to visible upconversion in Cs3Yb2Cl9:Tm3+,” J. Lumin. 63, 327–337 (1995). [CrossRef]

17.

M. Pollnau, D. R. Gamelin, S. R. Lüthi, and H. U. Güdel, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61, 3337–3346 (2000). [CrossRef]

18.

Y. G. Chen, Y. Liu, Z. G. Zhang, B. Aghahadi, G. Somesfalean, Q. Sun, and F. P. Wang, “Four-photon upconversion induced by infrared diode laser excitation in rare-earth-ion-doped Y2O3 nanocrystals,” Chem. Phys. Lett. 448, 127–131 (2007). [CrossRef]

19.

K. Krämer, D. Biner, G. Frei, H. Güdel, M. Hehlen, and S. Lüthi, “Hexagonal Sodium Yttrium Fluoride Based Green and Blue Emitting Upconversion Phosphors,” Chem. Mater. 16, 1244–1251 (2004). [CrossRef]

20.

X. Bai, H. W. Song, G. H. Pan, Y. Q. Lei, T. Wang, X. G. Ren, S. Z. Lu, B. Dong, Q. L. Dai, and L. B. Fan, “Size-Dependent Upconversion Luminescence in Er3+/Yb3+-Codoped Nanocrystalline Yttria: Saturation and Thermal Effects,” J. Phys. Chem. C 11113611–13617 (2007). [CrossRef]

21.

S. Sivakumar, J. M. van Veggel, and S. May, “Near-Infrared (NIR) to Red and Green Up-Conversion Emission from Silica Sol-Gel Thin Films Made with La0.45Yb0.50Er0.05F3 Nanoparticles, Hetero-Looping-Enhanced Energy Transfer (Hetero-LEET): A New Up-Conversion Process,” J. Am. Chem. Soc. 129620–625 (2007). [CrossRef] [PubMed]

OCIS Codes
(190.4180) Nonlinear optics : Multiphoton processes

ToC Category:
Nonlinear Optics

History
Original Manuscript: May 23, 2008
Revised Manuscript: June 30, 2008
Manuscript Accepted: July 2, 2008
Published: July 24, 2008

Citation
Guofeng Wang, Weiping Qin, Lili Wang, Guodong Wei, Peifen Zhu, and Ryongjin Kim, "Intense ultraviolet upconversion luminescence from hexagonal NaYF4:Yb3+/Tm3+ microcrystals," Opt. Express 16, 11907-11914 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-16-11907


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