JOURNAL OF APPLIED PHYSICS
VOLUME 89, NUMBER 5
1 MARCH 2001
Cooperative energy transfer in Yb3¿ –Tb3¿ codoped silica sol-gel glasses I. R. Martı´n Departamento de Fı´sica Fundamental y Experimental, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
A. C. Yanes Departamento de Fı´sica Ba´sica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
J. Me´ndez-Ramos Departamento de Fı´sica Fundamental y Experimental, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
M. E. Torres Departamento de Fı´sica Ba´sica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
V. D. Rodrı´gueza) Departamento de Fı´sica Fundamental y Experimental, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
共Received 19 September 2000; accepted for publication 21 November 2000兲 Optical properties of Yb3⫹ –Tb3⫹ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 m. The Tb3⫹ ions are excited by means of energy transfer processes from Yb3⫹ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3⫹ ions and their dependence on the excitation intensity at about 1 m has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3⫹ ions. An efficient backtransfer process is observed from Tb3⫹ ions towards Yb3⫹ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1344216兴
I. INTRODUCTION
the dopant concentration, so it is possible to obtain rare earth doped luminescent glasses with a wide range of compositions. Studies in rare earth doped sol-gel samples have shown that these ions have tendency to be segregated or form clusters rather than to be uniformly distributed through the glass.5,6 The clusters are normally undesirable because decrease the efficiency radiative by energy transfer processes. However, these transfer processes can strongly increase the upconversion efficiency. It would also be taken into account that the high-energy vibrations of OH groups in sol-gel materials reduce the efficiency of the luminescent ions. Due to this, most of the works have been devoted to rare earth ions with high energy gaps under the emitting levels, as Eu3⫹ and Tb3⫹. 7–9 So, yellow-orange upconverison emission has been observed in Yb3⫹ – Eu3⫹ codoped sol-gel silica glass by a cooperative nonresonant energy transfer process.10 But the Tb3⫹ ions are more interesting candidates to be excited by a cooperative process from Yb3⫹ ions. In this case the energy transfer is resonant and, moreover, the main upconversion emissions from the Tb3⫹ are at the blue and green spectral region, i.e., in shorter wavelengths than the Eu3⫹. On the other hand, blue upconversion emission has also been obtained in sol-gel silica glass doped only with Yb3⫹ ions due to cooperative processes and its potential application in three-dimensional displays has been considered.11
The upconversion processes have received great attention because of the possibility to obtain by this means solid state blue lasers pumped by near infrared diodes. Several mechanisms can produce efficient conversion from infrared to visible radiation in crystalline or glass matrix doped with rare earth ions. However, the most extensive work on upconversion has been done in Yb3⫹ sensitized rare earth systems that can basically present two mechanisms.1 The most efficient one involves successive transfer from Yb3⫹ to acceptor ions. But, if there are no intermediate levels of the activator that can receive a single transfer from a donor, a second mechanism denominated cooperative energy transfer becomes important. In this process two excited Yb3⫹ ions simultaneously transfer their energy to an activator ion that goes up to an excited level. The Tb3⫹ ions can be excited by this cooperative process from Yb3⫹ ions, so in different Yb3⫹ – Tb3⫹ codoped glasses and crystals it has been possible to obtain visible upconversion emission after excitation at 1 m.2–4 On the other hand, the sol-gel technique is a method to prepare oxide-based materials with improved properties for optoelectronic applications. Using this process oxide glasses can be prepared at lower temperatures respect to conventional melt methods. Moreover, this sol-gel method extends a兲
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© 2001 American Institute of Physics
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Martin et al.
J. Appl. Phys., Vol. 89, No. 5, 1 March 2001
In this work, upconversion processes are reported and analyzed in Yb3⫹ – Tb3⫹ codoped sol-gel silica samples after the gel to glass transition and under excitation at about 1 m.
TABLE I. Values for coefficients in Eq. 共1兲 for different interactions.
II. EXPERIMENT
The silica glasses were prepared by a sol-gel process. Single doped samples were doped with 4 mol % of Tb3⫹ or 4 mol % of Yb3⫹ and codoped samples were prepared with 0.5 mol % Tb3⫹ and 4 mol % of Yb3⫹ 共mol % referred to SiO2兲. Preparation involved primary mixing of proper amounts Tb共NO3兲3•5H2O and/or Yb共NO3兲3•5H2O 共Aldrich, 99.9%兲 in 89.66 mmol ethanol 共Merck pro analysis兲 with stirring for 30 min, 22.42 mmol tetraethyl ortho silicate 共TEOS, Aldrich, 99.999%兲, 89.66 mmol water 共Sigma-Aldrich, HPLC grade兲 and HCl, as a catalyst to adjust the pH value of the mixed solutions to about 2.0, was then added to the solutions. After 2 h of vigorous stirring, the resulting clear solutions were placed in sealed containers at 40 °C. These solutions converted to wet gels after 4–5 days. The wet gels were further allowed to remain at ambient temperature in containers having a perforation in the top. Monolithic transparent samples in the form of glassy plates of thickness about 2 mm and area about 150 mm2, were finally obtained after 60 days aging at room temperature. The dried gels were heated in air at 3 °C/h up to 800 °C and then maintained for 24 h in that temperature, to achieve the gel to glass conversion, and subsequently quenched to room temperature by removing them from the furnace to open air. All the spectroscopic measurements were done at room temperature and using the experimental equipment described in this section. The emission spectra were obtained by exciting the sample with light from a 300 W Xe arc lamp passed through 0.25 Spex 1681 monochromator. Fluorescence was detected through a 0.25 Spex 1680 double monochromator with a Hamamatsu photomultiplier either the R928 model for the visible or a R406 model for the near infrared. Spectra were corrected for instrument response. Emission decays from the 5 D 4 level of Tb3⫹ ions were obtained exciting with a Spex flash lamp. Upconversion spectra were obtained by exciting the samples with a pulsed Nd:yttrium–aluminum– garnet laser at 1064 nm or a diode laser at 980 nm. The temporal evolution of the emissions was recorded using a Tektronix 2432 digital storage oscilloscope controlled by a personal computer.
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S
a1
a2
b1
6 8 10
10.866 17.072 24.524
15.500 35.860 67.909
8.743 13.882 20.290
the following generalized expression of the Yokota and Tanimoto model13 for any kind of interaction 共S⫽6, 8, or 10兲 can be used14
冋
冉 冊 冊 册
t 4 3 I 共 t 兲 ⫽I 共 0 兲 exp ⫺ ⫺ C ⌫ 1⫺ 3 A S S 兲 3/S ⫻共 C 共DA t兲
冉
1⫹a 1 X⫹a 2 X 2 1⫹b 1 X
S⫺3/S⫺2
共1兲
with S 兲 ⫺2/S 1⫺2/S t , X⫽D 关 C 共DA 兴
共2兲
where is the intrinsic lifetime of the luminescent ions, S ⫽6,8,10,... depending on the interaction character 共dipole– dipole, dipole–quadrupole, quadrupole–quadrupole,...兲, C A is the acceptor concentration, ⌫(y) is the gamma function, (S) C DA is the donor-acceptor energy transfer parameter, and D is the diffusion parameter which characterizes the transfer between donors. The values for the a i and b i coefficients are presented in Table I. B. Upconversion processes
The cooperative energy transfer process among two donor ions (Yb3⫹) and an acceptor ion (Tb3⫹) is showed schematically in Fig. 1. After excitation of two donor ions, these can simultaneously transfer their energy to an acceptor ion, which goes up to an excited level. With this mechanism, the upconversion emission intensity from the A 2 level needs two photons and then increases with the square of the excitation intensity.
III. THEORETICAL INTRODUCTION A. Energy transfer processes
When the interaction between luminescent ions is not important, the decay of the luminescence can be fitted to a single exponential. However, when the ions concentration is large enough, energy transfer appears and the decay curves become nonexponential. If we consider multipole interaction between donors 共initially excited ions兲 and acceptors 共traps for the energy兲, then the expression derived by Inokuti and Hirayama12 can describe the decay curves. But if we also consider energy transfer among donors 共energy migration兲,
FIG. 1. Energy level diagram of Yb3⫹ and Tb3⫹ ions. The cooperative energy transfer process among two donors (Yb3⫹ ions) and an acceptor (Tb3⫹ ion) is indicated.
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J. Appl. Phys., Vol. 89, No. 5, 1 March 2001
If rapid migration among donors is assumed a rate equations model can describe the dynamics of the upconversion processes produced by cooperative transfer. The rate equations become 1 dY 2 ⫽ Y 1 ⫺ Y 2 ⫺WY 22 A 1 , dt D
共3兲
1 dA 2 ⫽⫺ A 2 ⫹WY 22 A 1 , dt A
共4兲
where Y i and A i correspond to the populations of the ith level of donors and acceptors, respectively, is the absorption cross section of donor ions, is the incident pumping flux, W is the cooperative energy transfer rate and D and A are the lifetimes of donors and acceptors, respectively. Neglecting the ground state depopulation and the transfer term in Eq. 共3兲 upon intrinsic processes, it is obtained for the steady state 2 W 共 兲 2 A 0 Y 20 , A 2⫽ A D
FIG. 2. Emission spectrum obtained at room temperature by exciting at 360 nm a sample codoped with 0.5 mol % of Tb3⫹ and 4 mol % of Yb3⫹ and with thermal treatment at 800 °C.
共5兲
where Y 0 and A 0 correspond to the donor and acceptor concentrations, respectively. It is interesting to emphasise in Eq. 共5兲 the expected quadratic dependence on the pump intensity. The temporal evolution of the upconversion emission after pulsed excitation at t⫽0 can be obtained solving the Eqs. 共3兲 and 共4兲. The solution is given by WA 0 Y 2 共 0 兲 2 A 2共 t 兲 ⫽ 关 exp共 ⫺2t/ D 兲 ⫺exp共 ⫺t/ A 兲兴 , 共6兲 1 2 ⫺ A D where Y 2 (0) is the initial population of excited donor ions. It is remarkable in Eq. 共6兲 that the shape of the transient A 2 (t) corresponds to a curve with a rise time D /2 and a decay time A .
the heat treatment, the dependence is showed in Fig. 3 for single Tb3⫹ doped samples. The results are similar to those by Guodong et al.,9 they found the fluorescence intensity of Tb3⫹ ions to increase with the heat treatment temperature, although with a slower increasing over 500 °C. The increasing of the lifetime observed after heating to 800 °C suggests that the gel to glass transition continues at this temperature. However, the obtained lifetime of 2.4 ms is close to the value about 3 ms obtained in silicate glasses.15 Therefore, samples heated to 800 °C will be studied in the following, moreover this can be considered a normal treatment in the works about sol-gel silica glasses. For Yb3⫹ ions a very short lifetime of 5 s is obtained after heating at 800 °C, whereas a typical value in silicate glasses would be about 1 ms.16 This parameter is strongly shortened by the nonradiative decays in the sol-gel glasses.
IV. RESULTS AND DISCUSSION A. Emission measurements and gel to glass transition
Silica sol-gel samples codoped with Tb3⫹ and Yb3⫹ ions show emission from these ions by excitation of Tb3⫹ ions to the 5 D 4 level or higher ones. The emission spectrum obtained with a sample codoped with 0.5 mol % of Tb3⫹ and 4 mol % of Yb3⫹ excited at 360 nm 共over the 5 D 3 level兲 is showed in Fig. 2. The sample had been heat treated at 800 °C for several hours and the spectrum was obtained at room temperature. Visible emission of the Tb3⫹ ions from the 5 D 4 level is observed together with a weak ultraviolet-visible emission 共400–450 nm兲 from the 5 D 3 level. The near infrared emission at about 1 m is due to the Yb3⫹ ions. These results indicate that after excitation a fast nonradiative decay to the 5 D 4 level is happening. On the other hand, the Yb3⫹ ions emission is caused by excitation of these ions by means of nonresonant energy transfer from the Tb3⫹ ions. These processes will be analyzed in the next section. The quantum efficiency and the lifetime of the emissions depend on the nonradiative decay rates and can be used to monitor the gel to glass transition. In this way, the lifetime of the 5 D 4 level of Tb3⫹ ions increases with the temperature of
B. Energy transfer from Tb3¿ to Yb3¿ ions
The high phonon energy in these samples allows an efficient nonresonant energy transfer from Tb3⫹ to Yb3⫹ ions
FIG. 3. Dependence of the 5 D 4 level lifetime of the Tb3⫹ ions in a sample doped with 4 mol % of these ions as function of the thermal treatment temperature.
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Martin et al.
J. Appl. Phys., Vol. 89, No. 5, 1 March 2001
FIG. 4. Decays of Tb3⫹ emission at 540 nm obtained at room temperature after excitation at 360 nm 共a兲 or 1064 nm 共b兲 a sample codoped with 0.5 mol % of Tb3⫹ and 4 mol % of Yb3⫹. The solid lines correspond to the fits to Eq. 共1兲. The inset shows in logarithmic scale the fits of the decay curve 共b兲.
in spite of the separation between the Tb3⫹ emission 共see Fig. 2兲 and the Yb3⫹ absorption at about 1 m. In this cross relaxation process an excited Tb3⫹ ion decays from the 5 D 4 level to any 7 F J level meanwhile an Yb3⫹ ion is excited to the 2 F 5/2 level. In samples single doped with Tb3⫹ ions 共0.5 or 4 mol %兲 the decay curves from the 5 D 4 level are exponential. But, in the Tb3⫹ – Yb3⫹ codoped samples, the decays are faster and nonexponential, see Fig. 4, due to this energy transfer Tb3⫹ →Yb3⫹. The quantum yield of this cross relaxation mechanism can be calculated by
⫽1⫺
兰 ⬁0 I 共 t 兲 dt
I共 0 兲
,
共7兲
where I(t) is the fluorescence intensity from the 5 D 4 level and is the lifetime of this level. A value of 0.91 is obtained for the sample codoped with 0.5 mol % of Tb3⫹ and 4 mol % of Yb3⫹ which indicates a very effective transfer towards Yb3⫹ ions. The Tb3⫹ decay curves have been fitted to the generalization of the Yokota–Tanimoto model, Eq. 共1兲 in the text. In this generalized model energy migration between donors is considered with whatever multipole interaction among donors and acceptors. The best fits are obtained for dipole– dipole interaction among Tb3⫹ and Yb3⫹ ions with an energy (6) ⫽2.0⫻10⫺40 cm6 s⫺1 transfer donor-acceptor parameter C DA ⫺13 and a diffusion parameter D⫽8.8⫻10 cm2 s⫺1.
C. Upconversion
The Yb3⫹ ion has a simple level diagram 共see Fig. 1兲 with only one excited state 2 F 5/2 , located at about 10 000 cm⫺1 above the ground state 2 F 7/2 , so it does not present visible fluorescence. However, different upconversion visible emissions are obtained by exciting at about 1 m (Yb3⫹ : 2 F 7/2→ 2 F 5/2) a silica sol-gel glass doped with 0.5
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FIG. 5. Upconversion emission spectrum obtained at room temperature by exciting at 1064 nm a silica sol-gel glass doped with 0.5 mol % of Tb3⫹ and 4 mol % of Yb3⫹ ions. All transitions come from the 5 D 4 level of the Tb3⫹ ions.
mol % of Tb3⫹ and 4 mol % of Yb3⫹ 共Fig. 5兲. These emissions can be clearly identified with transitions coming from the 5 D 4 level of the Tb3⫹ ions 共see Fig. 2兲. It would be remarked that emission from the 5 D 3 level is not observed, indicating that the excitation by transfer from Yb3⫹ just allows go up to the 5 D 4 level. In order to analyze the upconversion mechanism that produces these emissions from the Tb3⫹ ions, the dependence of the emission intensity on the excitation intensity at 1064 nm has been measured. From a log–log representation of the experimental data, a slope of 2.2 is obtained, close to a quadratic dependence. This result indicates that two photons are needed in the upconversion process, which is in agreement with a cooperative energy transfer mechanism. Additional information about this upconversion mechanism can be obtained analysing the temporal evolution of the upconversion emissions after pulsed excitation of the Yb3⫹ ions. The decay of the Tb3⫹ emission at 540 nm after Yb3⫹ excitation is compared in Fig. 4 with the decay after Tb3⫹ excitation at 360 nm, measurements were made at room temperature in a silica sol-gel glass codoped with Tb3⫹ and Yb3⫹ 共0.5 and 4 mol %, respectively兲. The temporal evolution of the Tb3⫹ upconversion emission after excitation of Yb3⫹ ions presents a rise and a decay, as predicted by Eq. 共6兲. The rise is very fast due to the short lifetime of the Yb3⫹ ions 共about 5 s兲. With respect to the decay, the experimental results cannot be fitted to Eq. 共6兲 because of the energy backtransfer processes from Tb3⫹ to Yb3⫹ ions, which are not taken into account in this expression. In these conditions, the Tb3⫹ emission decay can be analyzed assuming a pulsed excitation of these ions and using Eq. 共1兲. Moreover, in Fig. 4 it is outstanding that a faster decay is observed when the Tb3⫹ ions are excited by transfer from the Yb3⫹ ions than when they are directly excited at 360 nm. This behavior is consequence of the ‘‘correlation effects’’ 共in similar way to the transfer and backtransfer processes among Yb3⫹ and Er3⫹ ions observed in fluoroindate glasses兲,17 the Tb3⫹ ions directly excited by the laser are randomly distributed, meanwhile the ions excited by transfer from the Yb3⫹
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Martin et al.
J. Appl. Phys., Vol. 89, No. 5, 1 March 2001
ions are predominantly close to these and then they have a high backtransfer probability. As a consequence, a high value of 0.97 is obtained for the quantum yield of this process using the Eq. 共7兲, which is quite larger than the value calculated in the Sec. IV B. The parameters for the backtransfer can be calculated from the fit of the upconversion emission decay to Eq. 共1兲. The best fit is obtained for S⫽10 with an energy transfer parameter from Tb3⫹ to Yb3⫹ ions C AD⫽3.2 ⫻10⫺67 cm10 s⫺1 and a diffusion parameter between Tb3⫹ ions D⫽2.5⫻10⫺13 cm2 s⫺1. The parameter C AD is overestimated due to the correlation effect. Moreover, the high value for S would also be due to this effect, the short distance between ions in the Tb–Yb couples involved favors high order multipole interaction mechanism. Whereas, a dipole– dipole interaction character was obtained in Sec. IV B for the Tb3⫹ →Yb3⫹ transfer when the Tb3⫹ ions were excited directly by the flash lamp. The low value obtained for the diffusion parameter is in reasonably good agreement with the result presented in Sec. IV B. In order to compare the efficiency of the cooperative upconversion in Yb3⫹ – Tb3⫹ codoped silica sol-gel samples with results presented in other works,10,11 the quantum efficiency has been measured using a diode laser at 980 nm as excitation source focused in the sample. This efficiency is defined as the ratio between the emitted and the absorbed power. So, a value of 6.4⫻10⫺6 has been obtained for the efficiency of the emission 5 D 4 → 7 F 5 共540 nm兲 using an excitation power of 200 mW 共about 77 mW are absorbed by the sample兲. In similar sol-gel samples but doped only with Yb3⫹ ions a cooperative blue upconversion is observed with a lower efficiency of 2.8⫻10⫺7 共calculated for 77 mW of absorbed power from data in Ref. 11兲. Moreover, other upconversion emissions have been observed in Yb3⫹ – Eu3⫹ codoped sol-gel samples,10 but the upconversion efficiency of the emissions observed in the yellow-orange range coming from Eu3⫹ ions is even lower than in the case of the blue cooperative upconversion due to Yb3⫹ ions. V. CONCLUSIONS
Visible upconversion emissions, mainly blue and green have been observed in silica sol-gel glass codoped with Tb3⫹ and Yb3⫹ ions upon Yb3⫹ ions near infrared excitation at
about 1 m. The intense visible emissions, corresponding to the Tb3⫹ ions transitions 5 D 4 → 7 F J , are explained considering cooperative resonant energy transfer from two excited Yb3⫹ ions to a Tb3⫹ ion. This mechanism is supported by the upconversion emission dependence on the excitation intensity, which indicates that the process involves two photons. Moreover, the temporal evolution of the upconversion emission, after pulsed excitation, is in good agreement with a theoretical model considering a fast cooperative resonant energy transfer process with high backtransfer probability, with a high quantum yield for this process of 0.97, which limits the upconversion efficiency. In spite of this, the quantum efficiency of these upconversion emissions results to be higher than in single Yb3⫹ doped and Yb3⫹ – Eu3⫹ codoped sol-gel silica glass. ACKNOWLEDGMENTS
This work was partially supported by ‘‘Gobierno Auto´nomo de Canarias 共PI1999/100兲’’ and ‘‘Comisio´n Interministerial de Ciencia y Technologı´a 共PB98-0437兲.’’ J. C. Wright, Top. Appl. Phys. 15, 239 共1976兲. F. W. Ostermayer and L. G. Van Uitert, Phys. Rev. B 11, 4208 共1970兲. 3 R. S. Brown, W. S. Brocklesbey, W. L. Barnes, and J. E. Townsend, J. Lumin. 63, 1 共1995兲. 4 E. Martins, Cid B. de Arau´jo, J. R. Delben, A. S. L. Gomes, B. J. Da Costa, and Y. Messaddeq, Opt. Commun. 158, 61 共1998兲. 5 R. Campostrini, G. Carturan, M. Ferrari, M. Montagna, and O. Pilla, J. Mater. Res. 7, 745 共1992兲. 6 I. R. Martı´n, V. Lavı´n, J. Me´ndez-Ramos, F. Delgado, U. R. Rodrı´guezMendoza, V. D. Rodrı´guez, and A. C. Yanes, J. Alloys Compd. 共to be published兲. 7 M. Nogami and Y. Abe, J. Non-Cryst. Solids 197, 73 共1996兲. 8 M. Nogami and Y. Abe, Appl. Phys. Lett. 71, 3465 共1997兲. 9 Q. Guodong, W. Minquan, W. Mang, F. Xianping, and H. Zhanglian, J. Lumin. 75, 63 共1997兲. 10 G. S. Maciel, A. Biswas, and P. N. Prasad, Opt. Commun. 178, 65 共2000兲. 11 G. S. Maciel, A. Biswas, R. Kapoor, and P. N. Prasad, Appl. Phys. Lett. 76, 1978 共2000兲. 12 M. Inokuti and F. Hirayama, J. Chem. Phys. 43, 1978 共1965兲. 13 M. Yokota and O. Tanimoto, J. Phys. Soc. Jpn. 22, 779 共1967兲. 14 I. R. Martı´n, V. D. Rodrı´guez, U. R. Rodrı´guez-Mendoza, V. Lavı´n, E. Montoya, and D. Jaque, J. Chem. Phys. 111, 1191 共1999兲. 15 T. Hayakawa, N. Kamata, and K. Yamada, J. Lumin. 68, 179 共1996兲. 16 J. A. Paisner, S. S. Sussman, W. M. Yen, and M. J. Weber, Bull. Am. Phys. Soc. 20, 447 共1975兲. 17 I. R. Martı´n, V. D. Rodrı´guez, V. Lavı´n, and U. R. Rodrı´guez-Mendoza, J. Appl. Phys. 86, 935 共1999兲. 1 2
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