Upconversion spectra of Er/Tm codoped NaY(WO4)2 crystal excited by 974nm laser is investigated. Intensive 520nm/550nm and 800nm emissions are observed. Three-photon process at high pumping power is presented in the green upconversion. A model is proposed that the three-photon process is actualized by the energy indirect sensitization between two Er3+ ions and one Tm3+ ion, via excited state absorption and two cross relaxations. Two Er3+ ions totally absorb three 974nm photons, emitting one green photon, and Tm3+ ion plays the role of temporary energy storage in this process.
© 2006 Optical Society of America
Plenty of research has been done in the luminescence of the erbium doped materials. Usually two-photon phenomenon in green upconversion emission was observed, and in some cases the green emission showed a tendency of “saturation” at high pumping power by a 970~980nm band laser [1–4]. Three-photon phenomenon in 520 and 550nm band green upconversion luminescence has been reported in some erbium doped materials [5–7].
Some energy levels of Er3+ and Tm3+ are very close. For instance, the central wavelengths for 4I9/2 level of Er3+ and 3H4 level of Tm3+ are both around 800nm, and 4F9/2 level (656nm) of Er3+, 3F3 level (692nm) and 3F2 level (671nm) of Tm3+ are rather close, too. Thus the energy-transfer rate between Er3+ and Tm3+ ions increases when codoped . As a result, Er/Tm codoped materials own some interesting spectroscopic properties: several efficient energy transfer channels have been proposed according to the energy levels and testified by excitation and emission spectra [9–11]; photon avalanche upconversion was observed in Er/Tm codoped LiKYF5 crystals .
In this paper, we present the three-photon upconversion phenomenon in 550nm green luminescence of Er/Tm codoped NaY(WO4)2 (Er:Tm:NYW) crystal. Based on experimental spectroscopic properties and theoretical calculation, we develop a model that the three-photon process is accomplished by indirect sensitization between two Er3+ ions and one Tm3+ ion, via excited state absorption and two key cross relaxations.
Our Er:Tm:NYW crystal sample was grown by the Czochralski pulling method. The doped concentrations were 0.5% wt (Tm3+) and 1.5% wt (Er3+), respectively. The thickness of the crystal was 1.76mm. The index of refraction was 1.9. Both sides were well polished.
An adjustable (0–500mW) laser diode with a central wavelength of 974nm at room temperature was used to pump the sample. The laser was focused on the sample with a collimator. The upconversion emission spectrum of the sample, shown as Fig. 1, was measured using a Model F111AI Fluorometer.
3. Results and discussions
There was intensive green emission at 520nm/550nm band (Er3+: 2H11/2→4I15/2/4S3/2→4I15/2), and near-infrared emission at 800nm band (Tm3+: 3H4→3H6, Er3+: 4I9/2→4I15/2), but much weaker red emission at 656nm band (Er3+: 4F9/2→4I15/2). The emission peaks enhanced as pumping power increased, and the log-log dependency of 550nm green emission intensity on pumping power is shown in Fig. 2. At low pumping power (<200mW), the slope of the plots in Fig. 2 is 1.9, indicating a two-photon process. However, when pumping power is over 200mW, the slope is about 2.4, implying participation by some kind of three-photon process.
Energy level diagram and transition mechanism of Er3+ and Tm3+ are presented in Fig. 3.
Besides the 4I9/2→4I15/2 emission of Er3+, the energy transfer from Er3+ ions to Tm3+ ions contributes much to the 800nm band: First, Er3+ ions at ground state are excited to 4I11/2 level after absorbing a 974nm photon, and then a large part of them fall onto 4I13/2 level by non-radiative relaxation, while the rest climb to 4F9/2 level through excited state absorption, corresponding to process I in Fig. 3. Second, because the energy levels 4F9/2 (Er3+), 3F3(Tm3+) and 3F2 (Tm3+) are rather close, with the maximal phonon energy of this material12 about 840cm-1, which leads to efficient photon assistance, energy can be easily transferred from Er3+ ions on 4F9/2 level to Tm3+ ions on 3H6 level [9–11]. This is a cross relaxation process, that is, 4F9/2(Er3+) +3H6(Tm3+)→4I15/2 (Er3+)+3F3, 2(Tm3+), corresponding to process II in Fig. 3. Finally, the Tm3+ ions on 3F3, 2 levels relax to 3H4 level non-radiatively, and then fall back to ground state resulting in 800nm band luminescence. Population of Er3+ ions on 4F9/2 level decreased via this process, thus much weaker red emission at 656nm band was detected.
At low pumping power, the green emission is a two-photon process, presented as process III in the right part of Fig. 3. After consecutive absorption of two 974nm photons, the Er3+ ions are excited to 4F7/2 level, and then rapidly relax to 2H11/2 and 4S3/2 level, due to the small energy gap between these levels. In the end, the Er3+ ions fall back to 4I15/2 level resulting in 520nm and 550nm emission bands. This kind of two-photon process is so common that it has been widely reported, as we mentioned in starting section.
At high pumping power, the 550nm green emission is participated by large part of three-photon process. The mechanism is analyzed as follows.
The energy gap between levels 4I13/2 and 4S3/2 of Er3+ (about 11900cm-1) and the energy gap between levels 3H4 and 3H6 of Tm3+ (about 12500cm-1) are very close, meeting the energy-matching condition of cross relaxation. With increasing pumping power, ground state absorption for Er3+ ions is enhanced, and more Er3+ ions are pumped to 4I11/2 level. Due to the very short lifetime of 4I11/2 level, most Er3+ ions will fall onto 4I13/2 level, thus raising the population on this level. Additionally, the increasing pumping power leads to stronger emission in 800nm band, indicating rising population in 3H4 level of Tm3+.
From the above, we propose that there exists a strong cross relaxation between Er3+ and Tm3+ ions at high exciting power, that is, 4I13/2(Er3+)+3H4(Tm3+)→4S3/2(Er3+)+3H6(Tm3+), corresponding to process IV in Fig. 3. Taking process I, II, III and IV as a whole, one Er3+ ion on ground state is excited to level 4F9/2 after absorbing two pumping photons step by step, and then it transfers this energy to a nearby Tm3+ ion, and next the Tm3+ ion transfers this energy to another Er3+ ion on 4I13/2 level which has already absorbed one photon in process III. The latter Er3+ ion is excited to 4S3/2 level and finally results in 550nm green emission. In this way, two Er3+ ions (noted as E, E in Fig. 3) totally absorb three 974nm photons to emit one green photon, and the Tm3+ ion provides temporary energy storage in the middle. This is a three-photon indirect sensitization process. Similar two-photon indirect sensitization in Tm/Yb codoped material has been reported before .
In previous experiment of Er3+ single-doped NaY(WO4)2 (Er:NYW) crystal, there was no three-photon phenomenon observed, partly because the probability for exciting an Er3+ ion to 4S3/2 level via three-step consecutive absorption is generally tiny. In this sample the codoping of Tm3+ ions splits the absorption of three photons into two parts, two photons as one part and the rest one as another, hence increasing the efficiency of energy absorption.
However, judging from Fig. 3, there exist several other possible level pairs which also meet the energy-matching condition so that cross relaxations may occur between them. To investigate this problem further, here we list the six possible relaxations C1~C6 into Table I, including the two (C1 and C2) mentioned in the above three-photon-relay process. All the mismatching between the energy gaps of these selected level pairs are less than 840cm-1, the maximum phonon energy, so the cross relaxations can be treated as resonant and the energytransfer rate between these level pairs can be calculated by Dexter’s theory , as formula (1) shows:
Here, n is the index of refraction; gi, gk are the degeneracies of initial energy levels; R is the minimal distance between energy donors and accepters, and obviously 1/R3 is proportional to the relative concentration (in atomic percentage) of the doped ions; S0=∫g(ν)h(ν)dν is the integral of the line shape overlap between the emission transitions i→j and absorption transitions k→e, and is approximately to be S0≈1/(πΔν) when the line shapes are assumed to be Lorentzian; Δν is the larger bandwidth at FWHM between the absorption and emission transitions. Ωλ are intensity parameters which can be fitted using Judd-Ofelt’s theory [14, 15] according to the sample’s absorption spectra; |〈Si , Li , Ji ,|U (λ)‖Sj , Lj , Jj 〉|2 and |〈Sk , Lk , Jk ‖U (λ)‖Se , Le , Je 〉|2 are reduced matrix elements for the transitions, which can be obtained in documents .
In a previous work, we have measured the absorption and excitation spectra for this sample and fitted the values of Ωλ for Er3+ and Tm3+ ions .
Er3+: Ω2=11.5×10-20 cm2, Ω4=0.547×10-20 cm2, Ω6=1.49×10-20 cm2;
Tm3+: Ω2=7.91×10-20 cm2, Ω4=0.631×10-20 cm2, Ω6=1.16×10-20 cm2.
The results of our calculation for energy-transfer rates of these six cross relaxations are listed in Table 1.
Obviously the energy-transfer rate for C1 and C2 are much larger. Considering their initial levels, 3H6 (Tm3+) and 4I13/2 (Er3+), they are either ground state or the first excited state, which generally own the largest populations of all levels. Consequently the two cross relaxations are quite strong, providing substantial support for previous analysis. In addition, we note that C6, the cross relaxation between Er3+ ions, is much weaker compared to C2, so the role of Tm3+ ions as temporary storage could not be replaced by other Er3+ ions.
We have presented the three-photon phenomenon in 550nm green upconversion luminescence of Er:Tm:NYW crystal. Based on experimental spectroscopic properties and theoretical calculation, we developed an indirect sensitization model. One Tm3+ and Two Er3+ ions absorbed three pumping photons together, emitting one 550nm photon finally. In this mechanism, Tm3+ ions act as temporary energy storage in the middle.
This work is supported by the Program for New Century Excellent Talents, the Excellent Young Teachers Program and the Scientific Research Foundation for the Returned Overseas Chinese Scholars sponsored by Ministry of Education, P.R.C, the Natural Science Foundation of Tianjin City, and the research funding for the undergraduate students of Nankai University. We also thank Nankai University for the scientific starting funding. We are grateful to A. Sasha Burdick for reviewing the whole manuscript and checking the grammar.
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