Origin of near to middle infrared luminescence and energy transfer process of Er3+/Yb3+co-doped fluorotellurite glasses under different excitations

We report the near to middle infrared luminescence and energy transfer process of Er3+/Yb3+ co-doped fluorotellurite glasses under 980, 1550 and 800 nm excitations, respectively. Using a 980 nm laser diode pump, enhanced 1.5 and 2.7 μm emissions from Er3+:I13/2→4I15/2 and I11/2→4I13/2 transitions are observed, in which Yb3+ ions can increase pumping efficiency and be used as energy transfer donors. Meanwhile, Yb3+ can also be used as an acceptor and intensive upconversion luminescence of around 1000 nm is achieved from Er3+:I11/2→4I15/2 and Yb3+: F5/2→4F7/2 transitions using 1550 nm excitation. In addition, the luminescence properties and variation trendency by 800 nm excitation is similar to that using 1550 nm excitation. The optimum Er3+ and Yb3+ ion ratio is 1:1.5 and excess Yb3+ ions decrease energy transfer efficiency under the two pumpings. These results indicate that Er3+/Yb3+ co-doped fluorotellurite glasses are potential middle- infrared laser materials and may be used to increase the efficiency of the silicon solar cells.

O ver the past decades, Er 31 /Yb 31 co-doped materials have attracted interest because of its usefulness for near to middle infrared emissions [1][2][3][4] . Erbium ion is an ideal luminescent center for 1.5 and 2.7 mm emissions, which correspond to the 4 I 13/2 R 4 I 15/2 and 4 I 11/2 R 4 I 13/2 transitions respectively 5,6 . The Er 31 doped fiber amplifier is one of the most important devices used in the 1.5 mm wavelength optical communication window 7 , and 2.7 mm emission also concerns researchers because of its possible applications in medicine, sensing, military countermeasures, and in light detection and ranging [8][9][10] . The absorption band of the Er 31 : 4 I 15/2 R 4 I 11/2 transition around 980 nm characterize weak ground state absorption and the sensitization of the Er 31 ions using Yb 31 ions increase the pumping efficiency 3 , as shown in Fig. 1(left). The Er 31 /Yb 31 doped materials are used for effective energy transfer mechanisms for obtaining 1.5 and 2.7 mm emissions under 980 nm excitation, in which the Yb 31 ions are donors that transfer energy from the 2 F 5/2 level to the Er 31 : 4 I 11/2 level 11,12 . On the other hand, Yb 31 ions are acceptors when the Er 31 /Yb 31 doped materials are excited using a 1550 nm Laser Diode (LD) 2,13 , as shown in Fig. 1(middle). The ions on the 4 I 15/2 ground state absorb two photons to the 4 I 9/2 levels. Then the ions on the 4 I 9/2 level decay radiatively or nonradiatively to the 4 I 11/2 level and the 1 and 2.7 mm emissions occur. The intensive 1000 nm upconversion luminescence converted from 1550 nm IR light in the Er 31 /Yb 31 doped materials increase the efficiencies of Si solar cells because Si solar cells show highest efficiencies at 1000 nm wavelength, whereas only 60% of the visible light can be converted to electrons 14,15 . Meanwhile, the Er 31 ions can also be pumped directly to the 4 I 9/2 level under 800 nm excitation as shown in Fig. 1(right) and the luminescence properties should be similar to that under 1550 nm excitation theoretically. It is important to understand the luminous mechanism under different excitations for the Er 31 /Yb 31 co-doped glasses in order to obtain more luminous information.
As host material for near to middle infrared emissions, glass attracts much research and development interest due to its ease of fabrication and its use as diode-pumped high-power solid state laser hosts, sensors, and optical amplifiers [16][17][18][19] . At present, attention have mainly been paid to the 2.7 mm emission of Er 31 doped fluoride glasses 20 and 1 mm emission of Er 31 /Yb 31 doped crystals 13,21 . As well known, so far most of works about 2.7 mm emission materials have been done in fluoride (ZBLAN) glasses. The glass family is the most stable one among all fluoride systems reported so far. In the past decade, Er doped and Er/Pr co-doped ZBLAN flbers have been developed for obtaining higher power output. But, the T g of the ZBLAN is as low as 270uC which causes thermal effect. Additionally, because of the small value of DT, crystallization is an obstacle of fabricating high concentration ZBLAN fibers. These weaknesses limit the application of the ZBLAN in the future 22 . So it is important and challenging for researcher to find new mid-infrared materials. Fluorotellurite glass is a potential near to middle infrared laser material because it combines the advantages of fluoride and oxide glasses. Fluorotellurite glasses possess relatively low phonon energy among all the oxide glasses, a broad transmission window of 0.4,6 mm, and stable chemical and physical properties relative to fluoride glasses, such as easy fibering 23 . However, no works concern near to middle infrared emissions of Er 31 /Yb 31 doped fluorotellurite glass excited under different wavelengths.
A new kind of fluorotellurite glass was prepared using AlF 3 -based glass modified with TeO 2 . Our previous work has reported the good thermal stability, low phonon energy and wide high transmittance of the glass 24 . In this study, near to middle infrared emissions of Er 31 /Yb 31 doped glasses were measured under different excitations and the energy transfer processes between the two ions were determined. The optimum ratio of the two ions was chosen to obtain intensive 2.7 and 1 mm emissions. In addition, cross sections for the emissions and the energy transfer microparameters between the two ions were calculated.
The characteristic temperatures (temperature of glass transition T g and temperature of the onset of the crystallization peak T x ) of the samples were determined using a NetzschSTA449/C differential scanning calorimeter at the heating rate of 10 K/min. The densities and refractive indices of the samples were measured using the Archimedes method, with distilled water as the immersion liquid and the prism minimum deviation method respectively. Furthermore, the absorption spectra were recorded using a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer at the range of 300 nm to 2000 nm, and the emission spectra were measured using a Triax 320 type spectrometer (Jobin-Yvon Co., France). All the measurements were performed at room temperature. Fig. 2 shows the absorption spectra of the Er 31 -doped and Er 31 / 1.5Yb 31 co-doped samples at room temperature in the wavelength region of 400 nm to1600 nm. The introduction of 1.5 mol% Yb 31 greatly enhances the absorption coefficient at around 980 nm, resulting in the efficient absorption of the pump source at around 980 nm. The radiative transition parameters within the 4f n configuration of the Er 31 ions can be analyzed using the Judd-Ofelt (J-O) theory and can be accurately measured using absorption spectra 25,26 . The J-O parameters and radiative transition parameters (spontaneous transition probability A, branching ratio b, and calculated lifetime t) for the Er 31 : 4 I 11/2 R 4 I 13/2 transition of the present FE and FEY1.5 samples have been calculated, and are shown in Table 1. V 2 parameters indicate the amount of the covalent bond, and are strongly dependent on the local environment of the ion sites, whereas the V 6 parameter is related to the overlap integrals of the 4f and 5d orbits 27,28 . The higher V 2 of the codoped sample indicates a higher covalency and lower symmetry. The spectroscopic quality factor i.e. V 4 /V 6 is an important parameter to predict the stimulated emission in a laser active host 29 . The spectroscopy quality factor (1.31) in the co-doped sample is much larger than those reported in fluoride glasses 30 , indicating that the co-doped sample is a favorable optical material. The predicted spontaneous emission probability for the 2.7 mm emission is larger in the co-doped sample, which provides a better opportunity to obtain laser actions 31 . Figure 3 shows the emission spectra around 1.5 and 2.7 mm and the measured decay lifetimes of the 1.5 mm emission before and after Er 31 co-doped Yb 31 under 980 nm excitation. The intensities of the two emissions initially increase as Yb 31 ions increase, whereas the  The change of the decay lifetimes with the Yb 31 contents coincides with those of the emissions. The lifetime is an important factor for potential laser materials. The full width at half maximum (FWHM) 32 determines 1.5 mm laser materials. The larger bandwidth of this transition is suitable for tunable lasers that deliver relatively constant power over a wide wavelength range. The FWHM value in the EY1.5 glass in this study is about 55 nm, which is not only higher than those of silicate (34.8 nm) 33 and phosphate (46.0 nm) 33 but also higher than those of pure fluoride ZELAG (46 nm) 34 and tellurite glasses (53 nm) 35 . The emission cross section is an important parameter for 2.7 mm emission which can be calculated as 10,36,37

Results
where l is the wavelength. A rad is the spontaneous transition probability. I(l) is the emission spectrum, n and c are the refractive index and light speed in vacuum respectively. The emission cross section of the 4 I 11/2 R 4 I 13/2 transition of the EY1.5 sample is calculated to be 8.3 3 10 221 cm 2 , which is higher than those of Er 31 doped ZBLAN glass (5.4 3 10 221 cm 2 ) 29 , chalcohalide glass (6.6 3 10 221 cm 2 ) 29 , fluorophosphate glass (7 3 10 221 cm 2 ) 38 , and tellurite glass (6.1 3 10 221 cm 2 ) 39 . Figure 4 shows the emission spectra around 1 and 2.7 mm before and after Er 31 co-doped Yb 31 under 1550 nm excitation. The upcon-version luminescence bands centered at 980 nm is a two-photon process and originate from the Er 31 : 4 I 11/2 R 4 I 15/2 and Yb 31 : 4 F 5/2 R 4 F 7/2 transitions. After introducing Yb 31 , the line shapes of the emission from the co-doped samples significantly change and are similar to those recorded from other materials containing Tb 31 / Yb 31 and Pr 31 /Yb 31 40,41 , which indicates that the emission is probably due to the transition in the Yb 31 ions and the energy transfer process from the Er 31 : 4 I 11/2 to the Yb 31 : 2 F 5/2 level. The intensity of the emission is highest when the Er 31 and Yb 31 ratio is 151.15. The energy transfer process cannot be efficiently performed with excess Yb 31 ion content the content. The obvious 2.7 mm emission is observed in the Er 31 singly doped sample and it is hardly to be obtained in the co-doped samples which can be explained by the energy transfer from the Er 31 : 4 I 11/2 level to the Yb 31 : 2 F 5/2 level. Figure 5 shows the emission spectra around 1 and 2.7 mm before and after Er 31 co-doped Yb 31 under 800 nm excitation. The similar phenomenon has been observed as that under 1550 nm excitation. The 1 mm is enhanced significantly in the co-doped samples and the 2.7 mm emission is decreased with the increasing Yb 31 ions when the Yb 31 content is below 1.5 mol %, which demonstrate the Yb 31 ions accept the energy from the Er 31 ions and the 1 mm emission mainly comes from the Yb 31 : 4 F 5/2 R 4 F 7/2 transition. Figure 5 also shows the energy transfer process can proceed efficiently when the ratio of the Er 31 and Yb 31 ions is 151.5.

Discussions
As discussed above, the Yb 31 : 2 F 5/2 level can transfer energy to the Er 31 : 4 I 11/2 and the backward process can also occur. If energy of the emission transition of one RE 31 ion (called the donor) is equal or close to the energy of the absorption transition of the other RE 31 ion (called the acceptor). Energy transfer between rare earth ions can  where log I 0 = I is the absorptivity from absorption spectrum, l is the thickness of the glass and N is the ion density. The emission cross section can be obtained by using the McCumber equation 3 : where h is Planck's constant, K B is the Boltzmann constant, T is the temperature, E zl is the ground state manifold and the lowest stark level of the upper manifolds and Z u and Z l are partition functions of the lower and upper manifolds. Fig. 6 shows the absorption and emission cross sections of the Yb 31 and Er 31 ions. Figure (a) describes the energy transfer process from Yb 31 to Er 31 , which corresponds to the results under 980 nm excitation, and (b) describes the reverse process which corresponds to the results under 1550 and 800 nm excitations. The overlap between s a and s e is quite large, therefore, efficient energy transfer can be expected between the two ions.
Based on the obtained absorption cross section of the donor and the emission cross section of the acceptor, the probability rate of the energy transfer between the donor and the acceptor can be described as where H DA j j is the matrix element of the Hamiltonian perturbation between the initial and final states in the energy transfer process, and S N DA is the overlap integral between the m-phonon emission line shape of the donor ions (D) and the k-phonon emission line shape of the donor ions (A). In the case of weak electron-phonon coupling, S N DA can be approximated as where S DA (0, 0, E) represents the overlap integral between the zerophonon line shape of the donor emission ions and the absorption of the acceptor ions, and S 0 D , and S 0 A are the Huang-Rhys factors of the donor and acceptor ions, respectively. The probability rate of the energy transfer can be obtained using the following direct transfer equation: where C D-A is the energy transfer coefficient, and R is the distance of separation between the donor and acceptor, and the critical radius of the interaction can be obtained by the equation R 6 C~C D{A t D , and t D is the intracenter lifetime of the excited level of the donor. The expression for direct transfer (D-A) is expressed as: The microparameters of energy transfer from Yb 31 : 2 F 5/2 to the Er 31 : 4 I 11/2 and the reverse process are calculated using Eqs (4)- (7). The values are 2.06 3 10 239 and 2.12 3 10 239 cm 6 /s, respectively, and they both are independent of phonon in the quasiresonant process. These results show that the high energy transfer process efficiency between the two ions and the direction of the process are dependent on the excitations. The Er 31 /Yb 31 co-doped fluorotellurite glasses can be used to obtain 1.5 and 2.7 mm emissions under 980 nm excitation and 1000 nm upconversion luminescence under 1550 nm excitation.

Conclusion
In conclusion, Er 31