Spectroscopic properties of fluorophosphate glass (779683), страница 2
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As is shownin Fig. 5, the graph of emission overlaps that of absorptionpartly. When measuring the lifetime, after one Er3+ ion in4I13/2 returns to the ground state by emitting one photon, itabsorbs a 1.53 µm photon emitted by other Er3+ ion and isexcited to 4I13/2 state again. This process takes place repeatedly, and results in that the measured lifetime is higher than87it should be.
Radiative trapping is linear proportional to theoptical length, the concentration of Er3+ ions and the absorption cross section. For Yb3+ the absorption spectrum overlapsthe emission spectrum just like Er3+ except that the wave bandis different. Dai et al. investigated the radiative trapping ofYb3+ , and found that for Yb3+ doped glass with concentration of 14.47 × 1020 ions/cm3, under 980 nm excitation, whenthe thickness of the sample is 0.5 mm and the absorption crosssection is close to Er3+ ion in this glass, the measured lifetimewill be higher than the virtual about 20% [20]. So it can beconcluded that the influence of radiative trapping on the measured lifetime of Er3+ : 4I13/2 → 4I15/2 transition in this glass isno more than 20%, and the virtual lifetime of 1.53 µm fluorescence is no less than 7.36 ms.The fluorescence spectrum is shown in Fig.
7. The fullwidth at half maximum (FWHM) of it is 59 nm. The highFWHM is partly due to the radiative trapping. However, theThe exponential decay curve of Er3+ ions inthe fluorophosphate glass under 970 nm excitationFIGURE 6FIGURE 7 Fluorescence spectrum of the fluorophosphateglass in the range of 1425–1675 nm88Applied Physics B – Lasers and Opticsinfluence of radiative trapping could not be high, because thefluorescence exhibits a sharp peak, and shows no obviouslyred shift. The FWHM of the graph of stimulated-emissioncross section in Fig.
5 is 56 nm. Because the graph in Fig. 5 isbased on absorption spectrum and is not influenced by radiative trapping, the virtual FWHM should be about 56 nm, andis still higher than for most of the materials investigated [21–23]. The wide FWHM can be explained by the fact that theEr3+ ions can be in or out of the network of fluorophosphateglass structure [24, 25], and the surrounding environment ofEr3+ ions varies severely.
At the same time, the mixed anion coordination including F− and O2− contributes to thelinewidth as well [26]. Additionally, the composition containseight different cations except for Er3+ . These cations as subordinate ordination of Er3+ have different effects on Er3+ ionsand increase the Stark splits [27].The lifetime of 4I13/2 → 4I15/2 transition, is comparatively high for Er3+ doped glasses under such a high concentration. Generally there are two processes which lead toconcentration quenching in Er3+ doped glasses. One is cooperative upconversion, namely summation of photon energiesthrough energy transfer [28]. It is a process which directly results in the loss of at least one excited ion between two excitedEr3+ ions.
The process occurs between two neighbouring ionsin the 4I13/2 excited state. The energy from one of the ions istransferred to the other, leaving one ion in 4I15/2 ground stateand the other in 4I9/2 excited state. It also occurs between twoions that one is in 4I13/2 state and the other is in 4I11/2 state,or both ions are in 4I11/2 excited state. The efficiency of upconversion depends on matrix glass.
First, the matrix glassdetermines the efficiency of interaction of Er3+ ions. Second, the phonon energy of matrix glass influences heavily thepopulation of high energy levels by multiphonon relaxation,especially for the excited state whose energy is only one ortwo phonon higher than the lower state. The other quenchingprocess is the non-radiative relaxation due to water. OH vibrational frequencies typically occur in the range of 2.8 – 3.5 µm,which is much higher than other vibrational frequencies inthe glass.
Only two or three phonons concerning OH are usually required for non-radiative deexcitation of most Er3+ ionsin 4I13/2 excited state. Under high Er3+ concentration, thelifetime of 4I13/2 → 4I15/2 transition is very sensitive to thewater content, because under high concentration, the energymigration between Er3+ ions increases, and consequently theprobability of energy quenched by OH group increases.For the fluorophosphate glass prepared by us, the high lifetime of 1.53 µm fluorescence owes much to the low OH content, and also to the ineffective upconversion process whichis held back by the inefficiency of interaction of Er3+ ionsdetermined by matrix glass and the high phonon energy. Because the phonon energy is high, once Er3+ ions absorb energy of one photon and are excited to 4I11/2 from groundstate, they relax to 4I13/2 by multiphonon relaxation. Even oneEr3+ ion in 4I13/2 is excited to 4I9/2 or other excited statesby upconversion process, it can return to 4I13/2 by sequentialmultiphonon relaxation instead of transition to ground stateby upconversion process.
It should be point out that multiphonon relaxation plays no role in the non-radiative transitionof 4I13/2 → 4I15/2 , since the energy gap between 4I13/2 and4I15/2 is about five times of the phonon energy [29].Upconversion spectra under different excited power areindicated in Fig. 8. It is necessary to explain that under maximum excited power, 620 mW, no upconversion luminescence can be observed by naked eyes in the dark conditions.The green (549 nm) and red (666 nm) upconversion emissions are assigned to the 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2transitions respectively. The log–log plots of upconversion luminescence as a function of pump power are depicted in theinsert of Fig.
8. The intensity of luminescence exhibits nearlyquadratic dependence on the excitation power.σemi × τmea can be used to evaluate the gain properties andσemi × FWHM can be used to evaluate the bandwidth properties of Er3+ doped glasses. In Table 3 it can be found thatthe gain properties and bandwidth properties of the glass areclose to phosphate glass and better than germinate and silicateglasses.FIGURE 8 Upconversion spectra of the fluorophosphateglass under 970 nm excitation, the insert shows the log–logplots of upconversion luminescence as a function of pumppowerLIAO et al.4Spectroscopic properties of fluorophosphate glass with high Er3+ concentrationConclusionFluorophosphate glass with high Er3+ concentration was prepared.
The metaphosphate content in the composition is only 4 mol. %, but the phonon energy of glass,1290 cm−1 , is fairly high. The full width at half maximum isabout 56 nm and is wider than for most of the materials investigated, since the surrounding environment of Er3+ ionsvaries severely. Though the concentration of Er3+ is considerably high, the measured lifetime of 4I13/2 → 4I15/2 transition,contributed by the high phonon energy, inefficient interactionof Er3+ ions, and low water content, amounts to no less than7.36 ms. The gain properties and bandwidth properties of theglass are close to phosphate glass and better than germinateand silicate glasses.ACKNOWLEDGEMENTS The author would like to thank Ms.Ying Zhao for her help in the experiment.
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