0Spectroscopic properties and energy transfer parameters of Er3+- doped fluorozirconate and oxyfluoroaluminate glasses
Feifei Huang1,2, Xueqiang Liu1,2, Lili Hu1 & Danping Chen1
1Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China, 2Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China.
Er31- doped fluorozirconate (ZrF4-BaF2-YF3-AlF3) and oxyfluoroaluminate glasses are successfully prepared here. These glasses exhibit significant superiority compared with traditional fluorozirconate glass (ZrF4-BaF2-LaF3-AlF3-NaF) because of their higher temperature of glass transition and better resistance to water corrosion. Judd-Ofelt (J-O) intensity parameters are evaluated and used to compute the radiative properties based on the VIS-NIR absorption spectra. Broad emission bands located at 1535 and 2708 nm are observed, and large calculated emission sections are obtained. The intensity of 2708 nm emission closely relates to the phonon energy of host glass. A lower phonon energy leads to a more intensive 2708 nm emission. The energy transfer processes of Er31 ions are discussed and lifetime of Er31: 4Ii3/2 is measured. It is the first time to observe that a longer lifetime of the 4Ii3/2 level leads to a less intensive 1535 nm emission, because the lifetime is long enough to generate excited state absorption (ESA) and energy transfer (ET) processes. These results indicate that the novel glasses possess better chemical and thermal properties as well as excellent optical properties compared with ZBLAN glass. These Er31- doped ZBYA and oxyfluoroaluminate glasses have potential applications as laser materials.
SUBJECT AREAS:
MATERIALS FOR OPTICS
OPTICAL MATERIALS AND STRUCTURES
OPTICAL PHYSICS
Received 11 March 2014
Accepted 6 May 2014
Published 23 May 2014
Correspondence and requests for materials should be addressed to D.P.C. (dpchen@mail.
siom.ac.cn)
Rare-earth elements are of interest in several high-tech and environmental applications1-6. Over the past decades, Er31 has become one of the most interesting centers of research because of its 1.55 and 2.7 mm emissions from 4I13/2 r 4I15/2 and 4I11/2 R 4I13/2 transitions, respectively4,7-11. The Er31- doped fiber amplifier is one of the important devices used in the 1.5 mm wavelength optical communication window. Er31 waveguide laser and up-conversation laser operations have been achieved at room temperature12. The optical properties of Er31 are interesting because of their applications in infrared lasers operating at eye-safe wavelengths8,13. 2.7 mm emission is also becoming a concern for researchers owing to the strong absorption of radiation by water. It has potential applications in medicine, sensing, and military countermeasures, as well as in light detection and ranging14,15. Meanwhile, the maturity of laser diodes (LDS) accelerates Er31 development because of its efficient absorption at 800 or 980 nm.
Glasses known as convenient hosts for rare earth ions have been widely used because of their good mechanical and thermal stability, low synthesis cost, as well as possibility of pulling to fiber16. Er31- doped fluoride, chalco-genide, fluorophosphate, silicate, and heavy metal oxide (tellurite, germanate, and bismuthate) glasses have been investigated for applications in near- and mid-infrared (IR) regions14,17. Fluoride glasses are potential candidates for Er31 doped materials because of their low phonon energy and wide optical transmission window, ranging from UV to mid-IR18,19. The fluorozirconate system, notably the ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) glass composition, is one of the most stable systems against devitrification among fluoride glasses. However, Er31: ZBLAN fiber lasers have poor thermal properties (i.e., very low melting temperatures and high heat generation of Er31 actives ions) compared with those of near-IR silica-based fiber lasers, and the relatively large loss of ZBLAN fibers limits the usable length of the fibers, so further scaling up the power output is fundamentally difficult20,21. Thus, exploring effective fluoride glasses for host matrices becomes a challenge to researchers, for example the fluorozirconate system (ZBYA)22.
Fluoroaluminate glasses (AlF3-based glasses) have better chemical durability and enhanced mechanical strength than fluorozirconate glasses, which would thus be useful for optical applications23. However, some
devitrification problems are associated with these glasses. The addition of some oxides, especially Al(PO3)3 or TeO2, is effective to stabilize the glass state24. Oxyfluoroaluminate glasses containing low P or Te have potential applications as hosts for high-power glass lasers.
Several structural studies have revealed the basic structure of these glasses (ZBYA and oxyfluoroaluminate glasses)22,25. However, few investigations are available on the thermal, chemical, and the 1.5 and 2.7 mm emissions properties of these Er31- doped glasses. In this study, fluorozirconate glass (ZBYA) and oxyfluoroaluminate glasses containing low P or Te are successfully prepared. The thermal and chemical properties of these glasses are investigated. The absorption and emission spectra at near- and mid-IR regions are tested. Simultaneously, the spectroscopic properties, Judd-Ofelt theory analysis results, cross sections, and emission parameters of these glasses are discussed.
Experimental
The compositions of the glasses were ZrF4-BaF2-YF3-AlF3-1ErF3 (designated as ZBYA), 99(AlF3-YF3-CaF2-BaF2-SrF2-MgF2)-1Al(PO3)3-1ErF3 (designated as AYFP or FP) and 90(AlF3-YF3-CaF2-BaF2-SrF2-MgF2)-10TeO2-1ErF3 (designated as AYFT or FT). For comparison, fluorozirconate glass with composition of 100(ZrF4-BaF2-LaF3-AlF3-NaF)-1ErF3 (designated as ZBLAN) was prepared. The samples were prepared using high-purity ZrF4, AlF3, YF3, CaF2, BaF2, SrF2, MgF2, Al(PO3)3, TeO2 and ErF3 powders. Well-mixed 25 g batches of the samples were placed in platinum crucibles and melted at about 1100°C for 30 min. Then the melts were poured onto a preheated copper mold and annealed in a furnace around the glass transition temperature. The annealed samples were fabricated and polished to the size of 20 mm X 15 mm X 1 mm for the optical property measurements.
The characteristic temperatures (temperature of glass transition Tg and temperature of onset crystallization peak Tx) of the samples were determined using a NetzschSTA449/C differential scanning calorimetry at a heating rate of 10 K/min. The densities and refractive indices of the samples were measured through the Archimedes method using distilled water as an immersion liquid and the prism minimum deviation method respectively. Furthermore, the absorption spectra were recorded with a Perkin-Elmer Lambda 900 UV/ VIS/NIR spectrophotometer in the range of 300 nm to 1600 nm, and the emission spectra were measured with a Triax 320 type spectrometer (Jobin-Yvon Co., France). All the measurements were carried out at room temperature.
Results and discussion
Differential scanning calorimeter results. Fig. 1 shows the DSC results of the four samples in this study. Characteristic temperatures of Tg (temperature of glass transition), Tx (temperature of onset of crystallization), and Tp (temperature of peak of crystallization) are also marked in Fig. 1. Tg is an important factor for laser glass, higher values of the oxyfluoroaluminate glasses compared with those of fluorozirconate glasses and other reported glasses26 give glass good thermal stability to resist thermal damage at high pumping intensities. The glass criterion, AT = Tx — Tg introduced
ITi fi
544 °C
455 °C /\552°C / A
376°C f
ZBLAN 266 C 352°c/ V /
7. BY A 520°C/ fSlTX
FP V 435 °C
FT 434 "C
200 250 300 350 400 450 500 550 600 Temperature(°C)
Figure 1 | DSC curves of the present samples.
by Dietzel27,28, is often regarded as an important parameter for evaluating the glass forming ability. AT has been frequently used as a rough criterion to measure glass thermal stability. A large AT indicates strong inhibition of nucleation and crystallization. The glass formation factor of the materials is given by the parameter kg = (Tx — Tg)/(Tm — Tg), where Tm is the melting temperature of the glass29. Compared with AT, the parameter is more suitable in estimating the glass thermal stability. A larger kgl, imparts better forming ability of the glass. The glass forming ability can be estimated by these given characteristic temperatures. The existing stability criterion parameters AT and kg of the samples are shown in Table 1. These values are larger than those of fluoride and phosphate glasses30,31. These results indicate that the ZBYA and oxyfluoroaluminate glasses have better forming ability and thermal stability against crystallization.
Chemical stability. The chemical durability of the sample was measured as follows: First, the weighed sample (W1) was placed into the distilled water. Second, the sample was kept in a thermostatic water bath at 98°C for 1 h and then cooled and dried in a dying box at 70°C for 1 h. Finally, the dry sample was weighed again (W2). The chemical durability of glasses was evaluated using wi —w2
the value of AW% =-x 100%21. The boiled water treatment
process was repeated five times for each sample in this research. The results of the AW% are shown in Table 1. ZBLAN exhibits poorer resistance to water corrosion compared with the other samples, which coincides with the reported phenomenon32.
The transmittance spectra of the samples before and after water treatment are shown in Fig. 2. Figure 2(a) shows the transmittance spectra of the samples without any treatment. Transmittance can reach as high as 90%, whereas approximately 10% loss contains the Fresnel reflection dispersion, and glass absorption. The fluorozirco-nate glasses have a weak absorption band at about 4500 nm because
Table 1 | Physical, thermal, and chemical parameters of the present glasses
AW (mg/g)
P (g/cm3) N (X10 26/cm3) n 1 h 2 h 3h 4 h 5 h AT(°C) kgl
ZBYA 4.55 1.35 1.502 0 0 0 0.66 0 82 0.324
ZBLAN 4.38 1.46 1.499 38.0 6.9 11.1 3.8 9.1 86 0.257
FP 3.81 2.14 1.431 0 0 0 0.91 0.45 85 0.183
FT 3.94 2.13 1.482 0 0 0 0 0 93 0.254
Figure 2 | Transmittance spectra of the present glasses before and after water treatment (a) Transmittance spectra of all the samples before treatment (b) Transmittance spectra of ZBLAN after treatment (c) The curves of OH2 absorption coefficient of the ZBYA, FP and FT samples depend on the treatment time.
of CO2 absorption, and the oxyfluoroaluminate glasses possess an absorption band at about 4750 nm because of the vibration peak [XO]. However, these fluctuations do not influence the near- and mid-infrared emissions of Er31. The phonon energy can be inferred from the transmittance spectra, and large phonon energy increases the nonradiative decay rate. A higher nonradiative decay rate results in fewer radiative transitions and therefore less intense fluorescence bands33. The phonon energy calculated by this model is also presented in Figure 2 (a). ZBYA glass has the smallest phonon energy and the IR cut-off wavelength is at about 7 mm.
The transmittance spectra of the samples after water treatment are shown in Figures 2(b) and 2(c). The basic form of the spectra almost remains the same for ZBYA, FP, and FT samples. Only the absorption band at about 2900 nm caused by OH2 obviously changes. The OH2 in glass is related to the emission efficiency of rare-earth ions, because the residual OH2 groups will participate in the energy transfer of rare-earth ions and reduce the intensity of emissions10,20. The OH2 group content in the glass can be expressed by the absorption coefficient of the OH2 vibration band at 3 mm, which can be given by
«OH-~ ln (T/T0)/1 (1)
where l is the thickness of the sample, T0, and T are the transmitted and incident intensities respectively. Figures 2(c) describes the relationship between the OH2 absorption coefficient and the time of water treatment for ZBYA, FP, and FT samples. The OH2 absorption coefficients of the original samples are 0.055, 0.060, and 0.096 cm21,
respectively, which are significantly lower than some reported values of bismuthate glass17, germanate glass29, and fluorophosphates glass34. Some lower OH2 content glasses have also been reported35 recently and it is reported that the OH2 absorption coefficient should be < 2 cm21 to achieve optimum laser performance34. The values of the present glasses are far less than 2 cm21. Therefore, excellent transmission property provides these Er31- doped glasses with potential applications as laser materials. The OH2 absorption coefficient becomes larger for all the samples with increasing water treatment time, and ZBYA glass possesses best chemical stability according to Fig. 2(c). The ZBLAN sample has poor resistance to water corrosion. The spectra for the ZBLAN sample after water treatment are demonstrated alone in Fig. 2(b). After 1 h water treatment, the transmittance noticeably declines and the OH2 absorption coefficient approaches near infinity. Afterward, the ZBLAN glass becomes opaque at the mid-IR region.
Absorption spectra and calculation of optical parameters. Fig. 3 indicates the absorption spectra of the samples at room temperature in the wavelength region of 300 nm to 1600 nm. Absorption bands corresponding to the transitions starting from the 4I15/2 ground state to the higher levels 4I№2, 4In/2,4I9/2, 4F9/2, %/2, 2Hn/2, and 4F7/2 are labeled. The shape and peak positions of each transition for the Er31-doped glasses are very similar to those of other Er31- doped glasses36, indicating homogeneous incorporation ofthe Er31 ions in the glassy network without clustering and changes in the local ligand field. The
1000 1200 Wavelength(nm)
Figure 3 | Absorption spectra of the present samples.
absorption band around 980 nm indicates that these glasses can be efficiently excited by 980 nm LD.
Important spectroscopic and laser parameters of rare earth doped glasses have been commonly analyzed using the Judd-Ofelt the-ory37,38. Details of the theory and method have been well described earlier, so only the results will be presented in this section. The intensity parameters Vt of these Er31- doped glasses are calculated and shown in the Table 2. 8 presents the agreement between calculated and experimental values. The room-mean-square error deviation of intensity parameters is X1026, which indicates the validity of the Judd-Ofelt theory for predicting the spectral intensities of Er31 and the reliability of the calculations. Previous studies have revealed that V2 parameters are indicative of the amount of the covalent bond, and are strongly dependent on the local environment of the ion sites, whereas the V6 parameter is related to the overlap integrals of the 4f and 5d orbits39. Values of V4 and V6 also provide some information on the rigidity and viscosity of the hosts. However, compared with V2, which bears higher sensitivity to the chemical nature of the hosts, structural information carried by V4 and V6 values is marginal and sometimes inaccurate. An analysis of the values of V2 shows that the FP sample possesses lower covalence and higher symmetry. Compared with oxide glasses18, fluoride glasses have smaller V2 because an O22 ion possesses higher polarizability than an F2 ion.
The calculated predicted spontaneous transition probability (A), branching ratio (b) and radiative lifetime (trad) of certain optical transitions for Er31 - doped fluoride glasses are also shown in Table 2. The predicted spontaneous emission probabilities of Er31: 4I13/2 r 4I15/2 and 4I112 r 4I13/2 transitions are presented, which are much higher than reported values40. Higher spontaneous emission probability provides a better opportunity to obtain laser actions.
Fluorescence properties and energy transfer processes. Under 980 nm diode laser excitation, the 4I13/2 r 4I15/2 fluorescence around 1.5 mm and 4I11/2 r 4I13/2 fluorescence around 2.7 mm are obviously observed, as seen in Fig. 4. For the present samples, no shift in the wavelength of the emission peaks is observed, but the peak intensity is evidently different. Generally, the intensity of 1530 nm is opposite of that of 2710 nm for the same sample in this study. The fluorozirconate glasses possess more intensive 2708 nm emission owing to the lower phonon energy. The multi-phonon nonradiative decay rate is given by the well-known energy gap law41
Wn ~W0[1-exp(-hv/kT)]-n (2)
where Wn is the rate at temperature T, W0 is the rate at 0 K, n = DE/ hv, DE is the energy gap between the levels involved, v is the relevant
13 a _a
o. c o
t-4—
UK ■—
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0 1— 5 1— 3
0 .8 cn .8
'— 3. 8. .
8 ■— 5 3
oi x O >o
K <> "O ■-; K <> cn lo co (> co ■— CN
K CO ■— cn
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cn co" .0
. .5 0.
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io<> Oco KK c>o co<j ■—
cok kcn
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co ■— "O CO CO
co o ■— ■— ■— cnco coo "'to iocncoiocncn ioio cnco io
cn k ooao
<>lO ao
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0<> O -O
•— co -ok w^ w^ f CO ^ lO^r o
lOO ■— ^r^r ■— ^ x
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<>CNkco<>CN"o
CN CN CN CN CN CN O
\ \ \ \ \ \ ■ in in CI A n
4 4 4 4 4 4 °
o X <T
CN CN CO
Figure 4 | Emission spectra of the prepared samples: (a) 1.5 mm (b) 2.7 mm.
phonon's frequency. When DE is equal to or less than 4-5 times the high-energy phonons, the multi-phonon nonradiative relaxation with the emission of a few high-energy phonons becomes competitive with radiative processes. The energy gap between the 41ц/2 and 4Ii3/2 levels is about 3690 cm21, which is equal to 5-6 times the high-energy phonons of the oxyfluoroaluminate glasses and 6-7 times that of the fluorozirconate glasses. The multi-phonon nonradiative relaxation with the 2.7 mm emission of the oxyfluoroaluminate glasses has a larger probability than that of the fluorozirconate glasses, which leads to a much lower intensity of the 2.7 mm emission. The higher intensity of the 1.5 mm emission of the oxyfluoroaluminate glasses can be explained by the 4I13/2 level decay lifetime of the samples, which will be discussed below.
The upconversion spectra of the present samples are shown in Fig. 5(a). In this region, the green emissions at about 545 and 550 nm dominate. The green emission of the fluorozirconate glasses is stronger than that of the oxyfluoroaluminate glasses, which is similar to the emission of 2710 nm and opposite to that of the 1530 nm emission. To explain the relationship among the green emission, and the near- and mid-IR emissions, the energy level of Er31 is demonstrated in Fig. 5(b). Ions of the 4I15/2 state are excited to the 4I11/2 state by ground state absorption (GSA) when the prepared samples are pumped by a 980 nm LD. On the one hand, some ions in the 4I11/2 level undergo the energy transfer upconversion (ETU1) and excited stated absorption (ESA1) processes, thus contributing to the population of 4F7/2 level. Afterward, the excited energy stored in the
Figure 5 | (a) Upconversion spectra of the present glasses, (b)Energy transfer sketch of Er31- doped glasses when pumped at 980 nm.
Figure 6 | Decay curves of 1.5 mm emission from the Er31- doped presented glasses.
4F7/2 level decays nonradiatively to the next- lower 2H11/2 and 4S3/2 levels. The green emission can be attributed to the Er31: 4H11/2 — 4I15/2 and 4S3/2 — 4I15/2 transitions. Some may have the chance to decrease to the lower 4F9/2 energy through nonradiative decay, after which red emission (Er31:2F9/2 — 4I15/2) occurs. On the other hand, ions in the 4I11/2 level decay radiatively to the 4I13/2 with 2.7 mm emission or nonradiatively to the 4I13/2 level. Then the 1.5 mm emission occurs because of the 4I13/2 —> 4I15/2 transition.
Fig. 6 shows the experimental decays of the Er31: 4I13/2 level at room temperature of the present samples. The lifetime is an important factor for potential laser materials. All the samples show an exponential decay with lifetime of 10.09, 6.66, 4.91, and 4.06 ms, respectively, which are larger than those of tellurite glass (3.3 ms)42, bismuth based glass (1.8 ms)42, and borosilicate glass (2.0 ms)43. Difference exists between the values of lifetime that are measured and calculated because the measurement occurs at room temperature, but not at low temperature. The measured lifetimes of the fluorozirconte glasses are longer than those calculated ones owing to the serious self-absorption of the 4I13/2 level. The fluorozirconate glasses possess longer lifetime of Er31: 4I13/2 level but smaller intensity of 1.5 mm emission, which can be explained by that the lifetime is long enough to generate the ESA2 and ETU2 processes (as shown in Fig. 5(b)) and the ET between Er31 ions.
Cross sections and emission parameters. Beer-Lambert44 and Fuchtbauer-Ladenburg45 equations are commonly used to calculate the cross section. The difference is that the former calculates the absorption cross section based on the absorption spectra firstly,
whereas the latter calculates the emission cross section primarily based on the emission spectra and spontaneous transition probability. Both relate the absorption and emission cross sections through McCumber theory36. The equations are as follows:
Beer — Lambert equation : aa (1) ~
2.303log(Io/I)
where logIo/I is the absorbance from absorption spectrum, l is the thickness of the glass and N is the ion density.
Fuchtbauer — Ladenburg equation :
l^Arad
8pcn2 " pI(A)d(1)
where l is the wavelength, Arad is the spontaneous transition probability, I(l) is the emission spectrum, and n and c are the refractive index and light speed in vacuum respectively.
McCumber equation : ffe(1) ~ aa(1)
'Zl Zu
where h is Planck's constant, KB is the Boltzmann constant, T is the temperature, Ezl is the ground state manifold and the lowest stark level of the upper manifolds and Zu and Zl are partition functions of the lower and upper manifolds.
The absorption and emission cross sections of 1.5 mm for all present glasses are calculated using both methods. The results are shown in Table 3. The values calculated using BL method are larger than
Table 3 | Calculated emission and absorption cross section and effective line width around 1.5 and 2.7 mm of the present glasses obtained through both BL and FL equations
Er3+:4I,3/2 R 4115/2 Er3+:4I 11/2 R 4I13/2
Sabs (BL) Sem(BL) Sabs(FL) Sem(FL) Sem(FL)
(X10 21 cm2) (X10 21 cm2) (X10 21 cm2) (X10 21 cm2) Al (nm) tcal (ms) texp (ms) (X10 21 cm2) Al (nm) t cal (ms)
ZBYA 6.79 8.95 4.55 6.29 75.6 6.82 10.09 8.87 98.5 5.87
ZBLAN 5.79 7.26 4.84 6.56 77.3 6.44 6.66 10.03 90.8 5.50
FP 5.26 6.61 3.01 4.18 77.3 11.17 4.96 7.30 86.4 1 1.28
FT 5.23 6.98 4.01 5.56 73.6 8.15 4.06 8.81 88.1 7.17
those obtained using FL equation. Nevertheless, the same trend emerges, namely, the value of the absorption cross section is somewhat smaller than that of the emission cross section and the fluorozirconate glasses possess larger values compared with oxyfluoroaluminate glasses. To demonstrate the difference between the values calculated by the two equations, the cross sections at about 1.5 mm are described in Fig. 7 for the FP samples (similar spectra of other samples). The curves calculated from the FL equation seem smoother. FL may be more theoretically accurate because it is based on both the emission and the absorption spectra (the calculated spontaneous transition probability is based on the absorption spectra).
Full width at half maximum (FWHM) is a determiner for 1.5 mm laser materials46. The larger bandwidth of this transition is suitable for tunable lasers delivering relatively constant power over a wide wavelength range. The 1.5 mm emission from Er31- doped silicate glasses extensively used in the present study exhibit a narrow FWHM of about 30 nm, which limits their further applications47. The effective line width (A1ef) is reportedly more accurate in estimating the bandwidth ofthis transition than FWHM because the emission band is slightly asymmetric48. The effective line width (A1eff) is determined using the expression:
A1eff ~
I(1)d1 /In
Figure 7 | The calculated emission and absorption cross section spectra around 1.5 mm emission of FP glass through both BL and FL equations.
where Imax is the peak fluorescence intensity corresponding to 1eff (the peak fluorescence wavelength). The A1eff values of 1.5 mm emission are presented in Table 3. The effective line width values in the present glasses are higher than those of silicate (34.8 nm)49 and phosphate (46.0 nm)49, making these fluoride glasses promising candidates for broadband amplifiers in WDM systems.
As known, a figure of merit (FOM) for the amplifier bandwidth is the product FWHM X ae50, which can be inferred from Table 3. The
products of the samples are much higher than those of ZBLAN (30 pm2?nm) and Al/SiO2 (25 pm2?nm) glasses, which have been studied as potential EDFA hosts51. Meanwhile, the FOM for amplifier gain is usually defined as the product of stimulated emission cross section and lifetime (aem X texp). As far as the material aspects are concerned, a larger product of CTem X texp is desirable for an efficient fiber amplifier52. The product of ZBYA glass has an obvious advantage over Al/SiO2 (5.5 pm2?ms)51. These results show that Er31-doped fluoride glasses are promising candidate materials for 1.5 mm signal amplification.
Based on Fig. 4(b) and Equ.(3) to (6), the emission cross section and the effective line width of Er31: 2.7 mm are calculated, as shown in Table 3. The maximum emission cross section occurs at 2708 nm, and the values are above 7 X 10 21 cm2 for all samples, which are higher than the reported values of 0.45 X 10 2 20 cm2 in the YAG crystal45, 0.53 X 10 2 20 cm2 in the LiYF4 crystal53, 0.54 X 10 2 20 cm2 in the ZBLAN glass53, and 0.66 X 10220 cm2 in the chalcohalide
glass53.
Microparameters of energy transfer between Er31 ions. To optimize the 1.5 and 2.7 mm laser systems, a quantitative understanding of the energy processes of Er31:4I13/2 level in the present glasses is required. The relevant energy transfer microparameters are quantitatively analyzed by applying the method developed by Forster and Dexter54,55. The probability rate of energy transfer between donor and acceptor can be described as
H|2oN DA I SDA
where \HDA\ is the matrix element of the Hamiltonian perturbation between the initial and final states in the energy transfer process. SDA is the overlap integral between the m-phonon emission line shape of donor ions (D) and k-phonon emission line shape of donor ions (A). For the case of weak electron-phonon coupling, S^ can be approximated by
-(SDSA )
'(sDs,
A\N' 0 )
Sda(0,0,E)L
(N,AE/ft®0)
where SDa (0, 0, E) represents the overlap integral between the zero-phonon line shape of donor emission ions and the absorption of acceptor ions. S0D, and S0A are the Huang-Rhys factor of donor and acceptor ions, respectively. The probability rate of energy transfer can be obtained using the following direct transfer equation:
Wd-a(R) ~
(2p)4n2R6g|
-(2n+i)sm
mm ( n+i)
sDmis(l+)sAbs(1)d(1) (9)
Table 4 | Calculated interaction microscopic parameters CD-Afor4h3/2 level in the present glasses. The number # of phonons necessary to assist the energy transfer process is also indicated along with its contribution (%) Glass N (No. of phonons) (%) phonon assisted Transfer coefficient (10 39 cm6/s)
ZBYA 0 1 3.99
99.6% 0.4%
ZBLAN 0 1 4.53
99.7% 0.3%
FP 0 1 .94
99.8% 0.2%
FT 0 1 2.95
99.8% 0.2%
where CD-A is the energy transfer coefficient, R is the distance of separation between donor and acceptor, and the critical radius of the interaction can be obtained using the equation RC~Cd_atd, where tD is the intracenter lifetime of the excited level of donor. The expression for direct transfer (D-A) is then expressed by:
Cda —
(2p)4n2gDp
-(2n+i)sm
SU sAbs(1)d(1)
Energy transfer properties of 4I13/2 level in the present glasses are calculated using Eqs. (7) to(10) and are listed in Table 4. The results show that the energy transference of Er31: 4I13/2 level in the present glasses scarcely needs phonon assistance. The results can explain why fluorozirconate glasses possess longer life time of 1.5 mm but less intensive 1.5 mm emission. The lifetime of Er31: 4I13/2 is long enough for energy transfer between Er31 ions. Accordingly, the intensity of 1.5 mm emission is weakened.
Conclusion
In conclusion, Er31- doped fluorozirconate (ZBYA) and oxyfluoroaluminate glasses have been prepared in this study. The DSC curves of these glasses show better thermal stability in resisting thermal damage at high pumping intensities compared with ZBLAN. The water treatment experiments show that ZBLAN exhibits serious weight loss and becomes opaque in the IR region. However, these samples demonstrate better resistance to water corrosion. Low OH2 absorption coefficient and phonon energy provide these glasses with potential for applications as laser materials. The high spontaneous transition probability and large emission cross section prove the intense near- and mid-infrared emissions. The energy transfer processes of Er31 ions are discussed based on the upconversion, near-and mid- IR emissions spectra. The decay lifetime of Er31: 4I13/2 is measured and the energy transfer microparameters between Er31 ions are calculated. Therefore, the Er31- doped glasses in this study possess desirable thermal resistance properties and spectroscopic characteristics, which will be promising materials for infrared lasers and optical amplifiers.
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Author contributions
F.H. wrote the main manuscript text and coauthor X.L. checked up. D.C. and L.H. are responsible for the experiment. All authors reviewed the manuscript.
Additional information
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Huang, F.F., Liu, X.Q., Hu, L.L. & Chen, D.P. Spectroscopic properties and energy transfer parameters of Er31- doped fluorozirconate and oxyfluoroaluminate glasses. Sci. Rep. 4, 5053; DOI:10.1038/srep05053 (2014).
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