Skip to main content
NCBI home page
Scientific Reports logoLink to Scientific Reports
. 2014 May 23;4:5053. doi: 10.1038/srep05053

Spectroscopic properties and energy transfer parameters of Er3+- doped fluorozirconate and oxyfluoroaluminate glasses

Feifei Huang 1,2, Xueqiang Liu 1,2, Lili Hu 1, Danping Chen 1,a
PMCID: PMC5381381  PMID: 24852112

Abstract

Er3+- 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 Er3+ ions are discussed and lifetime of Er3+: 4I13/2 is measured. It is the first time to observe that a longer lifetime of the 4I13/2 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 Er3+- doped ZBYA and oxyfluoroaluminate glasses have potential applications as laser materials.

Rare-earth elements are of interest in several high-tech and environmental applications1,2,3,4,5,6. Over the past decades, Er3+ has become one of the most interesting centers of research because of its 1.55 and 2.7 μm emissions from 4I13/2 → 4I15/2 and 4I11/2 → 4I13/2 transitions, respectively4,7,8,9,10,11. The Er3+- doped fiber amplifier is one of the important devices used in the 1.5 μm wavelength optical communication window. Er3+ waveguide laser and up-conversation laser operations have been achieved at room temperature12. The optical properties of Er3+ are interesting because of their applications in infrared lasers operating at eye-safe wavelengths8,13. 2.7 μm 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 Er3+ 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. Er3+- doped fluoride, chalcogenide, 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 Er3+ 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, Er3+: ZBLAN fiber lasers have poor thermal properties (i.e., very low melting temperatures and high heat generation of Er3+ 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 μm emissions properties of these Er3+- 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 × 15 mm × 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, ΔT = Tx − Tg introduced by Dietzel27,28, is often regarded as an important parameter for evaluating the glass forming ability. ΔT has been frequently used as a rough criterion to measure glass thermal stability. A large ΔT indicates strong inhibition of nucleation and crystallization. The glass formation factor of the materials is given by the parameter kgl = (Tx − Tg)/(Tm − Tg), where Tm is the melting temperature of the glass29. Compared with ΔT, 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 ΔT and kgl 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.

Figure 1. DSC curves of the present samples.

Figure 1

Open in a new tab

Table 1. Physical, thermal, and chemical parameters of the present glasses.

        ΔW (mg/g)    
  ρ (g/cm3) N (×10−26/cm3) n 1 h 2 h 3 h 4 h 5 h ΔT(°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
Open in a new tab

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 the value of Inline graphic21. The boiled water treatment process was repeated five times for each sample in this research. The results of the ΔW% 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 fluorozirconate glasses have a weak absorption band at about 4500 nm because 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 Er3+. 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 μm.

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 OH− absorption coefficient of the ZBYA, FP and FT samples depend on the treatment time.

Figure 2

Open in a new tab

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 OH− obviously changes. The OH− in glass is related to the emission efficiency of rare-earth ions, because the residual OH− groups will participate in the energy transfer of rare-earth ions and reduce the intensity of emissions10,20. The OH− group content in the glass can be expressed by the absorption coefficient of the OH− vibration band at 3 μm, which can be given by

graphic file with name srep05053-m1.jpg

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 OH− absorption coefficient and the time of water treatment for ZBYA, FP, and FT samples. The OH− absorption coefficients of the original samples are 0.055, 0.060, and 0.096 cm−1, respectively, which are significantly lower than some reported values of bismuthate glass17, germanate glass29, and fluorophosphates glass34. Some lower OH− content glasses have also been reported35 recently and it is reported that the OH− absorption coefficient should be < 2 cm−1 to achieve optimum laser performance34. The values of the present glasses are far less than 2 cm−1. Therefore, excellent transmission property provides these Er3+- doped glasses with potential applications as laser materials. The OH− 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 OH− 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 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, and 4F7/2 are labeled. The shape and peak positions of each transition for the Er3+- doped glasses are very similar to those of other Er3+- doped glasses36, indicating homogeneous incorporation of the Er3+ ions in the glassy network without clustering and changes in the local ligand field. The absorption band around 980 nm indicates that these glasses can be efficiently excited by 980 nm LD.

Figure 3. Absorption spectra of the present samples.

Figure 3

Open in a new tab

Important spectroscopic and laser parameters of rare earth doped glasses have been commonly analyzed using the Judd–Ofelt theory37,38. Details of the theory and method have been well described earlier, so only the results will be presented in this section. The intensity parameters Ωt of these Er3+- doped glasses are calculated and shown in the Table 2. δ presents the agreement between calculated and experimental values. The room-mean-square error deviation of intensity parameters is ×10−6, which indicates the validity of the Judd-Ofelt theory for predicting the spectral intensities of Er3+ and the reliability of the calculations. Previous studies have revealed that Ω2 parameters are indicative of the amount of the covalent bond, and are strongly dependent on the local environment of the ion sites, whereas the Ω6 parameter is related to the overlap integrals of the 4f and 5d orbits39. Values of Ω4 and Ω6 also provide some information on the rigidity and viscosity of the hosts. However, compared with Ω2, which bears higher sensitivity to the chemical nature of the hosts, structural information carried by Ω4 and Ω6 values is marginal and sometimes inaccurate. An analysis of the values of Ω2 shows that the FP sample possesses lower covalence and higher symmetry. Compared with oxide glasses18, fluoride glasses have smaller Ω2 because an O2− ion possesses higher polarizability than an F− ion.

Table 2. J-O parameters of Ωt, radiative transition probability Аrad, branching ratio β, and lifetime of some selected levels τR of the present samples.

Level   ZBYA ZBLAN FP FT
Initial level End level Аrad (S−1) β τR (ms) Аrad (S−1) β τR (ms) Аrad (S−1) β τR (ms) Аrad (S−1) β τR (ms)
4I13/2 4I15/2 146.65 100% 6.82 155.21 100% 6.44 89.55 100% 11.17 122.73 100% 8.15
4I11/2 4I15/2 142.99 83.91% 5.87 152.88 84.15% 5.50 70.61 79.67% 11.28 115.92 83.11% 7.17
  4I13/2 27.41 16.09%   28.80 15.85%   18.02 20.33%   23.55 16.89%  
4I9/2 4I15/2 68.04 57.13% 8.4 85.01 60.97% 7.17 76.33 74.21% 9.72 59.18 59.25% 10.01
  4I13/2 49.04 41.18%   52.41 37.59%   24.79 24.10%   38.77 38.81%  
  4I11/2 2.01 1.69%   2.01 1.44%   1.74 1.69%   1.93 1.93%  
Ω (2,4,6)(×10−20) 3.01, 1.03, 1.64 3.27, 1.3, 1.75 1.53, 1.39, 0.95 3.1, 0.94, 1.35
δ 0.2 × 10−6 0.29 × 10−6 0.04 × 10−6 0.15 × 10−6
Open in a new tab

The calculated predicted spontaneous transition probability (A), branching ratio (β) and radiative lifetime (τrad) of certain optical transitions for Er3+- doped fluoride glasses are also shown in Table 2. The predicted spontaneous emission probabilities of Er3+: 4I13/2 → 4I15/2 and 4I112 → 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 → 4I15/2 fluorescence around 1.5 μm and 4I11/2 → 4I13/2 fluorescence around 2.7 μm 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

graphic file with name srep05053-m2.jpg

where Wn is the rate at temperature T, W0 is the rate at 0 K, n = ΔE/hv, ΔE is the energy gap between the levels involved, v is the relevant phonon's frequency. When ΔE 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 4I11/2 and 4I13/2 levels is about 3690 cm−1, 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 μm 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 μm emission. The higher intensity of the 1.5 μm emission of the oxyfluoroaluminate glasses can be explained by the 4I13/2 level decay lifetime of the samples, which will be discussed below.

Figure 4. Emission spectra of the prepared samples: (a) 1.5 μm (b) 2.7 μm.

Figure 4

Open in a new tab

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 Er3+ 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 4F7/2 level decays nonradiatively to the next- lower 2H11/2 and 4S3/2 levels. The green emission can be attributed to the Er3+: 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 (Er3+: 2F9/2 → 4I15/2) occurs. On the other hand, ions in the 4I11/2 level decay radiatively to the 4I13/2 with 2.7 μm emission or nonradiatively to the 4I13/2 level. Then the 1.5 μm emission occurs because of the 4I13/2 → 4I15/2 transition.

Figure 5. (a) Upconversion spectra of the present glasses, (b)Energy transfer sketch of Er3+- doped glasses when pumped at 980 nm.

Figure 5

Open in a new tab

Fig. 6 shows the experimental decays of the Er3+: 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 Er3+: 4I13/2 level but smaller intensity of 1.5 μm 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 Er3+ ions.

Figure 6. Decay curves of 1.5 μm emission from the Er3+- doped presented glasses.

Figure 6

Open in a new tab

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:

graphic file with name srep05053-m3.jpg

where Inline graphic is the absorbance from absorption spectrum, l is the thickness of the glass and N is the ion density.

graphic file with name srep05053-m4.jpg

where λ is the wavelength, Arad is the spontaneous transition probability, I(λ) is the emission spectrum, and n and c are the refractive index and light speed in vacuum respectively.

graphic file with name srep05053-m5.jpg

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 μm 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 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 μm 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).

Table 3. Calculated emission and absorption cross section and effective line width around 1.5 and 2.7 μm of the present glasses obtained through both BL and FL equations.

  Er3+: 4I13/2 → 4I15/2 Er3+: 4I11/2 → 4I13/2
σabs (BL) (×10−21 cm2) σem(BL) (×10−21 cm2) σabs(FL) (×10−21 cm2) σem(FL) (×10−21 cm2) Δλ (nm) τcal (ms) τexp (ms) σem(FL) (×10−21 cm2) Δλ (nm) τcal (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 11.28
FT 5.23 6.98 4.01 5.56 73.6 8.15 4.06 8.81 88.1 7.17
Open in a new tab

Figure 7. The calculated emission and absorption cross section spectra around 1.5 μm emission of FP glass through both BL and FL equations.

Figure 7

Open in a new tab

Full width at half maximum (FWHM) is a determiner for 1.5 μm 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 μm emission from Er3+- 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 (Δλeff) is reportedly more accurate in estimating the bandwidth of this transition than FWHM because the emission band is slightly asymmetric48. The effective line width (Δλeff) is determined using the expression:

graphic file with name srep05053-m6.jpg

where Imax is the peak fluorescence intensity corresponding to λeff (the peak fluorescence wavelength). The Δλeff values of 1.5 μm 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 × σe50, 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 (σem × τexp). As far as the material aspects are concerned, a larger product of σem × τexp 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 Er3+- doped fluoride glasses are promising candidate materials for 1.5 μm signal amplification.

Based on Fig. 4(b) and Equ.(3) to (6), the emission cross section and the effective line width of Er3+: 2.7 μm are calculated, as shown in Table 3. The maximum emission cross section occurs at 2708 nm, and the values are above 7 × 10−21 cm2 for all samples, which are higher than the reported values of 0.45 × 10−20 cm2 in the YAG crystal45, 0.53 × 10−20 cm2 in the LiYF4 crystal53, 0.54 × 10−20 cm2 in the ZBLAN glass53, and 0.66 × 10−20 cm2 in the chalcohalide glass53.

Microparameters of energy transfer between Er3+ ions

To optimize the 1.5 and 2.7 μm laser systems, a quantitative understanding of the energy processes of Er3+: 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

graphic file with name srep05053-m7.jpg

where |HDA| is the matrix element of the Hamiltonian perturbation between the initial and final states in the energy transfer process. Inline graphic 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, Inline graphic can be approximated by

graphic file with name srep05053-m8.jpg

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:

graphic file with name srep05053-m9.jpg

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 Inline graphic, where τD is the intracenter lifetime of the excited level of donor. The expression for direct transfer (D-A) is then expressed by:

graphic file with name srep05053-m10.jpg

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 Er3+: 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 μm but less intensive 1.5 μm emission. The lifetime of Er3+: 4I13/2 is long enough for energy transfer between Er3+ ions. Accordingly, the intensity of 1.5 μm emission is weakened.

Table 4. Calculated interaction microscopic parameters CD-A for 4I13/2 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 1.94
  99.8% 0.2%  
FT 0 1 2.95
  99.8% 0.2%  
Open in a new tab

Conclusion

In conclusion, Er3+- 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 OH− 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 Er3+ ions are discussed based on the upconversion, near- and mid- IR emissions spectra. The decay lifetime of Er3+: 4I13/2 is measured and the energy transfer microparameters between Er3+ ions are calculated. Therefore, the Er3+- doped glasses in this study possess desirable thermal resistance properties and spectroscopic characteristics, which will be promising materials for infrared lasers and optical amplifiers.

Footnotes

The authors declare no competing financial interests.

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.

References

  1. Il'ichev N. N. et al. Effective 2.5-μm ZnSe:Cr2+ laser with transverse laser pumping. Laser Phy, 20, 1091–1094 (2010). [Google Scholar]
  2. Wang H. et al. Optical properties of Dy3+ ions in sodium gadolinium tungstates crystal. J. Lumin. 126, 452–458 (2007). [Google Scholar]
  3. Lakshminarayana G. & Qiu J. Photoluminescence of Pr3+, Sm3+ and Dy3+-doped SiO2–Al2O3–BaF2–GdF3 glasses. J. Alloys Com. 476, 470–476 (2009). [Google Scholar]
  4. Burtan B. et al. Optical properties of Nd3+ and Er3+ ions in TeO2–WO3–PbO–La2O3 glasses. Opt Mater. 34, 2050–2054 (2012). [Google Scholar]
  5. Nawaz F. et al. Spectral investigation of Sm3+/Yb3+ co-doped sodium tellurite glass. Chin. Opt. Lett. 11, 061605 (2013). [Google Scholar]
  6. Xu Y. et al. Nanocrystal-enhanced near-IR emisssion in the bismuth-doped chalcogenide glasses. Chin. Opt. Lett. 11, 041601 (2013). [Google Scholar]
  7. Ming C. et al. Tm3+/Er3+/Yb3+ tri-doped TeO2-PbF2-AlF3 glass for white-emitting diode. Opt Commun. 304, 80–82 (2013). [Google Scholar]
  8. Rayappan I. A. & Marimuthu K. Structral and lumilnescence behavior of the Er3+ doped alkali fluorobarate glsses. J. Non-Cryst Solids. 367, 43–50 (2013). [Google Scholar]
  9. Guo R. et al. Optical transition probabilities of Er3+ ions in La2CaB10O19. Chem Phys Lett. 416, 133–136 (2005). [Google Scholar]
  10. Zhan H. et al. Intense 2.7 μm emission of Er3+ -doped water-free fluorotellurite glasses. Opt. Lett. 37, 3408–3410 (2012) [DOI] [PubMed] [Google Scholar]
  11. Li X. et al. Emission enhancement in Er3+/Pr3+-codoped germanate. Chin. Opt. Lett. 11, 121601 (2013). [Google Scholar]
  12. Maheshvaran K. et al. Strucural and luminescence studies on Er3+/Yb3+ co-doped boro-tellurite glasses. J. Alloys Com. 561, 142–150 (2013). [Google Scholar]
  13. Ter-Gabrielyan N. et al. Ultralow quantum-defect eye-safe Er: Sc2O3 laser. Opt. Lett. 33, 1524–1526 (2008). [DOI] [PubMed] [Google Scholar]
  14. Wei C. et al. Passively continuous-wave mode-locked Er3+-doped ZBLAN fiber laser at 2.8 μm. Opt. Lett. 37, 3849–3851 (2012). [DOI] [PubMed] [Google Scholar]
  15. Artjushenko V. G. et al. Medical applications of MIR-fiber spectroscopi probes. Biochemical and Medical Sensors. 2085, 137–142 (1993). [Google Scholar]
  16. Guo C. et al. High-power and widely tunable Tm-doped fiber laser at 2 μm. Chin. Opt. Lett. 10, 091406 (2012) [Google Scholar]
  17. Guo Y. et al. Intense 2.7 μm emission and structural origin in Er3+-doped bismuthate (Bi2O3-GeO2-Ga2O3-Na2O) glass. Opt. Lett. 37, 268–270 (2012). [DOI] [PubMed] [Google Scholar]
  18. Said Mahraz Z. A. et al. Concentration dependent luminescence quenching of Er3+-doped Zinc boro-tellurite glass. J. Lumin. 144, 139–145 (2013). [Google Scholar]
  19. Yin D. et al. Enhancement of the 1.53 μm fluorescence and energy transfer in Er3+/Yb3+/Ce3+ tri-doped WO3 modified tellurite-based glass. J. Alloys Com. 581, 534–541 (2013). [Google Scholar]
  20. Zhao G. et al. Effcient 2.7 μm emission in Er3+-doped bismuth germanate glass pumped by 980nm laser diode. Chin. Opt. Lett. 10, 091601–091603 (2012). [Google Scholar]
  21. Guo Y. et al. Er3+-doped fluoro-tellurite glass: A new choice for 2.7 μm lasers. Mater Lett. 80, 56–58 (2012). [Google Scholar]
  22. Lavin V. et al. Site selective study in Eu3+-doped fluorozirconate glasses. J. Lumin. 72, 437–438 (1997). [Google Scholar]
  23. Dakui D. & Fuding M. Glass formation and crystallization in AlF3-YF3-BaF2-CaF2-MgF2. J. Non-Cryst Solids. 168, 275–280 (1994) [Google Scholar]
  24. Yasui I. et al. The effect of addition of oxides on the crystallization behavior of aluminum fluoride-base glasses. J. Non-Cryst Solids. 140, 130–133 (1992). [Google Scholar]
  25. Rigout N. et al. Chemical and physical compatibilities of fluoride and fluorophosphate glasses. J. Non-Cryst Solids. 184, 319–323 (1995). [Google Scholar]
  26. Choi Y. G. et al. Emission properties of the Er3+:4I11/2 → 4I13/2 transition in Er3+- and Er3+/Tm3+ -doped Ge-Ga-As-S glasses. J. Non-Cryst Solids. 278, 137–144 (2000). [Google Scholar]
  27. Wang X. et al. Spectroscopic properties of thulium ions in bismuth silicate glass. Chin. Opt. Lett. 10, 101601 (2012). [Google Scholar]
  28. Tian Y. et al. 1.8 μm emission of highly thulium doped fluorophosphate glasses. J. Appl Phys. 108, 083504 (2010). [Google Scholar]
  29. Xu R. et al. 2.05 μm emission properties and energy transfer mechanism of germanate glass doped with Ho3+, Tm3+, and Er3+. J. Appl Phys. 109, 053503 (2011). [Google Scholar]
  30. Lebullenger R. et al. Systematic substitutions in ZBLA and ZBLAN glasses. J. Non-Cryst Solids. 161, 217–221 (1993). [Google Scholar]
  31. Rigout N. et al. Chemical and physical compatibilities of fluoride and fluorophosphate glasses. J. Non-Cryst Solids. 184, 319–323 (1995). [Google Scholar]
  32. Frischat G. H. et al. Chemical stability of ZrF4- and AlF3- based heavy metal fluoride glasses in water. J. Non-Cryst Solids. 284, 105–109 (2001). [Google Scholar]
  33. Yu X. et al. Infrared spectrum estimaion for maximum phonon energy in optical glasses. J. Dalian Polytechnic University. 27, 155–157 (2008). [Google Scholar]
  34. Tian Y. et al. Optical absorption and near infrared emissions of Nd3+ doped fluorophosphate glass. Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy. 98, 355–358 (2012). [DOI] [PubMed] [Google Scholar]
  35. Jiang X. et al. Fluorogermanate glass with reduced content of OH− groups for infrared optics. J. Non-Cryst Solids. 355, 2015–2019 (2009). [Google Scholar]
  36. Huang F. et al. Highly Er3+-doped ZrF4-based fluoride glasses for 2.7 μm laser materials. Appl opt. 52, 1399–1343 (2013). [DOI] [PubMed] [Google Scholar]
  37. Ding J. et al. Effect of P2O5 addition on the structural and spectroscopic properties of sodium aluminosilicate glass. Chin. opt. lett. 10, 071602 (2012) [Google Scholar]
  38. Tian Y. et al. 2 μm Emission of Ho3+-doped fluorophosphate glass sensitized by Yb3+. Opt Mater. 32, 1508–1513 (2010). [Google Scholar]
  39. Zhang Y. et al. Spectroscopic properties of Dy3+:Bi4Si3O12 single crystal. J. Alloys Com. 582, 635–639 (2014). [Google Scholar]
  40. Shinn M. D. & Sibley W. A. Optical transitions of Er3+ ions in fluorozirconate glass. Phys. Rev. 27, 6635–6648 (1983). [Google Scholar]
  41. Guo H. N. et al. Visible Upconversion in Rare Earth Ion-Doped Gd2O3 Nanocrystals. J Phys Chem B. 108, 19205–19209 (2004). [Google Scholar]
  42. Rolli R. et al. Eribium doped tellurite glasses with high quantum efficiency and broadband stimulated emission cross section at 1.5 μm. Opt Mater. 21, 743–748 (2003). [Google Scholar]
  43. Yang J. et al. Mixed heavy metal effect on emission properties of Er3+-doped borosicilate glasses. Chin. Opt. Lett. 1, 294–295 (2003). [Google Scholar]
  44. Xu R. et al. Spectroscopic properties of 1.8 μm emission of thulium ions in germanate glass. Appl Phys B. 102, 109–116 (2010). [Google Scholar]
  45. Zhuang X. et al. Enhanced emission of 2.7 μm from Er3+/Nd3+-codoped LiYF4 single crystals. Materials Science and Engineering: B. 178, 326–329 (2013). [Google Scholar]
  46. Jha A. et al. Rare-earth ion doped TeO2 and GeO2 glasses as laser materials. Progress in Materials Science. 57, 1426–1491 (2012). [Google Scholar]
  47. Ohishi Y. et al. Gain characteristic of tellurite-based erbium doped fiber amplifiber. Opt Letters. 23, 274–276 (1998) [DOI] [PubMed] [Google Scholar]
  48. Sasikala T. et al. Spectroscopic properties of Er3+ and Ce3+ co-doped tellurite glasses. J. Alloys Com. 542, 271–275 (2012). [Google Scholar]
  49. Ding Y. et al. Spectral properties of erbium-doped lead halotellurite glasses. Proc. SPIE. 166–173 (2000). [Google Scholar]
  50. Jayasimdadri M. et al. Er3+-doped tellurofluorophoaphate glasses for laser and optical ampliers. J. Phys. Condens. 17, 7705–7715 (2005). [Google Scholar]
  51. Shen S. et al. Tellurite glasses for broadband amplifiers and integrated optics. J. Am. Cream. Soc. 85, 1391–1395 (2002). [Google Scholar]
  52. Wei K. et al. Pr3+- doped Ge-Ga-S glasses for 1.3 μm optical fiber amplifiers. J. Non-Cryst Solids. 182, 257–261 (1995). [Google Scholar]
  53. Lin H. et al. Enhanced mid-infrared emisssions of Er3+ at 2.7 μm via Nd3+ sensitization in chalcohalide glass. Opt Letters. 36, 1815–1817 (2011). [DOI] [PubMed] [Google Scholar]
  54. Xu R. et al. 2.0 μm emission properties and energy transfer processes of Yb3+/Ho3+ codoped germanate glass. J. Appl Phys. 108, 043522 (2010) [Google Scholar]
  55. Tian Y. et al. Intense 2.0 μm emission properties and energy transfer Ho3+/Tm3+/Yb3+ doped fluorophosphate glasses. J. Appl Phys. 110, 033502 (2011). [Google Scholar]

Articles from Scientific Reports are provided here courtesy of Nature Publishing Group

RESOURCES