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Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids
journal homepage: www.elsevier.com/locate/jnoncrysol
Removal of hydroxyl routes enhancing 2.85 μm mid-infrared luminescence
in oxyfluorotellurite glass with high ZnF2 content
⁎
⁎
Bo Huang, Changfu Xu , Zhen Zhang, Chun yang Zang, Lizhong Sun
Hunan Provincial Key Laboratory of Thin Film Materials and Devices, School of Materials Sciences and Engineering, Xiangtan University, Xiangtan 411105, China.
A R T I C LE I N FO
A B S T R A C T
Keywords:
Removal of hydroxyl groups
TeO2-ZnF2 glass
2.85 μm mid-infrared luminescence
ZnF2
The near ~3 μm mid-infrared laser demonstrates real and potential applications in many military and civilian
areas, but the lack of host materials with nice optical quality and the overlapped vibrations of OH– hamper the
emission intensity. To eliminate the OH– absorption, ZnF2 was adopted in Yb3+/Ho3+ codoped binary TeO2PbF2 glass to partially till totally replace PbF2. The high ZnF2 concentration would raise the glass transition
temperature and enhance the thermal stability of matrix, especially enhance the intensity of 2.85 μm mid-infrared luminescence obviously. TeO2-ZnF2 glass with 45.5% ZnF2 original content has high ratio of the strength
parameter Ω4/Ω6 (=2.90) and possesses higher spontaneous transition probability (45.25 S−1) along with the
larger calculated emission cross section (0.96 × 10−20 cm2) corresponding Ho3+: 5I6 → 5I7 transition. Fourier
transmittance of infrared spectra revealed ZnF2 could reduce the OH– concentration in glass substantially, which
were favor of 2.85 μm mid-infrared emission. Our results indicated that the use of ZnF2 to effectively remove the
hydroxyl groups is an efficacious way to develop near ~3 μm mid-infrared optical glass with high efficient
luminescence and thermochemical reliability.
1. Introduction
Near ~3 μm mid-infrared light source became appealing attracted
for its military and civilian applications in various areas such as laser
guidance, infrared remote sensing, toxic gas detect and medical operation [1–4]. Rare earth doped oxyfluoride glass was an important
mid-infrared luminescent materials for its relatively low phonon energy
and thermochemical reliability, but their optical properties were limited by the level of impurities that the glass contained. Among of these
impurities, OH– contributes the major factor for its fundamental vibrations give rise to strong absorption broadband in the mid-infrared
region from 2500 to 5000 cm−1 (2–4 μm), which would quench the
radiative transitions of doped active centers (Ho3+, Er3+, Dy3+) and
render to notably decrease the luminescent efficiency and transmissivity [5–10].
Many technologies were adopted to remove the hydroxyl groups
including purifying the raw materials, various drying techniques, preparation process controlling and controlling the synthesis or fabrication
atmosphere. These technologies can be named as “removal of hydroxyl
groups” or “dehydration”, and can be divided into two categories:
physical dehydration and chemical dehydration. Usually, the two
techniques are often used in combination. Factually, OH– can be incorporated inside the glass matrix during the every stages of the glass
⁎
preparation. Besides the adsorbed water on the raw materials, the water
diffused in ambient atmosphere can invade into the glass matrix at all
stages of preparation and subsequent processing, and further destroy
the internal network structure of the glass. To avoid the effect of water
vapor and eliminate the residual of hydroxyl groups, some researchers
proposed dehydration in an ultradry atmosphere to dispose the raw
material [11, 12], and the mixed gas with N2 and O2 were used to purge
the molten coupled with stirring in furnace [13, 14]. Reactive atmosphere processing can reduce OH– residual substantially, which often
adopt some gases including Cl2, O2, HF, CCl4, and etc. to react with OH–
so as to achieve the purpose of removing OH−, but this method was
concerning complicate experimental equipment and uncontrollable
harmful gases [15–19]. In addition, this method can't remove the
content of OH– residual to low enough. In order to avoid the abovementioned disadvantages, anhydrous solid fluorides or chlorides were
employed to remove OH– and meanwhile as glass matrix composition.
For this protocol, CaF2, BaF2, ZnF2, ZnCl2, PbF2, LaF3 and etc. were
used as drying agent [7, 8, 10, 20–23]. Among of these fluorides, ZnF2
had been proved to be an effective reagent to remove the hydroxyl
groups and also be a nice composition of laser host materials, especially
as one composition of oxyfluoride glass hosts. However, ZnF2 was often
used as one of ternary composition glass for photoluminescent host, to
date as we know, there is no report about the mid-infrared
Corresponding authors.
E-mail addresses: xcf@xtu.edu.cn (C. Xu), lzsun@xtu.edu.cn (L. Sun).
https://doi.org/10.1016/j.jnoncrysol.2018.07.032
Received 23 April 2018; Received in revised form 4 July 2018; Accepted 16 July 2018
0022-3093/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Huang, B., Journal of Non-Crystalline Solids (2018), https://doi.org/10.1016/j.jnoncrysol.2018.07.032
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
B. Huang et al.
Table 1
Absorption coefficiency, density, dopant ions concentration, OH– content and the refractive rate (n) at 633 nm of as-prepared glasses with their thermal properties
partially.
Label
Molor compositions
Tg,°C
Tx,°C
ΔT.°C
α(OH−),cm−1
OH−,1019 cm−3
ρ,g/cm3
Ho3+ concentration
n
Zn-0
Zn-10
Zn-20
Zn-30
Zn-45.5
50TeO2–45.5PbF2–2.5YbF3-2HoF3
50TeO2–35.5PbF2-10ZnF2–2.5YbF3-2HoF3
50TeO2–25.5PbF2–20ZnF2–2.5YbF3-2HoF3
50TeO2–15.5PbF2-30ZnF2–2.5YbF3-2HoF3
50TeO2–45.5ZnF2–2.5YbF3-2HoF3
309.8
431
121.2
333.8
440.8
107.0
385.7
497.9
112.2
1.636
1.342
1.003
0.576
0.567
2.036
1.680
1.263
0.736
0.711
6.3468
6.0578
5.8636
5.6798
5.1260
3.7907 × 1020 cm−3
3.8921 × 1020 cm−3
4.0754 × 1020 cm−3
4.2999 × 1020 cm−3
4.5034 × 1020 cm−3
2.104
2.079
2.055
2.034
2.002
Raman spectrometer which was excited by 532 nm laser. The infrared
transmission spectrum of the sample was measured by Nicolet Is50 FTIR infrared spectrometer in the range of 2500–7500 nm. The infrared
fluorescence spectra were determined by Zolix OmniFluo-MIR980XT
spectrophotometer in the range of 1000–5500 nm with an InGaAs detector cooled with liquid nitrogen under the excitation of 980 nm LD.
luminescence of binary composition TeO2-ZnF2 glass [8, 11, 12, 14, 15,
17, 21, 22].
Now, 2 μm Ho3+ laser has been successfully applied to the surgery
operation with noncontact mode, especially the minimally invasive
surgery. Nevertheless, compared with 2 μm laser from Ho: YAG
(2.12 μm), near ~3 μm laser from Er: YSGG (2.79 μm) exhibited a
higher tissue ablation efficiency and precision with a relatively small
zone of coagulated tissue because the water absorption coefficient of
the 3 μm is over 100 times larger than 2 μm [24]. In fact, Ho3+ laser
centered at 2.85 μm overlaps better with vibrations of OH– groups
(3400 cm−1) comparing with Er3+ laser centered at 2.7 μm [6, 8, 24].
Rare earth ions doped lead-oxyfluotellurite glasses have been gained
much attention for their low phonon energy, high density, refractivity
and dispersivity, high infrared transmissivity, and high emission efficiency, but lead is harmful to the environment and its deposition in
human body can lead to a series of health problems, so lead-free glass is
the trend of future development. Based on the afore-mentioned ideas,
we would study a type of lead-free and environmental friendly Ho3+
doped oxyfluorotellurite glass with low phonon energy and high efficient 2.85 μm mid-infrared luminescence. In this paper, we have systematically studied the effect of ZnF2 content on the removal of OH– in
TeO2-PbF2 glass when PbF2 was partially up to totally substituted by
ZnF2 to form a new type lead-free TeO2-ZnF2 glass, and the final molar
ratio of TeO2 to ZnF2 was approximately to 1:1. The results of FTIR and
mid-infrared luminescent spectra indicated ZnF2 could reduce OH– effectively, and hence the emission intensity of the 2.85 μm mid-infrared
enhance obviously. Furthermore, this TeO2-ZnF2 glass presents relatively big spontaneous transition probability, highly emission, absorption and gain cross section at 2.85 μm.
3. Result and discussion
3.1. Thermal properties
Thermal stability is required for laser amplifier glass materials and
fibre drawing. The nucleation and the following microcrystal growth
should be avoided for the microcrystal incorporated inside glass matrix
would lead to optical transmission loss and light scattering. The results
of the light scattering would cause local thermal accumulation rendering permanent thermal damage. The forming ability and thermal
stability of glass can be evaluated by these three characteristic temperatures Tg, Tx and Tf, which are correspond to glass transition temperature, crystallization start temperature and the glass melting temperature, respectively, and these thermal data were listed in Table 1.
Several criteria were proposed to evaluate the glass stability in literature. Usually, because the glass melting temperature is difficult to exactly determined, the criterion ΔT = Tx-Tg introduced by Hrubý was
used to evaluate the glass forming ability and the crystallization possibility during thermal processing [25]. It is considered stable for fibre
drawing if ΔT of a glass is bigger than 100 °C [26]. In addition, the
larger ΔT indicates the better thermochemical stability and the higher
Tg means the glass can withstand higher ambient temperature.
So, the DSC measurements were carried out to evaluate the thermal
stability of the as-prepared glasses and the results of three glasses
named with Zn-0, Zn-20 and Zn-45.5 were presented in Fig.1 and
Table 1. ΔT of three samples are 121.2 °C (Zn-0),107 °C (Zn-20),112.2 °C
(Zn-45.5) respectively, all are higher than fluoroaluminate glass (73 °C)
[27] and ZBLA (74 °C) [28]. The Tg of PZ-45.5 even reached 385.7 °C.
Besides, as can be seen from the insets, when PbF2 is completely replaced by ZnF2, it presents nice transparency, indicating good thermal
stability and transparency of this oxyfluorotellurite glass matrix.
2. Experimental
Oxyfluorotellurite glass with the molar compositions of 50TeO2(45.5-x)PbF2-xZnF2–2.5YbF3-2HoF3 (x = 0,10,20,30,45.5), hereafter
labeled as Zn-0, Zn-10, Zn-20, Zn-30 and Zn-45.5, respectively, were
prepared by the conventional melt-quenching method. High purity reagents TeO2, PbF2. ZnF2, YbF3 and HoF3 powders were used as raw
materials and placed in a dry glove box filled with N2. Well-mixed 10 g
batch was melted in corundum crucible at 950 °C for 30 min, then the
molten was poured onto a preheated graphite moult and further annealed at 330 °C for 3 h to eliminate the internal stress, finally the glass
was cooled down slowly inside the furnace to room temperature. The
annealed glass sample was cut and polished to the size of
10 × 10 × 2 mm3.
Glass transition temperature (Tg) and crystallization temperature
(Tx) were recorded using a DSC204F1 Low-high temperature differential scanning calorimeter produced by the German NETZSCH company with the heating rate of 10 °C/min. The absorption spectra were
performed with Carry 5000 UV–Vis-NIRspectrophotometer in the range
of 175–3300 nm. The recractive index of glass for sample Zn-45.5 was
measured at three wavelengths of 633, 1309 and 1533 nm by using a
Metricon 2010 prism coupler, and the refractive index dependence on
wavelength was determined by Cauchy formula n = A + B/λ2, where A
and B were determined to be 2.002 and 5056 (nm2), respectively. The
Raman spectra were determined using a Renishaw Invia Microscopic
3.2. Raman spectra and structure analysis
Raman spectrum analysis utilizes the characteristics of scattered
light at different wavelengths of the excited light to explore the internal
structure and functional groups inside the glass matrix which influenced the performance of these glasses such as mid-infrared emission
property. Furthermore, it is usually used to measure the maximum
phonon energy of the solid luminescent materials. The Raman scattering spectra of three as-prepared glasses (Zn-0, Zn-20 and Zn-45.5)
-were measured in the spectral range 62–1200 cm−1 as shown in Fig.2a.
All glass samples were operated at the same condition for comparing
the frequency shift and variation. Each spectra of three glass samples
can be separated into three spectral bands: a low wavenumber region
(< 250 cm−1), an intermediate region (from 250 to 600 cm−1) and
high wavenumber region (> 600 cm−1). The band at the lower wavenumber region around 115 cm−1 can be assigned to the collective
2
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
B. Huang et al.
Fig. 1. DSC curve of three as-prepared oxyfluorotellurite glasses with different ZnF2 content (a: 0, b: 20%, c: 45.5%) and the corresponding digital photographs of
insets.
The addition of PbF2 or ZnF2 changed the coordination of Te from
[TeO4] trigonal bipyramid to a trigonal pyramid structure [TeO3]
through the intermediate [TeO3+1] unit and led to non-bridging oxygen
bonds increased. It is worth noting the intensity of Raman scattering
bands around 750 cm−1 increased with the content of ZnF2 increased,
which indicated the number of non-bridging oxygen increased. It is
considered that the radius of F− is comparable to that of O2– and one
oxygen may be replaced by two fluorine breaking the former glass
network, but in other hand, covalent Zn might capture the non-bridging
oxygen to form [ZnO4] as network generating body into the glass network. In parallel, the maximal phonon energy increased as the ZnF2
content increased and the maximum phonon energy of three samples
(Zn-0, Zn-20 and Zn-45.5) were about 745, 759 and 781 cm−1, respectively. Even so, the maximum phonon energy of matrix was still
less than borate glass (1270 cm−1), silicate glass (1100 cm−1) and
germanate glass (810 cm−1) [38–40]. Lower phonon energy would
reduce the multi-phonon relaxation, so the near- and mid-infrared lightemitting efficiency could be improved.
Ultrasmall nanocrystals or big ion clusters, which may be formed
during the process of quenching, precipitated from as-prepared glasses
(Zn-0, Zn-20 and Zn-45.5). It can be seen from the XRD patterns in Fig.2
(b) there two diffrarction peak overlapping with amorphous peaks, and
the size can be estimated with the diffraction at 28° about 1.5 nm according to Scherrer’ equation. The diffraction maybe originate from
PbF2 (JCPDS 76-1816), PbTeO3 (JCPDS 78-0448) and ZnTeO3 (JCPDS
72-1410) for the strongest lines lie at 29.17°, 28.10° and 28.97°, respectively, however, the strongest lines of ZnF2 (JCPDS 74-0918) and
modes of the heavy metal vibrational modes which were related to the
vibrations of TeeO, PbeO and ZneO [29–31], and its intensity decreased with the PbF2 content decreasing. When all PbF2 was replaced
by ZnF2, the scattering peak at 115 cm−1 decreased obviously.
Stretching vibrations of [TeO3+1] and [TeO4] can cause the scattering
peaks in the intermediate wavenumber region from 250 to 600 cm−1
[8, 21, 31, 32]. Due to the two unequivalent TeeO bonds, the antisymmetric bending vibrations of Te-O-Te linkages rendered the scattering peak centered at 417 cm−1 for Zn-45.5 glass sample, which was
related to the continuous network linkages build up by [TeO4] trigonal
bipyramids similar to the three-dimensional network of α-TeO2
[31–33]. The internal gap among the loose reticular formation of
[TeO4] trigonal bipyramids of tellurite glass can hold much network
external body such as metallic oxides or fluorides. So, the peaks observed around 340, 511 cm−1 for Zn-0 and 348 cm−1 for Zn-20 can be
ascribed to the vibrations of PbeF [34] which exists among [TeO4] gaps
and the glass matrix, and the peak at 348 cm−1 of Zn-20 shifts to right a
little for the addition of ZnF2 comparing with Zn-0. No Raman scattering characteristic peaks of ZnF2 were found when PbF2 was substituted by the addition of ZnF2 [35]. The highest wavenumber region
from 600 to 900 cm−1 was assigned to the stretching vibrations of nonbridging oxygen bonds associated with [TeO3+1], [TeO3] and [TeO4]
[8, 21, 31, 32, 36, 37]. Obviously, the introduction of PbF2 or ZnF2 as
glass network modifier led to the presence of [TeO3] trigonal pyramids
through an intermediate stage formation of [TeO3+1] polyhedra involving the increase of non-bridging oxygen and internal glass network
breaks.
Fig.2. (a) Raman spectra of three oxyfluorotellurite glasses with different ZnF2 content and (b) corresponding XRD pattern of sample TeO2-ZnF2 (Zn-0, Zn-20, Zn45.5).
3
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
B. Huang et al.
2.0118, 2.7444 and 0.9463× 10−20 cm2, respectively. Generally, J-O
parameters can reflect the information of rare earth ions host, in which
Ω2 is sensitive to the covalence and the ligand symmetry of host materials. The Ω2 value of Ho3+ doped TeO2-ZnF2 glass (Zn45.5) is smaller
than that of Ho3+ doped TeO2-PbF2 glass (Ω2 = 2.35 × 10−20 cm2)
[45], tellurite oxy-halide glass (Ω2 = 8.82 × 10−20 cm2) [46] germanotellurite glass (Ω2 = 4.252 × 10−20 cm2) [47], heavy-metal oxide
glass (Ω2 = 4.77 × 10−20 cm2) [48] and chalcogenide glass
(Ω2 = 6.98 × 10−20 cm2) [48], higher than that of Ho3+ doped fluoroaluminate glass (Ω2 = 1.59 × 10−20 cm2) [49] and fluoride glass
(Ω2 = 1.86 × 10−20 cm2) [50]. Obviously, Fluorine has higher electronegativity (4.0) compared to that of Oxygen (3.44) and Sulphur
(2.58), which makes HoeF bond more ionic than HoeO and HoeS. The
ligand ions can be F− or O2– or both of them in Zn-45.5 glass, so the
value of its Ω2 is smaller than that of oxide and chalcogenide glasses
and higher than that of fluoride glass and oxyfluoride glass with higher
F− concentration. The relative low Ω6 value means stronger ionic
bonding of Ho3+ and higher rigidity of TeO2-ZnF2 glass. It is worthy
noted that the Ω4/Ω6 determines the spectroscopic quality of the laser
host materials and higher Ω4/Ω6 value indicates favorable mid-infrared
emission. The TeO2-ZnF2 glass (Zn-45.5) with relative higher Ω4/Ω6
value (2.90) indicates nice mid-infrared emission quality.
The radiative transition probabilities of the excited levels of Ho3+
can be calculated with J-O parameters as following [41–44]:
TeO2 (JCPDS 78-1714) deviate more from 28°. In fact, the peak around
28° shifts right a little when all PbF2 was substituted by ZnF2 for the
strongest line of ZnTeO3 lies more right than that of PbTeO3, and another diffraction peak shifts left for ZnTeO3 has more stronger diffraction peaks between 30° to 45°3. So is can be deduced that the diffraction
peaks mainly originated from PbTeO3 and ZnTeO3, and trace PbF2 exist
in Zn-0 and Zn-20 glasses.
3.3. Absorption spectra and Judd-Ofelt analysis
The absorption spectra of Ho3+/Yb3+ codoped TeO2-ZnF2 glass at
room temperature in the range of 350–2200 nm are shown in Fig.4. The
absorption bands are mainly centered at 420, 452, 540, 650, 1165 and
1940 nm, which are assigned to the transition from the ground state 5I8
of Ho3+ to the excited states 5F1、5S2、5F3、5S2 + 5F4 and 5F5, respectively. In addition, it can be observed an intense broad absorption
band ranged from 900 to 1000 nm and centered at 980 nm which originated from the intense absorption of the 2F7/2 level to 2F5/2 level of
Yb3+. It is meant that the codoping of Yb3+ would be expected to
provide an efficient excitation channel.
The absorption spectra can be used to predict the spectroscopic
parameters of rare earth ions doped materials such as radiative transition probability, radiative lifetime of various transitions, fluorescence
branching ratio, even the relative intensity of emission bands through
the Judd-Ofelt theory [41–44]. The experimental oscillator strength
(Fexp) for each absorption band can be obtained according to the JuddOfelt theory as the following [41–44]:
fexp =
2.303mc 2
πe 2Ndλ2
A J ′→ J =
∫ OD (λ) dλ
8π 2mcν (n2 + 2)2
3h (2J + 1)
9n
∑
(1)
Smd =
hν
1
n 〈ψJ ‖L + 2S‖ ψ′J ′〉2
6mc (2J + 1)
N
h2
16π 2m2c 2
〈ψJ ‖L + 2S‖ ψ′J ′〉2
(2)
1
∑ A J ′→ J
J′
2
β J ′→ J =
A (ψ′J ′, ψJ )
∑ A (ψ′J ′, ψJ )
J′
(6)
(7)
(8)
(9)
The calculated radiative probability, branching ratio and transition
lifetime with used doubly reduced matrix elements are listed in Table 3.
The radiative transition probability (Arad) of Ho3+: 5I6 → 5I7 is
45.24 S−1, which is much bigger than that in fluoroindate (11 S−1),
fluoroaluminate (31.77 S−1), fluoride (31.43 S−1), gallate (28.59 S−1),
aluminate (24.92 S−1), fluorophosphate (34.11 S−1), germanate
(7.33 S−1), silicate (15.01 S−1), phosphate (19.21 S−1) glass [49–52],
and very close to that of lanthanum–tungsten–tellurite (41.2 S−1) glass
[53], but smaller than that of chalcogenide (113 S−1) glass [54]. Obviously, the relative bigger spontaneous radiative transition probability
would provide a better opportunity to achieve mid-infrared laser action
at 2.85 μm in this TeO2-ZnF2 (Zn45.5) glass.
(3)
Where ν is the wave-number between the initial state to the final state,
〈ψJ‖L + 2S‖ψ′J′〉 of Ho3+ can be expressed as {[(S + L + 1)2 − J2]
[J2 − (L − S)2]/4J}1/2 when ΔS = ΔL = 0, ΔJ = 0, ± 1. Judd-Ofelt
parameters Ωt (t = 2, 4, 6) can be calculated using a least squares fitting
approach through the experimental measured values of oscillator
strength for different transitions. The measured, calculated oscillator
strength values and J-O parameters are listed in Table 2. The root mean
square deviation between the measured (Fexp) and the calculated (Fcal)
osillator strengths can be calculated as [41–44]:
3.4. Mid-infrared luminescence and FTIR analysis
1/2
⎤
⎡
δ = ⎢∑ (Δf )2 /(p − 3) ⎥
⎦
⎣ i=1
Ωt 〈ψJ ‖U (t ) ‖ ψ′J ′〉2
The radiative lifetime and the fluorescent branching ratio are related to the radiative transition probilities by [41–44]:
Where h is Plank constant, n is the glass refractive index, (n + 2) /9n is
the local field correction for electric dipole transition, Ωt (t = 2, 4, 6) is
Judd-Ofelt intensity parameters and 〈ψJ‖U(t)‖ψ′J′〉 is the double reduced matrix elements of absorption transitions, which value can be
obtained from reference [44].
The magnetic dipole (Fmd) can be given by the following [41–44]:
fmd =
∑
t = 2,4,6
τrad =
2
2
Sed (ψJ , ψ′J ′) =
Ωt 〈ψJ ‖U (t ) ‖ ψ′J ′〉2
t = 2,4,6
(5)
where n(n + 2) /9 is the local field correction for electric dipole
transitions Sed and n3 for Smd. The Sed and Smd can be given by the
following, respectively [41–44]:
2
Where m is the mass of electron, c is the light velocity, e is the electron
charge, N is the concentration of the dopant rare earth ions, d is the
thickness of sample, and OD(λ) is the measured absorption optical
density.
The theoretical oscillator strength (Fcal) includes two parts: the
induced electric dipole (Fed) and magnetic-dipole (Fmd) 4f → 4f transitions from the initial state |S, L, J〉 to the final state |S′, L′, J′〉. The
electric dipole (Fed) can be expressed as [41–44]:
fed =
2
2
64π 4e 2
⎡ n (n + 2) Sed + n3Smd⎤
3
⎥
⎢
3hλ (2J + 1) ⎣
9
⎦
Fig. 4 (a) shows the 2.85 μm mid-infrared spectra of Ho3+/Yb3+
codoped oxyfluorotellurite glasses with different ZnF2 content under
the excitation of 980 nm diode laser. It is observed an intense broad
mid-infrared luminescent emission band ranged from 2700 to 3100 nm
and centered at 2.85 μm owing to Ho3+: 5I6 → 5I7 transition. The full
width at half maximun of these oxyfluorotellurite glasses can be
(4)
where p is the number of transitions for calculating the oscillator
strength. δ value is calculated to be 48.97 × 10−8 indicating that the
calculating process is reliable. Three J-O parameters Ωt (t = 2, 4, 6) are
4
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
B. Huang et al.
Table 2
J-O parameters, Oscillator strength emission and absorption cross section of TeO2-ZnF2 (Zn-45.5) glass.
I8→
5
λ (nm)
5
I7
1940
I6
1165
5
F5
650
5
S2, 5F4
540
5
G6
452
5
G5
420
Ω4/Ω6 = 2.90, δ = 48.97 × 10−8
5
Fex(×10−8)
fcal(×10−8)
Ω2
2.0118 × 10−20 cm2
168.3
99.1
339.8
440.6
1547.1
413.0
185.1
85.5
335.3
366.2
1545.4
447.0
Ω4
Ω6
Absorption cross section at 1936 nm
Emission cross section at 2030 nm
Absorption cross section at 2847 nm
Emission cross section at 2853 nm
2.7444 × 10−20 cm2
0.9463 × 10−20 cm2
0.52 × 10−20 cm2
0.84 × 10−20 cm2
0.86 × 10−20 cm2
0.96 × 10−20 cm2
This illustrated that when ZnF2 content was increased to a certain extent, the hydroxyl group in the glass matrix would be no longer reduced. It is known that the OH– content in the glass matrix can be
expressed by the absorption coefficient of OH– vibrations around 3 μm
[6]:
calculated to be bigger than 100 nm, which is much larger than that of
the chalcogenide and fluoroaluminate glasses [49, 55]. It is obviously
that the wider emission band would be favor of the applications in midinfared fibre amplifier and broadband tunable lasers. Interestingly,
though the rare earth ions doping condition kept the same, 2.85 μm
mid-infrared luminescent intensity was prominently enhanced with the
PbF2 gradually replaced by ZnF2, and it reached the maximum value
when all PbF2 was completely replaced by ZnF2.
Factually, according to the Raman spectra analysis, the phonon
energy of these glass matrixs increased with the content of ZnF2 increased that would not be favor of the 2.85 μm mid-infrared luminescence and it is contradictory with the increase of 2.85 μm mid-infrared
emission intensity. To elucidate the contradiction, the infrared transmission spectra of these oxyfluorotellurite glasses were measured and
the results were shown in Fig.3 (b). The maximal transmittances of
these glasses were reached 74%, and the transmitted infrared cutoff
wavelength was above 6200 nm. It could be observed an intense
characteristic absorption peak centered at 2946 nm (3394 cm−1) which
was ascribed to the strong absorption of hydrogen bonds and OH–
combined with non-bridging oxygen. Two weak peaks were located at
3425 nm (2920 cm−1) and 3905 nm (2850 cm−1) which can be ascribed to the stretching vibration of hydrogen bond in the glass internal
networks. Corresponding to the 2.85 μm mid-infrared luminescence, the
characteristic absorption intensity of OH– centered at 2946 nm decreased obviously with ZnF2 content increased. As the ZnF2 content
reached above 30%, the absorption intensity at 2946 nm varied little.
α (OH−) = ln(T0/ T )/ l
(10)
where l is the glass sample thickness, T is the transmittance at 2.85 μm
and T0 is the incident intensities, respectively. The results of the calculated absorption coefficient were 1.66, 1.37, 1.03, 0.60 and 0.58 for
Zn-0, Zn-10, Zn-20, Zn-30 and Zn-45.5, respectively. The concentration
of OH– groups can be estimated by the equation [8, 55]:
N (OH−) =
NA
α (OH−)
ε
(11)
where NA is the Avogadro number, and ε is the molar absorptivity of the
OH– groups remained in glass and here a value of 4.91 × 104 cm2/mol.
The values of OH– concentration and absorption coefficient for these
glasses were listed in Table 1. The considerable absorption coefficient
values of all glass samples meant much residual OH– incorporated in the
glass matrix. It is known that the OH– groups have great influence on
near ~3 μm mid-infrared emission since OH– incorporated in glass acts
as luminescence-quenching center of the active ions. It is noted from
Fig.5 that the absorption of OH– decreased obviously as the addition of
ZnF2 content increased, and the corresponding results were the 2.85 μm
emission intensity of of Ho3+/Yb3+ codoped oxyfluorotellurite glasses
Table 3
Doubly reduced matrix elements [44], calculated radiative probability, branching ratios, and lifetimes of Ho3+ in zinc oxyfluorotellurite glass.
Transition
λ (nm)
|U2|2
|U4|2
|U6|2
Sed (×10−20)
Aed (s−1)
Smd (×10−20)
Amd (s−1)
Arad (s−1)
β(%)
τrad (s)
I7 → I8
I6 → 5I8
5
I7
5
I5 → 5I8
5
I7
5
I6
5
I4 → 5I8
5
I7
5
I6
5
I5
5
F5 → 5I8
5
I7
5
I6
5
I5
5
I4
5
S2 → 5I8
5
I7
5
I6
5
I5
5
I4
5
F5
5
F4 → 5I8
5
I7
5
I6
5
I5
5
I4
5
F5
1940
1165
2850
895
1653
3920
756
1234
2172
4869
650
961
1448
2297
4348
544
755
1026
1391
1947
3527
536
741
1002
1342
1854
2947
0.0249
0.0087
0.0314
0
0.0028
0.0435
0
0
0.0023
0.031
0
0.0194
0.0113
0.0071
0.0001
0
0
0
0
0.0014
0
0
0
0.0011
0.0016
0.0002
0.1980
0.1344
0.0389
0.1324
0.0102
0.0226
0.1703
0
0.0034
0.0282
0.1237
0.4201
0.3309
0.1242
0.0281
0.0061
0
0
0.0240
0.0052
0.0302
0.0123
0.2385
0.1965
0.2574
0.1334
0.0241
0.0920
1.5231
0.6920
0.9295
0.0930
0.8896
0.5720
0.0076
0.1568
0.6639
0.9103
0.5701
0.4298
0.4972
0.1630
0.0036
0.2145
0.0553
0.1458
0.0968
0.2839
0.0050
0.7090
0.032
0.1704
0.4666
0.2576
0.0071
1.8603
0.7791
1.3061
0.1160
0.9095
1.0962
0.0072
0.1577
0.7103
1.2633
1.6924
1.3539
0.8341
0.2456
0.0203
0.2030
0.0523
0.2038
0.1059
0.3544
0.0385
1.3255
0.5696
0.8699
0.8109
0.3103
0.6575
86.80
168.60
21.76
55.63
77.82
8.092
5.76
32.52
30.87
5.75
2140.64
593.47
122.63
10.66
0.16
442.12
47.57
84.60
20.76
30.90
0.46
3020.87
548.36
387.83
177.12
31.34
13.51
0.952
44.40
23.48
100
78.84
21.16
36.63
51.23
12.14
7.27
41.07
38.99
12.67
74.65
20.7
4.28
0.37
0
70.63
7.6
13.52
3.32
4.94
0.07
72.11
13.09
9.26
4.23
0.75
0.56
0.00762
0.00468
1.410
131.20
168.60
45.24
55.63
77.82
18.44
5.76
32.52
30.87
10.03
2140.64
593.47
122.63
10.661
0.16
442.12
47.57
84.60
20.76
30.90
0.46
3020.87
548.36
387.83
177.12
31.34
23.63
5
5
5
5
1.402
0.941
0.493
10.35
4.28
10.13
0.00658
0.01263
0.00035
0.00165
0.00024
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
B. Huang et al.
Fig. 3. Absorption spectra of Ho3+/Yb3+ codoped TeO2-ZnF2 (Zn45.5) oxyfluorotellurite glasses.
Fig.4. (a) Mid-infrared luminescence and (b) FTIR spectra of Ho3+/Yb3+ codoped oxyfluorotellurite glasses with PbF2 replaced by different ZnF2 content.
Fig. 5. (a) Emission cross section line shape calculated by Füchtbauer-Ladenburg method and absorption cross section line shape calculated by McCumber relationship for Ho3+: 5I6 → 5I7 emission, (b) calculated gain cross section shape line for Ho3+ at 2850 nm laser emission.
6
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
B. Huang et al.
Fig.6. (a) Infrared emission spectra, (b) absorption and emission cross section of Ho3+: 5I7 → 5I8 transition, (c) gain cross section of Ho3+: 5I7 → 5I8 transition for
TeO2-ZnF2 glass (Zn-45.5) and (d) the energy transfer sketch of Ho3+/Yb3+ codoped TeO2-ZnF2 glass (Zn-45.5) pumped by the excitation of 980 nm diode laser.
Zn-0 and Zn-20 glasses. So, the addition of ZnF2 can release more free
F− ions reacting with hydroxyl routes to form the volatizing gas HF
from the glass melt, and finally rendering substantial decrease of OH–
groups resulted in the enhanced intensity of 2.85 μm mid-infrared
emission.
increased substantially. The emission intensity of Zn-45.5 glass at
2.85 μm is 2.85 times of that Zn-0 glass while all PbF2 was replaced by
ZnF2.
When all PbF2 was replaced by ZnF2, the OH– residuals were reduced to 34.92% of the original value. In fact, the addition of PbF2 can
remove the OH– in GeO2-PbO-Na2O-Ga2O3 glass system when PbO was
partially or totally replaced by PbF2 [7]. Our results revealed that the
addition of ZnF2 had a better effect on the removal of OH– groups.
According to the analysis of Raman results, it is hardly found the
characteristic vibrations of ZneF, but the Raman scattering of PbeF
could be easily found. Generally, the addition of fluoride would be
favor of removal of hydroxyl routes from the glass melt at the melting
temperature under dry atmosphere according to the equations:
2[≡Te − OH] + 2F− → HF ↑ + ≡Te − O − Te≡
3.5. Emisison, absorption and gain cross section
The mid-infrared spectra at 2.85 μm are selected to calculate stimulated emission and absorption cross section in order to estimate the
gain property of this TeO2-ZnF2 (Zn45.5) glass in this wavelength region. The stimulated emission cross section (σe) is calculated from the
emission spectra by using the Füchtbauer-Ladenburg Eq. [56]:
(12)
σe (λ ) =
As the volatilization of HF gas, metal cations were left as network
modifier. In the glass melt and the cooled glass, the glass consists of a
continuous random network of [TeO3], [TeO3+1] and [TeO4] polyhedra. For Zn2+ ions have much smaller radius (0.74 Å) than Pb2+ ions
(1.19 Å), it is easier to get into the gap of tellurium oxide polyhedra. On
the other hand, α-PbF2 (S. G: Pmnb(62), a = 3.898, b = 6.441,
c = 7.679) has the similar layered structure to PbTeO3 (S. G.: P41 (76),
a = b = 5.304, c = 11.9) and ZnTeO3 (S. G.: Pbca (61), a = 7.36,
b = 6.381, c = 12.32) at low temperature and therefore more Pb2+
ions can exist with F− ions as the coordination polyhedra in the cooled
glass. In fact, the XRD patterns proved ultrasmall PbTeO3 and ZnTeO3
nanocrystals (average size < 2 nm) exist in the as-prepared glasses.
Obviously, the similar layered structure α-PbF2 with PbTeO3 and
ZnTeO3 has more tendency to coexist with PbTeO3 and ZnTeO3 in the
Arad λ5I (λ )
8πn2c ∫ λI (λ ) dλ
(13)
Where Arad is the spontaneous radiative transition probability of Ho3+:
5
I6 → 5I7 transition, I(λ) is the emission intensity at the region of
2.85 μm. The absorption cross section (σa) can be can be calculated by
McCumber theory according to the emission cross section as [41, 57]:
σa (λ ) = σe (λ )
Zu
E − hc / λ ⎞
exp ⎛− ZL
Zl
kB T
⎠
⎝
⎜
⎟
(14)
Where kB is Boltzmann constant, T is the Kelvin temperature, EZl is the
zero-line energy (the energy separation between the lowest components
of the upper and lower states), and Zu/Zl is the partition function and
can be expressed as [57]:
7
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
B. Huang et al.
n
Zl
=
Zu
4. Conclusion
1 + ∑ exp(−E1j / kT )
j=2
m
1 + ∑ exp(−E2j / kT )
j=2
Yb3+/Ho3+ codoped 50TeO2-(45.5-x)PbF2-xZnF2–2.5YbF3-2HoF3
(x = 0,10,20,30,45.5) oxyfluorotellurite glass had been synthesized
with high temperature melt-quenching method, and intense mid-infrared fluorescence of 2.85 μm can be observed in these glasses. The
increasing of ZnF2 content would raise the glass transition temperature
and enhance the thermal stability of matrix, especially enhance the
emission intensity of 2.85 μm mid-infrared fluorescence obviously. ZnF2
could reduce the OH– concentration in glass substantially, which were
favor of the mid-infrared emission. Especially when ZnF2 substitutes
PbF2 completely, the TeO2-ZnF2 glass presents nice near- and mid-infrared emission, especially in 2.85 μm mid-infrared region. The TeO2ZnF2 glass presents high quality of mid-infrared luminescence with high
ratio of the strength parameters Ω4/Ω6 (=2.90) and higher spontaneous transition probability (45.25 S−1) along with the larger calculated emission cross section (0.96 × 10−20 cm2) corresponding Ho3+:
5
I6 → 5I7 transition.
(15)
Where Eij is the difference in energy between the jth and the lowest
component of level i and the fine energy level structure can referred to
[58]. Here, the value of Zu/Zl can be calculated as 0.94 and EZl is
3517.9 cm−1 for Ho3+: 5I6 → 5I7.
According to the calculated emission and absorption cross section of
Ho3+: 5I6 → 5I7 for TeO2-ZnF2 glass (Zn45.5) presented in Fig.5 (a), the
maximum values of σe and σa reach 0.96 and 0.86 × 10−20 cm2 at about
2850 nm and are listed in Table 2. Higher emission cross section indicates that better laser gain can be obtained in this TeO2-ZnF2 (Zn45.5)
glass, which value is higher than that of ZBLAN (0.50 × 10−20 cm2)
[59], silica-germanate (0.805 × 10−20 cm2) [56], germanate
(0.92 × 10−20 cm2) [6] glass, and very close to that of fluoride glass
(0.98 × 10−20 cm2) [60], but lower than that of fluoroaluminate
(1.91 × 10−20 cm2) [49], fluorotellurite (1.51 × 10−20 cm2) [61]
glasses. It is usually necessary to achieve big enough emission cross
section to provide high gain performance so as to result in better amplification behavior. In addition, the gain character of the amplifier
depends on the emission cross section and its full-width at half-maximun (FWHM). Here FWHM of emission and absorption cross section
for Ho3+: 5I6 → 5I7 transition are 81 and 68 nm, which means the
Ho3+/Yb3+ codoped TeO2-ZnF2 is anticipated to be an effective gain
host material for broadband amplifier and Ho3+/Yb3+ codoped laser in
mid-infrared region.
According to the calculated σe and σa, the wavelength dependence of
net gain is the function of population inversion for upper laser level for
the sake of determination of the gain property quantitatively. The gain
cross section can be calculated with:
G (λ ) = pσe (λ ) − (1 − p) σa (λ )
Acknowledgement
This work is supported by the National Natural Science Foundation
of China (Grant No. 11574260) and Opening foundation from key laboratory of low dimensional materials and its application technology,
Ministry of Education (KF20140201).
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where P is the population inversion given by the ratio between the
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8
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