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Broadband 2.84 μm luminescence properties and Judd–Ofelt analysis in Dy3+ doped ZrF4–BaF2–LaF3–AlF3–YF3 glass
Journal of Luminescence 132 (2012) 128–131
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Broadband 2.84 mm luminescence properties and Judd–Ofelt analysis in Dy3 þ doped ZrF4–BaF2–LaF3–AlF3–YF3 glass Ying Tian a,b, Rongrong Xu a,b, Lili Hu a, Junjie Zhang a,n a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2011 Received in revised form 27 July 2011 Accepted 8 August 2011 Available online 12 August 2011
2.84 mm luminescence with a bandwidth of 213 nm is obtained in Dy3 þ doped (ZrF4–BaF2–LaF3–AlF3–YF3) ZBLAY glass. Three intensity parameters and radiative properties have been determined from the absorption spectrum based on the Judd–Ofelt theory. The 2.84 mm emission characteristics and energy transfer mechanism upon excitation of a conventional 808 nm laser diode are investigated. The prepared Dy3 þ doped ZBLAY glass possessing high predicted spontaneous transition probability (45.92 s 1) along with large calculated emission cross section (1.17 10 20 cm2) has potential applications in 2.8 mm laser. & 2011 Elsevier B.V. All rights reserved.
Keywords: 2.84 mm luminescence ZBLAY glass Broadband
1. Introduction Recently considerable research efforts have been devoted to mid-infrared lasers operating around 3 mm due to their various applications, including military, remote sensing, atmosphere pollution monitoring and medical surgery [1–4]. Among various rare earth ions Dy3 þ is a natural candidate for 3 mm laser owing to the 6H13/2-6H15/2 transition, and several results have been reported on the near 2.8 mm emission in various types of Dy3 þ -doped crystals and non-oxide [5–8] glasses. Particularly, infrared emission of Dy3 þ has been obtained in lead oxide complex glass [9]. The reported Dy3 þ -doped materials are always excited by complicated and expensive lasers including a Ti3 þ :sapphire laser driven by an Ar þ laser [5,7], 1.3 mm Nd:YAG laser [8] and 1.1 mm Yb silica fiber laser [10], to obtain 2.8 mm emission. In pursuit of an efficient, cost-effective and durable solid state laser source, chemically stable host, cheap excitation (e.g. 808 nm laser diode) and efficient absorption are required. However, few results have been reported on 2.8 mm luminescence in Dy3 þ doped glasses pumped by common 808 nm laser diode (LD) at room temperature. ZrF4 based fluoride glasses are well known for their high solubility of rare earth ions and long fluorescence lifetime of the excited electronic states as well as their low nonlinear refractive index [11–15]. Furthermore, mid-infrared laser produced from fluoride fiber has been demonstrated in many cases [1,3,10,16].
n
Corresponding author. Tel.: þ86 21 5991 4297; fax: þ 86 21 5991 4516. E-mail address: [email protected] (J. Zhang).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.08.017
However, fluoride glasses suffer from poor chemical stability [17]. It is known that the addition of YF3 can increase the viscosity of the melt and enhance the stability of the glass against crystallization[18], so YF3 is introduced into the fluoride matrix. However, few results focused on the 2.8mm emission in Dy3 þ doped ZrF4–BaF2–LaF3–AlF3–YF3 (ZBLAY) glass and the corresponding optical properties. In this work, the spectroscopic properties and energy transfer mechanism relating to the 2.8 mm emission of ZBLAY glass pumped by 808 nm LD are investigated. Moreover, the Judd–Ofelt intensity parameters, spontaneous transition probability and stimulated emission cross section are calculated and discussed.
2. Experimental Glass with the molar composition 50ZrF4–33BaF2–17(LaF3 þ AlF3 þYF3)–1DyF3 was prepared by the tradition melt quenching method. The sample was prepared by using high-purity ZrF4, BaF2, YF3, AlF3, LaF3 and DyF3 powder. A batch of about 25 g was mixed homogenously and placed in a platinum crucible. It was melted at 900 1C for a time long enough to assure an approximately homogeneous distribution of the Dy3 þ dopants in the melt. The melt was poured on a preheated steel plate, which keeps the rare earth ions distribution as homogeneous as in the melt. The obtained glass was annealed for several hours to remove thermal strains and then allowed to cool to room temperature. The experimental procedures used in this work as well as high rare earth ions solubility of fluoride glass allow Dy3 þ doped ZBLAY glasses with good optical quality,
Y. Tian et al. / Journal of Luminescence 132 (2012) 128–131
transparency and homogeneity. Finally, the annealed sample was fabricated and polished to the size of 20 mm 10 mm 1 mm to be tested for the optical properties. The absorption spectrum has been recorded using a JASCO V-570UV/vis spectrophotometer in the range of 400–2000 nm. The mid-infrared transmission spectrum was obtained by using a Thermo Nicolet (Nexus FT-IR spectrometer) in the range of 2.5–10 mm. The photoluminescence spectrum was carried out with a Triax 320 type spectrometer (Jobin-Yvon Co., France) upon excitation at 808 nm. All the measurements were carried out at room temperature.
129
Table 1 J–O parameters Ot of Dy3 þ in various glasses.
Ot (10 20 cm2)
ZBLAY
ZBLA
ZBLAN
ZBLALi
O2 O4 O6
3.16 7 0.01 1.67 7 0.01 2.45 7 0.02 Present work
3.22 1.35 2.38 [24]
1.86 1.42 2.37 [25]
2.70 1.80 2.00 [26]
Ref.
Table 2 Radiative parameters for various selected excited levels of Dy3 þ in ZBLAY glass.
3. Results and discussion 3.1. Absorption spectrum and Judd–Ofelt analysis Fig. 1 shows the absorption spectrum of the 1 mol% Dy3 þ doped sample recorded at room temperature in the wavelength region of 400–2000 nm. The absorption bands corresponding to the transitions starting from the 6H15/2 ground state to higher levels 6H11/2, 6F11/2, 6F 9/2, 6F7/2, 6F5/2, 6F3/2 and 4F9/2 are labeled. Generally, the shape and peak positions of transitions in Dy3 þ doped ZBLAY glass are similar to those in other Dy3 þ -doped glasses [19,20], except small differences originating from different chemical nature of the host. The absorption band around 800 nm indicates that this glass can be excited efficiently by 808 nm LD. The inset of Fig. 1 presents the mid-infrared transmittance spectrum of Dy3 þ doped ZBLAY sample. As it can be seen, the transmittance reaches as high as 88% and extends up to 9.0 mm. It is noted that the prepared glass has high transmittance around typical H2O absorption band ( 3 mm). Therefore, excellent midinfrared transmission property provides ZBLAY glass potential application for 2.8 mm laser material. The absorption spectra were used for calculation of the Judd– Ofelt parameters, spontaneous emission transition probabilities, lifetimes and branching ratios, using the procedure as described previously [21–23]. Table 1 gives a comparison of Judd–Ofelt intensity parameters Ot (t¼ 2, 4, 6) in various glass hosts. The quality of the calculation is indicated by the root-mean-square deviation, which is equal to 0.3 10–6. This value represents a fairly good agreement between the measured and calculated oscillator strengths and reliable calculations.
Initial level
Terminal level
A (s 1)
6
6
45.92 9.04 80.09 5.46 25.37 61.17 0.00 25.27 83.64 723.07 0.86 1.22 22.40 236.20 911.74 0.00 0.27 2.16 10.79 32.73 18.68 1.66 0.61 0.31 7.28 28.87
H13/2 H11/2
6
6
H9/2
6
F11/2
6
F9/2
6
H7/2
6
H5/2
H15/2 H13/2 6 H15/2 6 H11/2 6 H13/2 6 H15/2 6 H9/2 6 H11/2 6 H13/2 6 H15/2 6 F11/2 6 H9/2 6 H11/2 6 H13/2 6 H15/2 6 F9/2 6 F11/2 6 H9/2 6 H11/2 6 H13/2 6 H15/2 6 H7/2 6 F9/2 6 F11/2 6 H9/2 6 H11/2 6
P A (s 1) 45.92 89.13
92.01
831.98
1172.41
64.63
38.73
b (%)
trad (ms)
100.00 10.15 89.85 5.94 27.58 66.49 0.00 3.04 10.05 86.91 0.07 0.10 1.91 20.15 77.77 0.00 0.42 3.34 16.70 50.64 28.90 4.29 1.57 0.80 18.79 74.55
21.78 11.22
10.87
1.20
0.85
15.47
25.82
According to C. K. Jørgensen and Reisfeld [24], O2 is strongly affected by covalent chemical bonding, while O6 is related to the rigidity of the medium in which ions are situated. From Table 1, it is found that the calculated O2 in present glass is comparable to that of ZBLA glass [25] and higher than that of ZBLAN glass [26]. It is known that O2 increases with the asymmetry of the local structure. So it can be inferred that there is a higher asymmetrical surrounding of rare earth ions in the present glass than that in ZBLAN glass. As is shown in Table 1, O4 and O6 of ZBLAY are close to those of other fluoride glasses [25–27], which indicates the rigidity of the ligand between these fluoride systems is similar.
3.2. Radiative properties
Fig. 1. Absorption spectrum of Dy3 þ doped sample. The inset is the mid-infrared transmittance spectrum of Dy3 þ doped ZBLAY sample.
The J–O intensity parameters (Ot) have been widely used to estimate certain radiative parameters and predict luminescence characteristics of certain transitions. Table 2 shows the calculated predicted spontaneous transition probability (A), total spontaP neous transition probability ( A), branching ratios (b) and radiative lifetime (trad) of the optical transitions for the Dy3 þ doped ZBLAY glass. From Table 2, it is noted that the predicted spontaneous emission probabilities for Dy3 þ :6H13/2-6H15/2 transition is 45.92 s 1, which is significantly higher than for ZBLA (19.5 s 1) [25]. Higher spontaneous transition probability provides the material with a better opportunity to obtain laser actions [28]. Thus, the present glass can be selected as a promising 2.8 mm laser material.
130
Y. Tian et al. / Journal of Luminescence 132 (2012) 128–131
3.3. Photoluminescence spectrum, energy transfer mechanism and emission cross section Fig. 2 shows the measured photoluminescence spectrum of the Dy3 þ doped ZBLAY glass pumped by 808 nm LD. As is shown in Fig. 2, intense emission with a peak at 2.84 mm due to the transition 6H13/2-6H15/2 can be observed. Moreover the emission spectrum of present glass is broad and smooth with a full bandwidth at half maximum (FWHM) 213 nm around 2.84 mm spectral region, which can be attractive fiber gain element capable in the ultra-short pulse laser generation. The involved energy transfer mechanisms are indicated in the inset of Fig. 2. Firstly ions of the 6H15/2 level are excited to the 6F5/2 state by ground state absorption (GSA) when the sample is excited by 808 nm LD. Then the excited energy stored in the 6F5/2 level decays non-radiatively to the next-lower levels. The upper laser level 6H13/2 is populated by the mentioned non-radiatively decay process. Finally, ions in the 6H13/2 level decay radiatively to the 6 H15/2 level with 2.84 mm emission. In order to better understand 2.84 mm luminescence features, the lifetime (tmea) for the 6H13/2 level in the present glass is important. The radiative lifetime reported in Table 2, the energy transfer rates, and the multiphonon emission rates calculated can be according to the following equation [29]: 1
tmea
¼ WR þWET þWCR þ WMPR ,
ð1Þ
where WR is the radiative transition rate; WMPR, WCR and WET are the rate of the multiphonon relaxation, cross-relaxation and energy transfer to impurities (i.e., OH ), respectively. Since the processing route was controlled to reduce the OH content and rare earth concentration in the present glass is not high, tmea becomes [30] 1
tmea
¼
1
tr
þWMPR ,
ð2Þ
where tmea and tr are the measured and the radiative lifetime calculated from the Judd–Ofelt analysis, respectively. The multiphonon relaxation rates WMPR(0) at 0 K from different levels can be described as [25] WMPR ð0Þ ¼ CeaDE ,
where a and C are positive-definite constants related to the characteristic of host material, andDE is the energy difference between the 6H13/2 and 6H15/2 levels. The values of constants C and a have been determined previously for fluorozirconate glass to be C ¼1.88 1010 s 1 and a ¼5.77 10 3 [23]. With increasing temperature T, WMPR(T) becomes [31] WMPR ðTÞ ¼ WMPR ð0Þ½1e_wmax =kT p ,
ð4Þ
where _wmax is the highest phonon energy and P¼ DE/_wmax. The multiphonon relaxation rate WMPR(T) and lifetime at 300 K were calculated to be 41.52 s 1 and 11.43 ms, respectively. WMPR of 6H13/2-6H15/2 in the present glass (41.52 s 1) is lower than that of the fluoride glass (65.7 s 1) [25]. However, the multiphonon relaxation processes will influence the 2.84 mm luminescence efficiency in the present glass; tmea (11.43 ms) of prepared glass is comparable to that of ZBLA glass (11.7 ms) [25] but larger than that of other fluoride glass (5 ms) [32]. The quantum efficiency of the 6H13/2-6H15/2 transition (ratio of tmea to the calculated lifetime tr using the Judd–Ofelt theory) was calculated to be 52.48%. The phonon sub-system plays a role in the kinetics features of Dy3 þ fluorescence, which includes many photoinduced anharmonic electron–phonon interactions [9,33–35]. Thus, the present Dy3 þ doped ZBLAY glass with high quantum efficiency is a promising material for 2.8 mm laser. High stimulated emission cross-section is extremely useful to determine the possibility to achieve laser effect [36]. According to the Fuchtbauer–Ladenburg theory, the emission cross section sem can be calculated as [37]
sem ¼
l4 Arad 8pcn2
R
lIðlÞ lIðlÞdl
ð5Þ
where l is the wavelength, Arad is the spontaneous transition probability, I(l) is the emission spectrum, n is the refractive index and c is light speed in vacuum. The peak of sem in Dy3 þ doped ZBLAY glass is 1.17 10 20 cm2, which is one order higher than other rare earth doped glass [38]. Thus, the present Dy3 þ doped ZBLAY glass would be an appropriate host material to achieve 2.8 mm laser due to higher emission cross-section.
4. Conclusion
ð3Þ Dy3 þ doped fluoride glasses with the chemical composition of 50ZrF4–33BaF2–17(LaF3 þAlF3 þYF3) are prepared by the meltquenching method. The room temperature luminescence spectrum at about 2.84 mm with large bandwidth 213 nm has been obtained when the present glass is pumped by conventional 808 nm LD. The Judd–Ofelt intensity parameters (Ot), radiative transition probability and branching ratios of Dy3 þ were calculated and discussed. The predicted spontaneous transition probability and emission cross section of Dy3 þ : 6H13/2-6H15/2 reach 45.92 s 1 and 1.17 10 20 cm2, respectively. The present investigations suggest that this Dy3 þ doped ZBLAY glass is a promising 2.8 mm laser material.
Acknowledgement This work is financially supported by National Natural Science Foundation of China (Nos. 60937003 and 50902137) and GF Innovation Project (Nos. CXJJ-11-M23 and CXJJ-11-S110). References Fig. 2. 2.84 mm emission spectrum for Dy3 þ doped ZBLAY glass. The inset is the Dy3 þ energy level diagram and energy transfer sketch map when pumped at 808 nm.
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Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Broadband 2.84 mm luminescence properties and Judd–Ofelt analysis in Dy3 þ doped ZrF4–BaF2–LaF3–AlF3–YF3 glass Ying Tian a,b, Rongrong Xu a,b, Lili Hu a, Junjie Zhang a,n a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2011 Received in revised form 27 July 2011 Accepted 8 August 2011 Available online 12 August 2011
2.84 mm luminescence with a bandwidth of 213 nm is obtained in Dy3 þ doped (ZrF4–BaF2–LaF3–AlF3–YF3) ZBLAY glass. Three intensity parameters and radiative properties have been determined from the absorption spectrum based on the Judd–Ofelt theory. The 2.84 mm emission characteristics and energy transfer mechanism upon excitation of a conventional 808 nm laser diode are investigated. The prepared Dy3 þ doped ZBLAY glass possessing high predicted spontaneous transition probability (45.92 s 1) along with large calculated emission cross section (1.17 10 20 cm2) has potential applications in 2.8 mm laser. & 2011 Elsevier B.V. All rights reserved.
Keywords: 2.84 mm luminescence ZBLAY glass Broadband
1. Introduction Recently considerable research efforts have been devoted to mid-infrared lasers operating around 3 mm due to their various applications, including military, remote sensing, atmosphere pollution monitoring and medical surgery [1–4]. Among various rare earth ions Dy3 þ is a natural candidate for 3 mm laser owing to the 6H13/2-6H15/2 transition, and several results have been reported on the near 2.8 mm emission in various types of Dy3 þ -doped crystals and non-oxide [5–8] glasses. Particularly, infrared emission of Dy3 þ has been obtained in lead oxide complex glass [9]. The reported Dy3 þ -doped materials are always excited by complicated and expensive lasers including a Ti3 þ :sapphire laser driven by an Ar þ laser [5,7], 1.3 mm Nd:YAG laser [8] and 1.1 mm Yb silica fiber laser [10], to obtain 2.8 mm emission. In pursuit of an efficient, cost-effective and durable solid state laser source, chemically stable host, cheap excitation (e.g. 808 nm laser diode) and efficient absorption are required. However, few results have been reported on 2.8 mm luminescence in Dy3 þ doped glasses pumped by common 808 nm laser diode (LD) at room temperature. ZrF4 based fluoride glasses are well known for their high solubility of rare earth ions and long fluorescence lifetime of the excited electronic states as well as their low nonlinear refractive index [11–15]. Furthermore, mid-infrared laser produced from fluoride fiber has been demonstrated in many cases [1,3,10,16].
n
Corresponding author. Tel.: þ86 21 5991 4297; fax: þ 86 21 5991 4516. E-mail address: [email protected] (J. Zhang).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.08.017
However, fluoride glasses suffer from poor chemical stability [17]. It is known that the addition of YF3 can increase the viscosity of the melt and enhance the stability of the glass against crystallization[18], so YF3 is introduced into the fluoride matrix. However, few results focused on the 2.8mm emission in Dy3 þ doped ZrF4–BaF2–LaF3–AlF3–YF3 (ZBLAY) glass and the corresponding optical properties. In this work, the spectroscopic properties and energy transfer mechanism relating to the 2.8 mm emission of ZBLAY glass pumped by 808 nm LD are investigated. Moreover, the Judd–Ofelt intensity parameters, spontaneous transition probability and stimulated emission cross section are calculated and discussed.
2. Experimental Glass with the molar composition 50ZrF4–33BaF2–17(LaF3 þ AlF3 þYF3)–1DyF3 was prepared by the tradition melt quenching method. The sample was prepared by using high-purity ZrF4, BaF2, YF3, AlF3, LaF3 and DyF3 powder. A batch of about 25 g was mixed homogenously and placed in a platinum crucible. It was melted at 900 1C for a time long enough to assure an approximately homogeneous distribution of the Dy3 þ dopants in the melt. The melt was poured on a preheated steel plate, which keeps the rare earth ions distribution as homogeneous as in the melt. The obtained glass was annealed for several hours to remove thermal strains and then allowed to cool to room temperature. The experimental procedures used in this work as well as high rare earth ions solubility of fluoride glass allow Dy3 þ doped ZBLAY glasses with good optical quality,
Y. Tian et al. / Journal of Luminescence 132 (2012) 128–131
transparency and homogeneity. Finally, the annealed sample was fabricated and polished to the size of 20 mm 10 mm 1 mm to be tested for the optical properties. The absorption spectrum has been recorded using a JASCO V-570UV/vis spectrophotometer in the range of 400–2000 nm. The mid-infrared transmission spectrum was obtained by using a Thermo Nicolet (Nexus FT-IR spectrometer) in the range of 2.5–10 mm. The photoluminescence spectrum was carried out with a Triax 320 type spectrometer (Jobin-Yvon Co., France) upon excitation at 808 nm. All the measurements were carried out at room temperature.
129
Table 1 J–O parameters Ot of Dy3 þ in various glasses.
Ot (10 20 cm2)
ZBLAY
ZBLA
ZBLAN
ZBLALi
O2 O4 O6
3.16 7 0.01 1.67 7 0.01 2.45 7 0.02 Present work
3.22 1.35 2.38 [24]
1.86 1.42 2.37 [25]
2.70 1.80 2.00 [26]
Ref.
Table 2 Radiative parameters for various selected excited levels of Dy3 þ in ZBLAY glass.
3. Results and discussion 3.1. Absorption spectrum and Judd–Ofelt analysis Fig. 1 shows the absorption spectrum of the 1 mol% Dy3 þ doped sample recorded at room temperature in the wavelength region of 400–2000 nm. The absorption bands corresponding to the transitions starting from the 6H15/2 ground state to higher levels 6H11/2, 6F11/2, 6F 9/2, 6F7/2, 6F5/2, 6F3/2 and 4F9/2 are labeled. Generally, the shape and peak positions of transitions in Dy3 þ doped ZBLAY glass are similar to those in other Dy3 þ -doped glasses [19,20], except small differences originating from different chemical nature of the host. The absorption band around 800 nm indicates that this glass can be excited efficiently by 808 nm LD. The inset of Fig. 1 presents the mid-infrared transmittance spectrum of Dy3 þ doped ZBLAY sample. As it can be seen, the transmittance reaches as high as 88% and extends up to 9.0 mm. It is noted that the prepared glass has high transmittance around typical H2O absorption band ( 3 mm). Therefore, excellent midinfrared transmission property provides ZBLAY glass potential application for 2.8 mm laser material. The absorption spectra were used for calculation of the Judd– Ofelt parameters, spontaneous emission transition probabilities, lifetimes and branching ratios, using the procedure as described previously [21–23]. Table 1 gives a comparison of Judd–Ofelt intensity parameters Ot (t¼ 2, 4, 6) in various glass hosts. The quality of the calculation is indicated by the root-mean-square deviation, which is equal to 0.3 10–6. This value represents a fairly good agreement between the measured and calculated oscillator strengths and reliable calculations.
Initial level
Terminal level
A (s 1)
6
6
45.92 9.04 80.09 5.46 25.37 61.17 0.00 25.27 83.64 723.07 0.86 1.22 22.40 236.20 911.74 0.00 0.27 2.16 10.79 32.73 18.68 1.66 0.61 0.31 7.28 28.87
H13/2 H11/2
6
6
H9/2
6
F11/2
6
F9/2
6
H7/2
6
H5/2
H15/2 H13/2 6 H15/2 6 H11/2 6 H13/2 6 H15/2 6 H9/2 6 H11/2 6 H13/2 6 H15/2 6 F11/2 6 H9/2 6 H11/2 6 H13/2 6 H15/2 6 F9/2 6 F11/2 6 H9/2 6 H11/2 6 H13/2 6 H15/2 6 H7/2 6 F9/2 6 F11/2 6 H9/2 6 H11/2 6
P A (s 1) 45.92 89.13
92.01
831.98
1172.41
64.63
38.73
b (%)
trad (ms)
100.00 10.15 89.85 5.94 27.58 66.49 0.00 3.04 10.05 86.91 0.07 0.10 1.91 20.15 77.77 0.00 0.42 3.34 16.70 50.64 28.90 4.29 1.57 0.80 18.79 74.55
21.78 11.22
10.87
1.20
0.85
15.47
25.82
According to C. K. Jørgensen and Reisfeld [24], O2 is strongly affected by covalent chemical bonding, while O6 is related to the rigidity of the medium in which ions are situated. From Table 1, it is found that the calculated O2 in present glass is comparable to that of ZBLA glass [25] and higher than that of ZBLAN glass [26]. It is known that O2 increases with the asymmetry of the local structure. So it can be inferred that there is a higher asymmetrical surrounding of rare earth ions in the present glass than that in ZBLAN glass. As is shown in Table 1, O4 and O6 of ZBLAY are close to those of other fluoride glasses [25–27], which indicates the rigidity of the ligand between these fluoride systems is similar.
3.2. Radiative properties
Fig. 1. Absorption spectrum of Dy3 þ doped sample. The inset is the mid-infrared transmittance spectrum of Dy3 þ doped ZBLAY sample.
The J–O intensity parameters (Ot) have been widely used to estimate certain radiative parameters and predict luminescence characteristics of certain transitions. Table 2 shows the calculated predicted spontaneous transition probability (A), total spontaP neous transition probability ( A), branching ratios (b) and radiative lifetime (trad) of the optical transitions for the Dy3 þ doped ZBLAY glass. From Table 2, it is noted that the predicted spontaneous emission probabilities for Dy3 þ :6H13/2-6H15/2 transition is 45.92 s 1, which is significantly higher than for ZBLA (19.5 s 1) [25]. Higher spontaneous transition probability provides the material with a better opportunity to obtain laser actions [28]. Thus, the present glass can be selected as a promising 2.8 mm laser material.
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Y. Tian et al. / Journal of Luminescence 132 (2012) 128–131
3.3. Photoluminescence spectrum, energy transfer mechanism and emission cross section Fig. 2 shows the measured photoluminescence spectrum of the Dy3 þ doped ZBLAY glass pumped by 808 nm LD. As is shown in Fig. 2, intense emission with a peak at 2.84 mm due to the transition 6H13/2-6H15/2 can be observed. Moreover the emission spectrum of present glass is broad and smooth with a full bandwidth at half maximum (FWHM) 213 nm around 2.84 mm spectral region, which can be attractive fiber gain element capable in the ultra-short pulse laser generation. The involved energy transfer mechanisms are indicated in the inset of Fig. 2. Firstly ions of the 6H15/2 level are excited to the 6F5/2 state by ground state absorption (GSA) when the sample is excited by 808 nm LD. Then the excited energy stored in the 6F5/2 level decays non-radiatively to the next-lower levels. The upper laser level 6H13/2 is populated by the mentioned non-radiatively decay process. Finally, ions in the 6H13/2 level decay radiatively to the 6 H15/2 level with 2.84 mm emission. In order to better understand 2.84 mm luminescence features, the lifetime (tmea) for the 6H13/2 level in the present glass is important. The radiative lifetime reported in Table 2, the energy transfer rates, and the multiphonon emission rates calculated can be according to the following equation [29]: 1
tmea
¼ WR þWET þWCR þ WMPR ,
ð1Þ
where WR is the radiative transition rate; WMPR, WCR and WET are the rate of the multiphonon relaxation, cross-relaxation and energy transfer to impurities (i.e., OH ), respectively. Since the processing route was controlled to reduce the OH content and rare earth concentration in the present glass is not high, tmea becomes [30] 1
tmea
¼
1
tr
þWMPR ,
ð2Þ
where tmea and tr are the measured and the radiative lifetime calculated from the Judd–Ofelt analysis, respectively. The multiphonon relaxation rates WMPR(0) at 0 K from different levels can be described as [25] WMPR ð0Þ ¼ CeaDE ,
where a and C are positive-definite constants related to the characteristic of host material, andDE is the energy difference between the 6H13/2 and 6H15/2 levels. The values of constants C and a have been determined previously for fluorozirconate glass to be C ¼1.88 1010 s 1 and a ¼5.77 10 3 [23]. With increasing temperature T, WMPR(T) becomes [31] WMPR ðTÞ ¼ WMPR ð0Þ½1e_wmax =kT p ,
ð4Þ
where _wmax is the highest phonon energy and P¼ DE/_wmax. The multiphonon relaxation rate WMPR(T) and lifetime at 300 K were calculated to be 41.52 s 1 and 11.43 ms, respectively. WMPR of 6H13/2-6H15/2 in the present glass (41.52 s 1) is lower than that of the fluoride glass (65.7 s 1) [25]. However, the multiphonon relaxation processes will influence the 2.84 mm luminescence efficiency in the present glass; tmea (11.43 ms) of prepared glass is comparable to that of ZBLA glass (11.7 ms) [25] but larger than that of other fluoride glass (5 ms) [32]. The quantum efficiency of the 6H13/2-6H15/2 transition (ratio of tmea to the calculated lifetime tr using the Judd–Ofelt theory) was calculated to be 52.48%. The phonon sub-system plays a role in the kinetics features of Dy3 þ fluorescence, which includes many photoinduced anharmonic electron–phonon interactions [9,33–35]. Thus, the present Dy3 þ doped ZBLAY glass with high quantum efficiency is a promising material for 2.8 mm laser. High stimulated emission cross-section is extremely useful to determine the possibility to achieve laser effect [36]. According to the Fuchtbauer–Ladenburg theory, the emission cross section sem can be calculated as [37]
sem ¼
l4 Arad 8pcn2
R
lIðlÞ lIðlÞdl
ð5Þ
where l is the wavelength, Arad is the spontaneous transition probability, I(l) is the emission spectrum, n is the refractive index and c is light speed in vacuum. The peak of sem in Dy3 þ doped ZBLAY glass is 1.17 10 20 cm2, which is one order higher than other rare earth doped glass [38]. Thus, the present Dy3 þ doped ZBLAY glass would be an appropriate host material to achieve 2.8 mm laser due to higher emission cross-section.
4. Conclusion
ð3Þ Dy3 þ doped fluoride glasses with the chemical composition of 50ZrF4–33BaF2–17(LaF3 þAlF3 þYF3) are prepared by the meltquenching method. The room temperature luminescence spectrum at about 2.84 mm with large bandwidth 213 nm has been obtained when the present glass is pumped by conventional 808 nm LD. The Judd–Ofelt intensity parameters (Ot), radiative transition probability and branching ratios of Dy3 þ were calculated and discussed. The predicted spontaneous transition probability and emission cross section of Dy3 þ : 6H13/2-6H15/2 reach 45.92 s 1 and 1.17 10 20 cm2, respectively. The present investigations suggest that this Dy3 þ doped ZBLAY glass is a promising 2.8 mm laser material.
Acknowledgement This work is financially supported by National Natural Science Foundation of China (Nos. 60937003 and 50902137) and GF Innovation Project (Nos. CXJJ-11-M23 and CXJJ-11-S110). References Fig. 2. 2.84 mm emission spectrum for Dy3 þ doped ZBLAY glass. The inset is the Dy3 þ energy level diagram and energy transfer sketch map when pumped at 808 nm.
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