Tm3+ codoped lead silicate glass for 2 μm laser materials

Optics and Laser Technology 97 (2017) 364–369

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Ho3+/Tm3+ codoped lead silicate glass for 2 lm laser materials Ning Wang, Ruijie Cao, Muzhi Cai, Lingling Shen, Ying Tian, Feifei Huang, Shiqing Xu, Junjie Zhang ⇑ College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China

a r t i c l e

i n f o

Article history: Received 1 April 2017 Received in revised form 14 June 2017 Accepted 18 July 2017

Keywords: Ho3+/Tm3+ co-doped Lead silicate glass Phonon energy Spectroscopy Laser materials

a b s t r a c t The mid-infrared emission of low phonon (963 cm
1) lead silicate glass system with Ho3+/Tm3+ co-doped has been investigated. Luminescence at 2.1 lm corresponding to 5I7 ? 5I8 transition in holmium was obtained by energy transfer between Tm3+ and Ho3+ ions. Energy transfer mechanism between them was analyzed. And the highest value of the luminescence intensity was obtained in glass co-doped with 1Tm2O3/0.3Ho2O3. The full width at half maximum of the (Ho3+/Tm3+) emission reached to 350 nm in 1Tm2O3/0.1Ho2O3 sample. Absorption and emission cross section have also been calculated and analyzed. The maximum emission cross section was 3.9  10
21 cm2 around 2.0 lm. And when P > 0.4, a positive gain can be obtained at wavelengths >1941 nm. Results demonstrated that the prepared Ho3+/Tm3+ codoped lead silicate glasses have excellent spectroscopic properties in mid-infrared wavelengths and can obtain high gain in fiber lasers. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Powerful and highly efficient fiber lasers at 2 lm region have shown promise as tools for a variety tasks in scientific and technical, such as precision medical laser surgery, eye-safe atmospheric sensing. Moreover, the laser operating in ultrafast pulse regime can also be devoted to time-resolved spectroscopy and as pumping source for the optical parametric oscillator (OPO) in the longer mid-IR regions [1–4]. The transitions of Tm3+: 3F4 ? 3H6 and Ho3+: 5I7 ? 5I8 have a corresponding peak emission wavelength around 2.0 lm, respectively, which indicate they are suitable for providing 2.0 lm emissions in fiber. Thulium (Tm)-doped fiber laser is an ideal candidate to generate a near 2.0 lm luminescent center which exhibits highpower and high efficiency characteristics. The thulium fiber laser can be excited with commercial diode laser pump sources emitting at 0.79 lm, and its output has a wide spectrum of laser gain tuning from at least 1.86 lm to around 2.09 lm [5]. Cross-relaxation (‘two-for-one’ excitation) between neighboring Tm3+ cations is especially resonant in silicate glass and can nearly double the slope efficiency [6]. To further extend the wavelength beyond 2 lm for a series of applications including medicine, remote sensing and the generation of longer wavelengths using nonlinear optics, a simple, efficient, and robust source of high-power radiation is required [7]. The transition from 5I7 to 5I8 of Ho3+ has a peak emission wavelength at 2.1 lm. Besides, compared to Tm3+, Ho3+ ion has longer ⇑ Corresponding author. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.optlastec.2017.07.025 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.

fluorescent lifetime and higher stimulated emission cross [8,9]. Thus, Ho3+ is clearly a good choice. However, since Ho3+ ion is lack of a corresponding ground absorption band around 808 or 980 nm, it cannot be pumped directly by a readily commercial diode lasers pump sources. Generally, Tm3+ or Yb3+ ion, which can exploit the commercial diode-pump able absorption, is co-doped as a sensitizer for Ho3+ ion, thus making it possible to produce emission at 2.1 lm, accompanying with the transition from 5I7 to 5I8 [10]. The energy transfers from Tm3+ and Yb3+ to Ho3+ ion are schemed in Fig. 1. Notably for the Tm3+/Ho3+ co-doped system, theoretically, the maximal quantum efficiency could be expected to 2 due to the cross relaxation (CR) of Tm3+ ions: 3H4+3H6 ? 3F4+3F4 [11]. It is worth nothing that there have some extrinsic defects in observed luminescence behavior of doped samples. Pumping at 808 nm using Tm3+ as the sensitizer would results in a large quantum defect and heat deposition for a 2 lm laser. The combination of large quantum defect heating and low emission quantum efficiency seems to produce a large heat load. Literature sources mention that a number of Ho3+/Tm3+ codoped glass systems have obtained radiation emissions in the region of 2 lm. These include silicate [12], phosphate [13], germanate [14], tellurite [15] glasses and so on. The silicate and phosphate possess robust mechanical resistance and good thermal stability which are required in constructing high-power fiber lasers. Nevertheless, these two systems are characterized by a high probability of non-radiative transfers caused by high phonon energy (1100–1200 cm
1) [16]. Because of that, an alternative glass of low vibrational frequency of bond oscillation is necessary. Germanate and tellurite glasses have the suitably lower phonon

N. Wang et al. / Optics and Laser Technology 97 (2017) 364–369

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Fig. 1. Schematic of energy transfer from Tm3+ to Ho3+ ions and Yb3+ to Ho3+ ions.

energy that provides the necessarily high transmission of the fiber, and are capable of hosting relatively great quantities of activator ions [17]. However, in spite that germanate glasses also have good mechanical qualities, maximum phonon energies of 900 cm–1 [18,19], it tends to microscopic segregation, crystallization during fiber drawing. What’s more, its raw materials are very high-cost. As for tellurite glasses, a consequence of weak structural bonds is a significantly lower mechanical strength which often makes it impossible to produce good quality optical fiber from them [15,20]. Silicate glasses remain to be the most successful fiber host materials. In order to further select a lower phonon energy as well as good thermal and mechanical parameters, we performed a composition of antimony-lead silicate glasses doped with Tm3+ and Ho3+ ions to obtain radiation emission of 1.7–2.2 lm.

performed on a Raman spectrometer (Renishaw inVia, space resolution <1 lm) with the excitation source of 532 nm. Characteristic temperatures were determined based on differential scanning calorimeter (DSC) measurements at the heating rate of 10 C/min performed using a NETZSCH DTA 404 PC differential scanning calorimeter. Absorption spectra measurements within the range from 300 to 2100 nm were taken using Perkin Elmer Lambda 900 UV/VIS/NIR double beam spectrophotometer (WalthamMA, resolution 0.1 nm). Fluorescence spectra of the samples were measured by a computer controlled FLS980 Spectrometer (Edinburgh Instruments, Highest Sensitivity >25,000:1 standard; >35,000:1 optional) with an 808 nm laser diode. TDS 3012C type Digital Phosphor Oscilloscope (100 MHz, 1.25GS/S) was used to measure and display the lifetime decay curves.

2. Experiments 2.1. Glass syntheses The glass system of (mol%) (100-x-y) (SiO2 + PbO + Al2O3 + Na2O + BaO + Sb2O3) doped with x mol% Tm2O3, y mol% Ho2O3 (where x = 0, 1; y = 0.1, 0.2, 0.3, 0.4) (where x = 0, 1; y = 0.1, 0.2, 0.3, 0.4) were melted in alumina crucible in Si-Mo resistance furnace in temperature of 1400 °C for 50 min. They were denoted as 0T-0.1H, 1T-0H, 1T-0.1H, 1T-0.2H, 1T-0.3H, 1T-0.4H, respectively. Analytical grades of SiO2, Pb3O4, Al2O3, Na2CO3, BaCO3, Sb2O5 and high purity (99.99%) Tm2O3 and Ho2O3 were used as the raw materials. Then the molten glasses were poured into the preheated stainless steel plate and exposed to the process of annealing in the temperature approximate to the transformation temperature (Tg) for 4 h. The annealed samples were finally cut and were polished with the dimensions of 10  10  1.2 mm3 for determining spectral properties. The photograph of polished samples was showed in the insert of Fig. 3. 2.2. Measurements The glass density was measured by using Archimedes’ liquidimmersion method in distilled water. The refractive indexes were measured by the prism minimum deviation method at three wavelengths, 633, 1311 and 1539 nm (Mectricon Models 2010/M, Routine index resolution of ±0.0005). The Raman spectrum was

3. Results and discussion 3.1. Raman spectrum The large nonradiative decay probability of excited rare earth ions by multi-phonon emission leads to low fluorescence quantum efficiency. For multi-phonon relaxation, one is predominantly interested in the highest frequency phonons in oxide glass. And the highest frequency phonons which are excited in the decay can be determined from Raman spectrum [21]. Fig. 2 shows the Raman spectrum of the matrix. It is clear that the largest phonon energy merely reaches to 963 cm
1, much lower than that of silicate glass (1080 cm
1) [22]. The lower phonon energy leads to a smaller of the nonradiative decay probability and thus be helpful to increase the fluorescence quantum efficiency. The inset shows the Raman bands in the frequency region from 800 to 1200 cm
1. It was seen that five peaks centered at 840, 882, 949, 1010, and 1048 cm
1 are observed by Gaussian fitting, which are attributed to the SiO4 tetrahedra with four, three, two, one non-bridging oxygen ion and with four bridging oxygens, respectively [23,24]. The centre of the strong Raman band of the lead silicate glass at 963 cm
1 seems to be too high for isolated SiO4 tetrahedra and some degree of polymerization of the tetrahedra is required [25]. Thus, lead oxide played the role of a modifier in our glass network and silicate network was depolymerized with the PbO doped, which are favor to improve the solubility of rare earth ions.

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Fig. 2. Raman spectrum of the matrix. The inset shows the Raman bands in the frequencyregion from 800 to 1200 cm
1 consisted of five peaks.

3.2. Absorption spectra and Judd–Ofelt analysis Fig. 3 presents absorption spectra of 1 mol% Tm2O3 singly doped, 0.1 mol% Ho2O3 singly doped as well as 1 mol% Tm2O30.1 mol% Ho2O3 co-doped samples in the wavelength region of 350–2200 nm. Due to the concentrations of Ho3+ is relatively low, so its absorption peaks are not obvious compared to Tm3+. The characteristic absorption bands corresponding to the transitions from the ground states to higher energetic levels of Tm3+ and Ho3+ ions are labeled in the spectra curves. In glass doped with thulium ions, the transitions from the 3H6 ground state to higher levels of 3F4, 3H5, 3H4, 3F2,3 and 1G4 are observed. Similarly, for holmium singly doped glass, five bands corresponding to transitions from the ground state 5I8 to the exited states 5I7, 5I6, 5F5, 5S2, 5F1 level are also identified. The Tm3+/Ho3+ co-doped sample can be efficiently pumped by 808 nm LD due to the intense absorption of Tm3+ at 792 nm. Table 1 illustrates basic parameters, i.e. the

Fig. 3. Absorption spectra of Tm3+ singly doped, Ho3+ singly doped and Tm3+/Ho3+ co-doped glasses. The inset shows the photograph of polished samples.

Table 1 Absorption parameters of singly doped glasses.

Dk

k (nm)

E (cm
1)

(Tm) H6 ? G4 3 F2,3 3 H4 3 H5 3 F4

446–494 648–713 748–833 1110–1251 1553–1809

470 683 792 1210 1724

21,374 14,150 12,720 8390 5811

(Ho) 5I8 ? 5F1 5 S2 5 F5 5 I6 5 I7

432–470 515–559 617–669 1131–1196 1915–2020

450 538 641 1148 1940

22,375 18,354 15,519 8614 5116

Transition 3

1

wavelength at the k maximum absorption transition, the spectral range Dk of the absorption band, and the average energy E. Judd–Ofelt (J-O) theory has been widely used for predicting the spectroscopic properties of the f-f transitions of trivalent rare earth ions [26,27]. According to the J-O theory, the transition intensities are characterized by three phenomenological parameters known as J-O intensity parameters Xt (t = 2, 4, 6). Three intensity parameters of Ho3+ in lead glass can be calculated by using the absorption spectra. And the Xt parameters for Ho3+ ions in several environments along with present work are collected in Table 2. Among the three Xt parameters, X2 is more sensitive to the chemical bonding between RE3+ ions and the local environment such as ligand anions and asymmetry [28]. The larger value of X2 indicates the stronger chemical bonding [29]. The X2 parameter is related to the covalence of the metal–ligand bond, whereas the parameters X4 and X6 indicate the rigidity of the host matrix in which the RE3+ ions are inserted [30]. Generally, they are inherent to the bulk properties such as viscosity and dielectric of the media and hardly sensitive to the matrix [31]. From Table 2, it can be found that value of X2 in the present glass suggests that Ho3+ ion experiences relatively higher covalence of the bond between Ho3+ ions and the surrounding ligand as well as lower symmetry of the coordination structure compared to other reported Ho3+ in fluoride and aluminum silicate. On the other hand, the covalence of the bond and asymmetry of this work are lower than these in tellurite and phosphate glass.

N. Wang et al. / Optics and Laser Technology 97 (2017) 364–369 Table 2 Comparisons of J–O intensity parameters Xt (10
20 cm2) (t = 2, 4, 6) of Ho3+ ions in various glass hosts. Glass

X2

X4

X6

Reference

Fluoride Tellurite Phosphate Aluminum silicate Lead silicate

2.78 5.26 6.28 3.14 4.88

1.39 2.28 1.03 3.04 2.64

1.23 2.18 1.39 0.94 0.44

[34] [35] [36] [12] This work

Some important radiative properties can be calculated by the use of values of Xt. The spontaneous transition probability Arad of Ho3+:5I7 ? 5I8 is 72.62 s
1, which is higher than that in two other previously reported silicate glasses (61.65 s
1 [32] and 64.96 s
1 [33]). Higher spontaneous radiative transition probability provides a better chance to achieve efficient laser actions. Besides, The calculated radiative lifetime of Ho3+:5I7 is 13.77 ms.

3.3. Luminescence properties and energy transfer The luminescence properties play key role in resolving the intrinsic/extrinsic defects, associated electronic transitions and gauge the surface or interface properties of the given material. It is the promising nondestructive technique which serves as a significant tool to probe the characteristic features of material and explore the leading edge technological applications for laser applications [37–40]. Fig. 4 shows the obtained luminescence spectra of the glasses pumped by 808 nm LD. As a result, luminescence lines in the range of 1600–2300 nm are observed. The Fig. 4 clearly illustrates the dependence of the emission intensity at the wavelength around 2.0 lm on the concentration of Ho3+ ions. The glass doped exclusively with Tm3+ ions are characterized by strong luminescence (the black line) with the peak at the wavelength of 1835 nm due to the strong transition of 3F4 ? 3H6. Introducing holmium ions to glass results in a decrease of the level of luminescence coming from Tm3+ ions; at the same time, a strong emission line appears in the region of 2.0 lm corresponding to the 5I7 ? 5I8 transition in the energetic structure of Ho3+ ions. And with the gradual increment of Ho2O3 concentration, an increase in the 5I7 ? 5I8 (Ho3+) transition luminescence is observed up until the molar concentration of Ho2O3 is up to 0.3 mol%. After this concentration is exceeded, a decrease in the emission intensity takes place, possibly as a result of diffusion transitions of the donor–donor type or concentration quenching [41]. It is worth not-

Fig. 4. Emission spectra of the obtained glasses doped withTm3+ as well as Tm3+ and Ho3+ ions exited by 808 nm LD.

367

ing that a significant broadening of the luminescence spectrum, caused by overlapping of the 3F4 ? 3H6 (Tm3+) and 5I7 ? 5I8 (Ho3+) emission transitions was observed in the 1 T-0.1H sample. The full width at half maximum (FWHM) of the (Tm3+/Ho3+) emission amounts to 350 nm, and for the glass singly doped with Tm3+ ions FWHM = 232 nm. Wider emission band might have potential application in mid-infrared fiber amplifier. The mechanism of energy transfer between the excited metastable levels of Tm3+ (3F4) and Ho3+ (5I7) ions is presented in Fig. 5. (1) Under commercial 808 nm LD pumping, the ions in the Tm3+:3H6 level jump to a higher 3H4 level via ground state absorption (GSA). (2) When the Tm3+ concentration is accumulated to some degree, the population of the 3H4 level enables transfer to lower energetic levels: 3H5 and 3F4. The 3H5 level in thulium is characterized by a short lifetime and its role in the relaxation process can be neglected, whereas the lifetime of the 3 F4 level is long; besides, cross relaxation process can be taken place between Tm3+ ions (3H4+3H6-3F4+3F4). And as a result it allows for reaching a high population density at this level. (3) Once 3F4 state is saturated, Tm3+ ions, on the one hand, return to 3H6 ground state with strong 1.8 lm emission via 3 F4 ? 3H6 transition. On the other hand, Tm3+ ions in 3F4 state transfer their energy to Ho3+:5I7 state via energy transferring (ET) (3F4+5I8-3H6+5I7). Once the 5I7 state is populated, the Ho3+ ions decay to 5I8 ground state producing strong 2.0 lm emission. 3.4. Cross sections and laser spectroscopic properties The absorption and emission cross sections are usually calculated to evaluate the possibility of achieving laser effects. According to the above measured absorption spectra shown in Fig. 3, the absorption cross sections (rabs) can be expressed as the following Beer–Lambert equation [12]:

rabs ðkÞ ¼

2:303 logðIo ðkÞ=IðkÞÞ Nl

ð1Þ

Fig. 5. The energy transfer diagrams of Tm3+/Ho3+ co-doped lead silicate glass pumped at 808 nm LD.

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where I0(k) and I(k) are the incident optical intensity, the optical intensity throughout the sample, respectively, N is the density of rare-earth ion (ions/cm3) and l is the polished sample thickness (cm). The emission cross sections (rem) can be calculated from the absorption section by using the McCumber formula [42].

rem ðkÞ ¼ rabs ðkÞ 

   Zl hc 1 1   exp
KT ko k Zu

ð2Þ

where Zl and Zu are the partition functions of the lower and the upper levels corresponding to the transitions of Ho3+: 5I7 ? 5I8 and Tm3+: 3F4 ? 3H6, T is room temperature (300 K), k is the Boltzmann constant and k0 is the wavelength for the transition between the lower Stark sublevels of the emitting multiplets and the lower Stark sublevels of the receiving multiplets [43]. Fig. 6 shows the calculated absorption and emission cross sections. Obviously, it can be seen that the overlap between the emission cross section of Ho3+ and the absorption section of Tm3+ is quite large indicating the existence of efficient energy transfer from Tm3+ to Ho3+: 3F4 (Tm3+) + 5I8 (Ho3+)-3H6(Tm3+) + 5I7(Ho3+). The calculated maximum for Ho3+: 5 I7 ? 5I8 transition of Ho3+ singly doped sample has a maximum of 3.9  10
21 cm2 at 2013 nm. This value is larger than the corresponding value 3.07  10
21 cm2 in 60SiO2-19CaO-5Na2O-15K2O glass [12] and 3.72  10
21 cm2 in 50SiO2-10Al2O3-25CaO-15SrO glass [44] but smaller than that in tellurite glass, germanate glass. For a laser medium, it is generally desirable to ensure that the stimulated emission cross section is as large as possible to provide high gain in fiber. Gain coefficient is another important parameter to estimate mid-infrared emission ability of fiber. To further determine the gain ability of Ho3+/Tm3+ co-doped lead silicate glasses, we calculate the gain coefficient G(k, P) of Ho3+ on the basis of and according to the following equation [45]:

Gðk; PÞ ¼ N½Prem ðkÞ
ð1
PÞrabs ðkÞ

ð3Þ

where N is the total concentration of Ho3+ ions and P represents the population inversion given by the ratio between the upper population of Ho3+:5I7 and the total Ho3+ concentration. Here, we choose the P is 0, 0.2, 0.4, 0.6, 0.8, and 1. The concentration of Ho3+ in the sample is 1.81  1020 ions/cm3. Fig. 6 illustrates the gain coefficient curve of the sample with different population inversion parameters P. The result is shown in Fig. 7. When P > 0.4, a positive gain can be obtained at wavelengths >1941 nm, which means a low pumping threshold of laser operation [46]. When P > 0.6, the gain coefficient is positive from 1850 nm to 2100 nm. These results suggest that the

Fig. 7. Gain spectra of Ho3+ in lead silicate glasses.

lead silicate glass is a promising host material to achieve midinfrared emission due to higher gain properties. Besides, the peak wavelength at which the maximum values of the gain curve occur shifts to longer wavelength as population inversion decreases, which is a typical feature of the quasi-three level laser system [47]. 4. Conclusions To summarize, a detailed description of the spectroscopic properties for the Ho3+/Tm3+ co-doped ions in lead silicate glasses was performed in the article. The Judd–Ofelt parameters, spontaneous emission probability, absorption and emission cross section, and gain properties have been calculated and analyzed. Results indicated that Ho3+ in the prepared glasses had high spontaneous radiative transition probability (72.62 s
1), large emission cross section (3.9  10
21 cm2). Moreover, in the wavelength range of 1600–2200 nm, a strong band and broad (FWHM = 350 nm) of luminescence caused by overlapping of the 3F4 ? 3H6 (Tm3+) and 5 I7 ? 5I8 (Ho3+) was observed. The experimental results suggested that Tm3+/Ho3+ co-doped lead silicate can be potentially used in 2.0 lm fiber lasers and provide a beneficial guide for midinfrared laser material. Acknowledgments This research was financially supported by the Chinese National Natural Science Foundation (Nos. 51372235, 51272243, and 51472225), Zhejiang Provincial Natural Science Foundation of China (No. LR14E020003), the Public Technical International Cooperation project of Science Technology Department of Zhejiang Province (2015c340009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Fig. 6. Absorption Tm3+:3F4 ? 3H6.

and

emission

cross

sections

of

Ho3+:5I7 ? 5I8

and

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