ARTICLE pubs.acs.org/JPCA
Structural Origin and Laser Performance of Thulium-Doped Germanate Glasses Rongrong Xu,†,‡ Lin Xu,†,‡ Lili Hu,† and Junjie Zhang*,† †
Key Laboratory of Materials for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, People's Republic of China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100039, People's Republic of China ABSTRACT: The structural origin and laser performance of thulium-doped germanate glasses have been studied. The investigation includes two main sections. The first part discusses the Raman spectroscopic and thermal stability of the host glass structure. The low value of the largest phonon energy (850 cm1) reduces the probability of nonradiative relaxation. The large emission cross section of the Tm3+ : 3F4 level (8.69 1021 cm2), the high quantum efficiency of the 3F4 level (71%), and the low nonradiative relaxation rate of the 3F4 f 3H6 transition (0.09 ms1) illustrate good optical properties of the germanate glass. In the second part, the room-temperature laser action from the thulium-doped germanate glass is demonstrated when pumped by a 790 nm laser diode. The maximum output power of 346 mW and slope efficiency of 25.6% are achieved.
1. INTRODUCTION Recently, 2 μm solid-state lasers based on glass hosts have been extensively investigated due to their important optical properties for applications in nonlinear optics,1 atmospheric sensing,2,3 and medical surgery eye-safe laser radar.4 Combining good optical quality and smooth emission spectra with broadband, rare earth ion doped glass represents an attractive option to realize laser emission with a short pulse width and wide tunable range.4,5 Thulium is a good choice for infrared domain applications because of its absorption at 790 nm, which is available using commercial laser diode (LD), and its interesting emission at ∼2 μm, suitable for a number of sensing applications in medicine.6,7 With a wide emission range related to the glass matrix, Tm3+-doped glass maintains the capacity of the emitting laser covering the region of 1.82 μm.1,4,8 For the best case, pumped with a Ti:sapphire laser, 190 mW of 2 μm laser output has been achieved in Tm3+-doped fluorogermanate glass with a slope efficiency up to 50%.8 However, the commercial pump source LD is preferable to the Ti:sapphire laser for the development of a solid-state laser system with the advantage of efficiency, compactness, low cost, and availability. This study covers a detailed structural analysis and develops a short cavity of LD-pumped oscillation. Germanate glass is chosen as the host matrix due to its combination of good thermal stability, chemical durability, low phonon energy, and high transparency in a wide wavelength range.9,10 Previous work has reported the characteristics of optical properties of barium gallogermanate (BGG) glass acting as an exit window for high-energy lasers in the nearinfrared wavelength region.9 It has been demonstrated that the properties of BGG glass can also be modified by adding/substituting other components such as La2O3.11,12 The introduction of the r 2011 American Chemical Society
F ion could reduce the content of OH groups in glass, which increases the emission intensity with an efficient energy transfer of rare earth ions.13
2. EXPERIMENT A. Sample Preparation. In this paper, the germanate glass is composed of, in mol %, GeO2 (6070%), Ga2O3 (1020%), BaO/BaF2 (1020%), Na2O (515%), and La2O3 (38%). The sample was doped with 1 mol % Tm2O3, and the glass was prepared by the traditional melt-quenching method with high-purity reagents as the raw materials. Before melting the powders in a SiC resistance electric furnace, the molar masses were weighted, mixed, and dried. The mixtures were stirred for 1 h with a platinum rod and then homogenized at 1400 °C in a platinum crucible in a furnace with a stream of dry air. The melt was poured onto a preheated brass mold and annealed for several hours around 600 °C in a muffle furnace; then, it was cooled down slowly by turning the power supply off. The two-face polished slab sample with the dimension of 10 20 1 mm3 was prepared for optical measurements. B. Structural and Optical Measurements. As a part of structural investigation on the host glass, thermal relaxation properties were measured using a differential scanning calorimeter (DSC). The characteristic temperatures of glass transition Tg, the onset crystallization peak Tx, and the top crystallization peak Tp were determined by NETZSCH STA 409PC/PG with Received: August 7, 2011 Revised: October 27, 2011 Published: November 15, 2011 14163
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Figure 1. Deconvolution of Raman spectra of undoped germanate glass using symmetric Gaussian functions. The plots show the measured spectrum, Gauss fitting of the measured data, and the sum of the peaks.
an accuracy of (1 °C. The structure of glass was analyzed via a Raman spectrum, which was measured with a FT Raman spectrophotometer (Nicolet MODULE) in the range of 100 1000 cm1. The absorption analysis of Tm3+ ions from 350 to 2100 nm was carried out with a Perkin-Elmer-Lambda 900UV/ VIS/NIR spectrophotometer. The emission spectrum of the glass was measured under excitation of a 790 nm LD and recorded on a Traix 320 type spectrometer (Jobin-Yvon Co., France) with the resolution of 1 nm. For the lifetime measurement, the fluorescence decay curve was recorded by a digital storage adaptor, and the lifetime was calculated by an exponential fitting method. All of the measurements were performed at room temperature.
3. RESULTS AND DISCUSSION A. Thermal Stability. The values of Tg, Tx, and Tp were found to be 638, 837, and 854 °C, respectively. The glass criterion,14 ΔT = Tx Tg, introduced by Dietzel, is often regarded as an important parameter for evaluating the glass-forming ability. The Hr criterion giving by Hruby, Hr = (Tx Tg)/(Tm Tp),15 where Tm is the melting temperature of glass, is also a worthwhile parameter. The glass formation factor of the materials is given by the formula kgl = (Tx Tg)/(Tm Tg), which is more suitable for estimating the glass thermal stability than ΔT. Larger kgl represents better forming ability of the glass. The existing ability criterion parameters Hr and kgl are 0.364 and 0.261, respectively. The values reveal that the germanate glass possesses good forming ability and chemical durability. The high power densities associated with pulsed lasers cause rapid local heating of the glass, which sometimes leads to spall, as well as local melting. To prevent this damage, it is necessary for the glass to obsess a high melting temperature Tm and transition temperature Tg.16 Obviously, with a high glass transition temperature Tg (638 °C), the germanate glass is desired and has a potential for high power laser application. B. Analysis of Vibrational Spectroscopic Data. The vibrational spectrum for the host matrix is shown in Figure 1. In this figure, the symmetric vibrational modes in the germanate glass have been analyzed and assigned. These assignments are derived from the data reported for other types of germanate glasses
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Figure 2. Normalized emission spectra of Tm2O3-doped germanate glass; the inset shows the energy levels of Tm3+.
containing modifying oxides and GeO2 quartz.17,18 The Raman peaks at low wavenumber a, b, and c, centered at 128, 166, and 292 cm1, respectively, represent the vibration of network-modifying cations in large interstitial sites. Intermediate frequency regions are dominated by a complicated envelope of overlapping peaks d, e, f, and g, centered at 473, 494, 523, and 574 cm1, respectively, that have been assigned to various vibrational modes involving Ge OGe and GaOGa bending. The high-energy Raman peaks h and i, peaked at 774 and 850 cm1, contribute to the stretching mode of GeO and GaO structure units in the environment of bridging and nonbridging oxygen. The largest phonon energy only extends to 850 cm1, which is smaller than that of the germanate glass reported (900 cm1).19 In rare-earth-doped glasses, the highest-energy phonons exercise the most influence in nonradiative relaxations because multiphonon decay occurs with the fewest number of phonons required to bridge the energy gap between two manifolds.20 Lower phonon energy could reduce the probability of nonradiative relaxation. The relation between radiative and nonradiative decay in rare earth ions is of considerable importance in the study of luminescence behavior of laser materials. The low vibrational energy (850 cm1) of the basic [GaO4] structural unit of the germanate glass reduces the nonradiative emission probability. The relative efficiencies of radiative and multiphonon relaxation to a large extent determine the intensity distribution of the luminescence spectrum. With low thulium doping concentration, the emission intensity of the 1.47 μm band is much stronger than that of the 1.8 μm band because of the high nonradiative decay rate caused by high phonon energy. The high doping concentration of thulium enhances the 3 F4 f 3H6 transition (1.8 μm) while suppressing the 3H4 f 3F4 transition (1.47 μm) because of cross-relaxation.21,22 A part of the energy of one ion in level 3H4 is transferred to another ion in the ground state, with both ions ending up in level 3F4. Figure 2 gives the normalized emission spectra of the prepared glass, and it shows that the intensity of 1.47 μm is quite lower than that of 1.8 μm. That means that the cross-relaxation rate (3H4, 3H6 f 3 F4, 3F4) is much higher than that of the spontaneous emission (3H4 f 3F4). The multiphonon relaxation rate constant, kmp, from a given excited state can be calculated from the energy gap law23,24 kmp ðTÞ ¼ kmp ð0Þ½1 epωmax =kT p 14164
ð1Þ
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noting that it can be advantageous to pump into either 3H5 or 3F4 levels of Tm3+ in order to reduce the quantum defect or avoid undesirable pump excited-state absorption (ESA).27 According to the absorption spectra, the absorption cross section can be calculated though the formulas σa(λ) = 2.303 log(I0/I)/NL, where log(I0/I) is the absorptivity, L is the thickness of the glass sample, and N is the concentration of the thulium ions. To provide high gain, it is generally desirable for the emission cross section of the laser glass to be as large as possible. The emission cross section was evaluated from the absorption cross section by the McCumber theory28 ! Zl EZL hcλ1 σ e ðλÞ ¼ σ a ðλÞ exp ð3Þ Zu kT
where kmp(0) is the multiphonon relaxation at 0 K and is given by kmp(0) = βeα(ΔE2pωmax) and parameters α and β are positivedefinite constants characteristic of the host glass, such as 4.6 103 cm and 6.1 107 s1 reported for germanate glass.23 The pωmax is the highest energy of the optical phonon,23 and ΔE is the energy difference between the relaxing and next lower state; the value between 3F4 and 3H6 is calculated to be 5658. Finally, p = ΔE/pωmax is the minimum number of phonons required to bridge the energy gap ΔE. The spontaneous radiative relaxation rate constant kr for a transition j f i is determined by25 krj f i ¼
2πe2 2 2 n v fi fj ε0 mc3
ð2Þ
where ε0 is the vacuum permittivity and c is the velocity of light. n and v are the refractive index of the glass and the frequency of the transition, respectively. f is the calculated total oscillator strength; for the 3F4 f 3H6 transition, the value is 7.63 106. The radiative decay fraction r is defined as r = kr/(kr + kmp), which does not consider the energy transfer process. The radiative (kr) and multiphonon (kmp) relaxation rate constants were calculated to be 497 and 0.755 s1, respectively, giving r = 0.9985. It reveals that the radiative relaxation rate of the 3F4 level is much higher than that of the multiphonon relaxation, which has the benefit of obtaining an intense emission of 1.8 μm from the thulium-doped germanate glass. C. Cross Section and Quantum Efficiency. The visNIR absorption measurement result is shown in Figure 3. Tm3+ is usually pumped into the 3H4 level due to the excited resource being readily available at ∼790 nm. The cross-relaxation process (3H4, 3H6 f 3F4, 3F4) makes it possible to obtain two ions in the upper laser level 3F4 for each pumping photon.26 It is worth
where Zu and Zl denote the partition functions of the upper and lower states, λ, k, and EZL are the wavelength of the transition, Boltzmann’s constant, and the energy of the so-called “zero line:, respectively. Quimby29 has studied the range of validity of the theory in relating absorption and emission cross sections and found the root-mean-square (rms) of the agreement between calculated and measured spectra to be 5% or smaller at room temperature. The peak emission cross section of Tm3+ at 1.85 μm is 8.69 1021 cm2. Table 1 shows the comparison of emission cross sections σe of the Tm3+/3F4 level in various germanate glasses. As seen, the σe of the prepared glass is larger than those of other thulium-doped germanate glasses. The large emission cross section may properly contribute to the large absorption coefficient. It suggests that the prepared glass with promising properties could be applied in a high power regime laser system. The comparison of spectroscopic properties of various Tm3+doped germanate glasses is also listed in Table 1. The product of the stimulated emission cross section and lifetime (σe τm) is the table of merit (FOM) for amplifier gain, which is an important parameter to characterize laser materials.30 Assuming that the FOM bandwidth is an indication of the achievable gain band, the obtained value for the prepared glass suggests that the glass could provide high gain at short wavelength around 2 μm. Quantum efficiency ηqe is defined as the ratio of the measured lifetime to the calculated one ηqe = τm/τrad, where τrad and τm are radiative and measured lifetimes, respectively.31 The quantum efficiency ηqe is governed by the nonradiative relaxation rate Wnr, as represented by Wnr = 1/τm 1/τrad.32 The value of Wnr = 0.09 ms1 is lower than those of the other two germanate glasses, indicating that nonradiative quenching is not active for the prepared glass. It will be demonstrated that optical glasses with high quantum efficiency ηqe are apt to obtain lasers. D. Laser Performance of the Germanate Glass. A traditional plane-concave laser cavity has been employed in the laser experiment. The central wavelength of the fiber pigtailed pumping LD
Figure 3. Absorption coefficient of the Tm2O3-doped germanate glass.
Table 1. Spectroscopic Properties of the Tm3+ : 3F4 Level in Various Thulium-Doped Germanate Glasses σe
σe τm
Nb (ions/cm3)
λ (nm)
τrad (ms)
τm (ms)
(1021 cm2)
(1021 cm2 ms)
ηqe (%)
GeO2BaOK2O33
2.50
1840
4.98
0.60
6.8
4.08
12
GeO2PbONb2O534
3.37
1820
1.77
0.74
7.7
5.70
42
0.79
GeO2BaO/CaONa2O/Li2O35
2.62
1865
4.05
4.29
5.7
24.24
100
0.01
GeO2BaO/BaF2Ga2O3Na2OLa2O3a
2.02
1853
4.31
3.06
8.7
26.59
71
0.09
glass composition
a
Wnr ms1) 1.47
Represents the present research. b N stands for the ion concentration. 14165
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Figure 4. Laser spectrum of Tm3+-doped germanate glass pumped by LD at 790 nm.
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longer wavelength, associated with the reabsorption effects in the thick gain medium. The dependence of the output power on the absorbed pump power is illustrated in Figure 5. In order to obtain high power, the copper mount is water-cooled and maintained at 10 °C. The maximum output power of 346 mW is obtained with the 3% transmission OC, and the slope efficiency is 25.6%. A similar differential slope efficiency of 21.6% with 5% transmis sion OC is also presented, following with a different threshold of 1.04 W and a lower output power of 263 mW. Further increasing the pump power generates a serious thermal lens effect, causing the cavity to become unstable, and it reduces the output power. Improving the thermal conductivity coefficient and reducing the thermal lens effect will be part of our next work. The laser performance of the Tm3+-doped germanate glass can be further improved by utilizing a longer slab glass with an optimum coupling system for better absorption. Compressing the pump beam radius inside of the germanate glass can also lower laser threshold.
4. CONCLUSION In summary, the structural origin, spectroscopic properties, and laser performance of the Tm3+-doped germanate glass have been investigated. Spectroscopic results show that a large emission cross section of 8.69 1021 cm2, a high quantum efficiency of the Tm3+ : 3F4 level of 71%, and a low nonradiative relaxation rate of the 3F4 f 3H6 transition of 0.09 ms1 provide a good opportunity to obtain laser action. Efficient operation of the diode-pumped germanate glass laser has been demonstrated. A slope efficiency of 25.6% and maximum output power of 346 mW have been obtained. The results obtained in this work have demonstrated that the Tm3+-doped germanate glass would be a promising material for efficient diode-pumped laser generation at 2 μm. ’ AUTHOR INFORMATION Corresponding Author 3+
Figure 5. The Tm -doped germanate glass laser; output power versus absorbed pump power for 3 and 5% OC.
(DILAS, Germany) is around 790 nm, which matches with the absorption of Tm3+ : 3H6 f 3H4. A spot size of 200 μm radius pumping in the germanate glass is coupled by two aspheric lenses. The absorption pumping efficiency is measured to be around 80%. The glass is fabricated to a thin slab with 7 mm length, and both end faces are polished. It is wrapped with high thermal conductivity indium foil and cooled by a copper mount. Two output couplers with 100 mm radius of curvature are employed with different transmissions of 3 and 5% at around 2 μm to optimize the laser output efficiency. When the absorbed pump power is at 0.87 W, laser output with the wavelength centered at 1995 nm and ranging from 1978 to 1999 nm is measured, as shown in Figure 4. Vibronic broadening of the luminescence of the thulium ion in germanate glass and the splitting of the energy level caused by electronelectron and electronhost interactions lead to a wide and smooth emission spectrum.36 This wide gain spectrum medium and the multimode oscillation of the short plane-concave geometry cavity contribute to the large FWHM (full wave at half-maximum) of the laser emission. Compared with the emission spectra obtained above, the laser spectrum measured is shifted toward a
*Address: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, No. 390 Qinghe Road, Jiading District, Shanghai, 201800, China. Tel: +86 21 5991 4297. Fax: +86 21 5991 4516. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (No. 51172252). ’ REFERENCES (1) Doualan, J. L.; Girard, S.; Haquin, H.; Adam, J. L.; Montagne, J. Opt. Mater. 2003, 24, 563–574. (2) Wu, J.; Jiang, S.; Qua, T.; Kuwata-Gonokami, M.; Peyghambarian, N. Appl. Phys. Lett. 2005, 87, 211118–211113. (3) Sugimoto, N.; Sims, N.; Chan, K.; Killinger, D. K. Opt. Lett. 1990, 15, 302–304. (4) Fusari, F.; Lagatsky, A. A.; Richards, B.; Jha, A.; Sibbett, W.; Brown, C. T. Opt. Express 2008, 16, 19146–19151. (5) Jaque, D.; Lagomacini, J. C.; Jacinto, C.; Catunda, T. Appl. Phys. Lett. 2006, 89, 121101. (6) Shepherd, D. P.; Brinck, D. J. B.; Wang, J.; Tropper, A. C.; Hanna, D. C.; Kakarantzas, G.; Townsend, P. D. Opt. Lett. 1994, 19, 954–956. (7) Lakshminarayana, G.; Qiu, J. J. Alloys Compd. 2009, 481, 582–589. 14166
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