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Investigation of the spectroscopic properties of Yb3+-doped yttrium lanthanum oxide transparent ceramic
Optics Communications 273 (2007) 238–241 www.elsevier.com/locate/optcom
Investigation of the spectroscopic properties of Yb3+-doped yttrium lanthanum oxide transparent ceramic Q.H. Yang a, J. Ding b
a,*
, H.W. Zhang a, J. Xu
b
a School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Sciences, Shanghai 201800, PR China
Received 2 April 2006; received in revised form 26 November 2006; accepted 27 December 2006
Abstract Spectroscopic properties of (Y0.9xLa0.1Ybx)2O3 transparent ceramic were studied. Two main absorption peaks of the specimen are centered at 940 and 970 nm, which are suitable for InGaAs laser diode pumping. The main emission peaks were located at 1032 and 1075 nm with larger emission cross-section and longer fluorescence lifetime than those of Yb:Y2O3. These properties of (Y0.9xLa0.1Ybx)2O3 transparent ceramic are favorable to achieve high efficiency and high power laser output. 2007 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 42.55.Xi Keywords: Yttrium lanthanum oxide; Ceramic laser; Transparent ceramics; Spectroscopic property; Yb3+ doped
1. Introduction Recently, Yb3+ doped laser materials have attracted considerable attention due to the development of highpower, high-efficient and high-temperature laser diode. Yb3+ ions have a very simple electronic structure with only two manifolds separated by about 1000 cm1. Therefore, there is no intrinsic process for concentration quenching. Owing to perceptible electron–phonon interaction, Yb3+doped materials have broad absorption in near-IR which is suitable for laser diode (LD) pumping. The broad luminescence band 2F5/2 ! 2F7/2 is also attractive for ultrashort pulse generation [1]. The cubic Y2O3 is a promising solid state laser material for trivalent lanthanide activators due to its several favorable properties, such as refractory nature, stability, ruggedness, optical clarity over a broad spectral region. The thermal conductivity of Y2O3 is twice as large as that of *
Corresponding author. Tel.: +86 21 56331687; fax: +86 21 56332694. E-mail addresses: [email protected] (Q.H. Yang), [email protected] (J. Xu). 0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.12.035
YAG, and their thermal expansion coefficients are very similar. It is, however, extremely difficult to fabricate high quality Y2O3 single crystal because of its high melting point (2430 C) and the polymorphic phase from C to a high temperature hexagonal phase H at about 2350 C [2–4]. In 1978, Rohde et al. successfully fabricated Y2O3 transparent ceramic by adding La2O3 as sintering aid. They utilized the phase transition from C to H at extremely high temperature (2150 C) to retard grain growth thereby allowing sintering to proceed until a low porosity high transparency body is achieved, then lowered temperature to a temperature where the H phase reconverts to C phase (1900 C) to make the grain growth and remove additional porosity [5]. In their sintering process, the B phase (monoclinic) of La2O3 will be formed, which is harmful to the laser output. Recently, we have investigated yttrium lanthanum oxide transparent ceramics and found that solid solution will be formed at relatively low temperature and the sintering temperature can be remarkably decreased by more than 500 C without any influence on the phase and optical transmission [6]. In this paper, we investigate the spectroscopic properties of Yb3+-doped yttrium lanthanum
Q.H. Yang et al. / Optics Communications 273 (2007) 238–241
239
with a spectrophotometer used Xe light as pump source (Model V-570, JASCO) at room temperature. The fluorescence spectra and fluorescence lifetime of the ceramics excited with 970 nm LD were measured with a spectrofluorimeter (Fluorolog-3, Jobin Yvon Spex, France) at room temperature. 3. Results and discussion
Fig. 1. X-ray diffraction profiles of the sintered ceramic.
oxide laser ceramics which are fabricated by low temperature sintering.
As shown in Fig. 1, XRD pattern of our ceramic shows the single C phase of Y2O3, which means that the La2O3 has completely dissolved into the lattice. Fig. 2 shows the photo of (Y0.8La0.1Yb0.1)2O3 transparent laser ceramics and total transmission of the specimens is above 80%. The absorption spectrum of specimen dope with 10 at.% Yb3+ is shown in Fig. 3. The absorption band of the specimens are rather broad and three main absorption peaks are centered at 902, 942 and 968 nm, respectively. Absorption cross-section (rabs) of Yb3+ determined by the expression below and FWHM at the LD pumped 940 and 970 nm are listed in Table 1:
2. Experimental rabs ¼ High purity Y2O3 (>99.99 wt%), Yb2O3 (>99.95 wt%) and La2O3 (>99.95 wt%) were used as starting materials. Specimens were fabricated by weighing the starting powders to achieve the La3+ content of 10 at.% in the yttrium lanthanum oxide ceramic. The contents of Yb3+ in the specimens were 5, 10 and 15 at.%, respectively. The mixed powder was compacted into 15-mm-diameter disks by isostatically cool pressing at 200 MPa. After sintered at 1450– 1600 C for 3–30 h under H2 atmosphere without pressure in molybdenum wire furnace, the specimens were cut and double polished with 1 mm in thickness for spectral analysis. Phase identification of the specimens was performed using an X-ray diffractometer (D/max-rC, Japan). Digital microscope (VHX, Keyence) was used to observe the microstructure. The absorption spectra were measured
2:303 logðI 0 =IÞ LN
ð1Þ
where log(I0/I) is optical density, N is concentration of active ions in unit volume and L is thickness of the sample. The broad absorption band of Yb3+:Y1.8La0.2O3, especially at 940 nm make it suitable to be pumped by 940 nm LD, where the FWHM is larger, hence the temperature control of LD is unnecessary [7]. The emission spectrum of the samples is shown in Fig. 4. Similar to that of Yb3+:Y2O3, three emission peaks corresponding to the transitions from the sublevels of 2F5/2 to the components of the 2F7/2 ground state are located at 976, 1032 and 1075 nm. According to the absorption spectrum and emission spectrum, the energy level diagram of Yb3+ in Y1.8La0.2O3 ceramic is calculated as shown in Fig. 5.
Fig. 2. (a) Total transmission of (Y0.8La0.1Yb0.1)2O3 transparent ceramic; (b) photograph of (Y0.8La0.1Yb0.1)2O3 transparent ceramic.
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Fig. 5. Energy level diagram of Yb3+ in Y1.8La0.2O3 ceramic.
Fig. 3. Absorption spectrum of (Y0.8La0.1Yb0.1)2O3 transparent ceramic.
Table 1 Absorption spectrum parameters of (Y0.8La0.1Yb0.1)2O3 transparent ceramics rabs (·1020 cm2) FWHM (nm)
940 nm
970 nm
0.54 15.2
0.89 6.9
Table 2 Comparison of the emission cross-section and upper-level lifetime of Yb3+ in different host Properties 20
rem (·10 ssa (ms) a
2
cm )
8 at.% Yb3+:Y2O3 [8]
10 at.% Yb3+:Y1.8La0.2O3
1030 nm
1076 nm
1032 nm
1075 nm
0.92 0.82
0.5 0.82
1.05 1.07
0.87 1.04
Dss = ±5 ls.
˚ ) [6] will cause the distortion of lattice and Y3+ (R = 0.89 A the changed crystalline field. According to the formula (2), the stimulated emission cross-section varies inverse with the upper-level lifetime, which means that the larger the fluorescence lifetime is, the smaller the emission cross-section is. But Yb3+:Y1.8La0.2O3 has both large stimulated emission cross-section and long fluorescence lifetime, which indicates that Yb3+:Y1.8La0.2O3 transparent ceramic have more excellent spectroscopic properties and it is potential to be used as a high efficient and high power laser diode pumped solid-state laser material. Fig. 6 shows the relationship between fluorescence lifetime and Yb3+ content in (Y0.9xLa0.1Ybx)2O3 (x =
Fig. 4. Emission spectrum of (Y0.8La0.1Yb0.1)2O3 transparent ceramics.
Emission cross-sections (rem) of Yb3+ ion determined by Fu¨chtbauer–Ladenburg (F-L) formula and fluorescence lifetimes (ss) of the upper-level are given in Table 2: rem ðkÞ ¼
1 1 k5 IðkÞ R 8pn2 c srad kIðkÞ dk
ð2Þ
where srad the radiation lifetime, c is the light velocity in vacuum, and n is the refractive index and I(k) is the emission intensity at wavelength k. Although the Yb3+ content in our specimen is higher than that in Y2O3, the emission cross-section and upperlevel lifetime of Yb3+:Y1.8La0.2O3 transparent ceramic are still larger than those of Yb3+:Y2O3 ceramic. It can be sug˚ ) and gested that radius difference between La3+ (R = 1.03 A
Fig. 6. Fluorescence lifetime vs. Yb3+ content in (Y0.9xLa0.1Ybx)2O3 (x = 0.05, 0.1, 0.15) transparent ceramic.
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241
transition, therefore, the fluorescence lifetime is obviously decreased with the increased of the Yb3+ content. 4. Conclusions
Fig. 7. Up-conversion fluorescence spectrum of (Y0.9xLa0.1Ybx)2O3 ceramic.
0.05, 0.1, 0.15) transparent ceramic at 1032 and 1075 nm. The lifetime of Yb3+ both in 1032 and 1075 nm are decreased with the increased of the Yb3+ content, which may be caused by the up-conversion in the high doping concentration specimens. Up-conversion fluorescence spectrum of the specimen excited with a 940 nm LD is shown in Fig. 7. It can be clearly found that there exist four up-conversion emission bands centered at around 488, 538, 549 and 656 nm, respectively. The emission at 485 nm in the spectrum is the second-order diffraction peak of the 970 nm excitation wavelength. When the material contains a large amount of Yb3+ ions, the visible cooperative luminescence caused by the simultaneous radiative relaxation of a pair of excited Yb3+ ions accompanied by the emission of a photon in the following manner: Yb(2F5/2) + Yb(2F5/2) ! 2Yb(2F7/2) + hm will appear in the emission spectrum. The up-conversion luminescence can result in the losses of excited energy and radiationless
(1) (Y0.9xLa0.1Ybx)2O3 transparent ceramics were fabricated at low sintering temperature under H2 atmosphere without pressure by conventional process. The absorption bands in (Y0.9xLa0.1Ybx)2O3 ceramics is broad and the absorption cross-sections at LD pumped 940 and 970 nm of the specimen doped with 10 at.% Yb3+ are 0.54 · 1020 and 0.89 · 1020 cm2, respectively. (2) The main emission peaks are located at 1032 and 1075 nm with emission cross-sections of 1.05 · 1020 and 0.87 · 1020 cm2 and fluorescence lifetimes of 1.17 and 1.04 ms, respectively. (3) The broad absorption, large emission spectra and long fluorescence lifetime of (Y0.9xLa0.1Ybx)2O3 transparent ceramics make it competitive as a high efficient and high power laser diode pumped solidstate laser material. References [1] K. Takaichi, H. Yagi, J. Lu, J. Bission, A. Shirakawa, K. Ueda, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 84 (2004) 317. [2] J. Kong, D.Y. Tang, B. Zhao, J. Lu, K. Ueda, H. Yagi, T. Yanagitani, Appl. Phys. Lett. 86 (2005) 161116-1. [3] H.X. Ma, Q.H. Lou, Y.F. Qi, J.X. Dong, Y.R. Wei, Opt. Commun. 246 (2005) 465. [4] J. Kong, D.Y. Tang, J. Lu, K. Ueda, H. Yagi, T. Yanagitani, Opt. Exp. 12 (2004) 3560. [5] W.H. Rhodes, L. Mass, United State Patent: 814,341, 1977. [6] Q.H. Yang, J. Xu, L.B. Su, H.W. Zhang, Act. Phys. Sin. 55 (2006) 1207. [7] J. Kong, J. Lu, K. Takaichi, T. Uematsu, K. Ueda, D.Y. Tang, D.Y. Shen, H. Yagi, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 82 (2003) 2556. [8] J. Kong, D.Y. Tang, J. Lu, K. Ueda, Appl. Phys. B 79 (2004) 449.
Investigation of the spectroscopic properties of Yb3+-doped yttrium lanthanum oxide transparent ceramic Q.H. Yang a, J. Ding b
a,*
, H.W. Zhang a, J. Xu
b
a School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Sciences, Shanghai 201800, PR China
Received 2 April 2006; received in revised form 26 November 2006; accepted 27 December 2006
Abstract Spectroscopic properties of (Y0.9xLa0.1Ybx)2O3 transparent ceramic were studied. Two main absorption peaks of the specimen are centered at 940 and 970 nm, which are suitable for InGaAs laser diode pumping. The main emission peaks were located at 1032 and 1075 nm with larger emission cross-section and longer fluorescence lifetime than those of Yb:Y2O3. These properties of (Y0.9xLa0.1Ybx)2O3 transparent ceramic are favorable to achieve high efficiency and high power laser output. 2007 Elsevier B.V. All rights reserved. PACS: 42.70.Hj; 42.55.Xi Keywords: Yttrium lanthanum oxide; Ceramic laser; Transparent ceramics; Spectroscopic property; Yb3+ doped
1. Introduction Recently, Yb3+ doped laser materials have attracted considerable attention due to the development of highpower, high-efficient and high-temperature laser diode. Yb3+ ions have a very simple electronic structure with only two manifolds separated by about 1000 cm1. Therefore, there is no intrinsic process for concentration quenching. Owing to perceptible electron–phonon interaction, Yb3+doped materials have broad absorption in near-IR which is suitable for laser diode (LD) pumping. The broad luminescence band 2F5/2 ! 2F7/2 is also attractive for ultrashort pulse generation [1]. The cubic Y2O3 is a promising solid state laser material for trivalent lanthanide activators due to its several favorable properties, such as refractory nature, stability, ruggedness, optical clarity over a broad spectral region. The thermal conductivity of Y2O3 is twice as large as that of *
Corresponding author. Tel.: +86 21 56331687; fax: +86 21 56332694. E-mail addresses: [email protected] (Q.H. Yang), [email protected] (J. Xu). 0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.12.035
YAG, and their thermal expansion coefficients are very similar. It is, however, extremely difficult to fabricate high quality Y2O3 single crystal because of its high melting point (2430 C) and the polymorphic phase from C to a high temperature hexagonal phase H at about 2350 C [2–4]. In 1978, Rohde et al. successfully fabricated Y2O3 transparent ceramic by adding La2O3 as sintering aid. They utilized the phase transition from C to H at extremely high temperature (2150 C) to retard grain growth thereby allowing sintering to proceed until a low porosity high transparency body is achieved, then lowered temperature to a temperature where the H phase reconverts to C phase (1900 C) to make the grain growth and remove additional porosity [5]. In their sintering process, the B phase (monoclinic) of La2O3 will be formed, which is harmful to the laser output. Recently, we have investigated yttrium lanthanum oxide transparent ceramics and found that solid solution will be formed at relatively low temperature and the sintering temperature can be remarkably decreased by more than 500 C without any influence on the phase and optical transmission [6]. In this paper, we investigate the spectroscopic properties of Yb3+-doped yttrium lanthanum
Q.H. Yang et al. / Optics Communications 273 (2007) 238–241
239
with a spectrophotometer used Xe light as pump source (Model V-570, JASCO) at room temperature. The fluorescence spectra and fluorescence lifetime of the ceramics excited with 970 nm LD were measured with a spectrofluorimeter (Fluorolog-3, Jobin Yvon Spex, France) at room temperature. 3. Results and discussion
Fig. 1. X-ray diffraction profiles of the sintered ceramic.
oxide laser ceramics which are fabricated by low temperature sintering.
As shown in Fig. 1, XRD pattern of our ceramic shows the single C phase of Y2O3, which means that the La2O3 has completely dissolved into the lattice. Fig. 2 shows the photo of (Y0.8La0.1Yb0.1)2O3 transparent laser ceramics and total transmission of the specimens is above 80%. The absorption spectrum of specimen dope with 10 at.% Yb3+ is shown in Fig. 3. The absorption band of the specimens are rather broad and three main absorption peaks are centered at 902, 942 and 968 nm, respectively. Absorption cross-section (rabs) of Yb3+ determined by the expression below and FWHM at the LD pumped 940 and 970 nm are listed in Table 1:
2. Experimental rabs ¼ High purity Y2O3 (>99.99 wt%), Yb2O3 (>99.95 wt%) and La2O3 (>99.95 wt%) were used as starting materials. Specimens were fabricated by weighing the starting powders to achieve the La3+ content of 10 at.% in the yttrium lanthanum oxide ceramic. The contents of Yb3+ in the specimens were 5, 10 and 15 at.%, respectively. The mixed powder was compacted into 15-mm-diameter disks by isostatically cool pressing at 200 MPa. After sintered at 1450– 1600 C for 3–30 h under H2 atmosphere without pressure in molybdenum wire furnace, the specimens were cut and double polished with 1 mm in thickness for spectral analysis. Phase identification of the specimens was performed using an X-ray diffractometer (D/max-rC, Japan). Digital microscope (VHX, Keyence) was used to observe the microstructure. The absorption spectra were measured
2:303 logðI 0 =IÞ LN
ð1Þ
where log(I0/I) is optical density, N is concentration of active ions in unit volume and L is thickness of the sample. The broad absorption band of Yb3+:Y1.8La0.2O3, especially at 940 nm make it suitable to be pumped by 940 nm LD, where the FWHM is larger, hence the temperature control of LD is unnecessary [7]. The emission spectrum of the samples is shown in Fig. 4. Similar to that of Yb3+:Y2O3, three emission peaks corresponding to the transitions from the sublevels of 2F5/2 to the components of the 2F7/2 ground state are located at 976, 1032 and 1075 nm. According to the absorption spectrum and emission spectrum, the energy level diagram of Yb3+ in Y1.8La0.2O3 ceramic is calculated as shown in Fig. 5.
Fig. 2. (a) Total transmission of (Y0.8La0.1Yb0.1)2O3 transparent ceramic; (b) photograph of (Y0.8La0.1Yb0.1)2O3 transparent ceramic.
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Q.H. Yang et al. / Optics Communications 273 (2007) 238–241
Fig. 5. Energy level diagram of Yb3+ in Y1.8La0.2O3 ceramic.
Fig. 3. Absorption spectrum of (Y0.8La0.1Yb0.1)2O3 transparent ceramic.
Table 1 Absorption spectrum parameters of (Y0.8La0.1Yb0.1)2O3 transparent ceramics rabs (·1020 cm2) FWHM (nm)
940 nm
970 nm
0.54 15.2
0.89 6.9
Table 2 Comparison of the emission cross-section and upper-level lifetime of Yb3+ in different host Properties 20
rem (·10 ssa (ms) a
2
cm )
8 at.% Yb3+:Y2O3 [8]
10 at.% Yb3+:Y1.8La0.2O3
1030 nm
1076 nm
1032 nm
1075 nm
0.92 0.82
0.5 0.82
1.05 1.07
0.87 1.04
Dss = ±5 ls.
˚ ) [6] will cause the distortion of lattice and Y3+ (R = 0.89 A the changed crystalline field. According to the formula (2), the stimulated emission cross-section varies inverse with the upper-level lifetime, which means that the larger the fluorescence lifetime is, the smaller the emission cross-section is. But Yb3+:Y1.8La0.2O3 has both large stimulated emission cross-section and long fluorescence lifetime, which indicates that Yb3+:Y1.8La0.2O3 transparent ceramic have more excellent spectroscopic properties and it is potential to be used as a high efficient and high power laser diode pumped solid-state laser material. Fig. 6 shows the relationship between fluorescence lifetime and Yb3+ content in (Y0.9xLa0.1Ybx)2O3 (x =
Fig. 4. Emission spectrum of (Y0.8La0.1Yb0.1)2O3 transparent ceramics.
Emission cross-sections (rem) of Yb3+ ion determined by Fu¨chtbauer–Ladenburg (F-L) formula and fluorescence lifetimes (ss) of the upper-level are given in Table 2: rem ðkÞ ¼
1 1 k5 IðkÞ R 8pn2 c srad kIðkÞ dk
ð2Þ
where srad the radiation lifetime, c is the light velocity in vacuum, and n is the refractive index and I(k) is the emission intensity at wavelength k. Although the Yb3+ content in our specimen is higher than that in Y2O3, the emission cross-section and upperlevel lifetime of Yb3+:Y1.8La0.2O3 transparent ceramic are still larger than those of Yb3+:Y2O3 ceramic. It can be sug˚ ) and gested that radius difference between La3+ (R = 1.03 A
Fig. 6. Fluorescence lifetime vs. Yb3+ content in (Y0.9xLa0.1Ybx)2O3 (x = 0.05, 0.1, 0.15) transparent ceramic.
Q.H. Yang et al. / Optics Communications 273 (2007) 238–241
241
transition, therefore, the fluorescence lifetime is obviously decreased with the increased of the Yb3+ content. 4. Conclusions
Fig. 7. Up-conversion fluorescence spectrum of (Y0.9xLa0.1Ybx)2O3 ceramic.
0.05, 0.1, 0.15) transparent ceramic at 1032 and 1075 nm. The lifetime of Yb3+ both in 1032 and 1075 nm are decreased with the increased of the Yb3+ content, which may be caused by the up-conversion in the high doping concentration specimens. Up-conversion fluorescence spectrum of the specimen excited with a 940 nm LD is shown in Fig. 7. It can be clearly found that there exist four up-conversion emission bands centered at around 488, 538, 549 and 656 nm, respectively. The emission at 485 nm in the spectrum is the second-order diffraction peak of the 970 nm excitation wavelength. When the material contains a large amount of Yb3+ ions, the visible cooperative luminescence caused by the simultaneous radiative relaxation of a pair of excited Yb3+ ions accompanied by the emission of a photon in the following manner: Yb(2F5/2) + Yb(2F5/2) ! 2Yb(2F7/2) + hm will appear in the emission spectrum. The up-conversion luminescence can result in the losses of excited energy and radiationless
(1) (Y0.9xLa0.1Ybx)2O3 transparent ceramics were fabricated at low sintering temperature under H2 atmosphere without pressure by conventional process. The absorption bands in (Y0.9xLa0.1Ybx)2O3 ceramics is broad and the absorption cross-sections at LD pumped 940 and 970 nm of the specimen doped with 10 at.% Yb3+ are 0.54 · 1020 and 0.89 · 1020 cm2, respectively. (2) The main emission peaks are located at 1032 and 1075 nm with emission cross-sections of 1.05 · 1020 and 0.87 · 1020 cm2 and fluorescence lifetimes of 1.17 and 1.04 ms, respectively. (3) The broad absorption, large emission spectra and long fluorescence lifetime of (Y0.9xLa0.1Ybx)2O3 transparent ceramics make it competitive as a high efficient and high power laser diode pumped solidstate laser material. References [1] K. Takaichi, H. Yagi, J. Lu, J. Bission, A. Shirakawa, K. Ueda, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 84 (2004) 317. [2] J. Kong, D.Y. Tang, B. Zhao, J. Lu, K. Ueda, H. Yagi, T. Yanagitani, Appl. Phys. Lett. 86 (2005) 161116-1. [3] H.X. Ma, Q.H. Lou, Y.F. Qi, J.X. Dong, Y.R. Wei, Opt. Commun. 246 (2005) 465. [4] J. Kong, D.Y. Tang, J. Lu, K. Ueda, H. Yagi, T. Yanagitani, Opt. Exp. 12 (2004) 3560. [5] W.H. Rhodes, L. Mass, United State Patent: 814,341, 1977. [6] Q.H. Yang, J. Xu, L.B. Su, H.W. Zhang, Act. Phys. Sin. 55 (2006) 1207. [7] J. Kong, J. Lu, K. Takaichi, T. Uematsu, K. Ueda, D.Y. Tang, D.Y. Shen, H. Yagi, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 82 (2003) 2556. [8] J. Kong, D.Y. Tang, J. Lu, K. Ueda, Appl. Phys. B 79 (2004) 449.