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Preparation and optical nonlinearities of transparent bismuth-based glass ceramics embedded with Bi2O3 microcrystals
Journal of Non-Crystalline Solids 256 (2010) 2786–2789
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Preparation and optical nonlinearities of transparent bismuth-based glass ceramics embedded with Bi2O3 microcrystals Feifei Chen ⁎, Tiefeng Xu, Shixun Dai, Qiuhua Nie, Xiang Shen, Xunsi Wang, Baoan Song College of Information Science and Engineering, Ningbo University, Ningbo 315211, China
a r t i c l e
i n f o
Article history: Received 7 December 2009 Received in revised form 24 August 2010 Available online 11 October 2010 Keywords: Heavy-metal oxides; Microcrystallinity; Non-linear Optics
a b s t r a c t Transparent glass ceramics based on Bi2O3–B2O3–TiO2 ternary glass system were prepared by controlled onestep heat treatment method. X-ray diffraction measurement confirms the formation of Bi2O3 phase crystals in glass network. Z-scan measurements at a wavelength of 750 nm were employed to investigate the third-order optical nonlinearities of these transparent glass ceramics. The results show that the nonlinear properties were significantly enhanced after crystallization process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In recent years, transparent glass ceramics [1] are intensively studied since these materials with uniformly dispersed microncrystals (MC) possess advantages over conventional glasses such as better thermal and mechanical stabilities which offer promise for many potential applications [2,3]. Particularly in the field of nonlinear optics [4,5], they have been widely accepted for their large third-order optical nonlinearities (TONL) induced by dielectric confinement and quantum mechanical confinement effects. On the other hand, heavy metal oxide (HMO) glasses [6–8] based on TeO2, Bi2O3, and PbO were also well-known to possess high TONL, which is due to their high refractive indexes. However, not many detailed studies of preparation and TONL properties of transparent glass ceramics based on HMO glasses were reported so far probably because of strong devitrification capacity of HOM glasses which makes the grain size hard to control and reduce optical transmittance by their high refraction or scatting losses [9]. Titanium dioxide (TiO2) is known as a good nucleating agent that can be used to precipitate MCs in glass network. Besides, TiO2 also plays a significant role in enhancing optical nonlinearity for the empty d-orbitals of Ti4+ ions [10,11]. In this paper, we report the preparation of transparent glass ceramics containing MCs of Bi2O3 phase in the Bi2O3–B2O3–TiO2 ternary glass system along with their TONL characteristics.
2.1. Host glass preparation
⁎ Corresponding author. Tel.: + 86 574 87600328; fax: + 86 574 87600976. E-mail address: [email protected] (F. Chen). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.048
Glass with composition in molar percent of 60Bi2O3–30B2O3– 10TiO2 was selected as host glass (labeled as TGC0) and prepared from reagent-grade Bi2O3, H3BO3, and TiO2. To ensure high optical homogeneity of host glass, 150 g mixture was ground well and melted in a platinum crucible at 1200 °C for 1 h. A transparent yellow, flat plate was obtained by pouring the liquid onto a stainless steel plate and pressed quickly with another one, then annealed at 350 °C for 24 h to relieve it of thermal stress. 2.2. Glass ceramics preparation Differential thermal analysis (DTA) measurement was conducted from room temperature to 900 °C for a chip of TGC0 sample weighing ≈20 mg with a uniform heating rate of 10 °C/min in order to find out the glass transition (Tg), crystallization onset (Tx) and ensure the heat treating temperature. As the DTA profile shows in Fig. 1, the obvious downward shift around 400 °C is due to the glass phase transition, and the transition temperature Tg is calculated to be 402 °C as seen in the inset. The endotherm peak at 413 °C is attributed to the glass softening point. The crystallization onset temperature Tx also indicated in the inset is 471 °C. The strong exotherm peak corresponding to the crystallization temperature Tc was found to occurr at 526 °C, and the following sharp endotherm peak at 618 °C is ascribed to melting. The thermal stability parameter ΔT (ΔT = Tx − Tg) is 69 °C lower than 100 °C suggesting strong crystallization ability [12] of this precursor. According to the data shown previously, it is appropriate to microcrystallize TGC0 between Tg and Tc. In general, heat treatment
F. Chen et al. / Journal of Non-Crystalline Solids 256 (2010) 2786–2789
2787
(a) 8000 7000
EXO
Tc Tx
6000
ENDO
Intensity
Tg
5000 4000 3000 2000
Tm
1000 300
450
600
750
25
20
900
35
40
temperature too close to exotherm band results in high crystallization rate and nonuniformity of crystals distribution, thus low treating temperature is desirable for the growth of grain with fine size and uniformity. After many experiments, the heat treating procedures were set to one-step process with the heating rate of 5 °C/min from room temperature, and then TGC0 was held at 48 °C above Tg, namely 450 °C for different durations in air. After the treatments, the specimens were rapidly cooled to room temperature, and labeled as TGC1, TGC2, TGC3 and TGC4 for treating times of 6, 7, 8 and 9 h, respectively.
c b a
(220)
(200)
(111)
(b)
Counts (arb. units)
Fig. 1. DTA profile of the host glass (TGC0) at the heating rate of 10 °C/min. Inset is the photograph of TGC0, TGC1, TGC2, TGC3 and TGC4 from left to right in turn.
30
2 θ (degrees)
Temperature(°C)
(311)
150
Bi2O3
25
30
35
40
45
50
55
60
2 θ (degrees) 2.3. Optical measurements Powder X-ray diffraction (XRD) technique using CuK radiation (Bruker AXS D8 Discover) is used to confirm the crystalline nature of the precursor and heat-treated samples. Attenuation spectra were measured over the range of 400–1000 nm with a Perkin-ElmerLambda 950UV/VIS/NIR spectrophotometer. The TONL were measured by a conventional Z-scan method using a 76 MHz repetition rate mode-locked Ti: Sapphire laser (Coherent Mira 900-D) with 200 fs pulse width at the wavelength of 750 nm, the detailed experimental setup was described in Ref. [8]. The power density is 1.48 GW/cm2, and the total power energy was set to be small (65 mW) in order to minimize thermo-optics effects [13]. All the measurements above took place at room temperature. 3. Results 3.1. X-ray diffraction studies To confirm the crystallization in glass network, grounded powders obtained from the polished samples were applied to X-ray diffractograms (XRD). Fig 2(a) shows the high-resolution XRD pattern of TGC0, and it is interesting to see the sharp diffraction peak at 28.6° which means that crystallization behavior had already occurred in the host glass. The XRD patterns in Fig. 2(b) illustrate the growth of crystals before and after heat treatments, and four Bragg peaks were clearly observed in the spectrum of TGC2 and indexed to a Bi2O3 (PDF No.: 76-2478) phase which agrees quite well with Sunahara's report [14]. When treating duration reached 9 h (TGC4), these peaks become stronger and sharper, and the decrease in the full width at half maximum (FWHM) of peak at 28.6° from 0.337 to 0.216° indicates the
Fig. 2. (a) High-resolution XRD pattern of TGC0; (b) XRD patterns of the samples (a. TGC0; b. TGC2; c. TGC4) and Bi2O3 phase.
growth of crystals which can be confirmed by Scherrer equation [15] as follows: L=
Kλ ω cosϑ
ð1Þ
where L is the mean size of crystallites, K is a constant for a cubic system equal to 0.899, λ is the wavelength of the radiation (0.154 nm), and ω is the full width at half maximum (FWHM) corresponding to the Bragg peak ϑ at 28.6°. The calculated L in the transparent TGC0 and TGC2 is 6 and 70 nm respectively, while L of the longest treated sample TGC4 increases over 98 nm which makes it opaque. These results confirm that the average size of Bi2O3 crystals exhibits an increasing tendency on the heat treatment time. 3.2. Optical attenuation spectra As we have known, optical losses of transparent glass ceramics are mainly originated from: 1. Rayleigh–Gans type scattering introduced by the formation of MCs [9]; and 2. Optical absorption of electron transitions from 6s level of Bi3+ ions to 2p level of O2− ions [16] at ultraviolet region. In the optical attenuation spectra (Fig. 3), the attenuation coefficient A of the samples exhibits a linear relationship (denoted by the dotted lines in Fig. 3) with the reciprocal fourth power of wavelength (λ− 4) at wavelengths longer than 480 nm and increases with heat-treated times, indicating the governed role of optical scattering which depends on the size and numbers of crystals.
2788
F. Chen et al. / Journal of Non-Crystalline Solids 256 (2010) 2786–2789
30
7.5
(a) 2.2
6.0
2.0
4.5
1.8
Normalized transmittance
40
(αhv) / (eVcm )
Attenuation coefficient(cm-1)
50
3.0
1.5
0.0
20
2.4
2.6
2.8
3.0
3.2
hv (eV)
10
0 3.5
TGC0 TGC2
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Wavelength-4 (nm-4× 10-11)
-25 -20 -15 -10
On the other hand, at wavelengths shorter than 480 nm, A values of the samples show a parabolic dependence on λ− 4 which suggests that optical absorption dominates the optical attenuations [17]. Further, the growth of Bi2O3 grains causes a red shifting tendency of the ultraviolet cut-off edge which could derive an optical band gap Eopg by the well-known Tauc law [18]: m αhν = B hν Eopg
ð2Þ
where α is the linear absorption coefficient, hv is the incident photon energy, B is the electronic transition constant and exponent m is the factor that depends on the type of electronic transition, in the present case, m = 2 [19] for the nonuniformity of the amorphous medium. As seen in the inset of Fig. 3, Tauc plots indicate that the Eopg gradually decreases from 2.65 to 2.58 eV with heat treatment duration increasing to 7 h. It can be interpreted in terms of the enlargement of Bi2O3 MCs which accompanied by charged defect centers create localized energy states around the Fermi-level [20] in glassy structure, leading to the narrowing of the optical band gap. Therefore, this behavior indicates that it is possible to modulate the band structure of these composites by controlling their grain sizes.
0
5
10
15
20
25
Z (mm)
(b) 1.8 TGC0
Normalized transmittance
Fig. 3. Attenuation spectra of the transparent glass ceramic samples (○ TGC0, ● TGC1, ▲ TGC2). The inset shows the corresponding Tauc plots.
-5
1.6
TGC2
1.4
1.2
1.0
0.8
-25 -20 -15 -10
-5
0
5
10
15
20
25
Z (mm) Fig. 4. (a) Close and (b) open aperture Z-scan curves of TGC0 and TGC2. The solid lines are the theoretical fitting, and the error rate is 20%.
comparable to that of chalcogenide glass [21] indicating good TONL performance of transparent Bi2O3–B2O3–TiO2 glass ceramics. 4. Discussion
3.3. Third-order nonlinearities Fig. 4(a) and (b) shows close (aperture linear transmittance S = 0.05) and open (S = 1) aperture Z-scan curves obtained from samples TGC0 and TGC2, respectively. The observed transmittance changes between the pre-focal (Z b 0) maximum and the post-focal (Z N 0) minimum in the closed Z-scan are due to the negative nonlinear indexes of refraction of both samples, while in the open Z-scan the enhanced optical transmittance near the focus is an indication of saturable absorption (SA) at high optical intensity. It is clear that TGC2 exhibits stronger Z-scan signals demonstrating its higher nonlinear refraction γ and nonlinear absorption coefficient β. According to the well established calculation procedures [8], the γ value is estimated to increase by 24% while the β is enhanced by a factor of ~3.31 after 7 h of heat treatment. The growth of Bi2O3 crystals from 6 to 70 nm in size is responsible for this tendency, because larger MCs would enhance the electronic interactions between adjacent grains when excited with femtosecond laser, which consequently caused significant enhancements of TONL. Further, as listed in Table 1, the absolute γ of present glass ceramics is larger than those obtained from our previously studied Bi2O3–B2O3–TiO2 non-crystallized glass [11] as well as lead [7] and tellurite [8] glasses reported recently, and
The nonlinear refraction γ of the present glass ceramics shows an opposite sign to the non-crystallized Bi2O3–B2O3–TiO2 glasses in our previous work [11] with similar testing conditions, thus it is reasonable to attribute this large difference to the precipitated Bi2O3 grains in glass network. Further, the present larger absolute γ values support this argument since MCs induced resonant-type TONL (size confinement effect) was reported to be much stronger than nonresonant TONL in a homogeneous glass [4,5]. The MCs which are limited by the deep three-dimensional confinement potential in glass Table 1 Summaries of optical parameters of TGC0, TGC2 and references for comparison. Samples
Eopg (eV)
γ (× 10− 17 m2/W)
β (×10− 11 m/W)
TGC0 TGC2 PBH6 [7] BWT3 [8] BBT3 [11] Ge35As15Se50 [21]
2.65 2.58 3.82 2.47 2.12 1.94
− 1.64 − 2.14 1.58 0.15 0.89 2.46
− 1.67 − 5.53 7.75 0.57 2.73 0.5
The Z-scan wavelengths λ in Refs. [7], [8], [11] and [21] are 532, 720, 800 and 1540 nm, respectively.
F. Chen et al. / Journal of Non-Crystalline Solids 256 (2010) 2786–2789
network work as middle energy states [22] and trap the electrons from the medium during the excitation, which could induce drastic nonlinear refractive behavior. Consequently, the Bi2O3 MCs occupied the main status of TONL over the glass medium which only experienced the relatively weak bound electronic effect. On the other hand, the thermo-optics effect might also influence the Z-scan results with 76 MHz pulse repetition rate of the laser source, but the negative γ values obtained in the present work indicate a negligible thermal component since it only exhibits a positive impact [23,24] on nonlinear refraction with excited wavelength (photon energy 1.66 eV) below the optical band gap, and the low laser power (65 mW) is responsible. On the other hand, since the nonlinear refractive behavior in both glass ceramic and host glass was dominated by the Bi2O3 grains, the nonlinear absorption can also be influenced by these MCs. The zerodimensional Bi2O3 MCs with size much smaller than the wavelength of incident laser (750 nm) could decompose the conduction and valence bands into a series of discrete energy levels, which caused saturated absorption at high incident optical intensity, namely bandfilling effect. A similar phenomenon was also reported by Xiang [25] in PbS MCs precipitated Na2O–B2O3–SiO2 glasses. 5. Conclusions A series of transparent glass ceramics containing Bi2O3 microcrystals was prepared by controlled heat treatment within the glass composition of 60Bi2O3–30B2O3–10TiO2 (in molar %). By employing the Z-scan method with femtosecond laser pulses, optical nonlinearities of the transparent glass ceramics were investigated at 750 nm. The results show that the appearance of Bi2O3 crystals changed the signs of nonlinear refraction γ and nonlinear absorption coefficient β, and these two parameters were further promoted by the growth of Bi2O3 grain size from 6 to 70 nm. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (Nos. 60978058, 60908032, and
2789
60972064), the National Natural Science Foundation of Zhejiang Province (Nos. R1101263 and Y1090996), the Scientific Research Foundation of Graduate School of Ningbo University, the K C Wong Magna Fund in Ningbo University and the Program for Innovative Research Team in Ningbo city.
References [1] M.J. Dejneka, C. Powell, N. Borrelli, D. Ouzounov, A. Gaeta, J. Am. Ceram. Soc. 88 (2005) 2435. [2] G.H. Beall, L.R. Pinckney, J. Am. Ceram. Soc. 82 (1999) 5. [3] X.L. Duan, D.R. Yuan, F.P. Yu, L.H. Wang, Appl. Phys. Lett. 89 (2006) 183119. [4] G.S. Maciela, Cid B. de Araujoa, A.A. Lipovskii, D.K. Tagantsev, Opt. Commun. 203 (2002) 441. [5] C.A.C. Bosco, E.L. Falcao-Filho, G.S. Maciel, L.H. Acioli, Cid B. de Araujo, A.A. Lipovskii, D.K. Tagantsev, J. Appl. Phys. 94 (2003) 6223. [6] H. Nasu, T. Ito, H. Hase, J. Matsuoka, K. Kamiya, J. Non-Cryst. Solids 204 (1996) 78. [7] T. Hashimoto, T. Yamamoto, T. Kato, H. Nasu, K. Kamiya, J. Appl. Phys. 90 (2001) 533. [8] Y.F. Chen, Q.H. Nie, T.F. Xu, S.X. Dai, X.S. Wang, X. Shen, J. Non-Cryst. Solids 35 (2008) 3468. [9] X.H. Zhang, H.L. Ma, J. Lucas, J. Non-Cryst. Solids 337 (2004) 130. [10] X.H. Zhu, Z.Y. Meng, J. Appl. Phys. 75 (1994) 3756. [11] T.F. Xu, F.F. Chen, S.X. Dai, Q.H. Nie, X. Shen, X.S. Wang, Physica B 404 (2009) 2012. [12] P. Becker, Cryst. Res. Technol. 38 (2003) 74. [13] M. Yin, H.P. Li, S.H. Tang, W. Ji, Appl. Phys. B 70 (2000) 587. [14] K. Sunahara, J. Yano, K. Kakegawa, J. Euro. Ceram. Soc. 26 (2006) 623. [15] V. Vassilev, S. Boycheva, C. Popov, P. Petkov, L. Aljihmani, B. Monchev, K. Kolev, J. Non-Cryst. Solids 351 (2005) 299. [16] I.I. Oprea, H. Hesse, K. Betzler, Opt. Mater. 26 (2004) 235. [17] A. Sakamoto, F. Sato, S. Yamamoto, J. Non-Cryst. Solids 352 (2006) 514. [18] J. Tauc, J. Non-Cryst. Solids 8–10 (1972) 569. [19] Y.L. Wang, S.X. Dai, F.F. Chen, T.F. Xu, Q.H. Nie, Mater. Chem. Phys. 113 (2009) 407. [20] R.A. Street, N.F. Mott, Phys. Rev. Lett. 35 (1975) 1293. [21] J.T. Gopinath, M. Soljačić, E.P. Ippen, V.N. Fuflyigin, W.A. King, M. Shurgalin, J. Appl. Phys. 96 (2004) 6931. [22] K.S. Song, R.T. Williams, Self-trapped Excitons, Springer-Verlag, Berlin, 1993. [23] K.Y. Tseng, K.S. Wong, G.K.L. Wong, Opt. Lett. 21 (1996) 180. [24] T.F. Tsay, B. Bendow, S.S. Mitra, Phys. Rev. B 8 (1973) 2688. [25] W.D. Xiang, S.S. Tang, X.Y. Zhang, X.J. Liang, J. Alloys Compd. 471 (2009) 498.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Preparation and optical nonlinearities of transparent bismuth-based glass ceramics embedded with Bi2O3 microcrystals Feifei Chen ⁎, Tiefeng Xu, Shixun Dai, Qiuhua Nie, Xiang Shen, Xunsi Wang, Baoan Song College of Information Science and Engineering, Ningbo University, Ningbo 315211, China
a r t i c l e
i n f o
Article history: Received 7 December 2009 Received in revised form 24 August 2010 Available online 11 October 2010 Keywords: Heavy-metal oxides; Microcrystallinity; Non-linear Optics
a b s t r a c t Transparent glass ceramics based on Bi2O3–B2O3–TiO2 ternary glass system were prepared by controlled onestep heat treatment method. X-ray diffraction measurement confirms the formation of Bi2O3 phase crystals in glass network. Z-scan measurements at a wavelength of 750 nm were employed to investigate the third-order optical nonlinearities of these transparent glass ceramics. The results show that the nonlinear properties were significantly enhanced after crystallization process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In recent years, transparent glass ceramics [1] are intensively studied since these materials with uniformly dispersed microncrystals (MC) possess advantages over conventional glasses such as better thermal and mechanical stabilities which offer promise for many potential applications [2,3]. Particularly in the field of nonlinear optics [4,5], they have been widely accepted for their large third-order optical nonlinearities (TONL) induced by dielectric confinement and quantum mechanical confinement effects. On the other hand, heavy metal oxide (HMO) glasses [6–8] based on TeO2, Bi2O3, and PbO were also well-known to possess high TONL, which is due to their high refractive indexes. However, not many detailed studies of preparation and TONL properties of transparent glass ceramics based on HMO glasses were reported so far probably because of strong devitrification capacity of HOM glasses which makes the grain size hard to control and reduce optical transmittance by their high refraction or scatting losses [9]. Titanium dioxide (TiO2) is known as a good nucleating agent that can be used to precipitate MCs in glass network. Besides, TiO2 also plays a significant role in enhancing optical nonlinearity for the empty d-orbitals of Ti4+ ions [10,11]. In this paper, we report the preparation of transparent glass ceramics containing MCs of Bi2O3 phase in the Bi2O3–B2O3–TiO2 ternary glass system along with their TONL characteristics.
2.1. Host glass preparation
⁎ Corresponding author. Tel.: + 86 574 87600328; fax: + 86 574 87600976. E-mail address: [email protected] (F. Chen). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.048
Glass with composition in molar percent of 60Bi2O3–30B2O3– 10TiO2 was selected as host glass (labeled as TGC0) and prepared from reagent-grade Bi2O3, H3BO3, and TiO2. To ensure high optical homogeneity of host glass, 150 g mixture was ground well and melted in a platinum crucible at 1200 °C for 1 h. A transparent yellow, flat plate was obtained by pouring the liquid onto a stainless steel plate and pressed quickly with another one, then annealed at 350 °C for 24 h to relieve it of thermal stress. 2.2. Glass ceramics preparation Differential thermal analysis (DTA) measurement was conducted from room temperature to 900 °C for a chip of TGC0 sample weighing ≈20 mg with a uniform heating rate of 10 °C/min in order to find out the glass transition (Tg), crystallization onset (Tx) and ensure the heat treating temperature. As the DTA profile shows in Fig. 1, the obvious downward shift around 400 °C is due to the glass phase transition, and the transition temperature Tg is calculated to be 402 °C as seen in the inset. The endotherm peak at 413 °C is attributed to the glass softening point. The crystallization onset temperature Tx also indicated in the inset is 471 °C. The strong exotherm peak corresponding to the crystallization temperature Tc was found to occurr at 526 °C, and the following sharp endotherm peak at 618 °C is ascribed to melting. The thermal stability parameter ΔT (ΔT = Tx − Tg) is 69 °C lower than 100 °C suggesting strong crystallization ability [12] of this precursor. According to the data shown previously, it is appropriate to microcrystallize TGC0 between Tg and Tc. In general, heat treatment
F. Chen et al. / Journal of Non-Crystalline Solids 256 (2010) 2786–2789
2787
(a) 8000 7000
EXO
Tc Tx
6000
ENDO
Intensity
Tg
5000 4000 3000 2000
Tm
1000 300
450
600
750
25
20
900
35
40
temperature too close to exotherm band results in high crystallization rate and nonuniformity of crystals distribution, thus low treating temperature is desirable for the growth of grain with fine size and uniformity. After many experiments, the heat treating procedures were set to one-step process with the heating rate of 5 °C/min from room temperature, and then TGC0 was held at 48 °C above Tg, namely 450 °C for different durations in air. After the treatments, the specimens were rapidly cooled to room temperature, and labeled as TGC1, TGC2, TGC3 and TGC4 for treating times of 6, 7, 8 and 9 h, respectively.
c b a
(220)
(200)
(111)
(b)
Counts (arb. units)
Fig. 1. DTA profile of the host glass (TGC0) at the heating rate of 10 °C/min. Inset is the photograph of TGC0, TGC1, TGC2, TGC3 and TGC4 from left to right in turn.
30
2 θ (degrees)
Temperature(°C)
(311)
150
Bi2O3
25
30
35
40
45
50
55
60
2 θ (degrees) 2.3. Optical measurements Powder X-ray diffraction (XRD) technique using CuK radiation (Bruker AXS D8 Discover) is used to confirm the crystalline nature of the precursor and heat-treated samples. Attenuation spectra were measured over the range of 400–1000 nm with a Perkin-ElmerLambda 950UV/VIS/NIR spectrophotometer. The TONL were measured by a conventional Z-scan method using a 76 MHz repetition rate mode-locked Ti: Sapphire laser (Coherent Mira 900-D) with 200 fs pulse width at the wavelength of 750 nm, the detailed experimental setup was described in Ref. [8]. The power density is 1.48 GW/cm2, and the total power energy was set to be small (65 mW) in order to minimize thermo-optics effects [13]. All the measurements above took place at room temperature. 3. Results 3.1. X-ray diffraction studies To confirm the crystallization in glass network, grounded powders obtained from the polished samples were applied to X-ray diffractograms (XRD). Fig 2(a) shows the high-resolution XRD pattern of TGC0, and it is interesting to see the sharp diffraction peak at 28.6° which means that crystallization behavior had already occurred in the host glass. The XRD patterns in Fig. 2(b) illustrate the growth of crystals before and after heat treatments, and four Bragg peaks were clearly observed in the spectrum of TGC2 and indexed to a Bi2O3 (PDF No.: 76-2478) phase which agrees quite well with Sunahara's report [14]. When treating duration reached 9 h (TGC4), these peaks become stronger and sharper, and the decrease in the full width at half maximum (FWHM) of peak at 28.6° from 0.337 to 0.216° indicates the
Fig. 2. (a) High-resolution XRD pattern of TGC0; (b) XRD patterns of the samples (a. TGC0; b. TGC2; c. TGC4) and Bi2O3 phase.
growth of crystals which can be confirmed by Scherrer equation [15] as follows: L=
Kλ ω cosϑ
ð1Þ
where L is the mean size of crystallites, K is a constant for a cubic system equal to 0.899, λ is the wavelength of the radiation (0.154 nm), and ω is the full width at half maximum (FWHM) corresponding to the Bragg peak ϑ at 28.6°. The calculated L in the transparent TGC0 and TGC2 is 6 and 70 nm respectively, while L of the longest treated sample TGC4 increases over 98 nm which makes it opaque. These results confirm that the average size of Bi2O3 crystals exhibits an increasing tendency on the heat treatment time. 3.2. Optical attenuation spectra As we have known, optical losses of transparent glass ceramics are mainly originated from: 1. Rayleigh–Gans type scattering introduced by the formation of MCs [9]; and 2. Optical absorption of electron transitions from 6s level of Bi3+ ions to 2p level of O2− ions [16] at ultraviolet region. In the optical attenuation spectra (Fig. 3), the attenuation coefficient A of the samples exhibits a linear relationship (denoted by the dotted lines in Fig. 3) with the reciprocal fourth power of wavelength (λ− 4) at wavelengths longer than 480 nm and increases with heat-treated times, indicating the governed role of optical scattering which depends on the size and numbers of crystals.
2788
F. Chen et al. / Journal of Non-Crystalline Solids 256 (2010) 2786–2789
30
7.5
(a) 2.2
6.0
2.0
4.5
1.8
Normalized transmittance
40
(αhv) / (eVcm )
Attenuation coefficient(cm-1)
50
3.0
1.5
0.0
20
2.4
2.6
2.8
3.0
3.2
hv (eV)
10
0 3.5
TGC0 TGC2
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Wavelength-4 (nm-4× 10-11)
-25 -20 -15 -10
On the other hand, at wavelengths shorter than 480 nm, A values of the samples show a parabolic dependence on λ− 4 which suggests that optical absorption dominates the optical attenuations [17]. Further, the growth of Bi2O3 grains causes a red shifting tendency of the ultraviolet cut-off edge which could derive an optical band gap Eopg by the well-known Tauc law [18]: m αhν = B hν Eopg
ð2Þ
where α is the linear absorption coefficient, hv is the incident photon energy, B is the electronic transition constant and exponent m is the factor that depends on the type of electronic transition, in the present case, m = 2 [19] for the nonuniformity of the amorphous medium. As seen in the inset of Fig. 3, Tauc plots indicate that the Eopg gradually decreases from 2.65 to 2.58 eV with heat treatment duration increasing to 7 h. It can be interpreted in terms of the enlargement of Bi2O3 MCs which accompanied by charged defect centers create localized energy states around the Fermi-level [20] in glassy structure, leading to the narrowing of the optical band gap. Therefore, this behavior indicates that it is possible to modulate the band structure of these composites by controlling their grain sizes.
0
5
10
15
20
25
Z (mm)
(b) 1.8 TGC0
Normalized transmittance
Fig. 3. Attenuation spectra of the transparent glass ceramic samples (○ TGC0, ● TGC1, ▲ TGC2). The inset shows the corresponding Tauc plots.
-5
1.6
TGC2
1.4
1.2
1.0
0.8
-25 -20 -15 -10
-5
0
5
10
15
20
25
Z (mm) Fig. 4. (a) Close and (b) open aperture Z-scan curves of TGC0 and TGC2. The solid lines are the theoretical fitting, and the error rate is 20%.
comparable to that of chalcogenide glass [21] indicating good TONL performance of transparent Bi2O3–B2O3–TiO2 glass ceramics. 4. Discussion
3.3. Third-order nonlinearities Fig. 4(a) and (b) shows close (aperture linear transmittance S = 0.05) and open (S = 1) aperture Z-scan curves obtained from samples TGC0 and TGC2, respectively. The observed transmittance changes between the pre-focal (Z b 0) maximum and the post-focal (Z N 0) minimum in the closed Z-scan are due to the negative nonlinear indexes of refraction of both samples, while in the open Z-scan the enhanced optical transmittance near the focus is an indication of saturable absorption (SA) at high optical intensity. It is clear that TGC2 exhibits stronger Z-scan signals demonstrating its higher nonlinear refraction γ and nonlinear absorption coefficient β. According to the well established calculation procedures [8], the γ value is estimated to increase by 24% while the β is enhanced by a factor of ~3.31 after 7 h of heat treatment. The growth of Bi2O3 crystals from 6 to 70 nm in size is responsible for this tendency, because larger MCs would enhance the electronic interactions between adjacent grains when excited with femtosecond laser, which consequently caused significant enhancements of TONL. Further, as listed in Table 1, the absolute γ of present glass ceramics is larger than those obtained from our previously studied Bi2O3–B2O3–TiO2 non-crystallized glass [11] as well as lead [7] and tellurite [8] glasses reported recently, and
The nonlinear refraction γ of the present glass ceramics shows an opposite sign to the non-crystallized Bi2O3–B2O3–TiO2 glasses in our previous work [11] with similar testing conditions, thus it is reasonable to attribute this large difference to the precipitated Bi2O3 grains in glass network. Further, the present larger absolute γ values support this argument since MCs induced resonant-type TONL (size confinement effect) was reported to be much stronger than nonresonant TONL in a homogeneous glass [4,5]. The MCs which are limited by the deep three-dimensional confinement potential in glass Table 1 Summaries of optical parameters of TGC0, TGC2 and references for comparison. Samples
Eopg (eV)
γ (× 10− 17 m2/W)
β (×10− 11 m/W)
TGC0 TGC2 PBH6 [7] BWT3 [8] BBT3 [11] Ge35As15Se50 [21]
2.65 2.58 3.82 2.47 2.12 1.94
− 1.64 − 2.14 1.58 0.15 0.89 2.46
− 1.67 − 5.53 7.75 0.57 2.73 0.5
The Z-scan wavelengths λ in Refs. [7], [8], [11] and [21] are 532, 720, 800 and 1540 nm, respectively.
F. Chen et al. / Journal of Non-Crystalline Solids 256 (2010) 2786–2789
network work as middle energy states [22] and trap the electrons from the medium during the excitation, which could induce drastic nonlinear refractive behavior. Consequently, the Bi2O3 MCs occupied the main status of TONL over the glass medium which only experienced the relatively weak bound electronic effect. On the other hand, the thermo-optics effect might also influence the Z-scan results with 76 MHz pulse repetition rate of the laser source, but the negative γ values obtained in the present work indicate a negligible thermal component since it only exhibits a positive impact [23,24] on nonlinear refraction with excited wavelength (photon energy 1.66 eV) below the optical band gap, and the low laser power (65 mW) is responsible. On the other hand, since the nonlinear refractive behavior in both glass ceramic and host glass was dominated by the Bi2O3 grains, the nonlinear absorption can also be influenced by these MCs. The zerodimensional Bi2O3 MCs with size much smaller than the wavelength of incident laser (750 nm) could decompose the conduction and valence bands into a series of discrete energy levels, which caused saturated absorption at high incident optical intensity, namely bandfilling effect. A similar phenomenon was also reported by Xiang [25] in PbS MCs precipitated Na2O–B2O3–SiO2 glasses. 5. Conclusions A series of transparent glass ceramics containing Bi2O3 microcrystals was prepared by controlled heat treatment within the glass composition of 60Bi2O3–30B2O3–10TiO2 (in molar %). By employing the Z-scan method with femtosecond laser pulses, optical nonlinearities of the transparent glass ceramics were investigated at 750 nm. The results show that the appearance of Bi2O3 crystals changed the signs of nonlinear refraction γ and nonlinear absorption coefficient β, and these two parameters were further promoted by the growth of Bi2O3 grain size from 6 to 70 nm. Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (Nos. 60978058, 60908032, and
2789
60972064), the National Natural Science Foundation of Zhejiang Province (Nos. R1101263 and Y1090996), the Scientific Research Foundation of Graduate School of Ningbo University, the K C Wong Magna Fund in Ningbo University and the Program for Innovative Research Team in Ningbo city.
References [1] M.J. Dejneka, C. Powell, N. Borrelli, D. Ouzounov, A. Gaeta, J. Am. Ceram. Soc. 88 (2005) 2435. [2] G.H. Beall, L.R. Pinckney, J. Am. Ceram. Soc. 82 (1999) 5. [3] X.L. Duan, D.R. Yuan, F.P. Yu, L.H. Wang, Appl. Phys. Lett. 89 (2006) 183119. [4] G.S. Maciela, Cid B. de Araujoa, A.A. Lipovskii, D.K. Tagantsev, Opt. Commun. 203 (2002) 441. [5] C.A.C. Bosco, E.L. Falcao-Filho, G.S. Maciel, L.H. Acioli, Cid B. de Araujo, A.A. Lipovskii, D.K. Tagantsev, J. Appl. Phys. 94 (2003) 6223. [6] H. Nasu, T. Ito, H. Hase, J. Matsuoka, K. Kamiya, J. Non-Cryst. Solids 204 (1996) 78. [7] T. Hashimoto, T. Yamamoto, T. Kato, H. Nasu, K. Kamiya, J. Appl. Phys. 90 (2001) 533. [8] Y.F. Chen, Q.H. Nie, T.F. Xu, S.X. Dai, X.S. Wang, X. Shen, J. Non-Cryst. Solids 35 (2008) 3468. [9] X.H. Zhang, H.L. Ma, J. Lucas, J. Non-Cryst. Solids 337 (2004) 130. [10] X.H. Zhu, Z.Y. Meng, J. Appl. Phys. 75 (1994) 3756. [11] T.F. Xu, F.F. Chen, S.X. Dai, Q.H. Nie, X. Shen, X.S. Wang, Physica B 404 (2009) 2012. [12] P. Becker, Cryst. Res. Technol. 38 (2003) 74. [13] M. Yin, H.P. Li, S.H. Tang, W. Ji, Appl. Phys. B 70 (2000) 587. [14] K. Sunahara, J. Yano, K. Kakegawa, J. Euro. Ceram. Soc. 26 (2006) 623. [15] V. Vassilev, S. Boycheva, C. Popov, P. Petkov, L. Aljihmani, B. Monchev, K. Kolev, J. Non-Cryst. Solids 351 (2005) 299. [16] I.I. Oprea, H. Hesse, K. Betzler, Opt. Mater. 26 (2004) 235. [17] A. Sakamoto, F. Sato, S. Yamamoto, J. Non-Cryst. Solids 352 (2006) 514. [18] J. Tauc, J. Non-Cryst. Solids 8–10 (1972) 569. [19] Y.L. Wang, S.X. Dai, F.F. Chen, T.F. Xu, Q.H. Nie, Mater. Chem. Phys. 113 (2009) 407. [20] R.A. Street, N.F. Mott, Phys. Rev. Lett. 35 (1975) 1293. [21] J.T. Gopinath, M. Soljačić, E.P. Ippen, V.N. Fuflyigin, W.A. King, M. Shurgalin, J. Appl. Phys. 96 (2004) 6931. [22] K.S. Song, R.T. Williams, Self-trapped Excitons, Springer-Verlag, Berlin, 1993. [23] K.Y. Tseng, K.S. Wong, G.K.L. Wong, Opt. Lett. 21 (1996) 180. [24] T.F. Tsay, B. Bendow, S.S. Mitra, Phys. Rev. B 8 (1973) 2688. [25] W.D. Xiang, S.S. Tang, X.Y. Zhang, X.J. Liang, J. Alloys Compd. 471 (2009) 498.