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Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dot-embedded silicate glasses
NOC-16492; No of Pages 4 Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dot-embedded silicate glasses Guoping Dong a, b,⁎, Guobo Wu a, b, Shaohua Fan a, b, Fangteng Zhang a, b, Yuanhao Zhang a, b, Botao Wu c, Zhijun Ma a, b, Mingying Peng a, b, Jianrong Qiu a, b,⁎⁎ a b c
State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 19 October 2012 Received in revised form 12 March 2013 Available online xxxx Keywords: Tunable photoluminescence; Optical glass; PbS quantum dots; Multi-wavelength; Optical amplification
a b s t r a c t Optical properties of PbS quantum dot (QD)-embedded silicate glasses prepared through post annealing were investigated. By modulating the heat treatment condition, tunable near-infrared (NIR) photoluminescence (PL) was obtained. Noticeable optical amplification was achieved at both 1330 nm and 1550 nm windows with similar amplification characteristics in single PbS QD-embedded glass, which indicated that broadband optical amplification is expected at the entire PL band of PbS QD-embedded glass. The PbS QD-embedded glass with multi-wavelength optical amplification is promising as the gain medium of broadband fiber amplifiers and tunable fiber lasers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Modern optical communication network has been rapidly developed since the invention of Er 3+-doped fiber amplifier (EDFA) [1,2]. Subsequently, the development of dense wavelength division multiplexing (DWDM) technology further promotes the progress of optical communication network based on rare-earth ion (such as Er3+, Tm3+, Pr3+ etc.)-doped fiber amplifier. However, because of the intrinsic spectroscopic characteristics of rare-earth ions, the operating waveband of rare-earth ion-doped fiber amplifier is restricted. Taking commercial EDFA as an example, although a series of novel technologies have been developed to expand the operating waveband of EDFA, it can only cover the C band (1530–1565 nm) and L band (1565–1605 nm) so far, which is much narrower than the whole optical communication window (1.1–1.7 μm) of commercial silica fiber. Therefore, to meet the higher requirement of speed and capacity of information transmission, it is urgent to improve the properties of the fiber amplifier. To expand the operating bandwidth of the optical communication network, it is necessary to find a novel gain medium, which can almost cover the whole optical communication window. Semiconductor quantum dots (QDs) have been widely investigated because of their unique optical and electronic properties [3–5]. By ⁎ Correspondence to: G. Dong, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. Tel.: + 86 20 87114235. ⁎⁎ Correspondence to: J. Qiu, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. E-mail addresses: [email protected] (G. Dong), [email protected] (J. Qiu).
controlling the size of QDs, we can conveniently tune the bandgap of QDs, and obtain tunable luminescence. Nowadays, researches on QDs are mainly focused on the II–VI, III–V and IV–VI group materials. Among them, it is well known that PbS QDs can provide tunable nearinfrared luminescence (about 1–2 μm) by controlling the size of QDs, which matches well with the whole optical communication window [5–19]. As a typical narrow bandgap (Eg = 0.41 eV) semiconductor, the exciton Bohr radius of PbS bulk is as large as 20 nm, which endows a strong quantum confinement effect that can be easily realized over a wide range of QD size [5]. Therefore, PbS QD is promising to act as a gain medium of broadband fiber amplifiers. Considering the potential fabrication of PbS QD-embedded fibers and fiber amplifiers, glass is probably the most suitable matrix for PbS QDs because of their high thermal, chemical, and mechanical stability, wide transmission range and excellent fiber-drawing ability [6–19]. Several works have been performed on PbS QD-embedded glasses. However, most of them are focused on the formation mechanism [10–13], tunable luminescence [7,18], optical nonlinear, saturable absorbers [14], etc. The optical amplification properties at the optical communication window, which are one of the most key factors to evaluate the potential application as a gain medium of fiber amplifiers, are scarcely investigated. Only a few works have characterized the optical amplification property at the 1.3 μm and 1.55 μm windows [6,19]. In our previous work, the optical amplification property at 1.33 μm and 1.55 μm was investigated separately in different PbS QD-embedded glass, and obvious optical amplification was detected at optical communication windows [19]. However, to evaluate the potential of PbS QDembedded glass applied in a broadband fiber amplifier, it is necessary
0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
Please cite this article as: G. Dong, et al., Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dotembedded silicate glasses, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
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G. Dong et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
to investigate the broadband optical amplification at the optical communication window. Herein, tunable near-infrared (NIR) luminescence is realized in PbS QD-embedded silicate glass by controlling the heat treatment condition. By choosing PbS QD-embedded glass providing matched luminescence in optical communication window as the representative sample, optical amplification properties are characterized by the two-wave mixing technology. Noticeable optical amplification at both the 1.3 μm and 1.55 μm windows is simultaneously obtained in single PbS QD-embedded silicate glass. 2. Experimental details and characterization In this work, 66SiO2–8B2O3–18K2O–4BaO–4ZnO–1.5PbS (in mol%) glass, which has been demonstrated as a good matrix to form PbS QDs, was used to prepare the PbS QD-embedded glass by heat treatment [7,19]. The matrix glass was prepared by the traditional melt-quenching method at 1400 °C for 60 min. Analytical grade SiO2, B2O3, K2CO3, BaCO3, ZnO and PbS were used as raw materials. After quenching on a stainless steel mold, the as-prepared glass was heated at 500–575 °C for 24 h to form the PbS QDs in the glass matrix. Then, the PbS QDembedded glass was cut into thin plates and optically polished for spectroscopic and amplification characterizations. The size, morphology and crystallization of PbS QDs in glass were characterized by high resolution transmission electron microscopy (HR-TEM, Philips-FEI, Netherlands) with an accelerating voltage of 300 kV. Absorption and photoluminescence (PL) spectra of the glass plate were performed on a Lambda 900 (Perkin Elmer, USA) spectrophotometer and a Triax 320 (Jobin Yvon, France) fluorescence spectrofluorometer, respectively. Optical amplification of PbS QD-embedded glass was performed by two-wave mixing configuration, which is similar to our previous works [19,20]. An 808 nm laser diode (LD) was used as the excitation source, while a 1330 nm (or 1550 nm) LD was used as the probe beam. The optical gain, g is estimated by g = I/I0, where I and I0 represent the intensity of the probe signal with and without an excitation source, respectively. All the measurements were performed at room temperature. 3. Results and discussion Fig. 1 shows the absorption spectra of glass heat treated at different temperatures for 24 h. For the as-prepared glass, no absorption band is detected between 500 nm and 2250 nm. When the glass is heated at 500 °C for 24 h, the absorption edge slightly shifts to the longer wavelength. However, there is still no obvious absorption band observed in the spectrum. When the heating temperature reaches 525 °C, besides
Fig. 1. Absorption spectra of glass heated at different temperatures for 24 h.
the red shift of the absorption edge, a weak absorption band centered at ~700 nm appears in the absorption spectrum. The absorption band is ascribed to the generation of electron and hole pairs induced by the excitation photon, which indicates that PbS QDs are beginning to form in the glass matrix heated at 525 °C. When the heating temperature further increases to 550 °C and 575 °C, the absorption band of PbS QDs gradually shifts to the longer wavelength, which confirms that the size of PbS QDs grows continuously with the increase of temperature. The intensity of the absorption band is also enhanced with the increase of temperature. This indicates that the number density of PbS QDs in glass matrix also increases when the heating temperature increases from 525 °C to 575 °C. According to previous works, the average size of PbS QDs in glass can be estimated by the following equation [21,22] 2 2 Eg ðRÞ ¼ Eg þ
! 2ħ2 Eg π2 ; R m
ð1Þ
where R is the average radius of PbS QDs, ħ is Planck's constant, m⁎ is the reduced mass, Eg(R) is the effective energy of PbS QDs, and Eg is the bandgap energy of bulk PbS semiconductor. For bulk PbS semiconductor, Eg = 0.41 eV and m⁎ = 0.085me, where me is the static electron mass [21]. Since PbS is a direct bandgap semiconductor, the effective energy of PbS QDs can be evaluated by the fitting of the absorption spectra [21,23]. The effective energy of PbS QDs in glasses heated at 525 °C, 550 °C and 575 °C is fitted to be 1.93 eV, 0.91 eV and 0.62 eV, respectively. Based on Eq. (1), the average radius of PbS QDs in glasses heated at 525 °C, 550 °C and 575 °C is estimated to be 1.4 nm, 3.2 nm and 5.8 nm, respectively. Fig. 2 illustrates the TEM image of PbS QD-embedded glass after heat treatment at 550 °C for 24 h. In the image, a large number of PbS QDs are formed in the glass matrix. The morphology of QDs is quasi-spherical, while the size of QDs is between 3 and 8 nm, with an average size of ~ 6 nm, which is consistent with the fitting result of the absorption spectra shown in Fig. 1(4). The relatively large size distribution of PbS QDs is probably due to the high doping concentration of PbS in glass preparation, which results in the inhomogeneous migration of S source and growth of PbS nuclei during PbS QD formation. The inset of Fig. 2 shows the HR-TEM image of an individual PbS
Fig. 2. TEM image of glass heated at 550 °C for 24 h. The inset shows the HR-TEM image of a single PbS QD.
Please cite this article as: G. Dong, et al., Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dotembedded silicate glasses, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
G. Dong et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
Fig. 3. PL spectra of glass heated at different temperatures for 24 h. An 808 nm LD was used as an excitation source.
QD. A group of lattice fringe with a space of ~0.21 nm is clearly observed in the image, which can be assigned to the (220) plane of cubic PbS crystals (JCPDF: 65-0132). The HR-TEM result confirms that the QDs formed in the glass matrix are of pure cubic PbS phase. Because of the formation of PbS QDs in the glass matrix, it is deduced that broadband PL will appear in the PbS QD-embedded glass. Fig. 3 shows the PL spectra of PbS QD-embedded glass under an excitation of 808 nm LD. Although PbS QDs are confirmed to begin forming in the glass heated at 525 °C, the PbS QDs are too small to be considered as PbS nuclei, and the PL band is too weak to be detected as shown in Fig. 3(1). Additionally, the mismatch of excitation wavelength is probably another factor resulting in the absence of a PL band (see Fig. 1(3)). When the glass is heated at 550 °C for 24 h, an obvious PL band centered at 1370 nm is obtained in the spectrum. The PL band covers the wavelength from 1100 nm to 1700 nm, with a full width at half maximum (FWHM) of 220 nm. When the heating temperature reaches 575 °C, the PL band shifts to 1705 nm, and the FWHM also broadens to 300 nm. The results of PL spectra confirm that with the increase of heating temperature, the PbS QDs grow gradually, which endows a strong quantum confinement effect on PbS QD-embedded
3
glass. The size distribution of PbS QDs in glass also broadens obviously, which is probably due to the inhomogeneous migration and growth of PbS QDs at higher temperature. Based on the results discussed above, it can be deduced that the formation characteristic of PbS QDs in glass (such as the beginning growth temperature, growth rate, density in glass matrix, size and size distribution of QDs, etc.) can be widely tuned by controlling the heat treatment condition. This will further tune the PL wavelength and bandwidth of PbS QD-embedded glass, which is significant for the application in broadband tunable fiber amplifiers. To evaluate the application as gain medium in fiber amplifier, optical amplification is one of the most important prerequisite conditions. Because of the good PL matching characteristic with 1330 nm window of PbS QD-embedded glass heated at 550 °C for 24 h (see the dotted line in Fig. 3), we choose this glass as the representative sample to characterize the optical amplification at the 1330 nm window. Fig. 4(a) shows the optical amplified signals at 1330 nm with different pump power values. It can be found that the intensity enhances gradually with the increase of power. Even when the pump power reaches 583 mW, no obvious signal saturation is detected. The optical gain (I/I0) at 1330 nm is collected as a function of pump power in Fig. 4(b), which indicates that the increase of optical gain vs. pump power is almost linear. The slope efficiency is as large as 1.06 W−1. From the PL spectrum of PbS QD-embedded glass heated at 550 °C for 24 h in Fig. 3(2), it can be observed that PL intensity at 1550 nm is still intense. So we attempt to characterize the optical amplification at the 1550 nm window of the same PbS QD-embedded glass. The optical gain under different pump power values is collected in Fig. 4(b). When the pump power exceeds ~300 mW, a noticeable optical gain is observed. The optical gain at 1550 nm also enhances linearly with the increase of pump power, and the slope efficiency is about 0.97 W−1, which is similar to the slope efficiency at 1330 nm. The similar optical amplification properties at both the 1330 nm and 1550 nm windows indicate that a broadband optical amplification is expected at the entire PL band of PbS QD-embedded glass. This allows them to be potentially applied in broadband fiber amplifiers. It should be pointed out that by controlling the heat treatment condition, doping concentration of PbS and glass composition, the characteristics of PbS QDs, such as the density in glass matrix, size and size distribution etc., can be widely tuned. This will further tune the PL property of PbS QD-doped glass, and enable them to match better with the whole optical communication window (1.1–1.7 μm).
Fig. 4. (a) The amplified signals at 1330 nm of PbS QD-embedded glass heated at 550 °C for 24 h. (b) Optical gains (I/I0) at 1330 nm (red) and 1550 nm (blue) of PbS QD-embedded glass heated at 550 °C for 24 h, which are collected as a function of pumping power. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: G. Dong, et al., Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dotembedded silicate glasses, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
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4. Conclusion In summary, PbS QD-doped glass was prepared by the heat treatment of as-prepared glass. By carefully controlling the heat treatment temperature, the size and size distribution of PbS QDs in the glass matrix can be tuned, which will result in tunable near-infrared photoluminescence. Because of the well matched photoluminescence with the optical communication window, we characterized the optical amplification at the 1330 nm and 1550 nm windows for PbS QD-doped glass heated at 550 °C for 24 h. Noticeable optical amplification was simultaneously detected at the 1330 nm and 1550 nm windows. Both optical gains enhanced linearly with the increase of pump power, with a slope efficiency of 1.06 W−1 and 0.97 W −1 at 1330 nm and 1550 nm, respectively. By the improvement of the formation and PL properties of PbS QDs in glass matrix, the PbS QD-doped glass can be potentially applied in broadband fiber amplifiers and tunable fiber lasers. Acknowledgments This work was financially supported by National Natural Science Foundation of China (51102096, 51072060, 50872123), Natural Science Foundation of Guangdong Province (S2011030001349), Open fund of State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, China Postdoctoral Science Special Foundation (201104350), the Fundamental Research Funds for the Central Universities (2011ZB0001, 2011ZZ0001, 2011ZP0002) and UIRT of Guangdong Province (1056111041).
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Please cite this article as: G. Dong, et al., Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dotembedded silicate glasses, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dot-embedded silicate glasses Guoping Dong a, b,⁎, Guobo Wu a, b, Shaohua Fan a, b, Fangteng Zhang a, b, Yuanhao Zhang a, b, Botao Wu c, Zhijun Ma a, b, Mingying Peng a, b, Jianrong Qiu a, b,⁎⁎ a b c
State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 19 October 2012 Received in revised form 12 March 2013 Available online xxxx Keywords: Tunable photoluminescence; Optical glass; PbS quantum dots; Multi-wavelength; Optical amplification
a b s t r a c t Optical properties of PbS quantum dot (QD)-embedded silicate glasses prepared through post annealing were investigated. By modulating the heat treatment condition, tunable near-infrared (NIR) photoluminescence (PL) was obtained. Noticeable optical amplification was achieved at both 1330 nm and 1550 nm windows with similar amplification characteristics in single PbS QD-embedded glass, which indicated that broadband optical amplification is expected at the entire PL band of PbS QD-embedded glass. The PbS QD-embedded glass with multi-wavelength optical amplification is promising as the gain medium of broadband fiber amplifiers and tunable fiber lasers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Modern optical communication network has been rapidly developed since the invention of Er 3+-doped fiber amplifier (EDFA) [1,2]. Subsequently, the development of dense wavelength division multiplexing (DWDM) technology further promotes the progress of optical communication network based on rare-earth ion (such as Er3+, Tm3+, Pr3+ etc.)-doped fiber amplifier. However, because of the intrinsic spectroscopic characteristics of rare-earth ions, the operating waveband of rare-earth ion-doped fiber amplifier is restricted. Taking commercial EDFA as an example, although a series of novel technologies have been developed to expand the operating waveband of EDFA, it can only cover the C band (1530–1565 nm) and L band (1565–1605 nm) so far, which is much narrower than the whole optical communication window (1.1–1.7 μm) of commercial silica fiber. Therefore, to meet the higher requirement of speed and capacity of information transmission, it is urgent to improve the properties of the fiber amplifier. To expand the operating bandwidth of the optical communication network, it is necessary to find a novel gain medium, which can almost cover the whole optical communication window. Semiconductor quantum dots (QDs) have been widely investigated because of their unique optical and electronic properties [3–5]. By ⁎ Correspondence to: G. Dong, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. Tel.: + 86 20 87114235. ⁎⁎ Correspondence to: J. Qiu, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. E-mail addresses: [email protected] (G. Dong), [email protected] (J. Qiu).
controlling the size of QDs, we can conveniently tune the bandgap of QDs, and obtain tunable luminescence. Nowadays, researches on QDs are mainly focused on the II–VI, III–V and IV–VI group materials. Among them, it is well known that PbS QDs can provide tunable nearinfrared luminescence (about 1–2 μm) by controlling the size of QDs, which matches well with the whole optical communication window [5–19]. As a typical narrow bandgap (Eg = 0.41 eV) semiconductor, the exciton Bohr radius of PbS bulk is as large as 20 nm, which endows a strong quantum confinement effect that can be easily realized over a wide range of QD size [5]. Therefore, PbS QD is promising to act as a gain medium of broadband fiber amplifiers. Considering the potential fabrication of PbS QD-embedded fibers and fiber amplifiers, glass is probably the most suitable matrix for PbS QDs because of their high thermal, chemical, and mechanical stability, wide transmission range and excellent fiber-drawing ability [6–19]. Several works have been performed on PbS QD-embedded glasses. However, most of them are focused on the formation mechanism [10–13], tunable luminescence [7,18], optical nonlinear, saturable absorbers [14], etc. The optical amplification properties at the optical communication window, which are one of the most key factors to evaluate the potential application as a gain medium of fiber amplifiers, are scarcely investigated. Only a few works have characterized the optical amplification property at the 1.3 μm and 1.55 μm windows [6,19]. In our previous work, the optical amplification property at 1.33 μm and 1.55 μm was investigated separately in different PbS QD-embedded glass, and obvious optical amplification was detected at optical communication windows [19]. However, to evaluate the potential of PbS QDembedded glass applied in a broadband fiber amplifier, it is necessary
0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
Please cite this article as: G. Dong, et al., Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dotembedded silicate glasses, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
2
G. Dong et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
to investigate the broadband optical amplification at the optical communication window. Herein, tunable near-infrared (NIR) luminescence is realized in PbS QD-embedded silicate glass by controlling the heat treatment condition. By choosing PbS QD-embedded glass providing matched luminescence in optical communication window as the representative sample, optical amplification properties are characterized by the two-wave mixing technology. Noticeable optical amplification at both the 1.3 μm and 1.55 μm windows is simultaneously obtained in single PbS QD-embedded silicate glass. 2. Experimental details and characterization In this work, 66SiO2–8B2O3–18K2O–4BaO–4ZnO–1.5PbS (in mol%) glass, which has been demonstrated as a good matrix to form PbS QDs, was used to prepare the PbS QD-embedded glass by heat treatment [7,19]. The matrix glass was prepared by the traditional melt-quenching method at 1400 °C for 60 min. Analytical grade SiO2, B2O3, K2CO3, BaCO3, ZnO and PbS were used as raw materials. After quenching on a stainless steel mold, the as-prepared glass was heated at 500–575 °C for 24 h to form the PbS QDs in the glass matrix. Then, the PbS QDembedded glass was cut into thin plates and optically polished for spectroscopic and amplification characterizations. The size, morphology and crystallization of PbS QDs in glass were characterized by high resolution transmission electron microscopy (HR-TEM, Philips-FEI, Netherlands) with an accelerating voltage of 300 kV. Absorption and photoluminescence (PL) spectra of the glass plate were performed on a Lambda 900 (Perkin Elmer, USA) spectrophotometer and a Triax 320 (Jobin Yvon, France) fluorescence spectrofluorometer, respectively. Optical amplification of PbS QD-embedded glass was performed by two-wave mixing configuration, which is similar to our previous works [19,20]. An 808 nm laser diode (LD) was used as the excitation source, while a 1330 nm (or 1550 nm) LD was used as the probe beam. The optical gain, g is estimated by g = I/I0, where I and I0 represent the intensity of the probe signal with and without an excitation source, respectively. All the measurements were performed at room temperature. 3. Results and discussion Fig. 1 shows the absorption spectra of glass heat treated at different temperatures for 24 h. For the as-prepared glass, no absorption band is detected between 500 nm and 2250 nm. When the glass is heated at 500 °C for 24 h, the absorption edge slightly shifts to the longer wavelength. However, there is still no obvious absorption band observed in the spectrum. When the heating temperature reaches 525 °C, besides
Fig. 1. Absorption spectra of glass heated at different temperatures for 24 h.
the red shift of the absorption edge, a weak absorption band centered at ~700 nm appears in the absorption spectrum. The absorption band is ascribed to the generation of electron and hole pairs induced by the excitation photon, which indicates that PbS QDs are beginning to form in the glass matrix heated at 525 °C. When the heating temperature further increases to 550 °C and 575 °C, the absorption band of PbS QDs gradually shifts to the longer wavelength, which confirms that the size of PbS QDs grows continuously with the increase of temperature. The intensity of the absorption band is also enhanced with the increase of temperature. This indicates that the number density of PbS QDs in glass matrix also increases when the heating temperature increases from 525 °C to 575 °C. According to previous works, the average size of PbS QDs in glass can be estimated by the following equation [21,22] 2 2 Eg ðRÞ ¼ Eg þ
! 2ħ2 Eg π2 ; R m
ð1Þ
where R is the average radius of PbS QDs, ħ is Planck's constant, m⁎ is the reduced mass, Eg(R) is the effective energy of PbS QDs, and Eg is the bandgap energy of bulk PbS semiconductor. For bulk PbS semiconductor, Eg = 0.41 eV and m⁎ = 0.085me, where me is the static electron mass [21]. Since PbS is a direct bandgap semiconductor, the effective energy of PbS QDs can be evaluated by the fitting of the absorption spectra [21,23]. The effective energy of PbS QDs in glasses heated at 525 °C, 550 °C and 575 °C is fitted to be 1.93 eV, 0.91 eV and 0.62 eV, respectively. Based on Eq. (1), the average radius of PbS QDs in glasses heated at 525 °C, 550 °C and 575 °C is estimated to be 1.4 nm, 3.2 nm and 5.8 nm, respectively. Fig. 2 illustrates the TEM image of PbS QD-embedded glass after heat treatment at 550 °C for 24 h. In the image, a large number of PbS QDs are formed in the glass matrix. The morphology of QDs is quasi-spherical, while the size of QDs is between 3 and 8 nm, with an average size of ~ 6 nm, which is consistent with the fitting result of the absorption spectra shown in Fig. 1(4). The relatively large size distribution of PbS QDs is probably due to the high doping concentration of PbS in glass preparation, which results in the inhomogeneous migration of S source and growth of PbS nuclei during PbS QD formation. The inset of Fig. 2 shows the HR-TEM image of an individual PbS
Fig. 2. TEM image of glass heated at 550 °C for 24 h. The inset shows the HR-TEM image of a single PbS QD.
Please cite this article as: G. Dong, et al., Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dotembedded silicate glasses, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
G. Dong et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
Fig. 3. PL spectra of glass heated at different temperatures for 24 h. An 808 nm LD was used as an excitation source.
QD. A group of lattice fringe with a space of ~0.21 nm is clearly observed in the image, which can be assigned to the (220) plane of cubic PbS crystals (JCPDF: 65-0132). The HR-TEM result confirms that the QDs formed in the glass matrix are of pure cubic PbS phase. Because of the formation of PbS QDs in the glass matrix, it is deduced that broadband PL will appear in the PbS QD-embedded glass. Fig. 3 shows the PL spectra of PbS QD-embedded glass under an excitation of 808 nm LD. Although PbS QDs are confirmed to begin forming in the glass heated at 525 °C, the PbS QDs are too small to be considered as PbS nuclei, and the PL band is too weak to be detected as shown in Fig. 3(1). Additionally, the mismatch of excitation wavelength is probably another factor resulting in the absence of a PL band (see Fig. 1(3)). When the glass is heated at 550 °C for 24 h, an obvious PL band centered at 1370 nm is obtained in the spectrum. The PL band covers the wavelength from 1100 nm to 1700 nm, with a full width at half maximum (FWHM) of 220 nm. When the heating temperature reaches 575 °C, the PL band shifts to 1705 nm, and the FWHM also broadens to 300 nm. The results of PL spectra confirm that with the increase of heating temperature, the PbS QDs grow gradually, which endows a strong quantum confinement effect on PbS QD-embedded
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glass. The size distribution of PbS QDs in glass also broadens obviously, which is probably due to the inhomogeneous migration and growth of PbS QDs at higher temperature. Based on the results discussed above, it can be deduced that the formation characteristic of PbS QDs in glass (such as the beginning growth temperature, growth rate, density in glass matrix, size and size distribution of QDs, etc.) can be widely tuned by controlling the heat treatment condition. This will further tune the PL wavelength and bandwidth of PbS QD-embedded glass, which is significant for the application in broadband tunable fiber amplifiers. To evaluate the application as gain medium in fiber amplifier, optical amplification is one of the most important prerequisite conditions. Because of the good PL matching characteristic with 1330 nm window of PbS QD-embedded glass heated at 550 °C for 24 h (see the dotted line in Fig. 3), we choose this glass as the representative sample to characterize the optical amplification at the 1330 nm window. Fig. 4(a) shows the optical amplified signals at 1330 nm with different pump power values. It can be found that the intensity enhances gradually with the increase of power. Even when the pump power reaches 583 mW, no obvious signal saturation is detected. The optical gain (I/I0) at 1330 nm is collected as a function of pump power in Fig. 4(b), which indicates that the increase of optical gain vs. pump power is almost linear. The slope efficiency is as large as 1.06 W−1. From the PL spectrum of PbS QD-embedded glass heated at 550 °C for 24 h in Fig. 3(2), it can be observed that PL intensity at 1550 nm is still intense. So we attempt to characterize the optical amplification at the 1550 nm window of the same PbS QD-embedded glass. The optical gain under different pump power values is collected in Fig. 4(b). When the pump power exceeds ~300 mW, a noticeable optical gain is observed. The optical gain at 1550 nm also enhances linearly with the increase of pump power, and the slope efficiency is about 0.97 W−1, which is similar to the slope efficiency at 1330 nm. The similar optical amplification properties at both the 1330 nm and 1550 nm windows indicate that a broadband optical amplification is expected at the entire PL band of PbS QD-embedded glass. This allows them to be potentially applied in broadband fiber amplifiers. It should be pointed out that by controlling the heat treatment condition, doping concentration of PbS and glass composition, the characteristics of PbS QDs, such as the density in glass matrix, size and size distribution etc., can be widely tuned. This will further tune the PL property of PbS QD-doped glass, and enable them to match better with the whole optical communication window (1.1–1.7 μm).
Fig. 4. (a) The amplified signals at 1330 nm of PbS QD-embedded glass heated at 550 °C for 24 h. (b) Optical gains (I/I0) at 1330 nm (red) and 1550 nm (blue) of PbS QD-embedded glass heated at 550 °C for 24 h, which are collected as a function of pumping power. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: G. Dong, et al., Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dotembedded silicate glasses, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010
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G. Dong et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
4. Conclusion In summary, PbS QD-doped glass was prepared by the heat treatment of as-prepared glass. By carefully controlling the heat treatment temperature, the size and size distribution of PbS QDs in the glass matrix can be tuned, which will result in tunable near-infrared photoluminescence. Because of the well matched photoluminescence with the optical communication window, we characterized the optical amplification at the 1330 nm and 1550 nm windows for PbS QD-doped glass heated at 550 °C for 24 h. Noticeable optical amplification was simultaneously detected at the 1330 nm and 1550 nm windows. Both optical gains enhanced linearly with the increase of pump power, with a slope efficiency of 1.06 W−1 and 0.97 W −1 at 1330 nm and 1550 nm, respectively. By the improvement of the formation and PL properties of PbS QDs in glass matrix, the PbS QD-doped glass can be potentially applied in broadband fiber amplifiers and tunable fiber lasers. Acknowledgments This work was financially supported by National Natural Science Foundation of China (51102096, 51072060, 50872123), Natural Science Foundation of Guangdong Province (S2011030001349), Open fund of State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, China Postdoctoral Science Special Foundation (201104350), the Fundamental Research Funds for the Central Universities (2011ZB0001, 2011ZZ0001, 2011ZP0002) and UIRT of Guangdong Province (1056111041).
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Please cite this article as: G. Dong, et al., Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dotembedded silicate glasses, J. Non-Cryst. Solids (2013), http://dx.doi.org/10.1016/j.jnoncrysol.2013.04.010