Space-selective precipitation of ZnO crystals in glass by using high repetition rate femtosecond laser irradiation Xi Du,1 Hang Zhang,1 Chen Cheng,1 Shifeng Zhou,1 Fangteng Zhang,1 Yongze Yu,1 Guoping Dong,1 and Jianrong Qiu1,2,* 1
Institute of Optical Communication Materials and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China 2 The China-Germany Research Center for Photonic Materials and Devices, Guangzhou 510640, China * [email protected]
Abstract: We report on three-dimensional (3D) precipitation of ZnO crystals inside a silicate glass by a 500 kHz femtosecond pulse laser. The precipitation and distribution of ZnO crystals in glass are confirmed and analyzed by Raman spectra and Raman mapping. Mirco- luminescence is observed in the laser modified region when excited by femtosecond pulse laser or Xenon lamp. The effect of laser average power on the precipitation of the ZnO crystals has also been investigated. The possibility of 3D optical data storage using the observed phenomena is demonstrated. ©2014 Optical Society of America OCIS codes: (320.2250) Femtosecond phenomena; (160.2750) Glass and other amorphous materials; (250.5230) Photoluminescence; (210.0210) Optical data storage.
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K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, and K. Hirao, “Photowritten optical waveguides in various glasses with ultrashort pulse laser,” Appl. Phys. Lett. 71(23), 3329 (1997). E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T.-H. Her, J. P. Callan, and E. Mazur, “Three-dimensional optical storage inside transparent materials,” Opt. Lett. 21(24), 2023–2025 (1996). J. Qiu, X. Jiang, C. Zhu, M. Shirai, J. Si, N. Jiang, and K. Hirao, “Manipulation of gold nanoparticles inside transparent materials,” Angew. Chem. Int. Ed. Engl. 43(17), 2230–2234 (2004). P. G. Kazansky, H. Inouye, T. Mitsuyu, K. Miura, J. Qiu, K. Hirao, and F. Starrost, “Anomalous anisotropic light scattering in Ge-doped silica glass,” Phys. Rev. Lett. 82(10), 2199–2202 (1999). Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). S. Kanehira, J. Si, J. Qiu, K. Fujita, and K. Hirao, “Periodic nanovoid structures via femtosecond laser irradiation,” Nano Lett. 5(8), 1591–1595 (2005). Y. Shimotsuma, M. Sakakura, P. G. Kazansky, M. Beresna, J. Qiu, K. Miura, and K. Hirao, “Ultrafast manipulation of self-assembled form birefringence in glass,” Adv. Mater. 22(36), 4039–4043 (2010). M. Sakakura, M. Shimizu, Y. Shimotsuma, K. Miura, and K. Hirao, “Temperature distribution and modification mechanism inside glass with heat accumulation during 250 kHz irradiation of femtosecond laser pulses,” Appl. Phys. Lett. 93(23), 231112 (2008). S. Eaton, H. Zhang, P. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13(12), 4708–4716 (2005). K. Miura, J. Qiu, T. Mitsuyu, and K. Hirao, “Space-selective growth of frequency-conversion crystals in glasses with ultrashort infrared laser pulses,” Opt. Lett. 25(6), 408–410 (2000). Y. Liu, B. Zhu, L. Wang, Y. Dai, H. Ma, G. Lakshminarayana, and J. Qiu, “Femtosecond laser direct writing of TiO2 crystalline patterns in glass,” Appl. Phys. B 93(2–3), 613–617 (2008). Y. Liu, B. Zhu, Y. Dai, X. Qiao, S. Ye, Y. Teng, Q. Guo, H. Ma, X. Fan, and J. Qiu, “Femtosecond laser writing of Er3+-doped CaF2 crystalline patterns in glass,” Opt. Lett. 34(21), 3433–3435 (2009). A. Stone, M. Sakakura, Y. Shimotsuma, G. Stone, P. Gupta, K. Miura, K. Hirao, V. Dierolf, and H. Jain, “Directionally controlled 3D ferroelectric single crystal growth in LaBGeO5 glass by femtosecond laser irradiation,” Opt. Express 17(25), 23284–23289 (2009). C. Fan, B. Poumellec, M. Lancry, X. He, H. Zeng, A. Erraji-Chahid, Q. Liu, and G. Chen, “Three-dimensional photo-precipitation of oriented LiNbO3-like crystals in silica-based glass with femtosecond laser irradiation,” Opt. Lett. 37(14), 2955–2957 (2012). Q. Luo, X. Qiao, X. Fan, and X. Zhang, “Near-infrared emission of Yb3+ through energy transfer from ZnO to Yb3+ in glass ceramic containing ZnO nanocrystals,” Opt. Lett. 36(15), 2767–2769 (2011). R. E. Service, “Materials science-Will UV lasers beat the blues,” Science 276(5314), 895 (1997). B. Ghaemi, G. Zhao, G. Jie, H. Xi, X. Li, J. Wang, and G. Han, “A study of formation and photoluminescence
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17908
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1. Introduction Femtosecond laser is a powerful tool for microfabrication in transparent materials due to the unique characteristics of ultrashort pulse and ultrahigh peak intensity [1–3]. Many novel phenomena and applications have been found based on the highly nonlinear interaction of the femtosecond laser and transparent materials [4–7]. Recently, high repetition rate femtosecond lasers have been widely used to induce crystallization within glasses due to their ability to rapidly and precisely deposit energy through nonlinear excitation and absorption. When a high repetition rate femtosecond laser is focused into glass, an increasing amount of energy will continuously accumulate in the modified region where a high temperature elevation occurs [8, 9]. When the local temperature reaches a certain temperature range for nucleation and crystal growth, crystals will be precipitated in the modified region. So far, various functional crystals such as β-BaB2O4 [10], TiO2 [11], CaF2 [12], LaBGeO5 [13] and LiNbO3 [14] have been space-selectively precipitated within glasses using high repetition rate femtosecond laser irradiation. ZnO has been widely investigated as a candidate for the development of ultraviolet (UV) laser, biosensor, field emission display, and solar cell owing to its direct wide-bandgap of 3.38 eV and extremely large binding energy of 60 meV [15, 16]. Based on
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17909
the potential applications of ZnO crystals, many studies have been carried out on ZnO crystals embedded glass ceramics prepared by conventional heat-treatment method or continuous laser irradiation [15, 17, 18]. ZnO crystals are precipitated in the whole glass without space selectivity, which prevents its integrated optical device application in 3D architecture. Space-selective precipitation of ZnO crystals in glass can be used for fabrication of a 3D optical data storage due to the controllable micro-luminescence of ZnO crystals. In this work, space-selective precipitation of ZnO crystals inside silicate glass induced by high repetition rate femtosecond laser pulses has been achieved. Raman spectra and Raman mapping are used to confirm the existence of ZnO crystals and examine the crystalline distribution in the modified region. We also investigate the influence of laser average power on the precipitation of ZnO crystals. Based on the controllable luminescence of ZnO crystals in glass, we demonstrate the possibility of 3D optical data storage in the glass. 2. Experiments A glass sample with the composition of 45SiO2-15Al2O3-25ZnO-15K2O (mol%) was prepared by conventional melt-quenching technique. Analytical grade reagents SiO2, Al2O3, ZnO, and K2CO3 were used as raw materials. A mixed batch 30 g in weight was mixed homogeneously in an agate mortar and then melted at 1600 °C for 2 hours in air and then cast onto a stainless steel plate. The glass sample was cut and then well polished for laser modification and optical measurements. A commercial femtosecond Yb-fiber laser system (FLCPA-02USCT11, Calmar Laser, Inc.) emitted 370 fs, 1030 nm laser pulses at a repetition rate of 500 kHz was employed for the space-selective precipitation of ZnO crystals. The laser beam was tightly focused by a microscope objective (50 × , NA = 0.8) into the glass sample that was fixed on a computer-controlled three dimensions XYZ stage. The crystalline distribution in glass was analyzed by a Raman spectrometer (Renishaw inVia) with a laser excitation source of 532 nm. The laser beam was focused at 100 μm beneath the glass surface for all Raman measurements. The Raman confocal diameter and the probe power are about 1 μm, 25 mW, respectively. The polarized Raman excitation was used in the experiments. Plotoluminescence excitation and emission spectra were measured by a fluorospectrometer (Edinburgh FLS920) using a 450 W Xenon lamp as excitation source. Luminescence spectra and second-harmonic generation (SHG) of the glass sample during the femtosecond laser irradiation were recorded by a spectrometer (Ocean Optics HR4000). In order to demonstrate the feasibility of 3D optical data storage by using controllable precipitation of ZnO crystals in glass, the luminescence imaging of ZnO crystals was carried out with a laser scanning confocal microscope (Carl Zeiss LSM710, Germany). All the measurements were performed at room temperature. 3. Results and discussion Micro-Raman spectra of the femtosecond laser-irradiated region at 100 μm beneath the glass surface are shown in Fig. 1(a). The irradiation duration was 2 minutes and the laser average power was 1300 mW. The plotted curves A to D denote the Raman spectra from four sampling points A to D, which respectively locate in different position of laser modified region marked with colored alphabet letters, as shown in the inset of Fig. 1(a).
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17910
Fig. 1. (a) Micro-Raman spectra of the femtosecond laser-irradiated region with different position, Raman spectrum of the heat-treated glass sample is added for comparison. (b) Micro-Raman mapping at the 437 cm−1 peak for the modified region.
The Raman spectrum of a glass sample heat-treated at 750 °C for 2 h (black curve in Fig. 1(a)), in which ZnO crystals were precipitated and comfirmed by X-ray diffraction (XRD) analysis [15], is added in Fig. 1 for comparison. The Raman spectrum of the heat-treated glass sample exhibits two sharp bands at 98 cm−1 and 437 cm−1, which can be assigned to the E2 (low) mode and E2 (high) mode for wurtzite ZnO crystals, respectively. The E2 (low) mode is related to the vibration of the heavy Zn sub lattice and the E2 (high) mode involves only the oxygen atoms [19]. Two characteristic Raman bands at 98 cm−1 and 437 cm−1 of ZnO crystals are also observed in both the center (point A) and at the surroundings (point B) of the laser modified region. On the contrary, no characteristic Raman bands due to ZnO crystals are observed in curve C (point C) and curve D (point D). The four Raman bands in the C and D spectra at 482, 570, 702 and 986 cm−1 can be ascribed to the Si-O-Si bending vibration, vibrational mode of Q3, Q1 and Q2 structure in [SiO4] tetrahedra, respectively [20]. The band at low frequency is characteristic of vibrations of silicate network itself [21]. The increase at low frequency in the C and D spectra is attributed to the increase of inelastic light-scattering in amorphous matter compared with the A and B spectra [22]. Moreover, the intensities of characteristic Raman bands at 98 cm−1 and 437 cm−1 for ZnO crystals are stronger at point B than point A. This result is consistent with the previous publications where femtosecond laser-induced crystals were mainly distributed at the surroundings of the focal center [23–25]. Raman mapping (i.e., analysis of x-y plane using confocal Raman microscopy) was used to make a detailed investigation in the definite spatial distribution of the precipitated ZnO crystals [Fig. 1(b)]. The intensity of the most intense characteristic Raman band at 437 cm−1 relative to the baseline was used as the detecting signal, which represents the distribution of ZnO crystals. The results indicate that the ZnO crystals content in the focal center is low. The ZnO crystals content gradually increases from the center of the modified region and reaches a maximum at 10 μm, and then remains constant when the distance from the center of the modified region increases from 10 μm to 20 μm, and finally decreases with further increasing the distance from the center of modified region. The distribution of the shell shape crystallization may correlate with the fluence of the laser irradiation and cooling conditions [26]. The black area [inset of Fig. 1(a)] (i.e., laser-induced ions diffusion zone) in the modified region is bigger than the crystallization zone, which is similar to the case of precipitation of β-BBO in glass [26]. The main reason is the diffusion of ions induced by using high repetition rate femtosecond laser. Many researches indicate that the Si4+ and O2are mainly located in the central region, K+ is primarily localized near the inner boundary, Zn2+ and Al3+ are mainly located in the area between the central region and the inner boundary [27, 28]. Therefore, the relative concentration of Zn2+ is low in the central region, which leads to the low precipitation of ZnO crystals. Crystallization occurs only the local temperature is proper for nucleation and crystal growth in the black area. Therefore, we can design the laser trace and select the appropriate laser parameters to get the expected two-dimensional (2D) or 3D structures based on crystalline distribution inside glass.
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Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17911
Figure 2(a) shows the excitation spectrum and emission spectrum of the glass sample before the femtosecond laser irradiation. Two wide emission bands centered at 408 nm and 465 nm are observed when excited at 259 nm. The blue emission bands can be attributed to the electron transition from the shallow level of defects to the valence band [29]. In the same way, the excitation spectrum and emission spectrum of the glass sample after the femtosecond laser irradiation is shown in Fig. 2(b). The focal depth of the laser irradiated area was 100 μm beneath the glass surface. Yellow emission is observed at about 582 nm when excited at 412 nm. The yellow emission may be attributed to the recombination of the excited electrons and deeply trapped holes [15]. Because the yellow emission was usually obtained in ZnO crystals or films that were prepared under oxygen-rich conditions [30, 31]. Therefore, space-selective yellow emission in glass using high repetition rate femtosecond laser irradiation can be achieved.
Fig. 2. Excitation and emission spectra of the unmodified (a) and the modified (b) region.
Figure 3(a) shows the Raman spectra of the glass sample modified region irradiated by femtosecond laser with different laser average power from 1000 mW to 1500 mW. The focal depth was 100 μm beneath the glass surface and irradiation duration was 2 minutes. At first, when the laser average power is 1000 mW, two sharp Raman bands at 98 cm−1 and 437 cm−1 of ZnO crystals cannot be observed in Fig. 3(a), indicating that no apparent ZnO crystals were precipitated under this irradiation condition. When the average power increases to 1100 mW, two typical Raman bands at 98 cm−1 and 437 cm−1 appear. When the laser average power increases further, the Raman intensities of the E2 (low) mode and E2 (high) mode are both gradually enhanced, as shown in Fig. 3(a). These phenomena indicate that ZnO crystals content in the modified region increases correspondingly, which can be attributed to the increased heat accumulation of the modified region [8]. Figure 3(b) shows the emission spectra measured during the femtosecond laser irradiation. An emission band with central wavelength at 590 nm is observed when the femtosecond laser power increases from 1000 mW to 1100 mW. With further increasing the laser average power, the intensity of yellow emission increases. These results agree well with the variation of the intensity of the Raman bands for ZnO crystals, as shown in Fig. 3(a). Therefore, the yellow emission of the ZnO crystals can be attributed to multiple-photon absorption during the femtosecond laser irradiation [32, 33]. In addition, because semiconductor ZnO crystals exhibit excellent second-order nonlinear optical property [34], SHG at 515 nm is also observed when the laser average power ranges from 1100 mW to 1500 mW [see Fig. 3(b)]. It indicates ZnO crystals are non-centrosymmetric and partially oriented. The result suggests that the variation of the SHG intensity is consistent with the emission band of ZnO crystals. These results indicate that the precipitation amount and emission intensity of ZnO crystals in glass can be tuned by changing laser average power.
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17912
Fig. 3. Raman spectra (a) and emission spectra (b) of the glass sample modified region induced by femtosecond laser with average power from 1000 mW to 1500 mW for 2 minutes.
Femtosecond laser has been used to induced void, nanograting, third-hamornic generation (THG) and luminescence change in glasses to realize 3D optical data storage [2, 5, 35, 36]. In the case of using femtosecond laser induced void, the signal-to-noise ratio (SNR) is low. In the present case, ZnO crystals can be space-selectively precipitated in glass using femtosecond laser irradiation, the ZnO crystals/glass volume ratio can be tuned and the controllable micro-luminescence of ZnO crystals can be achieved, which may have potential application in optical data storage with high SNR. Using the femtosecond laser-induced crystallization, a right angle-shaped bit pattern was recorded at a depth of 150 μm inside the glass. Figure 4 shows the luminescence intensity maps of the right angle-shaped bit pattern with 405 nm “Blu-ray” laser diode excitation. The driver light source of readout is commercial availability. ZnO crystallization bits can be read out clearly by detecting the yellow luminescence intensity. The SNR can be typically calculated by SNR (dB) = 20log[VS/VN], where VS is the signal amplitude and VN is noise average level. With the optimal power of the recording laser, an SNR of 22 can be obtained with 405 nm laser excitation, as is shown in Fig. 4(c). Further reduction in the threshold of laser power and exposure time for precipitation of ZnO crystals will be necessary for practical optical data storage application, and find promising applications in integrated optics.
Fig. 4. (a) Optical microscopic image of a right angle-shaped bit pattern written with femtosecond laser. (b) Readout of the pattern by luminescence imaging recorded with 1400 mW femtosecond laser. (c) Intensity distribution of the yellow emission for the pattern in (b). (d) Intensity distribution of the yellow emission for the pattern recorded with 1300 mW femtosecond laser. Scale bar, 50 μm.
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17913
4. Conclusion In summary, ZnO crystals have been space-selectively precipitated inside the silicate glass with the femtosecond laser irradiation. Raman spectra and Raman mapping indicate that ZnO crystals mainly locate in the area around the centre of the modified region. Microluminescence emission of ZnO crystals in glass has been successfully achieved, which is promising for fabrication of integrated optical device and 3D display. The ZnO crystals content increased with increasing average power. The observed phenomena are promising for the applications in 3D optical data storage with a high SNR and integrated optics. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51132004), Fundamental Research Funds for the Central Universities (Grant No. 2014ZP0001), Guangdong Natural Science Foundation (Grant Nos. S2011030001349), National Basic Research Program of China (2011CB808102).
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17914
Institute of Optical Communication Materials and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China 2 The China-Germany Research Center for Photonic Materials and Devices, Guangzhou 510640, China * [email protected]
Abstract: We report on three-dimensional (3D) precipitation of ZnO crystals inside a silicate glass by a 500 kHz femtosecond pulse laser. The precipitation and distribution of ZnO crystals in glass are confirmed and analyzed by Raman spectra and Raman mapping. Mirco- luminescence is observed in the laser modified region when excited by femtosecond pulse laser or Xenon lamp. The effect of laser average power on the precipitation of the ZnO crystals has also been investigated. The possibility of 3D optical data storage using the observed phenomena is demonstrated. ©2014 Optical Society of America OCIS codes: (320.2250) Femtosecond phenomena; (160.2750) Glass and other amorphous materials; (250.5230) Photoluminescence; (210.0210) Optical data storage.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, and K. Hirao, “Photowritten optical waveguides in various glasses with ultrashort pulse laser,” Appl. Phys. Lett. 71(23), 3329 (1997). E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T.-H. Her, J. P. Callan, and E. Mazur, “Three-dimensional optical storage inside transparent materials,” Opt. Lett. 21(24), 2023–2025 (1996). J. Qiu, X. Jiang, C. Zhu, M. Shirai, J. Si, N. Jiang, and K. Hirao, “Manipulation of gold nanoparticles inside transparent materials,” Angew. Chem. Int. Ed. Engl. 43(17), 2230–2234 (2004). P. G. Kazansky, H. Inouye, T. Mitsuyu, K. Miura, J. Qiu, K. Hirao, and F. Starrost, “Anomalous anisotropic light scattering in Ge-doped silica glass,” Phys. Rev. Lett. 82(10), 2199–2202 (1999). Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). S. Kanehira, J. Si, J. Qiu, K. Fujita, and K. Hirao, “Periodic nanovoid structures via femtosecond laser irradiation,” Nano Lett. 5(8), 1591–1595 (2005). Y. Shimotsuma, M. Sakakura, P. G. Kazansky, M. Beresna, J. Qiu, K. Miura, and K. Hirao, “Ultrafast manipulation of self-assembled form birefringence in glass,” Adv. Mater. 22(36), 4039–4043 (2010). M. Sakakura, M. Shimizu, Y. Shimotsuma, K. Miura, and K. Hirao, “Temperature distribution and modification mechanism inside glass with heat accumulation during 250 kHz irradiation of femtosecond laser pulses,” Appl. Phys. Lett. 93(23), 231112 (2008). S. Eaton, H. Zhang, P. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13(12), 4708–4716 (2005). K. Miura, J. Qiu, T. Mitsuyu, and K. Hirao, “Space-selective growth of frequency-conversion crystals in glasses with ultrashort infrared laser pulses,” Opt. Lett. 25(6), 408–410 (2000). Y. Liu, B. Zhu, L. Wang, Y. Dai, H. Ma, G. Lakshminarayana, and J. Qiu, “Femtosecond laser direct writing of TiO2 crystalline patterns in glass,” Appl. Phys. B 93(2–3), 613–617 (2008). Y. Liu, B. Zhu, Y. Dai, X. Qiao, S. Ye, Y. Teng, Q. Guo, H. Ma, X. Fan, and J. Qiu, “Femtosecond laser writing of Er3+-doped CaF2 crystalline patterns in glass,” Opt. Lett. 34(21), 3433–3435 (2009). A. Stone, M. Sakakura, Y. Shimotsuma, G. Stone, P. Gupta, K. Miura, K. Hirao, V. Dierolf, and H. Jain, “Directionally controlled 3D ferroelectric single crystal growth in LaBGeO5 glass by femtosecond laser irradiation,” Opt. Express 17(25), 23284–23289 (2009). C. Fan, B. Poumellec, M. Lancry, X. He, H. Zeng, A. Erraji-Chahid, Q. Liu, and G. Chen, “Three-dimensional photo-precipitation of oriented LiNbO3-like crystals in silica-based glass with femtosecond laser irradiation,” Opt. Lett. 37(14), 2955–2957 (2012). Q. Luo, X. Qiao, X. Fan, and X. Zhang, “Near-infrared emission of Yb3+ through energy transfer from ZnO to Yb3+ in glass ceramic containing ZnO nanocrystals,” Opt. Lett. 36(15), 2767–2769 (2011). R. E. Service, “Materials science-Will UV lasers beat the blues,” Science 276(5314), 895 (1997). B. Ghaemi, G. Zhao, G. Jie, H. Xi, X. Li, J. Wang, and G. Han, “A study of formation and photoluminescence
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17908
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1. Introduction Femtosecond laser is a powerful tool for microfabrication in transparent materials due to the unique characteristics of ultrashort pulse and ultrahigh peak intensity [1–3]. Many novel phenomena and applications have been found based on the highly nonlinear interaction of the femtosecond laser and transparent materials [4–7]. Recently, high repetition rate femtosecond lasers have been widely used to induce crystallization within glasses due to their ability to rapidly and precisely deposit energy through nonlinear excitation and absorption. When a high repetition rate femtosecond laser is focused into glass, an increasing amount of energy will continuously accumulate in the modified region where a high temperature elevation occurs [8, 9]. When the local temperature reaches a certain temperature range for nucleation and crystal growth, crystals will be precipitated in the modified region. So far, various functional crystals such as β-BaB2O4 [10], TiO2 [11], CaF2 [12], LaBGeO5 [13] and LiNbO3 [14] have been space-selectively precipitated within glasses using high repetition rate femtosecond laser irradiation. ZnO has been widely investigated as a candidate for the development of ultraviolet (UV) laser, biosensor, field emission display, and solar cell owing to its direct wide-bandgap of 3.38 eV and extremely large binding energy of 60 meV [15, 16]. Based on
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17909
the potential applications of ZnO crystals, many studies have been carried out on ZnO crystals embedded glass ceramics prepared by conventional heat-treatment method or continuous laser irradiation [15, 17, 18]. ZnO crystals are precipitated in the whole glass without space selectivity, which prevents its integrated optical device application in 3D architecture. Space-selective precipitation of ZnO crystals in glass can be used for fabrication of a 3D optical data storage due to the controllable micro-luminescence of ZnO crystals. In this work, space-selective precipitation of ZnO crystals inside silicate glass induced by high repetition rate femtosecond laser pulses has been achieved. Raman spectra and Raman mapping are used to confirm the existence of ZnO crystals and examine the crystalline distribution in the modified region. We also investigate the influence of laser average power on the precipitation of ZnO crystals. Based on the controllable luminescence of ZnO crystals in glass, we demonstrate the possibility of 3D optical data storage in the glass. 2. Experiments A glass sample with the composition of 45SiO2-15Al2O3-25ZnO-15K2O (mol%) was prepared by conventional melt-quenching technique. Analytical grade reagents SiO2, Al2O3, ZnO, and K2CO3 were used as raw materials. A mixed batch 30 g in weight was mixed homogeneously in an agate mortar and then melted at 1600 °C for 2 hours in air and then cast onto a stainless steel plate. The glass sample was cut and then well polished for laser modification and optical measurements. A commercial femtosecond Yb-fiber laser system (FLCPA-02USCT11, Calmar Laser, Inc.) emitted 370 fs, 1030 nm laser pulses at a repetition rate of 500 kHz was employed for the space-selective precipitation of ZnO crystals. The laser beam was tightly focused by a microscope objective (50 × , NA = 0.8) into the glass sample that was fixed on a computer-controlled three dimensions XYZ stage. The crystalline distribution in glass was analyzed by a Raman spectrometer (Renishaw inVia) with a laser excitation source of 532 nm. The laser beam was focused at 100 μm beneath the glass surface for all Raman measurements. The Raman confocal diameter and the probe power are about 1 μm, 25 mW, respectively. The polarized Raman excitation was used in the experiments. Plotoluminescence excitation and emission spectra were measured by a fluorospectrometer (Edinburgh FLS920) using a 450 W Xenon lamp as excitation source. Luminescence spectra and second-harmonic generation (SHG) of the glass sample during the femtosecond laser irradiation were recorded by a spectrometer (Ocean Optics HR4000). In order to demonstrate the feasibility of 3D optical data storage by using controllable precipitation of ZnO crystals in glass, the luminescence imaging of ZnO crystals was carried out with a laser scanning confocal microscope (Carl Zeiss LSM710, Germany). All the measurements were performed at room temperature. 3. Results and discussion Micro-Raman spectra of the femtosecond laser-irradiated region at 100 μm beneath the glass surface are shown in Fig. 1(a). The irradiation duration was 2 minutes and the laser average power was 1300 mW. The plotted curves A to D denote the Raman spectra from four sampling points A to D, which respectively locate in different position of laser modified region marked with colored alphabet letters, as shown in the inset of Fig. 1(a).
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17910
Fig. 1. (a) Micro-Raman spectra of the femtosecond laser-irradiated region with different position, Raman spectrum of the heat-treated glass sample is added for comparison. (b) Micro-Raman mapping at the 437 cm−1 peak for the modified region.
The Raman spectrum of a glass sample heat-treated at 750 °C for 2 h (black curve in Fig. 1(a)), in which ZnO crystals were precipitated and comfirmed by X-ray diffraction (XRD) analysis [15], is added in Fig. 1 for comparison. The Raman spectrum of the heat-treated glass sample exhibits two sharp bands at 98 cm−1 and 437 cm−1, which can be assigned to the E2 (low) mode and E2 (high) mode for wurtzite ZnO crystals, respectively. The E2 (low) mode is related to the vibration of the heavy Zn sub lattice and the E2 (high) mode involves only the oxygen atoms [19]. Two characteristic Raman bands at 98 cm−1 and 437 cm−1 of ZnO crystals are also observed in both the center (point A) and at the surroundings (point B) of the laser modified region. On the contrary, no characteristic Raman bands due to ZnO crystals are observed in curve C (point C) and curve D (point D). The four Raman bands in the C and D spectra at 482, 570, 702 and 986 cm−1 can be ascribed to the Si-O-Si bending vibration, vibrational mode of Q3, Q1 and Q2 structure in [SiO4] tetrahedra, respectively [20]. The band at low frequency is characteristic of vibrations of silicate network itself [21]. The increase at low frequency in the C and D spectra is attributed to the increase of inelastic light-scattering in amorphous matter compared with the A and B spectra [22]. Moreover, the intensities of characteristic Raman bands at 98 cm−1 and 437 cm−1 for ZnO crystals are stronger at point B than point A. This result is consistent with the previous publications where femtosecond laser-induced crystals were mainly distributed at the surroundings of the focal center [23–25]. Raman mapping (i.e., analysis of x-y plane using confocal Raman microscopy) was used to make a detailed investigation in the definite spatial distribution of the precipitated ZnO crystals [Fig. 1(b)]. The intensity of the most intense characteristic Raman band at 437 cm−1 relative to the baseline was used as the detecting signal, which represents the distribution of ZnO crystals. The results indicate that the ZnO crystals content in the focal center is low. The ZnO crystals content gradually increases from the center of the modified region and reaches a maximum at 10 μm, and then remains constant when the distance from the center of the modified region increases from 10 μm to 20 μm, and finally decreases with further increasing the distance from the center of modified region. The distribution of the shell shape crystallization may correlate with the fluence of the laser irradiation and cooling conditions [26]. The black area [inset of Fig. 1(a)] (i.e., laser-induced ions diffusion zone) in the modified region is bigger than the crystallization zone, which is similar to the case of precipitation of β-BBO in glass [26]. The main reason is the diffusion of ions induced by using high repetition rate femtosecond laser. Many researches indicate that the Si4+ and O2are mainly located in the central region, K+ is primarily localized near the inner boundary, Zn2+ and Al3+ are mainly located in the area between the central region and the inner boundary [27, 28]. Therefore, the relative concentration of Zn2+ is low in the central region, which leads to the low precipitation of ZnO crystals. Crystallization occurs only the local temperature is proper for nucleation and crystal growth in the black area. Therefore, we can design the laser trace and select the appropriate laser parameters to get the expected two-dimensional (2D) or 3D structures based on crystalline distribution inside glass.
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17911
Figure 2(a) shows the excitation spectrum and emission spectrum of the glass sample before the femtosecond laser irradiation. Two wide emission bands centered at 408 nm and 465 nm are observed when excited at 259 nm. The blue emission bands can be attributed to the electron transition from the shallow level of defects to the valence band [29]. In the same way, the excitation spectrum and emission spectrum of the glass sample after the femtosecond laser irradiation is shown in Fig. 2(b). The focal depth of the laser irradiated area was 100 μm beneath the glass surface. Yellow emission is observed at about 582 nm when excited at 412 nm. The yellow emission may be attributed to the recombination of the excited electrons and deeply trapped holes [15]. Because the yellow emission was usually obtained in ZnO crystals or films that were prepared under oxygen-rich conditions [30, 31]. Therefore, space-selective yellow emission in glass using high repetition rate femtosecond laser irradiation can be achieved.
Fig. 2. Excitation and emission spectra of the unmodified (a) and the modified (b) region.
Figure 3(a) shows the Raman spectra of the glass sample modified region irradiated by femtosecond laser with different laser average power from 1000 mW to 1500 mW. The focal depth was 100 μm beneath the glass surface and irradiation duration was 2 minutes. At first, when the laser average power is 1000 mW, two sharp Raman bands at 98 cm−1 and 437 cm−1 of ZnO crystals cannot be observed in Fig. 3(a), indicating that no apparent ZnO crystals were precipitated under this irradiation condition. When the average power increases to 1100 mW, two typical Raman bands at 98 cm−1 and 437 cm−1 appear. When the laser average power increases further, the Raman intensities of the E2 (low) mode and E2 (high) mode are both gradually enhanced, as shown in Fig. 3(a). These phenomena indicate that ZnO crystals content in the modified region increases correspondingly, which can be attributed to the increased heat accumulation of the modified region [8]. Figure 3(b) shows the emission spectra measured during the femtosecond laser irradiation. An emission band with central wavelength at 590 nm is observed when the femtosecond laser power increases from 1000 mW to 1100 mW. With further increasing the laser average power, the intensity of yellow emission increases. These results agree well with the variation of the intensity of the Raman bands for ZnO crystals, as shown in Fig. 3(a). Therefore, the yellow emission of the ZnO crystals can be attributed to multiple-photon absorption during the femtosecond laser irradiation [32, 33]. In addition, because semiconductor ZnO crystals exhibit excellent second-order nonlinear optical property [34], SHG at 515 nm is also observed when the laser average power ranges from 1100 mW to 1500 mW [see Fig. 3(b)]. It indicates ZnO crystals are non-centrosymmetric and partially oriented. The result suggests that the variation of the SHG intensity is consistent with the emission band of ZnO crystals. These results indicate that the precipitation amount and emission intensity of ZnO crystals in glass can be tuned by changing laser average power.
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17912
Fig. 3. Raman spectra (a) and emission spectra (b) of the glass sample modified region induced by femtosecond laser with average power from 1000 mW to 1500 mW for 2 minutes.
Femtosecond laser has been used to induced void, nanograting, third-hamornic generation (THG) and luminescence change in glasses to realize 3D optical data storage [2, 5, 35, 36]. In the case of using femtosecond laser induced void, the signal-to-noise ratio (SNR) is low. In the present case, ZnO crystals can be space-selectively precipitated in glass using femtosecond laser irradiation, the ZnO crystals/glass volume ratio can be tuned and the controllable micro-luminescence of ZnO crystals can be achieved, which may have potential application in optical data storage with high SNR. Using the femtosecond laser-induced crystallization, a right angle-shaped bit pattern was recorded at a depth of 150 μm inside the glass. Figure 4 shows the luminescence intensity maps of the right angle-shaped bit pattern with 405 nm “Blu-ray” laser diode excitation. The driver light source of readout is commercial availability. ZnO crystallization bits can be read out clearly by detecting the yellow luminescence intensity. The SNR can be typically calculated by SNR (dB) = 20log[VS/VN], where VS is the signal amplitude and VN is noise average level. With the optimal power of the recording laser, an SNR of 22 can be obtained with 405 nm laser excitation, as is shown in Fig. 4(c). Further reduction in the threshold of laser power and exposure time for precipitation of ZnO crystals will be necessary for practical optical data storage application, and find promising applications in integrated optics.
Fig. 4. (a) Optical microscopic image of a right angle-shaped bit pattern written with femtosecond laser. (b) Readout of the pattern by luminescence imaging recorded with 1400 mW femtosecond laser. (c) Intensity distribution of the yellow emission for the pattern in (b). (d) Intensity distribution of the yellow emission for the pattern recorded with 1300 mW femtosecond laser. Scale bar, 50 μm.
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17913
4. Conclusion In summary, ZnO crystals have been space-selectively precipitated inside the silicate glass with the femtosecond laser irradiation. Raman spectra and Raman mapping indicate that ZnO crystals mainly locate in the area around the centre of the modified region. Microluminescence emission of ZnO crystals in glass has been successfully achieved, which is promising for fabrication of integrated optical device and 3D display. The ZnO crystals content increased with increasing average power. The observed phenomena are promising for the applications in 3D optical data storage with a high SNR and integrated optics. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51132004), Fundamental Research Funds for the Central Universities (Grant No. 2014ZP0001), Guangdong Natural Science Foundation (Grant Nos. S2011030001349), National Basic Research Program of China (2011CB808102).
#213897 - $15.00 USD (C) 2014 OSA
Received 12 Jun 2014; revised 3 Jul 2014; accepted 4 Jul 2014; published 16 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.017908 | OPTICS EXPRESS 17914
