We report on space-selective precipitation of nonlinear optical crystals in an Er3+-doped BaO-TiO2-SiO2 glass by using a focused femtosecond laser with 800 nm, 250 kHz and 150 fs. An intense green emission due to upconversion luminescence is observed around the focal point of the femtosecond laser beam at the initial stage of the laser irradiation. A blue emission due to second harmonic generation begins to emerge from the irradiation region after 40 s irradiation. Micro-Raman spectra indicate that nonlinear optical crystals (Ba2TiSi2O8) are precipitated after the laser irradiation. The irradiation time for crystallization in Er3+-doped BaO-TiO2-SiO2 glasses is longer than that in BaO-TiO2-SiO2 glasses under the same irradiation conditions. The mechanisms responsible for the observed phenomena are discussed.
© 2007 Optical Society of America
Optically transparent glass-ceramics containing rare-earth ions  or nonlinear optical crystals [2, 3] have received considerable attention, because such materials have high potential applications in photonic devices such as phosphors  and ultrafast optical switches . From the viewpoint of practical application in integrated optics, it is important to fabricate active ions-doped transparent glass-ceramics with controlled patterns e.g. dots, lines, which can serve as laser waveguide, grating, and wavelength conversion devices. Laser irradiation can yield spatial structural modification and/or crystallization in glasses [4–9]. It is of particular interest for performing various optical function to form crystalline phases in glass through laser irradiation. Honma et al. used a continuous wave (cw) Nd:YAG laser at 1064 nm to induce nonlinear optical crystal line of SmxBi1-xBO3 on the surface of Sm2O3-Bi2O3-B2O3 glass . Because femtosecond lasers deliver ultrashort duration and high peak power pulses, they have been used to induce three-dimensionally crystalline phases in glass in the past decade [6–9]. When the femtosecond laser beam is focused, the power density at the focal point is high enough to induce non-linear absorption, resulting in the generation of a high-density electron plasma. As a result, structural modification including crystallization can be induced by the excess energy released from the plasma into the surrounding media . The electron plasma is generated only around the focal region where the peak power of the laser beam exceeds the threshold of non-linear absorption. The crystallization process utilizing a femtosecond-pulsed laser offer several advantages in the formation of three-dimensional crystalline patterns inside a transparent material such as a glass, compared with the crystallization processes relying on heat treatment or cw laser irradiation .
In this study, we used a focused femtosecond laser to induce Ba2TiSi2O8 crystals in an Er3+-doped BaO-TiO2-SiO2 glass. We observed green emission due to upconversion luminescence appearing at from the focal point of the laser beam in the initial stage of the laser irradiation, as well as bright blue light due to second harmonic generation emerging from the irradiation region after 40 s irradiation. The growth of Ba2TiSi2O8 crystals was confirmed through micro-Raman spectral analysis. Crystalline lines were written continuously in the glass by moving the focal point of the femtosecond laser beam.
The compositions of the glass samples used in this study were 33.3BaO-16.7TiO2-50SiO2-0.5Er2O3 and 33.3BaO-16.7TiO2-50SiO2 (mol%). The raw materials were 4N-purity grade BaCO3, TiO2, SiO2 and Er2O3. The batches were mixed thoroughly and melted in a Pt crucible at 1550 °C for 2 h in air. The melt was poured onto a steel plate and cooled down to room temperature. The obtained as-prepared glass samples were transparent. The samples were cut and polished into blocks 10 mm×10 mm×3 mm in size. A regeneratively amplified 800 nm Ti:sapphire laser (RegA 9000, Coherent Inc.) that emitted a train of 150 fs, 250 kHz mode-locked pulses was focused by a 100× objective lens (N. A. = 0.8), and the fluorescence spectra were recorded by a spectrometer (Ocean Optics: USB2000-VIS-NIR) during the femtosecond laser irradiation. Micro-Raman spectra were measured by a Raman spectrometer (Renishaw in Via) and a laser microscopy system with a 514 nm Ar+ laser excitation source. All the measurements were performed at room temperature.
3. Results and discussion
Figures 1(a) and 1(b) show photographs around the focal point of the laser beam during the femtosecond laser irradiation (average power: 950 mW) in the Er3+-doped glass sample. The laser beam was focused 100 μm below the glass surface. We first observed a green emission arising from upconversion luminescence of Er3+-ions in the irradiation region [Fig. 1(a)]. After irradiating for about 40 s, blue emission [Fig. 1(b)] started to appear in the irradiation region, and then the intensity of the blue emission increased with increasing irradiation time, and finally remained constant after 60 s irradiation. Femtosecond laser light with the same average power was focused in the 33.3BaO-16.7TiO2-50SiO2 glass sample at the same depth for comparison. White light emerged due to the self-phase modulation of the femtosecond laser in the focal point at the initial stage of the laser irradiation, and a bright blue emission started to appear from the irradiation region after 25 s irradiation. The time for the appearance of blue emission in the undoped glass was shorter than in the Er3+-doped glass. Figure 1(c) shows two emission spectra of the Er3+-doped glass during the femtosecond laser irradiation. After irradiation for about 60 s, the Er3+-doped glass exhibited three emission peaks at about 400, 525 and 546 nm. The emission at 400 nm was due to the second harmonic generation of the laser beam. While emissions at 525 and 546 nm, attributed to the upconversion luminescence of Er3+ were observed after irradiating for both 10 s and 60 s, no emission was observed at 400 nm for the glass sample after irradiating for 10 s.
Natural light and the polarized light micrographs of the Er3+-doped BaO-TiO2-SiO2 glass sample after femtosecond laser irradiation are shown in Fig. 2. The laser power and irradiation time was 750–950 mW and 1/125-64 s, respectively. Spherically induced dots due to the shock wave and thermal stress  were formed at the focal point of the laser beam. The dot size changed with laser power and irradiation time. Around the dots, the retardation is clearly observed, indicating that a refractive index change is induced around the dots. Bright green and blue emissions were observed from the irradiation region while writing lines through moving the focal point of the laser beam horizontally inside the glass at a scanning rate of 2 μm s-1. Figure 2(c) shows a photograph of the induced line under a polarized microscope.
Figure 3 shows the diameter of the femtosecond laser-induced dot in the Er3+-doped BaO-TiO2-SiO2 sample as function of the femtosecond laser irradiation time. The diameter of the dots varies from several μm to 16 μm. It is clear that the dot grew rapidly in the initial phase of the laser irradiation (≤ 2 s), and then reached their final size. At the initial stage of laser irradiation, the diameter of the femtosecond laser-induced dot increases linearly as a function of t1/3, indicating the thermal diffusion plays a dominant role in the formation of the dots .
Figure 4 shows the absorption spectrum of the Er3+-doped BaO-TiO2-SiO2 glass sample. The absorption spectrum consists of eight absorption peaks at about 800, 651, 542, 521, 487, 450, 406, and 378 nm, corresponding to the absorptions from the ground state 4I15/2 to the excited states 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2, 4F5/2, 2H9/2, and 4G11/2, respectively . Micro-Raman spectra were measured by a Raman spectrometer (Renishaw in Via) and a laser microscopy system with 514 nm Ar+ laser excitation. The Er3+-doped glass showed strong emissions while irradiated by the Ar+ laser, and the Raman spectrum was difficult to measured due to low signal to noise ratio. Here, we the used micro-Raman spectra of the undoped BaO-TiO2-SiO2 glass sample to confirm the precipitation of the crystals.
After the femtosecond laser irradiation, significant structural changes can be found in the focal point. Figure 5 shows micro-Raman spectra of the BaO-TiO2-SiO2 glass before irradiation and of the BaO-TiO2-SiO2 glass after femtosecond laser irradiation for 60 s. The broad band at 853 cm-1 in the original glass can be assigned to the stretching of the short Ti-O* bond (O* denotes an apical oxygen). The Ti-O- bonds (O- denotes a nonbridging oxygen), and terminal SiO3 groups, and the band at 320 cm-1 are most likely due to the v(Ba-O) mode . After irradiation for 60 s, the band at 853 cm-1 becomes narrow and the strongest peak at 860 cm-1 and assigned to the vibration of the short Ti-O bond, becomes more distinct. We also observe that the band at 320 cm-1 splits into several peaks at 226, 273, 343 and 373 cm-1 attributed to the translational and bending modes of the Si2O7 and TiO5 groups . Two peaks at 591 and 664 cm-1, assigned to the v(TiO4) and vs(Si-O-Si) modes gradually appeared with increasing irradiation time, indicating that the crystal structure consisting with corner-linked TiO5 pentahedra and pyrosilicate groups Si2O7 have formed . All the sharp peaks are in good agreement with the Raman spectrum of the Ba2TiSi2O8 single crystal [14, 15]. Therefore, we can conclude that Ba2TiSi2O8 crystals precipitated around the focal point of the laser beam after the femtosecond laser irradiation.
As can be seen in Fig. 4, the absorption spectrum of the Er3+-doped glass shows an absorption peak at about 800 nm. The glass sample showed a green emission in the focal point at the initial stage of the laser irradiation, which is due to upconversion luminescence in Er3+-doped glass. The emission spectrum of the Er3+-doped glass exhibits two emission peaks at about 525, and 546 nm [Fig. 1(c)] which can be attributed to the 2H11/2→4I15/2, and 2S3/2→4I15/2 transitions of Er3+, respectively . After irradiation for 40 s, a blue emission started to appear from the irradiation region. As seen from Fig. 1(c), the emission spectrum showed a sharp peak at 400 nm, which resulted from the frequency doubling of the 800 nm laser beam due to the precipitation of Ba2TiSi2O8 crystals, which showed a large second-order optical nonlinearity . We define the irradiation time of the appearance of blue light as the irradiation time required for crystallization. The required time for blue emission emerging from Er3+-doped BaO-TiO2-SiO2 glass sample was longer than that from the BaO-TiO2-SiO2 glass sample under the same condition. This can be attributed to the occurrence of upconversion luminescence of Er3+-ion, which drains apart of the energy of the femtosecond laser. Our recent investigations show that the irradiation tiome can be shorter than 0.1s for the precipitation of nonlinear optical crystals when we dope some nucleates into the glass, and that the dots can be smaller than 1μm. We can write three-dimensional dot patterns and the data (dots) can be read out in the form of intense second harmonic generation. Therefore, this technique is promising for three-dimensional optical memory.
In addition, by scanning three focused infrared lasers to the nonlinear optical crystals precipitated glass, we can get blue, green and red emissions near the focal point of the laser beams due to second harmonic generation, showing that this technique may also be useful for the solid state, full color three-dimensional display.
In conclusion, we have succeeded in the fabrication of Ba2TiSi2O8 crystalline dots and lines in 33.3BaO-16.7TiO2-50SiO2-0.5Er2O3 glasses using tightly focused femtosecond laser irradiation. The irradiation time required for crystallization in Er3+-doped BaO-TiO2-SiO2 glass was longer than that in BaO-TiO2-SiO2 glass under the same conditions, We attributed this difference to the occurrence of upconversion luminescence from Er3+-ions. During the femtosecond laser irradiation in Er3+-doped glass, an intense green emission was observed in the focal point at the initial stage of the laser irradiation. Blue emission started to emerge from the irradiation region after 40 s irradiation due to the precipitation of Ba2TiSi2O8 crystals with large second-order optical nonlinearity. It is possible to directly write crystalline dots and lines inside the glass by using the present technique. This technique paves the way to the formation of three-dimensional optical memories and three-dimensional solid state displays.
This work was financially supported by the National High-tech research and Development Program of China (G20060914). The authors appreciate Prof. G. Vienne of the Department of Optical Engineering, Zhejiang University, China for his careful reading and kind comments on the manuscript.
References and links
1. Y. Wang and J. Ohwaki, “New transparent vitroceramics codoped with Er3+ and Yb3+ for efficient frequency upconversion,” Appl. Phys. Lett. 63, 3268–3270 (1993). [CrossRef]
2. Y. Takahashi, Y. Benino, T. Fujiwara, and T. Komatsu, “Large second-order optical nonlinearities of fresnoite-type crystals in transparent surface-crystallized glasses,” J. Appl. Phys. 95, 3503–3508 (2004). [CrossRef]
3. M. Nogami, A. Kato, and Y. Tanaka, “Sol-gel preparation of CdSxSe1-x solid solution microcrystal-doped glasses,” J. Mat. Sci. 28, 4129–4133 (1993). [CrossRef]
4. R. Ihara, T. Honma, Y. Benino, T. Fujiwara, R. Sato, and T. Komatsu, “Writing of two-dimensional crystal curved lines at the surface of Sm2O3-Bi2O3-B2O3 glass by samarium atom heat processing,” Solid State Commun. 136, 273–277 (2005). [CrossRef]
5. 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, 3329–3331 (1997). [CrossRef]
6. 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, 408–410 (2000). [CrossRef]
7. Y. Yonesaki, K. Miura, R. Araki, K. Fujita, and K. Hirao, “Space-selective precipitation of non-linear optical crystals inside silicate glasses using near-infrared femtosecond laser,” J. Non-Cryst. Solids 351, 885–892 (2005). [CrossRef]
8. X. Jiang, J. Qiu, H. Zeng, and S. Zhu, “Femtosecond laser induced growth of LiNbO3 crystal in glass,” J. Inorg. Mater. 19, 935–938 (2004).
9. J. Qiu, X. Jiang, C. Zhu, M. Shirai, J. Si, N. Jiang, and K. Hirao, “Manipulation of gold nanoparticles inside transparent materials,” Angwe. Chem. Int. Ed. 43, 2230–2234 (2004). [CrossRef]
10. S. K. Sundaram, C. B. Schaffer, and E. Mazur, “Microexplosions in tellurite glasses,” Appl. Phys. A 76, 379–384 (2003). [CrossRef]
11. Y. Dai, B. Yu, B. Lu, J. R. Qiu, X. Yan, X. Jiang, and C. Zhu, “Thermal stress-induced birefringence in borate glass irradiated by femtosecond laser pulses,” Chin. Phys. Lett. 22, 2626–2629 (2005). [CrossRef]
12. S. Kawasaki, T. Honma, Y. Benino, T. Fujiwara, R. Sato, and T. Komatsu, “Writing of crystal-dots and lines by YAG laser irradiation and their morphologies in samarium tellurite glasses,” J. Non-Cryst. Solids 325, 61–69 (2003). [CrossRef]
13. S. Xu, G. Wang, S. Dai, J. Zhang, L. Hu, and Z. Jiang, “Infrared to visible upconversion in Er3+-doped lead oxyfluorosilicate glasses,” J. Lumin. 109, 187–192 (2004).
14. S. A. Markgraf, S. K. Sharma, and A. S. Bhalla, “Raman study of fresnoite-type materials: Polarized single crystal, crystalline powders, and glasses,” J. Mater. Res. 8, 635–648 (1993). [CrossRef]
15. Th. G. Mayerhöfer and H. H. Dunken, “Single-crystal IR spectroscopic investigation on fresnoite, Sr-fresnoite and Ge-fresnoite,” Vib. Spectrosc. 25, 185–195 (2001). [CrossRef]
16. Y. Takahashi, K. Kitamura, Y. Benino, T. Fujiwara, and T. Komatsu, “Second-order optical nonlinear and luminescent properties of Ba2TiSi2O8 nanocrystallized glass,” Appl. Phys. Lett. 86, 091110 (2005). [CrossRef]