1. Introduction

Optically transparent glass-ceramics containing rare-earth ions [1] or nonlinear optical crystals [2, 3] have received considerable attention, because such materials have high potential applications in photonic devices such as phosphors [1] and ultrafast optical switches [3]. 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 Sm_xBi_1-xBO₃ on the surface of Sm₂O₃-Bi₂O₃-B₂O₃ glass [4]. 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 [10]. 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 [4].

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.

2. Experiment

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

 

Fig. 1. Photographs around the focal region during femtosecond laser irradiation in Er³⁺-doped glass after (a) 10 s, and (b) 60 s, respectively. (c) Time dependence of emission spectra during femtosecond laser irradiation.

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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.

 

Fig. 2. Micrographs of the focal regions illuminated by (a) natural light and (b) polarized light after femtosecond laser irradiation (laser power: 750–950 mW, irradiation time: 1/125-64 s). (c) Polarized optical micrograph of the line written by femtosecond laser irradiation (laser power: 950 mW, scanning rate: 2 μm s-1).

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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 [11] 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.

 

Fig. 3. (a). Dependence between the diameter of femtosecond laser-induced dot and the laser irradiation time, and (b). correlation between the diameter of femtosecond laser-induced dots and the third root of the laser irradiation time t^1/3 in the Er³⁺-doped BaO-TiO₂-SiO₂ glass sample.

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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 t^1/3, indicating the thermal diffusion plays a dominant role in the formation of the dots [12].

 

Fig. 4. Absorption spectrum of the Er³⁺-doped BaO-TiO₂-SiO₂ glass sample.

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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 ⁴I_15/2 to the excited states ⁴I_9/2, ⁴F_9/2, ⁴S_3/2, ²H_11/2, ⁴F_7/2, ⁴F_5/2, ²H_9/2, and ⁴G_11/2, respectively [13]. 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.

 

Fig. 5. Micro-Raman spectra of the BaO-TiO₂-SiO₂ glass (0 s), and the focal region of the laser beam inside glass after femtosecond laser irradiation for 60 s.

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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 [14]. 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 [15]. 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 [14]. 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 ²H_11/2→⁴I_15/2, and ²S_3/2→⁴I_15/2 transitions of Er³⁺, respectively [13]. 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 [16]. 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.

4. Conclusions

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.