1. Introduction

In the past decades, femtosecond (fs) laser micromachining of transparent materials has been an active area of research because of the obvious advantages of fs laser pulses, such as ultrashort pulse duration and ultrahigh light intensity. Previous studies were mainly focused on the fs laser interaction with glasses or metals^[1-5]. However, few researches have been carried out on fs laser interaction with single crystals^[6-9]. Several new phenomena, such as self-organized void array along the propagation direction of the laser beam in bulk single crystal, have been found recently. They have promising applications in the fabrication of integrated photonic devices^[7-8]. Many micro-optical devices, e.g. optical waveguides, have also been fabricated in crystals using tightly focused fs laser^[9].

Dislocation is one of the most important researches on crystals because this phenomenon usually affects the electronic and optical properties of the crystals^[10-13]. Metallic nanowire bundles and filamentary structures with switching function have been successfully fabricated by using dislocation^[11-12]. Recently, using fs laser irradiation, Kanehira et al. reported optically produced cross-shaped patterns based on the local dislocations inside MgO single crystals^[13]. Nanovoid strings were also self-formed in the laser propagation direction by tightly focusing a single fs laser beam at a fixed depth inside the MgO crystal. The experimental results have offered a new technique to produce highly local dislocations. Though they have demonstrated the feasibility of applying fs laser micromachining to induce the formation of dislocation, there is still a long way to go for exactly controlling locally dense and directional dislocation bands in crystals.

In this paper, we optically induced microstructures (skew cross-shaped patterns) without catastrophic collapse in CaF₂ single crystal. This material is important for applications in photonics and optoelectronics because of its excellent mechanical, chemical and optical properties. A skew cross-shaped pattern array, different from the pattern array observed in MgO crystal, was successfully fabricated inside a single CaF₂ crystal. Furthermore, these optically induced local dislocations can be partially erased by additional irradiation of the fs laser pulses on their neighboring area.

2. Experimental

 

Fig. 1. Schematic diagram shows the formation of local dislocations by fs laser irradiation. The top facet of sample was (011) crystal plane. The high-density dislocations exit in crystallographic(11 1̄) and (1̄1 1̄) planes.

Download Full Size | PDF

Figure 1 schematically shows the generation of local dislocations by the fs laser irradiation. In the present work, the CaF₂ crystal sample was cut into a cube 5mm×5mm×5mm in size. In order to observe the sample from two orthogonal directions, the sample was four-facet polished. A regeneratively amplified 800-nm Ti: sapphire laser, with 120 fs pulse length and 1 kHz repetition rate, was used in this experiment. The pulse energy of the fs laser could be adjusted through neutral density (ND) filters and the number of laser pulses was controlled by an electronic shutter. The CaF₂ single crystal sample was put on an XYZ motorized stage, which was controlled by a computer. Unless otherwise noted, the laser beam parallel to the ^[011] direction of CaF₂ single crystal was focused into the sample through a high numerical aperture (NA=0.8) microscopic objective.

3. Results and discussion

 

Fig. 2. (a). Optical microscope image of CaF₂ (011) facet viewed from the top of the sample after irradiation; (b). TEM image of the photo-modified area induced by femtosecond laser inside CaF₂ crystal. The white stripe about 50 nm wide extends from the laser irradiation area corresponding to microcracks in the [11 1̄] direction; (c). Diffraction image of laser irradiated area marked by circle in Fig. 2(b); (d). High-resolution TEM image of the region around microcrack marked by the circle in Fig. 2(b); (e). The micrograph of lattice fringes of (11 1̄) plane corresponding to Fig. 2(d).

Download Full Size | PDF

As shown in Fig. 2(a), after the fs laser irradiation, a skew cross-shaped pattern consisting of a void surrounded by four dark straight lines appeared. The pulse energy and the number of pulses was 7.5 μJ and 16, respectively. The fs laser beam was focused 200 μm below the surface of the sample. The four dark lines were probably ascribed to the refractive index change due to the formation of dislocations. For high-resolution transmission electron microscopy (HRTEM) measurement, sample was thinned to a few tens of nanometers by focused ion beams. Figure 2(b) shows a typical TEM image of a photo-modified area inside the CaF₂ crystal induced by the fs laser pulses. As seen in Fig. 2(b), the white stripe about 50 nm wide extended from the laser irradiation area corresponding to the microcrack in the [11 1̄] direction. However, we confirmed that the length and the width of the white stripe were not consistent with the length and width of the dark lines from the optical microscope image in Fig. 2(a). This meant that the dark lines in Fig. 2(a) are not only consisted of microcracks. It is also noticed that electron diffraction pattern collected at the area of focal point shows the presence of polycrystalline phase at the center of the focus (Fig. 2(c)). Figure 2(d) shows a HRTEM image of the area around the microcrack (region marked by the circle in Fig. 2(b)). Figure 2(e) depicts the micrograph of lattice fringes of the (11 1̄) plane corresponding to Fig. 2(d). It is noted that a large number of line defects or dislocations exist unambiguously in this area. The sites where the dislocations are clearly present are shown by arrows in Fig. 2(e). A check on the (1̄1 1̄) plane also gave the same results. Based on the analysis above, we can conclude that the dark lines in Fig. 2(b) consist of microcracks with high density of dislocations. We speculate that shockwave induced by the fs laser plays an important role of the formation of dislocations. When the fs laser pulse with a pulse peak energy high enough was focused into the crystal, energy transfers from the laser pulse to the electrons through nonlinear ionization. Through phonon-mediated linear absorption, the nonlinearly excited electrons are further excited until they acquire enough kinetic energy to excite other bound electrons. Over a picosecond timescale, part of the optical energy absorbed by the electrons is transferred to the lattice. Usually a pressure or a shockwave occurs and separates from the hot focal volume within a couple of nanoseconds. Dislocations are induced along certain directions due to shockwave since the cleavage planes of CaF₂ crystal are most easily damaged^[14].

 

Fig. 3. (a) Average length of dark lines as a function of the pulse energy. The femtosecond laser beam was focused 200 μm beneath the surface of the sample, and the pulse number was 1000; (b) Average length of dark lines as a function of the number of femtosecond laser pulses. The linear focus of the objective was located 200μm below surface. The average length of dark lines was evaluated as the distance of dark lines from central dot in the skew cross-shaped pattern observed in the top-view image of the optical microscope.

Download Full Size | PDF

To investigate the influence of the pulse energy on the induced skew cross-shaped pattern, the pulse energy was adjusted from 1 to 60 μJ. The number of pulses was fixed to 1000. The average length of dark lines against the pulse energy of fs laser is plotted in Fig. 3(a). The fs laser beam was focused 200 μm beneath the surface of the sample. The average length of the dark lines almost increased monotonically with the increase in pulse energy. Figure 3(b) shows the typical average length of dark lines as a function of the number of fs laser pulses at a pulse energy fixed to 7.5 μJ, with the linear focus of the objective located 200 μm below the surface. The average length of the dark lines dramatically increases as the number of pulses increases from 5 to 125. Beyond 200 pulses, the increase of the average length of dark lines slows down apparently.

 

Fig. 4. (a). (c) Optical microscope images of CaF₂ (011) plane viewed from the top of the sample after irradiation with 500 pulses. The pulse energy was 7.5μJ and the linear focus of the objective located 200 μm beneath surface; (b) (d) Optical microscope images of CaF₂ (011) plane viewed from the top of the sample after another irradiation at neighboring area with the same laser condition. The position of second shot was indicated by the arrow in Figs. 4(b, d); (e-g) High-resolution TEM images of the region marked by circles; (h-j) Micrograph of lattice fringes of (11 1̄) plane corresponding to the region marked by the rectangle in Figs. 4(e-g).

Download Full Size | PDF

Figure 4 shows the shape change of an already existing local dislocation upon additional fs laser shot irradiation at a neighboring area. The pulse energy was 7.5 μJ, the number of pulses was 500 and the linear focus of the objective was located 200 μm beneath the surface. Firstly, the laser beam was tightly focused into a CaF₂ single crystal (011) plane with 500 laser pulses through a microscopic objective, and the top-view optical microscope image is shown in Fig. 4(a) where four dark lines were marked with Arabic numbers from 1 to 4, respectively. Then, keeping the laser focal depth fixed, we moved the laser focus to the neighboring area of the original irradiation location and irradiated the sample again with the same laser condition. Surprisingly, the second shot made the size of line 1 shortened and narrower, as illustrated in Fig. 4(b). This amazing phenomenon means the local dislocations inside CaF₂ single crystals has partly closed up without any visible scar left. To investigate the phenomena at different angles, we moved the laser focus to the other directions, and then found the line 1 and 3 changed, as shown in Figs. 4(c) and 4(d). It is worthy to note that changes always happen in the lines close to the second laser shot location while the lines away from the second shot location little change. Figure 4(e) shows a HRTEM image of the region not irradiated by the fs laser. Figure 4(h) shows the lattice fringes of the (11 1̄) plane corresponding to the region marked by the rectangle in Fig. 4(e). No apparent dislocations were observed (Fig. 4(h)). The HRTEM image corresponding to the dark lines is shown in Fig. 4(f). Figure 4(i) shows the lattice fringes of the (11 1̄) plane corresponding to the region marked by the rectangle in Fig. 4(f). The sites where dislocations are clearly present are shown by the arrows in Fig. 2(i). It can be seen clearly that a large number of line defects or dislocations existing unambiguously in the dark lines area after the fs laser irradiation. Figure 4(g) illustrates the HRTEM image of the recovered dark line region which is erased by further irradiation of the fs laser. Figure 4(j) depictes the lattice fringes of (11 1̄) plane corresponding to the region marked by the rectangle in Fig. 4(g). A majority of local dislocations disappear after another fs laser irradiation. A check on (1̄1 1̄) plane also gives the same results. We speculate that the effect of shock wave may lead to migration of part of the atoms and result in the disappearance of some dislocations.

 

Fig. 5. Optical microscope image of the array of skew cross-shaped patterns in the CaF₂ single crystal sample

Download Full Size | PDF

This phenomenon provides a new dislocation-controlling technique. To further utilizing this technique, we produce an array of skew cross-shaped patterns in the CaF₂ crystal, as shown in Fig. 5. The pulse energy was 7.5 μJ, the number of pulses was 500 and the linear focus of the objective was located 200 μm below the surface. This skew cross-shaped array pattern provided a new pattern array, consisted of highly dense local dislocations. It may be applied to the fabrication of integrated photonic devices, etc.

4. Conclusions

In summary, skew cross-shaped patterns due to local dislocations were induced inside the CaF₂ single crystal by the fs laser irrdiation. Optical microscope and HRTEM images have confirmed that the local dislocations can be partly erased by the additional femtosecond laser irradiation, which was probably due to the fs laser induced shockwave. A skew cross-shaped array pattern was written in the CaF₂ crystal. It is important to control the formation and disappearance of the local dislocations. This work offers a new method of producing controllable local dislocations by using fs laser pulses.