1. Introduction

When a femtosecond laser pulse is focused inside a bulk of transparent material, the intensity in a focal volume becomes high enough to produce microscopic structural modifications through nonlinear optical absorption. The modifications have been utilized as optical data-storage elements [1–3], waveguides [4–8], gratings [9], and couplers [8,10,11] inside a wide variety of transparent materials including glasses, crystals, and plastics. The submicrometer-damages that are produced by femtosecond laser pulses with high numerical-aperture (NA) lenses were demonstrated to be cavities or voids surrounded by densified materials [1,2]. Glezer et al. described the mechanism of the formation of the void as follows: When the laser pulse is tightly focused, hot electrons and ions explosively expand from the focal volume into the surrounding material due to the very high energy density produced by the femtosecond optical breakdown. We have shown that after producing a void or bubble in bulk glass by irradiation with intense femtosecond laser pulses it could be moved by shifting the laser focus slightly in front of the void and irradiating again [3]. We found we could move the void only in the direction opposite to the laser propagation direction up to 5 μm. We explained the mechanism as follows: Femtosecond laser irradiation produces a void in the glass. When a subsequent laser pulse if focused slightly in front of the void, it vaporized the material there. This hot material fills in the old void, leaving a new void slightly upstream from the original.

In this paper we show that even without moving the laser focus, the void moves upstream a few micrometers after irradiation with subsequent laser pulses in the bulk of calcium fluoride and silica glass. We find that the voids move a maximum distance of approximately 2 μm. We also demonstrate the lateral movement of the void along the axis perpendicular to the beam propagation axis by shifting the focal region of the laser pulses.

2. Experimental Setup

 

Fig. 1. Schematic of experimental setup for creation by femtosecond laser pulses and in situ observation of voids. ND, HWP, and P denote neutral density filter, half-wave plate, and polarizer, respectively. OB1 and OB2 indicate objective lenses. L1 and L2 indicate lenses.

Download Full Size | PDF

The experiments were performed with a regeneratively amplified Ti:sapphire laser system (Spectra-Physics, Inc., Hurricane), which produces 130-fs, 800-nm pulses. Figure 1 shows a schematic of the experimental setup. We operated the laser system at a repetition rate of 1 Hz. The beam was magnified by the help of a concave lens L1 with a focal length f of negative 60 mm and a convex lens L2 (f = 300 mm). The central part of the beam was truncated by a circular aperture (diameter, 5 mm) to give uniform spatial profile. The pulse energy was controlled by a half-wave plate and a polarizer. The linearly polarized laser pulses were tightly focused by a high-NA microscope objective to induce localized structural changes or optical damage. We used a 50×, 0.55-NA achromatic objective (Olympus, ULWD MPlan 50). Compensation for the dispersion in the optical system was accomplished by predispersing the pulses with a grating compressor in the amplifier of the laser system. We will report in the following the experiments with 3-mm-thick plate of calcium fluoride at room temperature, however, similar effects were also observed with amorphous silica glass. The four-sides of the samples were optically polished. To visualize sideview of the structural change in situ, optical images of the damages or voids were observed from the direction perpendicular to the optical axis by a transilluminated optical microscope. We used a 50×, 0.55-NA achromatic objective to observe voids in situ. We also made the observation by a 100×, 0.8-NA achromatic objective. Therefore, the spatial resolution of the observing system was approximately several hundred nanometer.

3. Movement of Voids by Irradiation of Laser Pulses

We investigated the movement of voids by changing the number of laser shots under fixed pulse energy. We created voids in the bulk calcium fluoride at a depth of 200 μm beneath the surface. The energy threshold for the observable permanent structural change with the microscope was 386 nJ/pulse by a single shot in the microscope. The threshold for optical breakdown and structural change in bulk calcium fluoride for 100-fs laser pulses has been shown to be around 50 nJ/pulse [11]. The energy in our experiment exceeds the threshold reported in Ref. 11 by a factor of 6. The possible explanations are is the temporal distortion of femtosecond laser pulses [12,13], spherical aberration in the focal point, and surface roughness of the polished samples. Figure 2 shows an optical image of movement of voids observed in the yz plane under illumination by unpolarized halogen lamp. The energy was 386 nJ/pulse. The dark spot that was observed in the optical microscope was previously shown to be a cavity or hole [1–3]. The beam propagation direction of the femtosecond laser pulses (+ z direction) was left to right in the plane of the image. We first created a void by a single shot of the laser pulse and translated the sample along y axis by 10 μm and irradiated two laser pulses at the fresh site. We translated the sample again and increased the number of laser pulse to be irradiated. The number of laser shots was varied from 1 shot to 5 shots. As the number of laser shots increased, the void moved toward -z direction. After structural changes occurs, we observe the scattering light of the laser pulses when subsequent pulses irradiate the sample. Figure 2 shows that the void moved approximately by 2 μm in the -z direction by successive irradiation of laser pulses without shifting the focal point upstream direction. This direction of movement coincides with that of the mechanical translation of a void reported by our group [3]. From the image contrast, the size of a void seems to increase shot by shot. This result is the same when we translated the focal point in Ref. 3. We confirm the void moves in -z direction by successive laser shots.

 

Fig. 2. Optical movement of a void under irradiation by successive laser shots. Side view of void was observed under illumination unpolarized halogen lamp.

Download Full Size | PDF

Figure 3 (a) shows the movie of the movement of voids at the energy of 386 nJ/pulse with the number of laser pulses. In Fig. 3, the small circular spot in the left of each figure indicates the absolute position in the images. As we increased the number of laser shots, the shape of void seemed to be elongated along z direction and then the second void was visible around the original location of first void. Figure 3 (b) shows the movement of a void at the energy of 299 nJ/pulse with the number of laser pulses. We set the energy that was incident on the objective lens less than the threshold for the permanent observable structural change. At this energy the structural change was seen after 22 shots. Then the void moved toward the incident direction and the distance of movement terminated after 54 pulses at 1.7 μm. The distance of the movement was plotted every two pulses with three different incident energy in Fig.4. Figure 4 demonstrates that the void moved with the number of laser pulses and that the positions of the void can be controlled within sub-micrometers by changing the number of laser pulses and the energy.

 

Fig. 3. (2MB) Optical movement of a void under successive irradiation of laser shots. Energy : (a) 386 nJ/pulse and (b) 299 nJ/pulse, respcetively. The small circular spot in the left of each figure indicates the absolute position in the images. The number of shots is indicated upper right. [Media 1] [Media 2]

Download Full Size | PDF

 

Fig. 4. Distance of movement of a void. ☐denotes the shape becomes elliptical along the optical axis..

Download Full Size | PDF

Let us discuss the mechanism of the motion of the void. When a femtosecond laser pulse is tightly-focused, at the focus, the laser intensity becomes high enough to induce nonlinear absorption of the laser energy by the material through multi-photon and avalanche photo-ionization process, resulting in optical breakdown and the formation of a high-density plasma. This hot plasma explosively expands into the surrounding materials. A microexplosion occurs inside the material and results in a permanently damaged region of the void[1,2]. Then the question arises why the void moves. High-intensity region of the focal volume extends upstream from the point where the void is made about one confocal parameter (approximately 2μm for the 0.55-NA focusing condition). Subsequent pulses thus have enough intensity to produce optical breakdown slightly upstream from the void that was produced by the first pulse. Breakdown is most likely to occur near the front interface of the void. Several groups have demonstrated the existence of defects or color centers that are produced by femtosecond laser pulses in glasses [4,14,15]. There is no report on the existence of defects in calcium fluoride, however, there are likely defects and easily-ionized surface states near the interface that enhance the breakdown process. When optical breakdown occurs at this front interface, the debris of the new one fills in the old void. Then we observed that the void moves upstream direction. This process continues to move the void upstream until the laser intensity drops too low to produce breakdown. This happens about one confocal parameter away from the laser focus, explaining the maximum void movement of about 2 μm. The only difference between this work and the previously published results is that, in previous work, we deliberately translated the laser focus to produce breakdown in another region, while in the present work, the region where breakdown is produced moves on its own during irradiation by successive pulses without shifting the laser focus. When we deliberately shifted the focal point, the distance of the motion by a single shot can be controlled by the amount of shift of focal point.

4. Lateral Movement of Voids

We demonstrate, to the best or our knowledge, the lateral movement of a void along the axis perpendicular to the beam propagation axis for the first time. Figures 5 show optical images of movement of voids in CaF₂ observed in the yz plane under illumination by a halogen lamp. The energy was 386 nJ/pulse. We created two voids for comparison and set the focal point of the objective lens to the upper in the figure. Then we created a void by a single shot (Fig.5 (a)). We shifted the focal point of the focusing objective along the +y direction by 1 μm and irradiated a laser pulse (Fig. 5 (b)). We repeated the same procedures. The void moves as shown in Fig. 5 (c) and (d). Figures 5 show that the void moves along the axis perpendicular to the beam propagation axis. After the irradiation of five pulses, one spot separated into two. We could not move the void in the + z direction. If we focus the beam at the rear of the void, the beam seriously suffers from scattering owing to the void and the surrounding area. The shift of the focus and the irradiation enables us to change the position of the void three-dimensionally in transparent materials, except pushing forward the void.

 

Fig. 5. Lateral movement of a void perpendicular to the beam propagation axis. The void moves by 2 μm along the direction perpendicular to optical axis.

Download Full Size | PDF

5. Conclusion

We demonstrated that irradiation of femtosecond laser pulses move a microscopic bubble inside crystalline calcium fluoride and amorphous silica glass. The microscopic observation revealed that the bubble moves against the direction of propagation of laser pulses as far as 2 microns. This movement is most likely due to the production of easily-ionized defect at the interface of the void. It is likely that easily-ionized defect states, surface state, and color centers that probably produced near the interface cause optical breakdown at the interface for subsequent laser pulses. Once we see that optical breakdown occurs preferentially at this front interface of the void, the void moves upstream direction. We also demonstrated the lateral movement of the void along the axis perpendicular to the beam propagation axis by shifting the laser focus. This movement technique will lead to the 3-D reconfigurable optical storage. The ability to move voids allows fine tuning of optical microstructures that are written within transparent materials.