Ultrafast lasers can be used to write microscopic features in glass, including waveguides (see Laser Focus World, April 2000, p. 73). If the femtosecond laser pulses are strong enough, the resulting features consist of micron-sized voids surrounded by compacted glass. Such features, once formed, are immovable—or so one would think. A group of Japanese researchers at Sumitomo Heavy Industries (Kanagawa), Osaka University, and the Osaka National Research Institute have shown that such voids can be seized and pulled along as if they were on a leash, and can even be made to merge.
To tug the voids, the researchers exploit the same laser pulses that were used to fabricate the voids, merely reducing the pulse energy. In the experiment, eight successive 130-fs, 800-nm pulses from a 1-kHz pulse regenerative amplified Ti:sapphire laser were sent through a 0.55-numerical-aperture microscope objective into silica glass to create each 0.9-µm-diameter void. The researchers created voids in evenly spaced lines 300 µm beneath the surface of the glass. Next, weaker pulses were focused onto a void and the focal spot moved along the optical axis, dragging the void along with it. Voids were moved as much as 5 µm. The researchers found they could move the voids in only one direction along the optical axis, and not at all laterally. A visible trajectory of the void after movement implied that the glass underwent a structural change, perhaps akin to that used to write waveguides. The researchers also demonstrated that two voids could be merged into one larger void.
To verify that the dark spots seen via optical microscope were indeed voids, the researchers created a laser-ablated line on the surface of the glass along a plane that included a row of voids, then broke the glass apart. Viewing of the cleaved voids under a scanning electron microscope confirmed the voids' existence and size.
Calculations showed that the initial temperature reached at the laser focus inside the glass was 106 K, with the residual temperature rise after one pulse being 10°C. The temperature in a region with a diameter of several microns around the void exceeded the 1150°C annealing point of the glass. After illumination, a rapid decrease in temperature in the region caused a phase change. To investigate thermal effects and stability, the researchers heated the void-containing glass to 1150°C for 1 hour. The voids remained, undergoing no structural change. However, the residual structural change in the trajectory created by movement of the voids disappeared. The researchers attribute the creation of the voids to plasma formation or thermal quenching of the melted region, but could not pin down the details of the mechanism.
The ability to move voids within glass makes void-based rewritable optical memories possible, say the researchers. In addition, the technique will allow the modification and fine-tuning of optical microstructures written within glass.
