Laser pulses move voids in glass

Feb. 1, 2001
Ultrafast lasers can be used to write microscopic features in glass, including waveguides (see Laser Focus World, April 2000, p. 73).

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 immovableor 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.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

Sponsored Recommendations

Request a free Micro 3D Printed sample part

April 11, 2024
The best way to understand the part quality we can achieve is by seeing it first-hand. Request a free 3D printed high-precision sample part.

How to Tune Servo Systems: The Basics

April 10, 2024
Learn how to tune a servo system using frequency-based tools to meet system specifications by watching our webinar!

Motion Scan and Data Collection Methods for Electro-Optic System Testing

April 10, 2024
Learn how different scanning patterns and approaches can be used in measuring an electro-optic sensor performance, by reading our whitepaper here!

How Precision Motion Systems are Shaping the Future of Semiconductor Manufacturing

March 28, 2024
This article highlights the pivotal role precision motion systems play in supporting the latest semiconductor manufacturing trends.

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!