Femtosecond laser offers big benefits

Jennifer L. Morey

CAMBRIDGE, Mass. -- A group of Harvard University researchers has demonstrated a technique for blasting miniature holes in transparent materials that could mean big advances in a number of photonic applications. Though in its initial stages, the technique could increase optical data storage memory and enable more precise tissue ablation.
In operation, 100-fs pulses from a regeneratively amplified Ti:sapphire laser generate microexplosions by focusing 50 to 200 µm below the surface of a transparent material at an intensity as high as 10¹⁹ W/m². The result is a 200- to 250-nm-diameter void in the center of the material, surrounded by a region of compacted material. The researchers tested fused silica, BK 7 optical glass, plastic, quartz, sapphire and diamond, but noted that other transparent materials can be used as well.
Based on preliminary observations, scientists attribute the small diameters to a strong self-focusing element: In essence, the laser forms its own lens and focuses itself.

Varied applications
According to research team member Chris Schaffer, this method could be used to create compact discs with up to 100 layers of information vs. the traditional one or two layers -- a huge increase in memory. The ability to write 3-D objects with submicrometer precision could also be useful in creating diffractive optical elements, photonic bandgap materials and patterned fiber gratings.
In addition, the research could have implications for human tissue, such as the eye. Shorter pulses would produce a more precise "scalpel" and less collateral damage within the tissue.