- Lasers and Chemical Etch Make Tiny Holes in Glas
Scientists at the Natural Research and National Research councils, both in Ottawa, have shown that femtosecond laser pulses, together with a chemical etching process, can be used to fabricate periodic microstructures in glass. They think that the technique could be applicable to the production of two- and three-dimensional photonic-bandgap crystals.
The fabrication of submicron structures in glass is of interest because it can lead to new approaches to making photonic-bandgap crystals. Currently, most of those crystals are two-dimensional "holey fibers," which researchers fabricate by creating a preform, usually of individual glass tubes, and then drawing a fiber from the preform using standard optical fiber techniques.
When femtosecond laser pulses are focused into silica glass, a nonlinear interaction in the glass physically alters its refractive index in the affected region. If the modified glass subsequently is removed by chemical etching, a very small, well-defined hole is the result. It is not well understood what change in the modified glass makes it more sensitive to chemical etching, but the result is effective and repeatable.
The Canadian scientists used a Ti:sapphire laser that generated 30- to 40-fs pulses at 800 nm at a 100-kHz repetition rate to create a long, highly tapered, modified region in the glass sample, with the dimension of the tapered tip as small as 100 nm. To do so, they focused the laser to a point just above the sample's bottom surface. The threshold for material modification was about 40 nJ, or 1 MW of peak power for the 40-fs pulses. After the sample was exposed to 4000 shots from the laser, etching with diluted hydrofluoric acid for a number of minutes selectively removed the modified material in the submicron-size tapered region.
This scanning-electron microscope image shows two holes in fused silica separated by 1.4 µm. The researchers created them using a femtosecond laser and a chemical etching process. The cause of the ridges inside the holes is not clear but may be related to the etching process.
In one such run, the two 750-nm-deep holes were separated by approximately 1.4 µm (see figure). In this case, the average laser power was 75 mW at 100 kHz, or a peak power of 20 MW.
The scientists said that the 1.4-µm spacing between the holes is not the limit of the fabrication technique. They are investigating approaches to reducing the diameter and spacing of the holes, and in recent results, they obtained 0.5-µm-diameter holes separated by 1 µm.
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