The science of laser materials processing can be summed up as pointing a laser at something and seeing what happens. What happens, though, can be any of a number of things, from melting the material to vaporizing it completely. Since lasers were first developed, one of the challenges has been to refine them so that they can be used precisely and cause as little damage as possible to the surrounding material. A research team led by physics professor Robin Marjoribanks has recently developed a technique to do just that, with applications ranging from surgery to fuel injectors.

Part of Marjoribanks’ research is studying how ultrafast lasers interact with matter, like glass or organic tissues. Whether a laser cuts, melts, or does nothing to the material depends on how the laser delivers its energy. The fluence of a laser (the energy delivered to a material per unit area) can depend on several factors, like the intensity of the laser, its wavelength (manifested in the laser’s colour), how long the laser is shone on the material and the shape of the pulse.

These last two variables have become important with the development of ultrafast pulse lasers—those that emit picosecond (one trillionth of a second) pulses at short intervals (nanoseconds, or one billionth of a second apart), as opposed to continuous train lasers, which emit light at a constant rate.

Materials processing at the speed of light

The birth of ultrafast lasers really happened in the late eighties, when researchers discovered a phenomenon called magic mode locking. “They called it magic mode locking ’cause at first they didn’t know how the heck it worked,” said Marjoribanks.

The researchers made the discovery while trying to build a tunable laser—one whose wavelength (and therefore the colour) could be changed.

“They aligned the laser, and it turns out that when they aligned the laser, they didn’t do a perfect job, and so it didn’t quite lase, but then there was a vibration in the room, knocking on the table or something, [which caused] a little fluctuation in one of the mirrors, so for a second the laser was aligned, and the light began to build up.”

When the laser was back out of alignment, it should have stopped, but instead the high intensity light that had built up while the laser was aligned kept going to give a high intensity pulse of light.

The phenomenon was caused by refraction, the bending of light when it hits a different medium. The intensity of the light was so high—about 100,000,000 watts/sq. cm (compared with sunlight, which is about .05 watts/sq. cm)—that the index of refraction became larger. When the high energy pulse passed through a prism, it bent at a steeper angle and aligned the laser. This technique allowed the researchers to isolate one high-intensity pulse to focus on materials.

It all comes back to fluence—the way energy is delivered to the material. In a continuous train laser, energy is often delivered in a lower-intensity constant stream. This has caused difficulties in the development of lasers for medical uses, especially for cutting precise layers of tissue. With traditional lasers, by the time a hole has been cut into the tissue, the energy from the laser has dissipated in the form of heat into the surrounding cells, damaging them and causing swelling. By contrast, with ultrafast lasers, the energy delivered to the cells vaporizes them, while very little energy is converted into heat.

Marjoribanks recalls one story from the University of Michigan, where a researcher was working with a very powerful laser. “The researcher at one point looked down to see whether the laser was working properly, and he got eye damage from that. His eye filled up with blood, and they whipped him off to the Kellogg Eye Institute.

The resident who was on duty said, ‘Wow, this is really cool. You should see what your retina looks like. There’s this beautiful cut across your retina.’”

Lasers for the new millennium

At U of T, Marjoribanks’ research group is studying the use of pulsed lasers in photonics research. In 1997, Anton Oettl, an undergraduate exchange student, was using a pulse train laser to drill holes in glass. Unlike the ultrafast laser described above, the one at U of T emits a series of high-intensity picosecond pulses at intervals of several nanoseconds. Researchers then select one pulse to shine on the material being processed. One of the limitations of using single pulses on glass is that after about three pulses the glass will crack. This is a problem, since it means that there is a limit to how deep you can drill a hole into glass before it cracks.

Because the holes being drilled are very, very small (usually on the scale of a micrometre, or a millionth of a metre) the researchers decided to create landmarks in the glass. “Instead of selecting one tiny little pulse, we just let the whole pulse train run, and we figured it would blast the heck of the material, and it would be really ugly,” Marjoribanks said.

The actual results were surprising. “Instead of it shattering the glass all to pieces, it turned out to make this really nice hole.”

The hole that the researchers had made turned out to be deeper and more regular than the ones created with single pulses.

Marjoribanks believes the speed of the pulses didn’t give the glass time to cool down, and the heat that the laser leaves behind between pulses softened up the glass, making it far less likely to crack. He theorizes that this ability to drill precise and relatively deep holes might have applications in things like fuel injectors or photonics circuits (which use light rather than electricity to work).

As well as photonics research, Marjoribanks has also been working in collaboration with Dr. Lothar Lilge, a biophysics professor at U of T, to find biological applications for the technique that will allow for less invasive procedures.

“This way of delivering fluence, we think, is a new wrinkle in the way in which ultrafast lasers can produce remarkable results,” he said.