Ultra Cool, Ultra-fast Lasers

Learn how ultra-fast lasers help scientists observe ultra-fast events on an Alumni Weekend tour.

They may sound like something out of a science-fiction movie, but ultra-fast lasers are real and they're here at the U of A. Writer Hayley Dunning talks to Dr. Frank Hegmann, about his fascinating Alumni Weekend physics tour "Ultra-fast Lasers: Observing nature in a trillionth of a second."

Frank Hegmann

"They're like hyped-up green laser pointers. They work on the same principle except that these are much more expensive, and much more powerful."

Frank Hegmann is describing two of the laser sources in his ultra-fast spectroscopy lab. The green glow gives way to a hint of red as a crystal ramps up the laser to the infrared part of the spectrum; a terahertz pulse. Such intense light is needed to test the response of nano-materials over extremely short periods of time: picoseconds. Picoseconds are one trillionth of a second-one picosecond is to one second as one second is to 32,000 years. Firing two pulses of the laser picoseconds apart produces an "excitation pulse" and a "probing pulse"-the first entices electrons to move and the second detects how they have moved through the material.

"The pulse moves like a motorboat in the water; the track it leaves behind is another wavelength of light that travels at a different speed, just like waves in water travel slower than the motorboat travelling in it. You create essentially a shockwave," Hegmann says.

Materials can be probed at any wavelength, and Hegmann's lab contains a dizzying array of mirrors and lenses arranged to tune the laser, in a technique called ultra-fast laser spectroscopy. Focusing the beam, like the sun through a magnifying glass, creates an intense spark that hangs in the air like a small thunderbolt. De-focusing it can create light any colour of the rainbow. Cranking up the intensity, Hegmann says, is like cranking up the volume on a stereo system. At a certain volume, the speakers no longer respond as you would expect; they reach the edge of their range and create reverberations and new frequencies. In the same way, exciting electrons with an ultra-fast laser source pushes them to the edge of their fields and causes them to behave non-linearly.

The technique is used to fundamentally understand how excitations occur in materials, what path they take, and how they return to their original state. Such methods can be used for example to track the path of electrons in solar cells, and only by understanding their behavior at the nano-scale can we hope to improve their efficiency.

The lightning bolt spark of a focused laser beam.

However, Hegmann hopes to push the use of ultra-fast lasers even further. By coupling them with scanning probe instruments, the best optic tools for imaging materials at the molecule-scale, he and his team hope to be able to look at the response of materials at a resolution never realized before. Imaging tools have very good spatial resolution; they can see individual atoms, but poor temporal resolution; they are slower and cannot watch the sequence of excitations in a material unfold. The ultra-fast lasers have the opposite problem, so linking the two together could allow researchers to record the movement of pulses across a single nano-crystal, rather than a film of crystals currently imaged by ultra-fast laser spectroscopy.

This is what Hegmann's new lab, the $3 million ultra-fast nano-tools lab, is attempting to do, and after a few years of set-up, is starting to produce promising results. The set-up is unique in the world, with several laser sources running at the same time sending pulses to different optical instruments. This approach will hopefully discover which instrument could be successfully coupled with the laser sources and produce images of material changes at the molecule scale and picosecond range. Already there have been some hints that the technique could work using a scanning tunneling microscope, and excitement is growing among the team.

"What we had planned to do in the proposal is starting to take form, which is amazing - because we weren't even sure it was going to work. This is trying to push the frontiers of what can be done. If we can do this it would be the first time it's ever been done."