At the Central Laser Facility, our high-power lasers have reached powers as large as 10,000 times the output of the National Grid. This incredible capability is increasing even further with developing projects such as Vulcan 20-20 and EPAC! But how do we safely reach and sustain these huge intensities? The answer lies in a brilliant technique created by Canadian laser physicist and Nobel Prize winner Donna Strickland.
Strickland began her career with a degree in engineering physics at McMaster University, hoping to learn more about lasers and electro-optics specifically. In her class, she was one of only three women in a class of twenty-five. She then pursued a Doctorate in Philosophy at The Institute of Optics (University of Rochester). Together with her supervisor Gerard Mourou, her doctoral project was focused on solving a key problem in high-power laser physics.
High-power lasers create powers on the order of gigawatts, terawatts, and even petawatts (10^15 W). This allows them to replicate some of the most extreme conditions in our Universe. However, if a laser reaches intensities in the realm of gigawatts per square centimetre and above, it tends to ‘self-focus’, which damages the components used to amplify (add power to) the laser. This previously set a limit on how far we could amplify a laser, even though it was theoretically possible. Strickland and Mourou’s project set out to find a way of amplifying the laser without the looming concern of this damage.
Strickland at the Ground Breaking Ceremony for EPAC at the CLF.
In 1985 they published their proposal – Chirped Pulse Amplification (CPA).
A laser is defined as a coherent beam of a single colour of light. However, even one colour has a tiny spectrum inside of it - a mini-rainbow if you like. CPA works by spreading this small spectrum in space, much like a prism creates a rainbow. If done correctly this also stretches the laser pulse in time. This stretch maintains the same energy in the laser pulse but reduces its power by increasing the time the energy is spread over. This process is called a ‘chirp’. Once this is done, we can add more and more energy to the laser, amplifying whilst still at a lower overall power.
CPA then compresses the laser pulse back to its original length, returning it to near the original timeframe and boosting the laser power. All that energy gained through the amplification stage is now compressed into an astronomically powerful pulse of light.
It’s this innovative technique that has allowed us to achieve high powers (now petawatts) here at the CLF with lasers such as Vulcan and Gemini and now EPAC. Thanks to CPA we can recreate supernovae in the lab, accelerate particles in a matter of centimetres, and fuse atoms with laser power.
The method has also made such systems smaller, helping to bring forward a new era of laser science with ‘tabletop terawatt lasers’. And aside from its significance in groundbreaking fundamental physics, the technique allows for ultra-precise beams which are used in micromachining tiny parts such as those in your phone, or even in laser eye surgery to cut a patient’s cornea.
CPA earned Strickland and Mourou the Noble Prize in 2018 (shared with Arthur Ashkin for optical trapping). This made Strickland the third women ever to be awarded the prize in Physics, and the first in 52 years.
Since her work on CPA, Strickland has worked with the National Research Council of Canada, Princeton University, and the University of Waterloo, where she became their first-full time female Professor in Physics. In 2020, she visited the CLF to host the Ground Breaking Ceremony for EPAC! At this event, which tied in with International Day of Women and Girls in Science, Strickland said, "Science education helps develop skills in problem solving and critical thinking necessary to address some of the world's biggest challenges... when we encourage girls and women to engage with science, they bring more diversity to science and fresh perspectives that can only help in finding innovative solutions."
Currently, she is leading an ultrafast laser group that investigates non-linear optics and is pushing the field into the infrared and ultraviolet regions of light. She is also investigating how high-power lasers could be used to cure presbyopia, where the eye loses the ability to focus on near-field objects.
