16 Dec 2018

Nobel Lectures

Preceding the Nobel Prize ceremony on Monday December 10th, the Laureates presented their Nobel Lectures on Saturday December 8th in a packed Aula Magna of the Stockholm University.

After the lecture given by René-Jean Essiambre on behalf of Laureate Arthur Ashkin, Donna Strickland entered the stage and presented the audience with an excellent introduction to the basics of laser physics and challenges of reaching high powers. Her lecture was filled with anecdotes, like the one about the start of her PhD research with Gérard Mourou: ‘Gérard gave me a paper by Stephen Harris of Stanford University. He got this idea that lasers were sort of stuck to the infrared. But we wanted this same type of radiation in high energy photons, maybe even into the extreme ultraviolet. He had come up with ideas to increase the amount of photons being grabbed by one atom, leading to the release of a photon with fifteen times the energy. Gérard asked me to think about this paper and see what I wanted to do with it for my PhD. I came up with an idea to come to nine photons being absorbed by nickel. Never got to that idea though. But that was what I needed an intense laser for in the first place.’

Copyright Nobel Media, photo: Alexander Mahmoud

Strickland explained how CPA worked, and what other research she conducted during her time at Rochester University. ‘Because even though inventing CPA turned out to be enough to win a Nobel Prize, I knew I needed to do some actual physics with it in order to obtain my PhD,’ she joked.

Passion for extreme light
After her lecture, Gérard Mourou entered the stage to talk about his passion for extreme light. ‘What strikes me about light is the variety of things you can do with it. You can slow down and cool down atoms, as is done in laser cooling and trapping. But you can also do the exact opposite: you can accelerate particles like electrons and even protons very close to the speed of light.’

In his talk, Mourou showed to have remained the visionary he proved to be in the eighties. ‘What you have to bear in mind is that by using CPA, we can achieve laser powers that represent a light power which corresponds to ten million Eiffel towers on the tip of your finger. That means we are entering new regimes of physics. Currently, we are working on petawatt laser systems. But there is no reason we could not go to even higher intensities. And then light will start interacting with vacuum, creating matter and antimatter out of nothing.’

The French physicist always has had an eye out for applications. Quite literally, as he illustrated with the story on how high-energy lasers entered the field of eye surgery: ‘One day, a student of mine, who is here in the audience today, got a laser beam in his eye. So we went to the hospital with him. The ophthalmologist –- who by the way is also present here today – examined his eye and said: ‘That is strange: the damage in your retina is perfect.’ And that is where lasers entered ophthalmology. To date, tens of millions of eye surgery procedures have been carried out.’

The CPA technique not only enables the amplification of laser light to extreme powers, but it also enables the amplifiers to be much smaller. And that makes extreme lasers very suitable for a whole range of applications. ‘With extreme lasers, we will soon be able to accelerate electrons and protons to the same energies as is done in CERN. With lasers however, we only need about a football field of space to achieve the Tera electron volts. In the future, when we will use solids instead of gases to accelerate the particles from, we will be able to achieve the same level of acceleration on a fingertip.’

Mourou dedicated a large part of his talk to the fields extreme lasers are relevant for. High-power lasers are of great use for science, especially in the fields of materials science, nuclear physics, and laser astrophysics and cosmology. But since laser-driven acceleration of particles enables ‘table-top’ particle accelerators, the technique also has many societal applications, he explained. ‘Think for example of proton therapy to treat cancer. Since protons do not burn the tissue between the radiation source and the tumor, you can destroy cancer cells much better with far less side effects than with conventional radiation therapy. Laser-driven accelerators could also be used to produce radionuclides, either for diagnostic imaging or for local radiation therapies, where the radionuclides are directly injected into the tumor.’

The best is yet to come
The Nobel Laureate ended his talk with an outlook towards a future application he is very passionate about. ‘Together with my colleagues, I am working on mitigating nuclear waste. By shooting neutrons directly into radioactive material, we hope to turn them into different isotopes with much shorter half-lives, decaying much more rapidly. Extreme light is capable of generating the largest fields, the largest accelerations, the largest temperatures and the largest pressures on earth. It carries the best hopes and opportunities for the future of science and society. I am confident that the best is yet to come!’

Watch the Nobel Lectures here: