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Posted on 01/07/2021 by Benjamin Skuse

Illuminating Laser Physics

Pobytov/iStockimage

Despite the Agora Talk with John Hall, Gérard Mourou and Donna Strickland on day 4 of #LINO70 being plagued by almost every videoconferencing difficulty under the Sun, it was worth persevering. This was because the audience was treated to a rare insight into how two important but seemingly disparate areas of laser physics are in fact intimately linked.

Though Hall, Strickland and Mourou all come from the laser physics community, their fields of research couldn’t be farther apart. Strickland and Mourou shared half of 2018 Nobel Prize in Physics for jointly inventing a new technique to create ultrashort high-intensity laser pulses in the 1980s. Called chirped pulse amplification, the technique stretches short laser pulses to reduce their power, then amplifies them, and finally compresses them back down. Chirped pulse amplification is used today in everything from proton therapies that treat deep-tissue tumours to machining the cover glass used in smartphones.

Meanwhile, Hall and Theodor Hänsch were awarded half the 2005 Nobel Prize in Physics for their fundamental contributions to the development of laser-based precision spectroscopy, in particular Hall’s invention of the optical frequency comb around the year 2000. An optical frequency comb is a special laser source originally developed to count the cycles from optical atomic clocks and used more broadly in high-precision metrology.

“High-intensity laser physics is all about making as short a pulse as possible,” said Strickland. “This is the opposite of high-resolution spectroscopy.” So what do the two fields have in common? The mode-locked laser.

Short Bursts of Light

As our high-school physics teachers taught us, laser light is unlike any natural light. Lasers produce an intense light beam where all the light waves have similar wavelengths and all their peaks are lined up, or in phase. Within the laser’s resonant cavity, the light bounces between opposing mirrors, leading to the formation of standing waves or modes. A regular laser allows these modes to oscillate independently from one another. In contrast, a mode-locked laser forces each mode to operate with a fixed phase between it and the other modes. As a result, these modes periodically constructively interfere with one another, allowing the laser to produce pulses of powerful laser light of short duration.

Laser
Gérard A. Mourou, Donna Strickland and moderator Rainer Blatt

The mode-locked laser was the obvious starting point for Mourou and Strickland to go on to produce the shortest and most intense laser pulses up to that time. But for Hall, its relation to his specialism –precision spectroscopy – was far from clear initially.

In his talk, Hall described the situation in 1999, when his team was “trying to make the lasers the most stable that they can possibly be. […] But then there were people such as Gérard Mourou who were saying ‘let’s make the laser as rapidly changing as possible’.” And this got Hall thinking.

Using a similar type of mode-locked laser as Mourou and Strickland had used in their work, Hall built a spectrum of light split up into a very fine series of pulses at equally spaced intervals of frequency that resembled the teeth of a comb. An unknown frequency could then be determined by relating it to one of the frequencies along the ‘frequency comb’.

As Strickland neatly summarised: “Whether you want a short pulse or you want all the pulses so that you can have this great frequency comb, you’re going to start with the mode-locked laser”.

Extreme Light for Thorium Power

Changing tack, Mourou’s talk aimed to show how ‘extreme light’ derived from the latest high-intensity laser physics research can help the planet in transitioning from fossil fuel-based energy to a cleaner way forward. “The biggest existential problems that this planet is facing is a problem of energy,” he said. “We’ve found a way to solve it by using a very, very short burst of light which is in the femtosecond regime; a millionth of a billionth of a second.”

These short pulses could be used to produce clean and efficient thorium-based nuclear energy. Using a GW power plant as an example, Mourou said that 3 million tonnes (or equivalently, 100 trains with 100 cars) of coal are needed to power a big city for a year. This drops to 300 tonnes if using uranium. But a thorium-based power plant would only require 1 tonne of raw material.

What’s more, a thorium nuclear power plant would not have to be based on traditional nuclear fission. “By using this phenomenally big light pressure, you can do fission or fusion,” he said. “Using extreme light, I think we have now a new way to really look at energy production in a safe and abundant way.”

Benjamin Skuse

Benjamin Skuse is a professional freelance writer of all things science. In a previous life, he was an academic, earning a PhD in Applied Mathematics from the University of Edinburgh and MSc in Science Communication. Now based in the West Country, UK, he aims to craft understandable, absorbing and persuasive narratives for all audiences – no matter how complex the subject matter. His work has appeared in New Scientist, Sky & Telescope, BBC Sky at Night Magazine, Physics World and many more.