This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is a test of copying the entire original post, from October 2018, including the images that appeared with it.

The 2018 Nobel Prize in Physics was announced this morning “for groundbreaking inventions in the field of laser physics.” This is really two half-prizes, though: one to Arthur Ashkin for the development of “optical tweezers” that use laser light to move small objects around, and the other to Gérard Mourou and Donna Strickland for the development of techniques to make ultra-short, ultra-intense laser pulses. These are both eminently Nobel-worthy, but really aren’t all that closely related, so I’m going to split talking about them into two separate posts; this first one will deal with the Mourou and Strickland half, because I suspect it’s the less immediately comprehensible of the two, and thus probably needs more unpacking.

What Mourou and Strickland did was to develop a method for boosting the intensity and reducing the duration of pulses from a pulsed laser. This plays a key role in all manner of techniques that need really high intensity light, from eye surgery to laser-based acceleration of charged particles (sometimes touted as a tool for next-generation particle accelerators), or really fast pulses of light such as recent experiments looking at how long it takes to knock an electron out of a molecule. This kind of enabling of other science is exactly the kind of thing that the Nobel Prize ought to recognize and support, so I think this is a great choice for a prize.

The technique Mourou and Strickland invented, as part of Strickland’s thesis research, is called “chirped pulse amplification,” and relies heavily on one of the central facts of wave physics, namely that making a pulse with a short duration in time requires a wide spread in frequency (and vice versa). They exploit this frequency spread in a clever way to circumvent the limits imposed by the fact that too-high intensity can damage the crystals used to amplify laser pulses.

But why does a short pulse need a wide range of frequencies? You can see this by looking at what happens when you start adding pulses of light with slightly different frequencies. The figure below shows a single frequency wave at the bottom, with combinations of 2, 3, and 5 slightly different frequencies above .

When we add a second frequency, the single smooth wave breaks up into a series of “beats,” regions where there’s some wave behavior separated by regions where the two different frequencies cancel each other out. This is a familiar phenomenon to anybody who’s ever tried to tune two similar musical instruments: when they’re trying to play the same note but not quite in tune, you hear an irritating pulsing tone, that gets slower as you bring the two into tune.

As you add more frequencies, the general beat structure remains, but the width of the region with obvious wave behavior gets smaller. This is a very general phenomenon relating to waves, and applies to anything with wave nature: sound waves, light waves, even the matter waves associated with quantum-mechanical particles. If you add together lots of waves with slightly different frequencies, you end up with a series of narrow pulses where you see intense wave activity, separated by wide regions where not much is happening.

You can use these physics to make a pulsed laser pretty easily, simply by finding a gain medium that will amplify light over a broad range of frequencies. One common such medium is titanium atoms embedded in a sapphire crystal, which will let you amplify light over a range extending from the visible spectrum across a huge swathe of the near infrared. Bang one of these Ti:sapph crystals in between two mirrors, pumps some energy into the system, and you can get a “mode-locked” laser in which the presence of a bunch of different frequencies of light leads to short pulses of light with each pulse containing a wide range of frequencies.

You can look at these in two complementary ways: one measurement you can make is to look at the overall intensity as a function of time, in which case you see a very short pulse. The other is to look at the intensity as a function of frequency, in which case you see a wide spread of different frequencies, each with a little bit of intensity.

If you want to make a really intense ultra-short laser pulse, you quickly run into the problem that there’s only so much any one amplifier crystal can take. When the intensity of light gets to great, the material can be damaged, and that limits what you can do with one of these lasers.

This problem seems insurmountable if you think of the pulse only in the intensity-versus-time sense, when the amplifier is getting All The Frequencies at once. If you look in intensity-versus-frequency, though, you can see that none of the individual frequencies contributing to the pulse have enough intensity on their own to pose a problem. So, the trick Mourou and Strickland figured out is to separate those out, so the amplifier has to deal with only a few frequencies at a time.

There are several waves of doing this– the cartoon provided to the media by the Royal Swedish Academy of Sciences (all the way up at the top of this post) shows it being done with a pair of diffraction gratings to spread out the different frequencies so that some follow a longer path than others. You can also send the pulse through a length of optical fiber, in which some frequencies of light travel faster than others (this is why you can use glass prisms to study the spectrum of light or make iconic album covers). Either way, you end up with a longer laser pulse in which the high-frequency light arrives first while the lower-frequency light straggles in some time later. This is referred to as a “chirped pulse,” because the chirp of a bird has the same sort of frequency structure: high frequency at the start, low at the end (or vice versa).

The chirped pulse gets you a longer duration, but more importantly, it spreads out the intensity so that it’s always below the damage threshold for the amplifier. Then you can safely boost the intensity of each of the individual frequencies in the pulse, which leaves you with a more intense but longer pulse. Then you just reverse the chirping process, using a pair of diffraction gratings to make the high-frequency light on the leading edge travel a slightly longer path than the low-frequency light on the trailing edge, in such a way that all the frequencies arrive at the same time, but now with many times the intensity.

Careful use of this technique can get you pulses of femtosecond duration with crazy intensities– 10^25 watts per square centimeter or thereabouts. These enormously intense fields can do all sorts of violent and interesting things to matter, opening a huge range of possibilities.

Chirped-pulse amplification is one of those extremely clever tricks that’s easier to say in words than to do in the lab, so it’s impressive that Mourou and Strickland were able to make it work. And having demonstrated it, lots of other people started imitating and refining the technique, which has found applications all over physics, and even in medicine. For that, they richly deserve the Nobel Prize.

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There is, of course, another important feature of this year’s prize, namely that Donna Strickland is the first woman in 55 years to share a Nobel Prize in Physics, and only the second woman not named “Marie Skłodowska Curie.” So, in addition to being excellent and prize-worthy science, this award is also a long-overdue step toward correcting that shameful history. I focused on the physics above, because there’s no shortage of coverage hitting the “first woman in far too long” angle, but it’s an important development, so I can’t not mention it.