Lots of interesting stuff happens really fast. Think about a chemical reaction, for instance. The rate of reactions might be slow, but each individual reaction proceeds quickly. This is because a chemical reaction is, essentially, the shuffling of electrons between different atoms, and electrons are fleet of foot.

Generally, if you want to watch something this fast happen, you use what is called pump-probe spectroscopy, in which one short pulse of light initiates an action while another measures the result. A critical requirement for pump-probe spectroscopy is control over the pulses, something that is difficult to achieve in the X-ray regime. This is why a new paper from Physical Review Letters is a promising development.

Pump-Probe

In pump-probe spectroscopy, the pump is a strong laser pulse that sets a reaction (or action of some kind) in motion. After some delay, a gentler probe pulse measures the state of the thing you just kicked. Repeat this for varying delays between pump and probe and you build up a picture of the trajectory a reaction might take.

For spectroscopy to work, the pump and probe need to be short and sharp, otherwise, you won't know exactly when you kicked the reaction off or when the probe measured the result. Long pulses simply smear the data out, blurring the picture that you are trying to take.

For the types of lasers that I used to work with, you could observe events that only lasted a few dozen femtoseconds (10-15s). Which, frankly, was pretty amazing. For the cool kids who had access to what was then some pretty special kit, events down to a few femtoseconds could be observed.

But none of the lasers that made this possible had the right wavelength to image the structure of a molecule and how that structure changes. That imaging requires lasers that produce X-rays, which is light with a wavelength that is about 1,000 times shorter than visible light. The X-rays bounce off individual atoms in a molecule, and these scattered light waves are added together as they exit the sample.

If the molecules are ordered—ie, if they're part of a crystal—in some directions, the light adds up in phase to create a bright spot. If the molecules are ordered in other directions, light adds up out of phase to create a dark area. The pattern of bright and dark spots is created by the spacing of individual atoms within a molecule, and so the pattern can be used to determine the precise arrangement of those atoms. This is called X-ray diffraction and is used to determine things like the structure of proteins.

Molecular and crystal structures can and do change in response to reactions, which changes this diffraction pattern. But, to see the patterns, you need a very short burst of X-rays. Enter the free electron laser.

Doing the electron wiggle

Every bit of light that exists in the Universe is due to only one thing: the deceleration of a charged particle. Most of this deceleration happens naturally as electrons gain and lose energy, which causes the electrons to absorb or emit photons. This happens within atoms as electrons change how they are distributed around an atomic nucleus. So, for a lot of light we see—from fires or fluorescent tubes etc—the colors (or wavelength) of light is beyond our control.

If you liberate an electron from an atom, though, many more possibilities open up. An electron can be made to emit light by decelerating it, and, if you can control the deceleration, then you can control the wavelength.

This is essentially how a free electron laser works. You obtain a bunch—and I mean that literally—of electrons. Accelerate said bunch to vast speeds so that the electrons have lots of energy. Then pass the bunch through a series of magnets so that the electrons continuously oscillate. These oscillations are a form of acceleration. Each cycle of oscillation corresponds to the emission of a single wavelength of light. The electron emits a photon of that wavelength, loses energy, and slows.

The oscillations are produced by a set of regularly spaced magnets, called a wiggler.

No dancing solo

Arranging the electrons to produce light is not that simple, though. If each electron doesn't emit light in synchrony, then the intensity of the light will never increase. While some electrons will emit together and their light will add up in phase to be brighter, there are just as many electrons that emit out of phase with each other so that their light will destructively interfere. Essentially, if the electrons are not dancing in time, then the X-ray photons emitted by some electrons will be absorbed by others, and nearly no light will come out at the end of your very expensive wiggler.

That is why the electrons are collected into a bunch all traveling with very nearly the same speed. The physical separation between electrons is what determines whether the light is emitted in phase or out of phase. The shorter the separation, the brighter the light will be at the end of the wiggler.

This also means that free electron laser light pulses are naturally rather short, but still as long as 50fs, which is not really short enough. Essentially, the bunch is compact enough to ensure that the radiation is all emitted together to make a bright light source, but not short enough to result in a very short laser pulse.

Your laser judges your dancing

Because electrons repel each other, making the bunch shorter is actually not realistic. So the solution proposed by researchers is to select a subset of the electrons to contribute to the laser pulse.

To do this, they take advantage of a normal laser near the start of the free electron laser. Called the heating laser, it's used to tailor the speed of the electrons in the bunch so that the bunch will stick together throughout the wiggler. Normally, this is just an ordinary laser pulse with the right energy and duration to give the electrons the kick that they need to ensure that they all contribute to X-ray production for the entire length of the wiggler.

But the heating laser can also be used to disrupt the electron bunch and ensure that lasing never occurs. Essentially, the heating laser can cause all the electrons to end up traveling at different speeds so that they can't emit radiation in phase with each other. In this case, the researchers used it to ensure that most of the electrons didn't contribute to the pulse.

The heating laser pulse shape was tuned to select a certain part of the electron bunch to contribute to laser. A simple version of pulse shaping is to modify the laser pulse so that it produces two intense peaks in time (a double pulse). As the electron bunch passes through the laser pulse, the first peak disrupts the energy spread of the front of the electron bunch; the second peak disrupts the back—the middle is left untouched.

The result is that only the middle section of the bunch emits light coherently, while the rest emits and absorbs light randomly, with no net light emission. The researchers tested this method on a real free electron laser. Using a heating laser pulse shape that was more sophisticated than the one described above, the free electron laser pulse was shortened from around 50fs to just 10fs.

One unfortunate consequence of this technique is that, by excluding electrons from contributing to the light output, you reduce the brightness of the laser beam as well. In this case, the output power was reduced to just 15 percent of its normal value. Although these are very bright X-ray pulses, I doubt that they have so much energy to spare that they can afford to throw away 85 percent of it.

This research, however, is in its early days. Gaining control of these short pulses for visible and infrared lasers has led to important new tools for understanding chemical reactions. Pulse shaping and control were keys to understanding the details of photosynthesis, for instance. I expect that similar control over ultrashort X-ray pulses will lead to similar advances. And, we can't leave without a bad pun: there is a bright future in X-ray pulse shaping.

Physical Review Letters, 2016, DOI: 10.1103/PhysRevLett.116.254801