Research is like any other human endeavor, as subject to trends and fads as the fashion industry. Everyone wants to jump on the latest new thing. In the world of optics, that means photonics. I'll explain photonics in a second, because it's cool and everyone should be able to talk knowledgeably about photonics to their older relatives.

Photonics involves carefully structuring materials to bend light to the experimenter's will. But photons don't always cooperate. They're a bit like ants—while one photon doesn't do much, several photons carry off all your breadcrumbs and threaten the honey, and the entire photon colony will repossess your fridge, contents included. In other words, photonics labs are filled with the burnt remains of experiments because careless researchers cranked up the laser power.

This is kind of sad, because photonic crystals are incredibly useful, and the world of high power lasers is missing out on all the cool tricks that have been developed by the photonics community. Until now, that is.

Imagine a spherical photon

The idea behind photonic crystals originates with the behavior of electrons in crystals. Think of it like this: the wavelength of light is rather long, while the spacing between atoms is very small. So, when light travels through a material, it doesn't really notice the individual atoms. Instead, it feels the average effect of all the atoms spread out over a single wavelength. The effect of the atoms on the light wave is what we call the refractive index, which allows us to focus light and such-like, but not a lot more.

By comparison, electrons have a wavelength that is about the same as the distance between atoms, so they "feel" every atom. When an electron tries to travel through a material, it will scatter off of atoms. And, because it is also a wave, it will interfere with itself and other electrons. The result is that electrons with certain energies don't exist in some materials because they destructively interfere with themselves.

This creates a band gap. Electrons can have energies above and below the band gap of a material, but not in the band gap. This basic phenomenon underpins the entire electronics industry.

A photonic crystal forces light to experience the same phenomena that electrons go through. We do this by physically structuring a material. For example, if you drill holes in a material every hundred nanometers or so, then light will scatter off these holes. The spacing between the holes is about the same as the wavelength of the light, so each photon will interfere with itself. Hence, certain wavelengths of light cannot exist in the material; if you shine that color of light on a photonic crystal, it will be reflected.

This has some strange effects. For instance, a fluorescent dye, sitting in a photonic crystal, will alter the color it emits so that the wavelength is either above or below the photonic band gap.

As with electronics, the idea is to create defects in the crystal that accomplish interesting things. These defects can, for example, guide light around very tight corners, confine light in highly localized regions, and all sorts of other things. But one consequence of confining light is that it is very bright in the region where it is confined. If it is bright enough, it destroys the material that makes up the photonic crystal.

So the photonic crystal club only has members with very puny lasers.

Destroying a material to create a crystal

To get around this problem, researchers have proposed a way to make a photonic crystal from a material that has already been destroyed. Said material is a plasma, which is created when you rip electrons from their atoms. The charged ions and the electrons then float around each other, generating currents, electric and magnetic fields, and all sorts of interesting effects.

In this case, researchers propose to use what we in the field term "a big-ass laser" to generate a structured plasma. A pulse of light is split into two, and both pulses are sent into a plasma in opposite directions. Where the pulses collide, they interfere to create a series of bright and dark spots. Not a lot happens in the bright or dark spots. But, in the transition region between the two, the sharp change from dark to bright drives the electrons into the bright areas.

The result is that the plasma has large electron-density variations on the scale of the wavelength of the light pulses. And since the material used to make the density variations is already decomposed, you can use it to manipulate a laser with any power you like.

The temporary photonic crystal is used to manipulate a third laser pulse that is incident on the plasma. But this doesn't last for long. Plasmas behave like very strange fluids. And, as you may have noticed, it is very difficult to stop a fluid from evening out its density variations. Furthermore, as the electrons lose energy, they recombine with the surrounding ions to create neutral atoms. So, you create your photonic crystal, use it immediately, let it fade away, and then recreate it whenever it's needed.

A nice thing about this is that it doesn't have to be the same photonic crystal every time. If you change the angle between the two colliding pulses, their wavelength, or their intensity profile, you get different photonic crystals. So one can even imagine creating defect structures that guide a highly energetic laser pulse around a corner or confine it briefly at a location within the crystal.

Destroying the indestructible

An odd property of the photonic crystal is that the laser you want to control with it can also modify the crystal itself, even to the point of washing out the structure. This may turn out to be rather useful. High-powered laser pulses are very difficult to make, and one of the issues is that you often get a main laser pulse—the one you are actually interested in—preceded by an unwanted smaller pulse.

Small is relative in this case: think of the relationship between Jupiter (the main pulse), Neptune (the unwanted pulse), and Earth (the experimental hardware you want to use the laser on). Although the unwanted pulse is small compared to the main pulse, it is still large enough to disrupt your Earth-sized experiment.

Now, if we pass the laser pulse train through one of these temporary photonic crystals, we can separate the two pulses. The unwanted pulse reflects off the crystal because we choose the spacing between high-density regions such that the wavelength of light experiences destructive interference. In reflecting off the photonic crystal, it also destroys the structure, allowing the main pulse to travel through unaffected. And, because it was all done by a plasma, you can do it over and over again without any trouble.

Even though I'm in it purely for the wow factor, this is not just a cool idea—it has some practical applications. Although most of those applications will be purely scientific, I can imagine that manufacturers of lasers for welding and other high-powered applications may eventually see some interesting ways to use this idea as well.

Physical Review Letters, 2016, DOI: 10.1103/PhysRevLett.116.225002