Thu. Mar 23rd, 2023
Crystal growth, imaged using a CARS microscope.
Enlarge / Crystal growth, imaged using a CARS microscope.

Research, like any other human endeavor, is 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 moment because it’s cool and everyone should be able to talk about photonics with their older relatives in a knowledgeable way.

Photonics involves carefully structuring materials to bend light to the will of the experimenter. But photons don’t always cooperate. They’re a bit like ants – while one photon doesn’t do much, several photons will take all of your breadcrumbs and threaten the honey, and the whole photon colony will repossess your fridge, including the contents. In other words, photonics labs are filled with the burnt remains of experiments because careless researchers have ramped up 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 developed by the photonics community. Until now, that is.

Imagine a spherical photon

The idea behind photonic crystals originates from the behavior of electrons in crystals. Think of it this way: the wavelength of light is quite long, while the distance 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 atoms spread over a single wavelength. The effect of the atoms on the light wave is called the refractive index, which allows us to focus light and the like, but not much more.

By comparison, electrons have a wavelength roughly equal to the distance between atoms, so they “feel” each atom. When an electron tries to travel through a material, it will scatter from atoms. And because it is also a wave, it will interfere with itself and other electrons. The result is that electrons of certain energies don’t exist in some materials because they interfere destructively 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 fundamental phenomenon is at the root of the entire electronics industry.

A photonic crystal forces light to experience the same phenomena that electrons undergo. We do this by physically structuring a material. For example, if you drill holes in a material every hundred nanometres, the light will be scattered by these holes. The distance between the holes is approximately equal to the wavelength of light, so each photon will interfere with itself. Therefore, 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 example, a fluorescent dye contained in a photonic crystal changes the color it emits so that its wavelength is above or below the photonic band gap.

As with electronics, the idea is to create defects in the crystal that create interesting things. For example, these defects can direct light through very tight corners, restrict light to highly localized areas, and all sorts of other things. But a consequence of limited light is that it is very bright in the area where it is limited. If it’s 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 make 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. That material is a plasma, which is created when you rip electrons from their atoms. The charged ions and electrons then float around each other, creating currents, electric and magnetic fields and all sorts of interesting effects.

In this case, researchers propose using what we call “a big laser” in the field to generate a textured plasma. A light pulse is split in two and both pulses are sent in opposite directions in a plasma. Where the pulses collide, they interfere to create a series of bright and dark spots. Not much happens in the light or dark spots. But in the transition region between the two, the sharp change from dark to bright drives the electrons toward the bright regions.

As a result, the plasma has large variations in electron density on the scale of the wavelength of the light pulses. And since the material used to make the density variations has already been decomposed, you can use it to manipulate a laser of whatever power you want.

The transient photonic crystal is used to manipulate a third laser pulse that strikes the plasma. But this won’t last long. Plasmas behave like very strange liquids. And, as you may have noticed, it is very difficult to prevent a liquid from evening out its density variations. In addition, as the electrons lose energy, they recombine with the surrounding ions to create neutral atoms. So you make your photonic crystal, use it immediately, let it fade, and then recreate it when you need to.

The 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 you can even imagine faulty structures forming that guide a highly energetic laser pulse around a corner or briefly trap it at a location in the crystal.

Destroy the indestructible

A strange 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 can prove to be quite useful. Powerful laser pulses are very hard to make, and one of the problems is that you often end up with a main laser pulse – the one you’re 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 transient photonic crystals, we can separate the two pulses. The unwanted pulse bounces off the crystal because we choose the spacing between high-density regions so that the wavelength of light experiences destructive interference. By reflecting off the photonic crystal, it also destroys the structure, allowing the main pulse to continue unaffected. And because it’s all done through a plasma, you can do it over and over again with no problem.

While I’m in it purely for the wow factor, this isn’t just a cool idea – it has some practical uses too. While most of those applications will be purely scientific, I imagine manufacturers of lasers for welding and other power applications will eventually see some interesting ways to use this idea as well.

Physical assessment letters2016, DOI: 10.1103/PhysRevLett.116.225002

By akfire1

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