Plasmons, Shiny Metals, and Stained Glass

Remember plasmas, the phase of matter where atoms are ripped apart into electrons and nuclei? Plasmas are primed for strong electromagnetic interactions with the world around them, because of all the loose charged particles. They can be used to etch down into surfaces and catalyze chemical reactions, though the ions in a plasma won’t necessarily react with every form of matter they come across. And you can actually use an electromagnetic field on its own to contain a plasma, because of the plasma’s sensitivity to electromagnetic force. The most common design for a fusion reactor, the tokamak, uses a doughnut-shaped field to contain a plasma.

That’s how plasmas work at the macroscale, but it’s the individual charged ions in the plasma which react to electromagnetic force. Their interactions sum to a larger observable phenomena, which emerges from nanoscale interactions. But interestingly, the collective interactions of these ions can actually be approximated as discrete entities, called quasiparticles. We’ve talked about quasiparticles before, when we talked about holes which are quasiparticles defined by the absence of an electron. But in plasmas, the collective motion of the ions can also be considered as a quasiparticle, called a plasmon. Each individual ion is responding to its local electromagnetic field, but the plasmon is what we see at a larger scale when everything appears to be responding in unison. A plasmon isn’t actually a particle, just a convenient way to think about collective phenomena.

Plasmons can be excited by an external electromagnetic stimulus, such as light. And actually, anyone who has looked up at a stained glass window has witnessed plasmonic absorption of light! Adding small amounts of an impurity like gold to glass results in a mixture of phases, with tiny gold nanoparticles effectively suspended in the amorphous silica that makes up glass. Gold, like many metals, has a high electron density, and the electrons effectively comprise a plasma within each nanoparticle. When light shines through the colored glass, some wavelengths are plasmonically absorbed and others pass through. Adding a different metal to the glass can change the color, and so can different preparations of the glass that modify the size of the included nanoparticles. So all the colors in the window shown below are due to differing nanoparticles that plasmonically absorb light as it passes through!

Now you might ask, what determines which wavelengths of light pass through and which don’t? In the case of stained glass, it has to do with the size of the nanoparticles and the metal. But more generally, plasmas have a characteristic frequency at which they oscillate most easily, called the plasma frequency. The plasma frequency depends on several fundamental physical constants, including the mass and charge of an electron, but notably it also depends on the density of electrons in the plasma. For nanoparticles, the size of the particle also affects the response frequency. The practical upshot of the plasma frequency, though, is that if incident light has a frequency higher than the plasma frequency, the electrons in the plasma can’t respond fast enough to couple to the light, and it passes through the material. So the material properties that dictate the plasma frequency also determine whether light will be absorbed or transmitted.

For most metals that aren’t nanoscale, the plasma frequency is somewhere in the ultraviolet range of the electromagnetic spectrum.  Thus, incident visible light is reflected by the free electron plasma in the metal, right at the surface of the material. And that’s why metals appear shiny!

Plasma Displays and Plasma Physics

You may have noticed one big technology missing from my recent post on how displays work: I didn’t talk about plasma displays! I wanted to have more space to discuss what plasma actually is before getting into the detail of how the displays work, and that’s what today’s post is about.

Plasmas are usually considered a state of matter. But whereas order and density distinguish the other states of matter from each other—solids are dense and ordered, liquid are dense and disordered, and gases are disperse and disordered—for plasma there is another factor that is important. Plasmas are disperse and disordered like gases, but they are also ionized. Whereas a non-ionized gas consists of atoms, in an ionized gas the negatively charged electrons have been stripped from the positively charged atomic nuclei and both are moving freely through the gas. The electrons and nuclei are both called ions, to indicate that they carry a charge. Remembering the attractive force that oppositely charged particles experience, it might seem like a plasma would be pretty short-lived! Electrons and nuclei form stable atoms together because that is a low-energy configuration, which means it’s very appealing for the plasma to recombine into regular atoms. And in fact that’s what happens if you let it cool down, but if you keep the plasma temperature high, the ions are more likely to stay separated. In fact, how many of the atoms are ionized depends roughly on the plasma temperature. Hotter plasmas often have nearly all of their atoms broken apart and ionized, whereas cooler plasmas may be only partly ionized. But the more ions you have, the more electromagnetic interactions occur within the plasma because of all the free charge, and this is what makes plasmas behave differently from non-ionized gases.

A hot gas of ions may sound somewhat removed from the quotidian phases of solid, liquid, and gas. But actually, plasma is the most common phase of luminous matter in the universe, prevalent both in stars and in the interstellar medium. (I say luminous matter here to distinguish from dark matter, which seems to make up more total mass than the matter we can see, and whose phase and nature are both unknown.) There are also lots of examples of plasmas here on Earth, such as lightning bolts, the northern lights, and the neon that lights up a neon sign. You may have noticed that these are all light-emitting phenomena;  the high energy of the ions means that they have many lower energy states available to jump to, and those energy level changes often involve emitting a photon to produce visible light.

So how can plasma be controlled to make a display? Illumination comes from tiny pockets of gas that can be excited into a plasma by applying an electrical current, and each pixel is defined by a separate plasma cell. For monochrome displays, the gas can be something like neon which emits light that we can see. But to create multiple colors of light, various phosphors are painted in front of the plasma cells. The phosphors absorb the light emitted by the plasma and emit their own light in a variety of colors (this is also how color CRT displays work). Plasma displays tend to have better contrast than LCDs and less dependence on viewing angle, but they also consume quite a bit of energy as you might expect from thinking about keeping the ions in the plasma separated.

There are a lot of other cool things about plasmas, like how they can be contained by electromagnetic fields and how they are used in modern industrial processing to etch semiconductors and grow nanomaterials. But for further reading I definitely recommend the wikipedia article on plasmas.