The field of science that I am in is called plasmonics and has only been a “field” for a couple of decades. Plasmonics, in short, is the study of how nanoscale metal particles interact with light. While small metal particles have been around as long as metal- so, well, a few billion years- our ability to understand and manipulate their properties is a much more recent development. Walk into any scientific presentation on plasmonics and you can make a pretty sizable wager that you will see, nice and large on the introductory slide, this image:
Maybe then, while slightly confused, you might proceed to check the date on your phone to see if you entered into a time warp that spit you out in the middle of December… because why else would a scientist show you a picture of matching holiday-themed wine chalices decorated with what appears to be naked Santa unsuccessfully pruning his unruly garden vines?
Well fear not, it is only August. You still have several months left to finish your holiday shopping. What’s more, that festive pair of goblets is not a pair at all. It is, in fact, the same cup…
The Lycurgus Cup. *cue the ahhhs in three-part harmony*
The Lycurgus Cup has been dated back to the 4th century Roman Empire. It depicts Lycurgus, the temperamental king of Thrace and vehement opposer of Dionysus (a.k.a. Bacchus), the god of wine from Greco-Roman mythology. As the story goes, Lycurgus banishes Dionysus from his kingdom, leading to a series of unfortunate events that sends the land into utter despair and angers the citizens. Dionysus then decides to torture Lycurgus for his bad behavior, with the help of Ambrosia- a nymph who has been transformed into a vine. Thus, we find Lycurgus in the unfortunate situation of being completely tangled in Ambrosia’s herbaceous web while Dionysus schools him silly.
Because let’s face it- anyone who screws with the god of wine is a friend to no one.
The glass cup belongs to a class of Roman containers called cage cups, named due to the raised nature of the illustrations around the cup. The construction of a cage cup during that time would have required remarkable workmanship from only the most skilled glass cutters, so they are thought to have been reserved for very wealthy patrons. Only a handful of cage cups have ever been recovered from ancient Roman ruins, and the Lycurgus Cup stands apart from them in a class of its own as the most technically ornate piece of glasswork to come from that period.
There is the obvious difficulty of chiseling into a glass blank (1) without shattering it and (2) with enough detail to accurately depict the shame on Lycurgus’ face for toying with everyone’s wine distributor. Researchers have already narrowed out the possibility that the cup was crafted using a mold through scanning electron microscope images that detect high energy backscattered electrons off the edges of the cup details. The images can give z-contrast information (they can distinguish between the atomic masses of different atoms) which show mechanical abrasions by metal tools used for shaping. (I couldn’t find a re-postable source of the BSE images, but you can find them, along with a comprehensive study of the cup in this article.)
But more impressive than the expert craftsmanship that was chiseled by hand, wait- let’s just dwell on that for one more second…
WOW. Ok, moving on.
More impressive than the expert craftsmanship of the cup is the medium on which it is crafted on. The glass exhibits dichroism, meaning that it interacts with certain wavelengths of light differently than others, so it can appear to be two different colors. When you shine light on the outside of the cup (such as a flash from a camera), the cup appears green. But if you place the light source inside the cup, the glass appears red. This is caused by the presence of a trace amount of gold and silver nanoparticles embedded in the glass.
When a metal nanoparticle interacts with light, the electrons on the surface of the nanoparticle absorb the energy of the light, causing them to oscillate back and forth. This is called a surface plasmon resonance. Depending on several factors such as size and composition, a metal nanoparticle will only interact with certain wavelengths (i.e. colors) of light. Most metals exhibit these properties when shrunken down to the nanoscale, but gold and silver are very special because they interact with the narrow range of wavelengths in the electromagnetic spectrum that we can detect with our eyes.
The cup appears green when you shine light on the outside of the cup because the gold-silver nanoparticles in the glass respond to the green wavelengths of light around 540 nm. When the plasmon is excited by green light, the electrons slosh around for a tiny fraction (try 10 trillionths) of a second, then they decay and produce a photon of the same color. This cool property of plasmons has led to a wide spread of modern day applications, from cancer therapy agents to engineered plasmonic pixels.
When the cup is illuminated from the inside, you are actually seeing light that is transmitted through the glass because it doesn’t have enough energy to interact with the nanoparticles. Red light has a lower energy (longer wavelength) than green light, so it passes right on through.
So, why is the Lycurgus Cup so rare? Did 4th century glassmakers have a working knowledge of plasmonics and the tunable optical properties of metal nanoparticles? Probably not. Most likely, there were trace amounts of metallic impurities in the sand that was melted down to make glass. Ancient Roman glassmakers often added elemental antimony during heating, which would react with various impurities and give the glass the right opacity. In a serendipitous moment, the added antimony reduced the gold and silver impurities into nanoparticles of the right size, composition and concentration to interact strongly with green light. Scientists today can synthesize the same nanoparticles with relative ease:
But it wasn’t so easy for the ancient Romans. As it turns out, this might have been the only piece of glass made with this degree of plasmonic awesomeness. Other pieces of Roman glass show some dichroism, but none with the radiance of the Lycurgus Cup. From both an artistic and scientific point of view, it really is one-of-a-kind.