College of Engineering News

As with any good event, we hope to have open communication while hosting Art in Science. In short, we will do our best to maintain transparency- and what in our lives is iconic for being transparent so much as glass?

green beads

Dichroic beads, Wikimedia

Glass is a material that has a wide variety of uses, from the practical drinking glasses to beautifully crafted opalescent stained glass windows to optical fibers. Today’s post focuses on glass with dichroic optical properties : where the glass has two (“di”) or more distinct behaviors depending on the angle or color of light with which it interacts, usually relating to the color (“chroic”) that can be observed.

Dichroic Glass

Dichroic glass is named because the glass changes colors depending on the lighting conditions. In modern times, this effect is created using the same metals, metal oxides and/or nitrides that are used to color glass, but in a different treatment. Instead of incorporating the coloring agents into the bulk glass, thin layers of coloring agent are applied to the glass. Because there are multiple nanoscale layers, interactions of dichroic glass with different angles of light will create different colors, giving the glass a distinct iridescent quality.

Schematic of Filter Cube

Filter Cubes in Fluorescence Microscopy

The most common scientific application of dichroic optical effects is in fluorescence microscopy as filter cubes. Filter cubes in fluorescence microscopy are composed of two filters and a dichroic mirror. Similarly to dichroic glass, a dichroic mirror is made by applying thin layers to a glass.

During fluorescence microscopy the light enters through the first filter, known as an excitation filter, which only allows one color of light to pass. The excitation filter is calibrated to the color of light with which the sample will have the greatest interaction, or excitation. Then, this first color of light bounces off of the dichroic mirror and interacts with the sample. The sample momentarily absorbs this light, dissipates some of the light energy, and emits the light with a lower energy color. This second color of light is separated from other light by the second filter in the filter cube, known as the emission filter, passes through the dichroic mirror, and can then be observed by the microscope’s user. Dichroic mirrors make fluorescence microscopy possible because of their ability to selectively reflect and transmit different colors of light.

The Lycurgus Cup

Although it was not created using thin films, the most famous piece of glass with a dichroic optical effect is the Lycugrus Cup which is on display at the British Museum. The Lycurgus Cup is a carved glass cup from the Roman era depicting the myth of King Lycurgus who expelled the god Dionysus from his kingdom and persecuted his followers. One of Dionysus’ followers is later transformed into a vine that traps Lycurgus. While the history is very interesting, the most notable feature of the Lycurgus cup is the dichroic optical effect.

red and green cup

The Lycurgus Cup viewed under transmitted (left) and reflected (right) light, British Museums

When viewed under reflected light the Lycurgus cup appears green. Conversely, when the Lycurgus is viewed under transmitted light it appears red. This effect is a result of incorporating trace nanoscopic gold and silver particles in the glass when it was forged. While this effect is spectacular, it is under debate whether the incorporation of nanoparticles was intentional or merely a happy result of contamination. Mistakes or otherwise, glass has been used in expressions of art throughout history, but today, we have viewed it in a different light.

Until next time.