The Irony of the Moth Eye
As promised in my previous post, this time I will actually delve into the nitty gritty of my research.
These days, many scientists are learning about complex topics in science by studying biological features of organisms we find in nature. For example, my research group has studied tiny microscopic surface structures found on the eye of a moth to learn more about light-matter interactions.
Indeed, this creepy little guy has a lot to teach us; and you could say it’s all in the eyes. It turns out that surfaces with these little bumps, which I will hereby just call motheyes, cause surfaces to transmit lots of light and reflect very little of it. That’s why moths have them in the first place: they can see better at night, and they won’t reflect glint back that predators could see. The Gordon group has developed a surface modification protocol, based on colloidal lithography and plasma etching, to create synthetic motheyes on inorganic substrates such as silicon. We can thus produce very useful, highly transmissive optics.
My research poses a different view on transmittance. Instead of shining light through a surface, let’s instead consider when light is emitted from within the surface in the form of thermal (blackbody) radiation. This is the radiation of light given off by any object above 0K; hotter objects, like red-hot iron, give off lots of visible light. Cooler objects, like human bodies, give off weaker infrared radiation, as does a silicon wafer at, say, 250ºC. Because of the same properties that lend motheyes high transmissivity, it stands to reason that adding motheye structures to silicon surfaces would allow more thermal radiation to “escape” from the surface of the object. What I aim to answer in my project is exactly how much of a difference in radiation we see, and if we observe preferential emission at certain wavelengths – a phenomena we observe with transmission. To determine these things, we make measurements on how much IR light is given off by structured and unstructured surfaces and compare what happens at different wavelengths.
I made a device that heats up and aligns our sample with our instrument, a Fourier transform infrared spectroscope (FTIR), without picking of the thermal signatures from the heater surrounding the sample or the maximum radiation reference (a jet black cone of graphite that emits close to the most an object theoretically can). Here’s a picture of the final setup:
We measure the spectrum of light coming off of our samples using the infrared spectroscope above on the right, and through what I will hand-wavingly call the “magic of optics and math,” we get a graph that looks like this:
This is a distribution of the intensity of light given off by each sample at different wavelengths. We can use this to characterize the emissive behaviors of different motheye surfaces. This structure, for example, boosts emission of light from about 5um to 15um, but then emits the same amount of light as unstructured silicon.
The ability to not only boost thermal emissions, but boost them selectively with respect to wavelength has some pretty neat implications. Besides a deeper understanding of how light and matter interact, this process can be used in passive radiative cooling, imaging technologies, and – maybe the biggest sell – efficient lighting. And with that said, the irony : technology that will brighten our lives… inspired by an insect that lives in the dark.