Plasmonic filters in nature — a follow-up

Just a quick follow-up on my previous post depicting surface plasmons. I came across a well prepared pop-science video about butterflies under a scanning electron microscope. Notice how pouring isopropanol over the butterfy’s wings changes the wavelength they reflect. Destin describes the “losing of color” occurring due to light not being able to penetrate the nanoholes of the butterfly, which is partly true due to the reflections by the liquid medium. However, what also happens is that the isopropanol modulates the oscillation frequency of the free unbound electrons of the “material” of the butterfly’s nanohole wing, therefore reducing/modifying the coupling between the incident photons and interfering electrons. And doubtlessly, also all sorts of other second order “ref-lec-to-rac-tive” effects. Notice the difference between the brown/blue hole arrays and their diameter.



An idea about a quick investigation that comes to mind: what if one measures the energy of the reflected (filtered) light back and compares it with the energy coming from incident light for the very same filter bandwidth? How efficient are the butterflies’ nanohole arrays compared to man-made ones? Most likely the answer is not that straightforward, as man-made filters are designed for optimized transmission coefficient, while butterflies use nanohole arrays to reflect light to attract/protect themselves to/from other species. It may also be highly likely that there’s already tons of investigations conducted on the butterfly metamaterial topic.

One last thing that I came across some time ago. Similar nanohole patterns are observed in when anodizing aluminium and etching it consecutively with e.g. a fine ion etcher. Here’s a preview on the topic: A visible metamaterial fabricated by self-assembly method.