Photonics in nature: Part III
It was just another night of summer, not so hot though, but rather fresh since some showers of rain fell down in Dresden, when I nicely catched up with many bright spots flying all over one of the fields of the “Großer Garten” (great garden). After realizing they were fireflies, I could not resist on going after them while thinking on how is that such a tiny insect can produce light so efficiently without blowing up itself if its glowing would produce heat at the same time as light, since the energy that is not transformed (in light for example, as the case of opto-electronic devices), is wasted as heat.
Doing a small research, I found out how they do it: Fireflies make light within their bodies via a process called bioluminescence. To do so, this wonderful insect have specialized cells in their abdomen containing a chemical named “luciferin” that produces the enzyme „luciferase“. Here, the luciferin produces light, and the luciferase acts as a catalyst, therefore, it speeds up the reaction and allows oxygen to combine with the luciferin. This reaction produces photons (light) as follows: The luciferin-luciferase reaction creates a very excited molecule, when oxygen comes into place, that excited molecule goes from “excited” back to a “steady” state releasing the energy in form of light.
In other words, the firefly’s glowing is controlled by gating oxygen to light emitting cells, controlling the start and end of the chemical reaction and therefore, the start and end of its light emission. So, if oxygen is available the light is on, otherwise is off.
Now, how do fireflies manage to control this ON/OFF switching?
When fireflies inhale air, oxygen is transported from outside the body to the interior cells within the tracheoles. In the abdomen, oxygen molecules are bound on the surface cells and trapped by the mitochondria (organelle found in cells, in which biochemical processes of respiration and energy production happen), but also nitric oxide (important molecule that helps to regulate biological processes not only in animals but also in humans) is produced, which binds to the mitochondria in the light emitting cell. This binding forces the mitochondria release the trapped oxygen, allowing its further flow into the light organ where it combines with the other chemicals needed to produce the bioluminescent reaction. Since nitric oxide breaks down very quickly, as soon as the chemical is no longer being produced, the oxygen molecules are again trapped by the mitochondria and are not available for the production of light.
Another interesting fact is that the light is emitted through the cuticle, a part of the insect’s exoskeleton. Since light travels through the cuticle more slowly than it travels through air, this “speed” mismatch means that a proportion of the light is reflected back into the firefly’s abdomen, decreasing the glow’s intensity. Some fireflies‘ cuticles has a slanted roof shingles shape, this surface morphology, however, act as anti-reflective nanostructures that can help minimize internal reflections, meaning that more light escapes to the outside once again, as shown on the figure.
Turns out that a team of researchers used this last firefly mechanism to make light emitting devices more efficient while searching for an optimal light-extracting surface derived from the morphology of the firefly lantern , . Mother nature gave us another hint of how to improve our technologies if we approach the optimization of a device performance based on the natural behavior of any specie of the natural kingdom.
 Annick Bay, Nicolas André, Michaël Sarrazin, Ali Belarouci, Vincent Aimez, Laurent A. Francis, and Jean Pol Vigneron, „Optimal overlayer inspired by Photuris firefly improves light-extraction efficiency of existing light-emitting diodes,“ Opt. Express 21, A179-A189 (2013)
 Annick Bay, Peter Cloetens, Heikki Suhonen, and Jean Pol Vigneron, „Improved light extraction in the bioluminescent lantern of a Photuris firefly (Lampyridae),“ Opt. Express 21, 764-780 (2013)