The fun thing about real science is that you keep learning stuff. Not just the latest news, but whole new things that you hadn't even thought about before.
This fall I was pleased—and quite amazed—to read about potential applications for changing the color of photons [1].
First—huh? What does that mean? How is this even possible?
And second—whoa! This is so cool!
The basic idea is that what we see as the color of photons is the different energies of photons. Many physical processes work for a limited range of energies, i.e., colors. Other photons, the "wrong color", are scattered away from the target or do not interact in the desired way. So, the more photons of the "right color", the better the process will work from our point of view.
So, Stanford Professors Schloemer and Congreve says, "let's change the color of photons", to get more of the ones we want.
OK…wow!
The basic idea is to combine two low-energy photons to get a single high-energy photon. This is called "upconversion", which would be a good name for a band. (None dare call it magic!)
"We have upconverted two low-energy photons into one-high energy photon. In terms of colors, that means we can take two red photons and turn them into a blue one, for example, or take two infrared photons and turn them into a yellow one."
(From [1])
When it can be made to work, upconversion boosts some otherwise unused photons into energy ranges that can be used. For example, the researchers are interested in increasing the output of photovoltaic materials by upconverting photons into the ranges that generate electricity.
Photovoltaic cells are made of semiconductor materials that respond to certain photons and generate electrons. PV cells are generally only 15-20% efficient, i.e., they only generate 20% of the electrons they would if all the sunlight was converted to electricity.
"The single largest source of this loss is a mismatch between the colors of incoming light and the colors of light that can be used by a solar cell."
(From [1])
Basically, for any given material, only some of the photons in sunlight will be in the energies that will produce PV effects. The rest will pass through without interacting, or will interact in ways that end up generating waste heat but no useful electrons. If some of the wasted photons can be changed into photons in the useful colors, the same material will generate that much more electricity for the same sunlight.
Voila!
PV is not the only photochemistry where this trick may be useful. Many applications need to focus light and control undesirable absorption. Schloemer and Congreve describe different scenarios, including 3D printing.
In these scenarios, low power red or IR lasers to penetrate to the desired target, where upconversion generates higher energy blue light that triggers curing or other chemistry.
Cool!
- Tracy H. Schloemer and Daniel N. Congreve, The Practical Power of Fusing Photons, in IEEE Spectrum - Semiconductors, September 10, 2023. https://spectrum.ieee.org/photon-upconversion
PS. Some great ideas for band names
"upconversion"
Fusing Photons
triplet-triplet annihilation
the triplet excited state
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