Making color: When two red photons make a blue photon
Color is strange, mainly due to perception. Setting aside complex brain processes, what we see is the result of light absorption, emission, and reflection. Trees appear green because atoms inside the leaves are emitting and/or reflecting green photons. Semiconductor LED brake lights emit single color light when electrical current passes through the devices.
Here’s a question: Can scientists generate any color of light? The answer is not really, but the invention of the laser in 1960 opened new doors for this endeavor. An early experiment injected high-power laser light through quartz and out popped a different color. This sparked the field of nonlinear optics and with it, a new method of color generation became possible: frequency conversion.
Not all crystals can perform this trick and only through careful fabrication of certain materials is frequency conversion possible. In a result published in Nature Communications, scientists demonstrate a new microstructure that does what’s called second harmonic generation (SHG), where the output light has twice the frequency as the input. This new device is a factor of 1000 smaller than previous frequency converters.