How to create multiple frequencies of light simultaneously: When the high-frequency optical laser beam hits the semiconductor material — in this case, gallium arsenide nanostructures — it creates an electron-hole pair called an exciton. The electron is negatively charged, and the hole is positively charged, and the two are bound together by their mutual attraction.
“The very strong, low-frequency free electron laser beam rips the electron away from the hole and accelerates it...”
“As the low-frequency field oscillates, it causes the electron to come careening back to the hole.” The electron has excess energy because it has been accelerated, and when it slams back into the hole, the recombined electron-hole pair emits photons at new frequencies — up to 11 in their experiment.
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