Fluorescence is pretty easy and cheap, we use it all the time. Yellow fluorescent material is added to blue LEDs to make white LEDs; blue fluorescent material i

Fluorescence is pretty easy and cheap, we use it all the time. Yellow fluorescent material is added to blue LEDs to make white LEDs; blue fluorescent material is added to paper, clothing, and laundry detergent to make whites appear brighter under UV light. Shifting frequency the other direction is much harder; some materials exist that can double the frequency of light (known as second harmonic generation), but it's not as cheap as fluorescence. They are used in green laser pointers to turn infrared light into green light.