Submitted by SurprisedPotato t3_10utol4 in askscience
If one browses an online store for laser pointers, there's an abundance of red, green and blue ones - but not yellow or cyan or other spectral colours.
Is there an actual engineering reason why yellow and cyan are harder to make than green or blue?
ellipsis31 t1_j7f4giw wrote
Infrared and red laser pointers are direct laser diodes (put current in and light squirts out). Green and blue laser pointers are made by using an infrared laser diode to pump a nonlinear optical crystal which doubles the frequency (halves the wavelength... however you want to say it) to put out photons with a new color. That doubling leaves a gap of wavelengths that are more difficult to access.