This is caused by a tech called Second harmonic generation, or frequency doubling.
"A process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of the initial photons. In some cases, almost 100% of the light energy can be converted to the second harmonic frequency. These cases typically involving intense pulsed laser beams passing through large crystals, and careful alignment to obtain phase matching. In other cases, like second harmonic imaging microscopy, only a tiny fraction of the light energy is converted to the second harmonic."