[fengazer]

Visible light photons have more energy each. The shorter the wavelength, the higher the energy of the photon.

Light with a short enough wavelength (UV-B, for example) has enough energy to damage the DNA in our skin and increase our risk of skin cancer.

[p1esk]

If that's the case, how is it possible that low energy photons absorbed by carbon nanotubes can generate high energy photons?

[skosch]

Because it's not a one-to-one conversion. See e.g. https://en.wikipedia.org/wiki/Second-harmonic_generation