Does nonlinear optics also create harmonics as with nonlinear electronics?
Yes, this is how frequency-doubled lasers work (eg. 532 nm green laser pointers, which are generated as a harmonic from a 1064 nm Nd:YVO4 laser by a nonlinear KTP crystal).
Yes, this is how frequency-doubled lasers work (eg. 532 nm green laser pointers, which are generated as a harmonic from a 1064 nm Nd:YVO4 laser by a nonlinear KTP crystal).