But because the electron is in an anharmonic potential (black curve), the electron motion is not sinusoidal. The three arrows show the Fourier series of the motion: The blue arrow corresponds to ordinary (linear) susceptibility, the green arrow corresponds to second-harmonic generation, and the red arrow corresponds to optical rectification. (When there is no oscillating force, the electron sits at the potential minimum, but when there is an oscillating force, it is, on average, further to the right, by an amount shown by the red arrow.)