Hi guys I'm reading a section in a book about non-linear optics. They have an expression for the polarization for the second harmonic field. They say that this expression has a linear and a non-linear contribution at frequency 2ω, but they don't explain this any further. I am a little confused about this statement, because we have light at frequency ω going into the medium. This light rocks the electrons at ω (linear) and at 2ω (non-linear). So when we write an expression for the polarization of the electrons oscillating at 2ω, then where does the linear term come from? I'm really confused about this, so I would be very happy, if you could solve this mystery. Best, Niles.