Hi guys(adsbygoogle = window.adsbygoogle || []).push({});

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.

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# Laser: Second harmonic generation

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