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Re: harmonics, anharmonic ...

John Denker wrote:

Optical frequency doubling definitely exists. Have you seen those green
diode lasers? They are really infrared lasers, doubled.

.... *) In the optical doubling crystal, some of the atoms are held
in the crystal by highly anharmonic potentials.

It is the nonlinearity that creates the frequency multiplication effect.

The way in which "nonlinearity" is responsible for the appearance
of harmonics is clear in electronics. A diode, for example, will
turn the pure harmonic signal into a rectified signal which is the
sum many frequencies (plus d.c.). What is nonlinear here? The
relation between V and I.

But what do we mean by the "nonlinearity" when we refer to
atoms doubling a laser frequency ? What versus what is nonlinear?
Is the term nonlinearity appropriate in this case?

Ludwik Kowalski