Re: harmonics, anharmonic ...

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

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