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Re: LASERS



On Tue, 4 May 1999, Ludwik Kowalski wrote:

I would also welcome somebody's explanations, even a semi
quantitative one. The matching of f is easy to justify on the basis of
conservation of energy. By why is polarization of "induced photons"
exactly the same as the polarization of the incoming wave? And why
do we have this perfect match of phases? It suggests that moments
of emission always correspond to an exactly well defined distance
between the incoming photon and the atom to be deexcited? I know
that the classical terminology of "exact moment and exact distance"
are not appropriate but I am using it to ask questions, not to answer
them.

Here's some pure speculation (I've been keeping my eyes out for similar
explanations in trustworthy references, but they never mention this.)

What if each exited atom in the laser medium ABSORBED the incoming photon,
and then re-radiated a pair of identical photons? Rather than appealing
to EM-fields to explain the triggered photon emission, or assuming that
the proximity of a nearby photon can mysteriously trigger the atom to emit
an identical one, we would then have a fairly sensible explanation
(although perhaps not one which is correct in a QM sense.)

If laser atoms absorb the stimulating photons, then they would emit photon
pairs, and this photon-pairing phenomenon would be measurable. I've seen
mention of "photon pairing" in the literature on lasers. I think there
are some photochemical reactions which behave differently if two
closely-spaced photons are absorbed. But I've yet to see anyone come out
and state that laser atoms ABSORB the incoming photons.

The idea of absorbtion would also make sense in terms of transparency. If
we describe transparancy and optical index as being a case where the atoms
of the material absorb photons and then re-radiate them after a delay,
then Stimulated Emission is simply a special case of the physics of
transparent materials. If glass normally absorbs/radiates photons
coherently, and occasionally absorbs a photon entirely (it's not 100%
transparent), then a pumped laser material would absorb/radiate photons
and behave transparently, but its atoms would occasionally emit TWO
photons after absorbing just one.

How do we explain why glass is transparent, and why the atoms of the glass
emit the absorbed photons in a manner which is phase-locked with the
incoming beam, and why the frequency, direction, and polarization of the
outgoing photons is identical with the incoming ones? I don't quite know.
However, a good, intuitive explanation of transparency MIGHT contain the
explanation of why Stimulated Emission is the way it is.

I realize that my explanation above is a bit too longwinded for Ludwik's
small handout. Mostly this is my response to a little niggling issue that
caused me confusion as a student (and which in later years I never saw
discussed in any textbook.) I suspect that I am probably not unique,
and others besides myself have wondered about this issue.

No, you are not alone, William. Perhaps somebody will help us. By
the way, my original handout was distributed today, together with
several other messages of this thread. I assume that what is posted
on phys-L can be shown to others without asking for a permission.

I'm glad to have any of my messages spread around. I forget: do the
authors in public forums own the copyrights on all of their messages? Or
is the material legally in the public domain?

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