M3L17c.txt

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The next subject is saturation.
Now, in this chapter I want to talk
about saturation in general.
I want to discuss monochromatic light,
but also broadband light.
And I want to introduce concepts of saturation intensity,
of absorption cross section, certain things which
I find extremely useful if I want
to understand what happens when light interacts with a system.
And we will find-- I will, just sort of to whet your appetite,
I will sort of show you that the absorption cross
section of a two level system is independent
whether you have a strong or weak transition.
Some people think the cross section should be,
but there is a difference which is
important between monochromatic and broadband light.
But in the end, the concepts are very simple.
I should say sometimes I feel it's almost too
simple to present it in class.
On the other hand, if I don't present it,
I can't make a few comments and guide you through.
So my conclusion now is at least for now, I show you
some prepared slides, and I sort of step you through, and make
a few annotations and point out certain things.
We have already partially transitioned
to teach you this material for homework assignment.
This week's homework assignment, which is due on Wednesday,
is almost completely on saturation.
And I will make a few comments where what I present you today
is an extension or different from what