M5L23m.txt

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Our next topic is lasing, or gain, without inversion.
I want to show you what was actually a rather
recent accomplishment that this common belief that you need
population in the excited state which
is larger in the ground state for optical gain and lasing
is not true.
Well, it's true for a two-level system.
But it's not true when you have more than two levels.
And I'll discuss it here for the case of a three-level system.
And this is a really insight which
came as recent as in the '80s.
One reference is, the person who pioneered a lot of work
on three-level systems and provided a lot of insight
was Steve Harris at Stanford.
And this was a paper in the late '80s which
enunciated the possibility of having
lasing without inversion.
And you know, when you are a graduate student like me
who was educated earlier, you start saying, gee whiz, that's
not what I learned in class.
But of course, it's often the small [? sprint ?].
It's the assumption for a two-level system.
So the secret behind lasing without inversion
is that you can have cancellation of absorption
by interference.
In other words, this belief that you need inversion
comes that you want that the stimulated emission is
stronger than the absorption.
But if you have a three-level system, you have to be careful.
We have interference effect.
And what I want to show you is that we
can create a situation where we have destructive interference
for absorption.
But we don't have destructive interference
for stimulated emission.
So therefore, we can now afford to have more population
in the ground state.
Because the absorption out of the ground
state does not have to be suppressed
by minimizing the population in the ground state.
We have an interference effect which suppresses absorption
from the ground state population.
And therefore, the ground state population
can be larger than in a two-level system.

I could simply tell you, you can hide population
in the dark state, end of discussion.
And you get the basic idea.
But the concept of lasing without inversion
is richer than that.
It's connected to the dark state, but there is more to is.
So that's why I want to spend the rest of today's class