Optical density of man atomic ensemble and linewidth

[Niles]

Optical density of an atomic ensemble and its linewidth

Hi

I've been thinking about something for a while now. If I take a two-level atom, completely at rest, then I can probe it with a quasi-resonant laser. The absorption of the atom is a nice Lorentzian, nothing fancy here.

However, now say I take 10¹¹-10¹² atoms. The specific number doesn't matter, all I am trying to do is to probe the system in a regime where the optical density has increased very much. For the sake of clarity I assume the atoms to be completely independent and at rest. No collisions/dephasing/... occur between them.

If I now probe this system, will I still see an absorption Lorentzian with the same FWHM as in the single-atom case? Or will the transition be broadened somehow?

The reason why I believe it should be broadening is because I keep thinking about, e.g., my table (or any other macroscopic object, a pencil, calculator, ...). It has an optical depth that is very large, so the absorption profile is also extremely broad. I was wondering if the same holds for an independent atomic ensemble.

I've been trying to look at the Optical Bloch Equations to see if such a behavior could occur, but it is not immediately clear that it is so. Is my analogy wrong?


Niles.

 

[eljose79]

Dear Niles,

Thank you for your thought-provoking question. The answer to your question is yes, the absorption profile of an atomic ensemble will be broadened compared to that of a single atom. This phenomenon is known as the homogenous broadening effect and is a result of the interaction between the atoms in the ensemble.

In an atomic ensemble, the atoms are not completely independent and at rest. They interact with each other through dipole-dipole interactions, which can lead to the broadening of the absorption profile. This is similar to the broadening effect observed in macroscopic objects, where the atoms/molecules are not completely independent and interact with each other.

The Optical Bloch Equations can indeed be used to describe this behavior. They take into account the interactions between the atoms and can predict the broadening of the absorption profile in an atomic ensemble. This broadening can be quantified by the linewidth, which is related to the optical density of the ensemble.

In fact, the broadening of the absorption profile is often used to study the properties of the atomic ensemble. By measuring the linewidth, scientists can gain information about the interactions between the atoms and the properties of the ensemble as a whole.

I hope this helps answer your question. Please let me know if you have any further inquiries.