Friday, January 27, 2023

Cavities and tuning physics

I've written before about cavity quantum electrodynamics.  An electromagnetic cavity - a resonator of some kind, like your microwave oven chamber is for microwaves, or like an optical cavity made using nearly perfect mirrors - picks out what electromagnetic modes are allowed inside it.  In the language of photons, the "density of states" for photons in the cavity is modified from what it would be in free space.  Matter placed in the cavity, e.g. an atom, then interacts with that modified environment, even if the cavity is not being excited.  Instead of thinking about just the matter, or just the radiation by itself, in the cavity you need to include the light-matter interaction, and you can end up with states called polaritons that are combinations of matter + radiation excitations.  There are various flavors of polaritons, as there are different kinds of cavities as well as different kinds of matter (atoms vs. excitons, for example).

I just heard a nice talk by Angel Rubio about recent advances in applying cavity effects to both chemistry and materials properties.  For a recent discussion of the former, you can try here (pdf file).  Similar in spirit, there is a great deal of interest in using cavity interactions to modify the ground states (or excited states) of solid materials.  Resonantly altering phonons might allow tuning of superconductivity, for example.  Or, you could take a material like SrTiO3, which is almost a ferroelectric, and try to stabilize ferroelectricity.  Or, you could to take something that is almost a spin liquid and try to get it there by putting it in a cavity and pumping a little.

It's certainly interesting to ponder.  Achieving this in practice is very challenging, because getting matter-cavity couplings to be sufficiently large is not easy.  Never the less, the idea that you can take a material and potentially change something fundamental about its properties just by placing it in the right surroundings sounds almost magical.  Very cool to consider.


Pu said...

Regarding the realization of such ideas, microwave cavities coupling with superconducting emitters should be possible. Otherwise, it seems very challenging if not possible.

Anonymous said...

Isn't there something inherently missing here? Cavities tend to couple to modes that couple to light. But most of the interesting modes in condensed matter don't couple to light because of symmetry (superconducting amplitudon) or wavelength mismatch (CDW soft modes). How do you could those things to a cavity?

Douglas Natelson said...

Anon, you are certainly correct that there must be some coupling mechanism. The couplings in some systems are indirect. The work by Cavalleri on light-driven superconductor response involves coupling to Raman active structural modes; Galitski is looking at coupling other modes ( As you say, though, not everything couples to EM modes of a cavity.