Half light, half matter
Place an exciton inside a microcavityand tune the cavity's mode close to the exciton energy. If they couple strongly enough, you no longer have two separate things — instead you have two new hybrid modes called exciton-polaritons: superpositions of light and matter.
The hallmark of strong coupling is the anti-crossing: as you sweep the cavity through the exciton, the two would-be crossing curves repel each other and form an upper and lower polariton branch.
See the gap open
The size of the gap is 2g, twice the coupling strength. Drag it below the exciton linewidth and you lose strong coupling; drag it well above and the branches separate clearly.
The two dashed lines are the bare cavity mode (slanted) and the bare exciton (flat). When they would cross they instead repel — the gap at zero detuning is the Rabi splitting 2g, the fingerprint of strong light–matter coupling.
Where it shows up in the simulator
Look for the cavity dip approaching the exciton energy in the reflectance plot. Instead of a single dip you'll see two, kissing but not crossing — that's the polariton signature.
- Polaritons = strongly coupled exciton + cavity mode.
- Hallmark = anti-crossing of size 2g in the spectrum.