Two mirrors, lots of bouncing
Two parallel mirrors trap light between them. The wave bounces back and forth, and at certain wavelengths the round-trip phase is exactly an integer number of 2π: the wave reinforces itself and the cavity becomes resonant. We call those wavelengths cavity modes.
Move the modes
Stretch the cavity and the comb of allowed wavelengths squeezes together. Make the mirrors better and the peaks get narrower.
Modes appear where 2 n L = m λ. Stretch the cavity and the comb of peaks squeezes together; better mirrors (higher R) make each peak narrower.
Why we built one
A microcavity with the CrSBr layers tucked inside is what makes the experiment sensitive: instead of a faint absorption peak we see a sharp dip in R(E) wherever a cavity mode and the exciton meet — and a tell-tale anti-crossing when they do.
- Fabry-Pérot cavity = two mirrors trapping a standing wave.
- Modes at 2 nL = m λ; spacing shrinks for longer cavities.
- Better mirrors → narrower, deeper resonances.