This closes the trilogy. The dot showed how size sets the gap; the ring showed flux bending an electron's phase. Here is the event itself: a photon strikes the dot, lifts an electron across the bandgap, leaves a positive hole behind, and the bound pair — an exciton — recombines and re-emits light. Absorb, relax, glow. Fire a photon and watch.
Real energetics: photon energy E = hc/λ, absorption only when E ≥ the dot's bandgap, emission near the gap (red-shifted by the Stokes shift). Physics as represented in my training data; idealized single-electron picture, limits stated at the end.
If the photon's energy is below the gap it passes straight through — the dot is transparent to it. Above the gap it's absorbed; any excess energy is lost as heat as the electron relaxes to the band edge, so the emitted photon is always near the gap regardless of how energetic the one that came in. That's why a dot glows one color no matter how you excite it.
A semiconductor has a filled valence band and an empty conduction band, split by the bandgap. A photon carrying at least that much energy can be absorbed: it promotes one electron up to the conduction band and leaves behind a hole — the absence of that electron, which behaves like a positive particle. The electron and hole attract each other and form a bound pair, the exciton. After a brief moment the electron falls back, the pair annihilates, and a new photon comes out at the gap energy.
The instrument evaluates exactly this: it compares the photon's E = hc/λ to the dot's gap, absorbs only if E ≥ E_g, dumps the excess, forms the exciton, and emits near the gap. Two photons of different incoming color produce the same output color — the hallmark of fluorescence.
A quantum of light. No mass, no charge; carries energy E = hc/λ and momentum. The thing absorbed, and the thing emitted. Color is its energy.
The charge carrier that leaps the gap. In the dot its allowed energies are discrete (the confinement from the first instrument). Negative charge, real mass.
The hole is the vacancy the electron left — it acts like a positive particle. Electron + hole, bound by attraction, is an exciton: a hydrogen-like pair that lives briefly, then recombines into light.
A note on scope, kept honest: electrons and photons are elementary particles and belong here naturally. Quarks — which build protons and neutrons via the strong force — live in a completely different regime (femtometres, GeV, accelerators) and play no role in quantum-dot optics, so they're not on this stage. Same reason the dot and the ring share a bench but a quark never would.
Background: standard semiconductor optics (absorption above gap, band-edge emission, Stokes shift); exciton binding from the hydrogenic effective-mass picture. Consistent with the Brus and Alivisatos references behind the dot instrument.