For lasing, you need to sustain photon density in the cavity so the active region can amplify. This requires a waveguide — layers with dimensions on the order of the photon wavelength that confine light via total internal reflection. Higher refractive index core surrounded by lower cladding, same principle as an optical fibre.
Conveniently, in semiconductors . So a p-i-n structure with high cladding around a low active region naturally forms both an electronic heterojunction (for carrier confinement) and an optical waveguide (for photon confinement). This is the separate confinement heterostructure (SCH) — the QWs sit inside a wider core layer that confines carriers and photons to the same region.
However, QW thickness (~5-10 nm) is far smaller than the optical mode (~hundreds of nm), so the confinement factor is small. Optimising is a central part of laser design.
Design trade-offs:
- Refractive index contrast between core and cladding — higher contrast gives stronger confinement but is limited by available lattice-matched materials.
- Core/SCH thickness — thick enough to confine the mode, but not so thick that the field spreads away from the QWs, reducing .
- Cladding thickness — must be thick enough to prevent the mode leaking into the highly doped contact layers, which are very lossy.
- Cladding doping — need enough for good carrier injection through the p-i-n junction, but doping introduces free-carrier absorption losses. Direct trade-off.
The optimal design depends on target wavelength and QW structure, so it’s an iterative process. More advanced geometries (ridge waveguides, DFBs, VCSELs) add lateral confinement and wavelength selectivity on top of this basic vertical waveguide structure.