Quantum wells with zero valence-band offset: Drastic enhancement of forbidden excitonic transitions

Forbidden $e1\mathrm{hh}3\mathrm{}$ and $e1\mathrm{lh}3\mathrm{}$ exciton transitions (where $\mathrm{hh}$ and $\mathrm{lh}$ are heavy-hole and light-hole, respectively) appear in magnetooptical spectra of quantum wells that undergo a type-I-type-II band-alignment transition, here for ordinary (In,Ga)As/GaAs multiple quantum wells and a single CdTe quantum well with a (Cd,Mn)Te semimagnetic barrier exhibiting a giant Zeeman effect. The oscillator strength becomes so strong in a field of 11.25 T that vacuum-field Rabi splitting is clearly seen in reflection spectra when the $e1\mathrm{lh}3\mathrm{}$ exciton transition couples to the two-dimensional optical mode in a microcavity.