Observation of electromagnetically induced transparency and measurements of subband dynamics in a semiconductor quantum well

The phenomenon of electromagnetically induced quantum coherence is demonstrated between three confined electron subband levels in a quantum well which are almost equally spaced in energy. Applying a strong coupling field, two-photon-resonant with the 1–3 intersubband transition, produces a pronounced narrow transparency feature in the 1–2 absorption line. This result can be understood in terms of all three states being simultaneously driven into “phase-locked” quantum coherence by a single coupling field. We describe the effect theoretically with a density matrix method and an adapted linear response theory. Tuning the excitation laser to the 1–2 absorption energy allows the electron subband momentum distributions to be measured with the sample close to intersubband inversion. These are modelled to yield scattering rates and non-equilibrium phonon densities.