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.

Efficient (∼1%) second harmonic generation, resonantly enhanced near λ=8.6 μm, has been observed in asymmetric double multi-quantum well structures. We used (i) edge-emitting waveguide geometry where the phase matching was achieved by incorporating a separate multiple quantum well region which modifies (via the Kramers–Kronig relation) the dispersion of light and (ii) 45° wedge multi-bounce geometry where the phases of second harmonic waves generated at sequential bounces were synchronized by changing the angle of incidence.

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Midinfrared optical pumping of electrons from the ground (n=1) to the first excited (n=2) subband of the quantum well produces a strongly nonthermal electron distribution which is probed spectroscopically. Two sharp induced absorption peaks appear, associated with electrons which have scattered from the upper subband via longitudinal optical (LO) phonon emission and absorption. The presence of the phonon absorption channel evidences the importance of nonequilibrium LO phonon populations (nph∼1), and the impact on the nonradiative intersubband scattering rates in quantum cascade laser devices is explored.

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