We use the phase difference of two lasers with equal frequencies for the control of spontaneous emission in a four-level system. Effects such as extreme spectral narrowing and selective and total cancellation of fluorescence decay are shown as the relative phase is varied.

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.

We study the effects of excitons in nonlinear optical rectification in one-dimensional semiparabolic quantum dots. We consider the cases that the electron and the hole are confined in semiparabolic potentials (i) with the same oscillator frequency and (ii) with the same width. In the first case we present approximate analytical results in the strong confinement regime and find a closed-form relation between the nonlinear optical rectification coefficient of an exciton and the nonlinear optical rectification coefficient when only one electron exists in the structure. In the second case we use the Hartree-Fock approximation and the potential morphing method and present results for the nonlinear optical rectification coefficient in all confinement regimes.

We show that a three-level $\Lambda$-type atom interacting with a classical standing-wave field resonantly coupling one transition and a weak probe laser field resonantly coupling the second transition can be localized provided the population of the upper state is observed.

We analyze the interaction of N laser fields with a $\mathrm{}(N+1\mathrm{})\mathrm{}$-level quantum system. A general analytic expression for the steady-state linear susceptibility for a probe-laser field is obtained and we show that the system can exhibit multiple electromagnetically induced transparency, with at most $N-1$ transparency windows occurring in the system. The group velocity of the probe-laser pulse can also be controlled.