Quantum well `quasimolecules' with nonlinear optical properties and new heterostructures with bound states in the continuum

In this paper we review the nonlinear optical properties associated with intersubband transitions in AlInAs/GaInAs coupled quantum wells grown by MBE. Structures with giant nonlinear susceptibilities χ(2) (2ω) and χ(3)(3ω) (compared to the bulk constituents of the quantum wells) have been designed and demonstrated. They exhibit large linear Stark shifts of the intersubband transitions which have been used to efficiency tune the nonlinear susceptibilities. The second-order nonlinear susceptibility |χ(2)(2ω)| exhibits a peak as a function of the electric field corresponding to the energy levels made equally spaced via the Stark effect. In a three-coupled-well structure, triply resonant third harmonic generation has been observed. The corresponding |χ(3ω)(3)|(10-14 (m/V)2 at 300 K and 4×10-14 (m/V)2 at 30 K) is the highest measured in any material. The equivalent of multiphoton ionization of a molecule has also been investigated in this structure. In the last section we discuss new quantum well structures which exhibit highly localized states above the potential well. This is achieved using as barriers quarter-wave stacks. In the superlattice limit the latter behave as Bragg reflectors and the localized resonance becomes a bound state.