Coupled quantum well semiconductors with giant electric field tunable nonlinear optical properties in the infrared

Coupled quantum wells present unique opportunities for engineering new semiconductors with large optical nonlinearities associated with intersubband transitions in the infrared. In this paper we report an in depth study of these properties in the AlInAs/GaInAs heterostructure material system grown by molecular beam epitaxy. We show that by judicious control of the tunnel coupling between wells and of the thickness of the latter one can design the wavefunctions and the energy levels in such a may that these new structures behave as quasi-molecules with extremely large dipole matrix elements and strongly field tunable nonlinear optical properties. 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 efficiently 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 being made equally spaced via the Stark effect. In a three-coupled-well structure triply resonant third harmonic generation has been observed. This process is associated with four equally spaced bound states. 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 third order noniinear susceptibility in any material. The equivalent of multiphoton ionization of a molecule has also been investigated in this structure. Electrons are photoexcited to a continuum resonance above the barrier via a three-photon transition enhanced by intermediate energy levels. The effect of this resonance on χ⁽³⁾(3ω) as the electric field is varied is also investigated. Finally, appropriate figures of merit are discussed