Model System for Optical Nonlinearities: Asymmetric Quantum Wells

Abstract

Thanks to the recent progresses in epitaxy techniques, it is now possible to grow alternating layers of semiconductors with different band gap energies with a thickness control down to one atomic layer. It is now well established that these systems behave, as far as the electron motion is concerned, as a succession of potential barriers and wells. If the width of the potential well is less than the deBroglie wavelength of the electrons in the material (e.g. less than ≈15 nm in GaAs), the motion of the electrons may be considered, at sufficiently low temperature, as quantized in the direction normal to the growth axis¹. The electrons are quantized into subbands where their wavefunctions in the growth direction have the form of envelope functions with an extension equal to the well width, i.e. in the few nanometer range. Electromagnetic waves may induce electronic transitions between these subbands. The dipole matrix elements associated to these intersubband transitions (ISBT) have the same order of magnitude as the quantum well width leading to extremely large absorption. These large absorptions have been observed for the first time in GaAs/AlGaAs multiquantum wells (MQW) by West and Eglash².