Coupled Quantum Wells for Optical Modulation

Abstract

The use of quantum wells in semiconductor materials to confine carriers in one dimension has led to the creation of many optoelectronic devices; among these are quantum well lasers and modulators.1,2 Within quantum wells, the lowest energy optical absorption edge is associated with an extended state consisting of extended election and hole states which are correlated through coulombic attraction, In semiconductors, such as GaAs, these are referred to as Wannier excitons. The electrostatic confinement imposed by the ‘barriers’ of the quantum well compress the radial extent of the exciton wavefunction in one dimension. If quantum confinement energies are much larger than the coulomb binding energies, then exciton absorption is roughly proportional to the overlap of electron and hole wavefunctions. Simply put, the confined system enhances this overlap through the compression of the electron and hole wavefunctions, thus increasing absorption. This large resonant absorption is typically modulated through the quantum-confined Stark effect (QCSE) in which an applied electric field shifts all of the quantized electron and hole states and moves the resonant absorption edge to lower energies (referred to as a red-shift). A region which was initially below the absorption edge (at a wavelength λ1) and had low absorption will have a larger absorption as the exciton energy is red-shifted from its zero bias wavelength (λ0) toward λ1. Simultaneously, the electric field creates a spatial separation of the electrons and holes. This spatial separation decreases the electron-hole envelope wavefunction overlap and reduces peak absorption. Hence, the total absorption change (Δα) at λ1 is a fraction of the maximum absorption at λ0 with zero applied field.KeywordsHole StateExciton AbsorptionCouple QuantumRadial ExtentBias LevelThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.