Theory and experiments of coherent photon coupling in semiconductor nanowire waveguides with quantum dot molecules

Abstract

We present a quantum optics theory, numerical calculations, and experiments on coupled quantumdots in semiconductor nanowire waveguides. We first present an analytical Green function theory tocompute the emitted spectra of two coupled quantum dots, treated as point dipoles, fully accountingfor retardation effects, and demonstrate the signatures of coherent and incoherent coupling througha pronounced splitting of the uncoupled quantum dot resonances and modified spectral broadening.In the weak excitation regime, the classical Green functions used in models are verified and justifiedthrough full 3D solutions of Maxwell equations for nanowire waveguides, specifically using finite-difference time-domain techniques, showing how both waveguide modes and near-field evanescentmode coupling is important. The theory exploits an ensemble-based quantum description, and andan intuitive eigenmode-expansion based Maxwell theory. We then demonstrate how the molecularresonances (in the presence of coupling) take on the form of bright and dark (or quasi-dark) reso-nances, and study how these depend on the excitation and detection conditions. To go beyond theweak excitation regime, we also introduce a quantum master equation approach to model the non-linear spectra from an increasing incoherent pump field, which shows the role of the pump field onthe oscillator strengths and broadening of the molecular resonances, with and without pure dephas-ing. Next, we present experimental photoluminescence spectra for spatially-separated quantum dotmolecules (InAsP) in InP nanowires, which show clear signatures of pronounced splittings, thoughthey also highlight additional mechanisms that are not accounted for in the dipole-dipole couplingmodel. Two different approaches are taken to control the spatial separation of the quantum dotmolecules, and we discuss the advantages and disadvantages of each.