Propagation and Scattering of Exciton-Polaritons in Nitride-Based Multiple Quantum Wells

Abstract

We show theoretically that disorder-induced scattering of light between « bright » and « dark » exciton-polariton modes governs the coherent optical response of GaN/AlGaN quantum wells (QWs) on a femtosecond scale. The incident light essentially excites the "bright" modes, they emit the most part of light radiatively during first few hundreds of femtoseconds. The rest of the coherent signal, i.e. the temporal behaviour on a scale from 1 to few picoseconds, comes from the "dark" modes, populated due to the elastic scattering processes in the system, and mixed so that they form two bulk-like polariton branches. The pico-second scale optical response of GaN/AlGaN QWs is essentially incoherent because of a very efficient scattering of exciton-polaritons by a structural disorder. INTRODUCTIONRandom potential fluctuations strongly affect the confined exciton states in GaN/AlGaN and GaN/InGaN quantum wells (QWs) thus reducing the advantages of these structures for opto-electronics applications [1,2]. The disorder arises mainly due to random polarization fields [3,4] and is manifested by a huge Stokes-shift and inhomogeneous broadening of the exciton lines. Recently, it has been shown experimentally [5] and theoretically [6,7] that the radiative coupling between excitons confined in different QWs in multiple quantum well (MQW) structures may reduce effectively the disorder effect on the exciton. This effect known as the vertical motional narrowing (VMN) effect manifests itself in the increase of the decay-time of the time-resolved reflection signal from MQWs as a function of the number of wells. It results from the averaging of the disorder potential in a MQW structure by extended exciton-polariton modes that occupy entire the structure.The interplay between exciton-polaritons and potential disorder is a subject of our present study. We analyze the behavior of polariton eigen-modes in MQWs as a function of exciton inhomogeneous broadening, and compare it with calculated frequency-and timeresolved transmission spectra. The total number of exciton-polariton eigen-modes in a system of N QWs equals N. One can distinguish between "bright" polariton modes whose oscillator strength exceeds the single-QW exciton oscillator strength 0 Γ and "dark" polariton modes