Saturation of Band‐Tail Absorption in Multiple Quantum Wells and Its Link with the Nonlinear Index Saturation

Abstract

A pump and probe measurement of the nonlinear transmission reveals that the origin of the saturating behaviour of the nonlinear index 6 1 1 ,~ observed in the band tail of GaAs/GaAIAs multiple quantum wells is due to the saturation of the photoexcited carrier density.It has been discovered recently that the dispersive nonlinearity of MQW exhibits a strong saturating behavior [I], which constitutes a serious limitation in the design of optical bistable etalons. Indeed, epitaxial methods allow the fabrication of nonlinear interferometric structures incorporating distributed Bragg reflectors and GaAs multiple quantum well (MQW) nonlinear media in a single crystal [2]. This results in very compact nonlinear microcavities, with typically 5 pm overall thickness. Optical bistability is observed in these structures at mW optical power, with a high contrast in the reflection mode [l]. These attractive properties are obtained using the large excitonic-resonant nonlinear refractive index Sn,, of GaAs MQWs at room temperature [3], which involves the photoexcitation of electron-hole pairs, initially created in the low-energy tail of the excitonic and band-to-band optical transitions (Urbach's tail), and subsequently thermalized in higherenergy states through interaction with the lattice vibrations. In the course of our studies of such bistable microcavities, it was found that, due to the short nonlinear medium length (typically 100 QWs with a 20 nm periodicity), optical bistability could only be observed in the highest finesse cavities [4], which suggested that the available nonlinear phase shift was limited by some saturation mechanism.In the nonsaturating regime, Sn,, is simply proportional to the intensity I , and it is well known that n, defined as &,,(I) = n,l is proportional to the linear absorption coefficient a0 [5]. It is thus possible to define a figure of merit nJao which gives information about the nonlinear efficiency of the material. When the nonlinearity saturates, it is no longer possible to rely on such a model. It is therefore of great interest to investigate how the saturation parameters vary with the absorption.Recently a direct measurement of the nonlinear index of semiconductors embedded in a Fabry-Perot cavity, showed a clear saturation behavior of the nonlinear index change Sn,, as a function of intracavity power P [6]. The experimental data were found to fit very well with a saturation law PIPS Srz,,(P) = Sn, -~.