RAPID COMMUNICATIONS PHYSICAL REVIEW B 96, 041303(R) (2017) Determining the nature of excitonic dephasing in high-quality GaN/AlGaN quantum wells through time-resolved and spectrally resolved four-wave mixing spectroscopy M. Gallart, 1 , * M. Ziegler, 1 O. Crégut, 1 E. Feltin, 2 J.-F. Carlin, 2 R. Butté, 2 N. Grandjean, 2 B. Hönerlage, 1 and P. Gilliot 1 1 IPCMS UMR 7504 CNRS - Université de Strasbourg, 23 rue du Lœss, BP 43, F-67034 Strasbourg Cedex 2, France 2 Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland (Received 26 April 2017; published 24 July 2017) Applying four-wave mixing spectroscopy to a high-quality GaN/AlGaN single quantum well, we report on the experimental determination of excitonic dephasing times at different temperatures and exciton densities in III- nitride heterostructures. By comparing the evolution with the temperature of the dephasing and the spin-relaxation rate, we conclude that both processes are related to the rate of excitonic collisions. When spin relaxation occurs in the motional-narrowing regime, it remains constant over a large temperature range as the spin-precession frequency increases linearly with temperature, hence compensating for the observed decrease in the dephasing time. From those measurements, a value of the electron-hole exchange interaction strength of 0.45 meV at T = 10 K is inferred. DOI: 10.1103/PhysRevB.96.041303 I. INTRODUCTION AND CONTEXT Four-wave mixing (FWM) spectroscopy is an extremely powerful nonlinear optical pump-probe spectroscopy tech- nique giving access to the decay of the time-integrated diffracted signal with increasing delay between the exciting pulses. In semiconductors, such decay provides a direct measure of the excitonic phase coherence, and the resulting characteristic decay time of the FWM intensity is the so- called dephasing or coherence time T 2 . It is connected to the homogeneous linewidth broadening γ (also called the dephasing rate) by the relationship T 2 = 2¯ hγ −1 [1]. The FWM technique has been applied to a wide range of material systems, including bulk GaAs and GaAs/AlGaAs quantum wells (QWs), where excitonic coherence was shown to be mainly determined by excitonic collisions. As an illustration, the influence of exciton- and free-carrier density, their interaction with phonons via a temperature increase, and the role of exciton localization were all explored. For three-dimensional (3D) and two-dimensional (2D) excitons, the dephasing time was found to lie in the picosecond range in this material family [2,3], whereas an ultralong dephasing time of several hundred picoseconds was reported in InGaAs/GaAs quantum dots [4] showing that wave-vector relaxation due to collisions is the dominant dephasing mechanism. It was also used to determine the binding energy of biexcitons in ultrahigh- quality GaAs/AlGaAs QWs [5]. The nonlinear optical prop- erties of C 60 were also accessed using this technique [6], and, more recently, FWM spectroscopy has been successfully used to probe the population and coherence dynamics of excitonic transitions in monolayers of transition-metal dichalcogenides [7]. In III nitrides, some pioneering works [8,9] focused on the optical coherence of excitons in GaN epilayers, but, to date, despite their technological importance for optoelectronics and high-power high-frequency electronics, data concerning III-nitride quantum heterostructures are lacking, even though such information would prove extremely useful to understand the spin physics of excitons in these systems. * Corresponding author: mathieu.gallart@ipcms.u-strasbg.fr Indeed, regardless of the spin-relaxation process at play, it always involves a relaxation of the wave vector. It is usually accepted that the spin relaxation of the exciton as a whole is the main cause of its spin relaxation in QWs [10]. It originates from a long-range electron-hole exchange interaction that is enhanced by quantum confinement along the growth direction. At finite wave vector K, the spin-exchange interaction couples the optically active |+1〉 and |−1〉 excitons. The coupling terms in the Hamiltonian act as an effective magnetic field about which the exciton pseudospin precesses with a frequency . If K is kept fixed, the direction of the effective magnetic field remains constant while the precession leads to spin relaxation when a scattering event occurs that modifies the direction of the K vector. Eventually, two limiting cases can be considered, depending on the ratio between the collision time τ C of the excitons and the inverse of the precession frequency : If τ C > −1 , the spin-relaxation time τ S is proportional to the collision time τ C , but, on the contrary, if τ C < −1 , the spin relaxation is hindered by motional narrowing and τ S can be much longer than τ C . As will be discussed hereafter, measuring the dephasing time T 2 of excitons will give valuable information on their collision time τ C and thus on spin relaxation. In this Rapid Communication, we report on the experimen- tal determination of the excitonic dephasing time, measured at different temperatures and excitonic densities, in a high-quality GaN/AlGaN QW by means of FWM spectroscopy. We also provide a comparison of the evolution with temperature of both the dephasing- and the spin-relaxation rate. II. SAMPLE AND EXPERIMENTAL SETUP The studied sample is a high-quality low Al content single GaN/AlGaN QW grown by metal-organic vapor phase epitaxy on a c-plane sapphire substrate. The template is composed of a standard 3 μm thick GaN buffer layer and a 200 nm thick Al x Ga 1−x N layer. A single GaN QW with a nominal thickness of 2.6 nm was then deposited and capped with a 50 nm thick Al x Ga 1−x N layer with x = 5%. More details on the structural and optical characterization of this sample can be 2469-9950/2017/96(4)/041303(5) 041303-1 ©2017 American Physical Society