Dynamics of exciton formation and relaxation in GaAs quantum wells

Abstract

%'e show that excitons form with a time constant~~20 ps following the creation of electron-hole pairs by subpicosecond optical excitation. The excitons are initially formed in large-wave-vector states. At low temperatures, these nonthermal excitons relax in =400 ps to the K =0 states, which couple directly to light by interaction with other excitons and acoustic phonons. This leads to a slow rise of exciton luminescence and an unusual dependence of this rise time on temperature, excitation density, and excitation energy.The optical properties of excitons in quantum wells have been the subject of intense research in recent years for fundamental' and applied reasons. Many fundamental properties of excitons have been investigated by using ultrafast laser spectroscopy. For example, the dynamics of exciton ionization and the ac Stark effect of excitons induced by an intense optical field ' have been investigated using excite-and-probe spectroscopy. The homogeneous linewidth of excitons, and the influence of temperature and various collisions on this linewidth, have been investigated by four-wave-mixing experiments. The recombination dynamics of excitons has been investigated by time-resolved luminescence spectroscopy.In spite of this intense interest in excitons, one important aspect of excitons, the dynamics of formation of bound states of excitons following photoexcitation of electron-hole pairs, has remained essentially unexplored.The recent results of Kusano et al. are largely dominated by extrinsic effects. The complex process of formation of intrinsic excitons has been identified as an important problem since the introduction of the concept of excitons in solids, but has not been addressed either experimentally or theoretically. Photoexcited electron and hole (either from a geminate or a nongeminate pair) can form an exciton by interaction with acoustic and optical phonons, and also by carrier-carrier interactions. The relaxation of photoexcited pairs within the bands proceeds simultaneously with the exciton formation process. Also, the excitons can be initially formed in the ground as well as excited states, and in the singlet as well as triplet states (corresponding to the orthohydrogen and parahydrogen states). Furthermore, the excitons are very likely created with a large total momentum wave vector E, corresponding to the center-of-mass motion of excitons in the quantumwell planes. The relaxation of these nonthermal excitons into the singlet K =0 state (the only state that can directly couple to the photons) must also be considered. It is clear that an understanding of these aspects of excitons is a fundamental importance in the physics of elementary excitons in solids.We present in this article the first results on the dynamics of exciton formation and relaxation, which provide insight into many facets of exciton physics that have not been considered before. All results presented in this letter deal with intrinsic excitons. We probed exciton formation dynamics in GaAs quantum wells as a function of temperat...

Determination of the Exciton Formation in Quantum Wells from Time-Resolved Interband Luminescence

“…A double question has then been debated for more than 10 years in the literature: first how do free electron hole pairs bind into excitons and second does indeed the luminescence at very short time proceed from bound excitons. A brief survey of the literature allows to find that experimentalists have reported formation times ranging from less than 10 ps up to about 1 ns [7,8,9, 10] and theoretical values range from 100 ps [11,12,13] to over 20 ns [14]. Clearly, the origin of this spreading in the reported values lies in the poor sensitivity of the experiments used in general to probe the exciton formation process, except for the case of the recent terahertz absorption experiments [10].…”

“…Clearly, the origin of this spreading in the reported values lies in the poor sensitivity of the experiments used in general to probe the exciton formation process, except for the case of the recent terahertz absorption experiments [10]. On the theoretical side, binding of an electron hole pair into an exciton requires, at low temperatures, the emission of an acoustic phonon, which brings long formation time due to the small coupling of acoustic phonons to excitons.The long formation time of excitons, together with the observation of luminescence at the exciton energy at the shortest times [7,15] led Kira et al [16] to introduce the idea that a free electron hole plasma, properly including Coulomb correlation effects, should give rise to luminescence at the exciton energy, without any exciton population. Although this proposed interpretation is currently largely questioned (see for example Hannewald and Glutsch et al [17]), the dominance of the excitonic transition at the shortest time has not received a sensible explanation yet.…”

“…The long formation time of excitons, together with the observation of luminescence at the exciton energy at the shortest times [7,15] led Kira et al [16] to introduce the idea that a free electron hole plasma, properly including Coulomb correlation effects, should give rise to luminescence at the exciton energy, without any exciton population. Although this proposed interpretation is currently largely questioned (see for example Hannewald and Glutsch et al [17]), the dominance of the excitonic transition at the shortest time has not received a sensible explanation yet.…”

Excitonic Photoluminescence in Semiconductor Quantum Wells: Plasma versus Excitons

“…1 For a long time, photoluminescence (PL) at the spectral position of the 1s exciton resonance has been considered as evidence for the existence of excitons. The rise of the 1s PL after nonresonant excitation of a semiconductor was interpreted as buildup of an excitonic population [1,2,3,4,5,6], and the PL decay was used to describe exciton recombination [7,8]. However, recently a microscopic theory predicted that PL at the 1s resonance can also originate from correlated plasma emission [9].…”

Optical Processes in Quantum Wells

“…From the theory viewpoint, optical processes provide an excellent source for gaining knowledge of the electron dynamics as well as an excellent testing ground of our understanding. From the experiment viewpoint, recent rapid progress in the time-resolved spectroscopic (7) measurements in semiconductor heterostuctures makes possible very direct measurements of the time evolution of certain electronic processes [2][3][4][5][6][7]. Yet, interpretation of the measurements requires a good theoretical framework.…”