Ultrafast light-polarization dynamics in semiconductor microcavities M.D. Martõ Ân a , L. Vin Äa a, * , E.E. Mendez b a Departamento de Fõ Âsica de Materiales C-IV, Universidad Auto Ânoma de Madrid, Cantoblanco, E-28049 Madrid, Spain b Department of Physics and Astronomy, SUNY at Stony Brook, Stony Brook, NY 11794-3800, USA Abstract We have studied the dynamics of the light emitted by a semiconductor microcavity paying special attention to the time- evolution of the degree of polarization, `, of the photoluminescence. ` depends strongly on the excitation power-density, an abrupt increase occurs when the emission becomes stimulated. Furthermore, we have found that a ®nite time is needed to reach the highest value of spin orientation, in contrast with the case of excitons in quantum wells where the spins are aligned almost instantly after a pulsed excitation. The faster emission dynamics of the polaritons that undergo stimulated emission, as compared with that of the polaritons with opposite spin, produces a very fast and ef®cient reversal of the polarization in the nonlinear regime. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 71.36.1c; 78.45.1h; 78.47.1p; 71.35.Gg Keywords: A. Quantum wells; D. Optical properties; D. Spin dynamics; E. Time-resolved optical spectroscopies Semiconductor electronic devices rely on the precise control of electronic charge, and in general the fact that the electrons also have a spin is ignored in practice. However, the scattering processes for electrons depend on their spin state. Recently, interest in electronic spin polari- zation in solid-state systems has grown fuelled by the possi- bility of producing ef®cient photoemitters with a high degree of polarization of the electron beam, creating spin memory devices and spin transistors as well as exploiting the properties of spin coherence for quantum computation. The control of the spin is very important for the develop- ment of new data storage and processing methods: a new ®eld, known as `spintronics', deals with the possibility of manipulation of electronic spin to read and write infor- mation through magnetism [1±8]. The properties of spin polarized carriers in bulk semi- conductors, their generation, characterization and the mechanisms responsible for the spin relaxation have been profusely studied in the past [9±12]. The spin relaxation, i.e. the change in the spin state, takes place through scattering with phonons or impurities, as a consequence of the change in momentum together with the spin-orbit coupling. This process for electrons in the conduction band of bulk semi- conductors appears from the mixing with ®nite momentum states of the valence band: Elliott±Yafet EY) [13,14]and Dyakonov'± Perel' DP) [15±17]mechanisms. In the case of a simultaneous existence of electrons and holes, after an optical excitation, this relaxation is due to the exchange interaction between both kinds of carriers, which is known as the Bir±Aronov±Pikus BAP) mechanism [18]. The scattering by magnetic impurities is similar to that described by the BAP: the role of the holes is taken in this case by the localized spin in the magnetic impurity. Experimental investigations on the spin dynamics in low- dimensional semiconductors have ¯ourished in the last decade. In these systems, the electronic properties, compared to those of bulk semiconductors, are strongly modi®ed by quantum size effects, and new, additional scattering mechanisms appear both for carrier transport and for the spin, due to the spatial con®nement of the carriers [19,20]. Different techniques, such as time-resolved photoluminescence and `pump and probe' spectroscopy have been used to study the spin dynamics [21±23]. In these experiments, the in¯uence of the temperature, the doping level, the excitation power, etc. on the different spin-¯ip mechanisms is investigated in great detail. Their Solid State Communications 119 2001) 259±270 0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-109801)00115-6 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Tel.: 134-91-397-4782; fax: 134-91- 397-8579. E-mail address: luis.vina@uam.es L. Vin Äa).