We demonstrate that the optical orientation of excitons can be coherently controlled and directly observed in a resonant time resolved photoluminescence experiment. The optical dephasing time of excitons, their longitudinal and transverse spin relaxation times, and their radiative lifetime, are measured with strictly the same experimental conditions. The measurements rely on the linear response of the crystal. [S0031-9007(97)04222-1]
We report on spin quantum beats of excitonic kind in the time-resolved photoluminescence of quantum wells in a magnetic field. When this field is perpendicular to the growth direction, conditions for the manifestation of the electron or exciton spin precession in the circularly polarized components of the excitonic luminescence are obtained. These results lead to a direct measurement of the electron-hole exchange energy of the 2D exciton and give important insights into the exciton properties. [S0031-9007 (97)02326-0] PACS numbers: 78.55.Cr, 71.35.Cc When two energetically closely spaced transitions are excited with a short optical pulse, the two induced polarizations in the medium oscillate with their slightly different frequencies. Their interference manifests in a modulation of the net polarization, the so-called quantum beats (QB). In semiconductors, QB from excitons have been seen in resonance fluorescence [1,2], optical absorption [3], degenerate-four-wave mixing [4], or linear birefringence [5] experiments.Recently, QB have been reported in the time-resolved free exciton photoluminescence (PL) of type I quantum well (QW), when a magnetic field perpendicular to the growth axis is applied [6,7]. These QB, which are observed on a time scale of a few hundreds of picoseconds, are interpreted by Heberle et al. [6] in terms of Larmor precession of the electron spins around the axis of the magnetic field. The corresponding pulsation v, which directly reveals the electron spin splitting "v g e m B B, allows the determination of the electron Landé g factor g e . However, at low temperature, when the laser excitation is resonant with the heavy-hole exciton (XH), the electron is bound into an exciton. This raises a fundamental question. On the ground of [8], in which all the spin relaxation processes were ignored, it is expected that QB in the excitonic photoluminescence should occur with a pulsation V " 21 p d 2 1 ͑"v͒ 2 , where d is the electron-hole exchange energy which splits the XH 1s quadruplet into the radiative and the nonradiative pairs of states. As a matter of fact, the authors of Ref.[6] did not observe any change in the linear dependence of the oscillation frequency on the applied magnetic field for resonant or nonresonant excitation conditions, high or low excitation density, or higher temperatures (T up to 200 K) at which free electrons and holes certainly prevail. Moreover, this linear dependence was verified at the smallest field values ͑B 0.12 T͒. This lack of excitonic manifestation is surprising.We have performed time-resolved PL on type I GaAs and GaInAs undoped QW grown by molecular beam epitaxy along the z axis, in a transverse magnetic field (x axis). The samples, which are immersed in liquid helium at 1.7 K, are mounted in Voigt configuration. They are excited with 1.2 ps pulses from a mode-locked Ti:sapphire laser with a repetition rate of 82 MHz. The laser beam is circularly polarized ͑s 1 ͒. The exciton luminescence components of opposed helicities I 1 and I 2 are detected by the up-conver...
The pulsation measurement of electron spin quantum beats by time resolved photoluminescence in a magnetic field perpendicular or at 45 • to the growth axis yields the transverse and longitudinal components of the electron Land é g factor in GaAs quantum wells (QW). The anisotropy of the g factor is thus determined as a function of the well width. No difference is found between the components of the g factor parallel and perpendicular to the growth axis for a 12 nm wide QW, whereas a clear anisotropy is measured for narrower QW.
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