Abstract

In recent years, many interests are focused on the spin relaxation in the semiconductor hetero structures such as the quantum wells, since the spin relaxation time is much faster than the carrier life time. In the quantum wells, the degeneracy between the heavy hole and the light hole excitons is lifted, so that the one spin state in the conduction- and valence-band state can be excited selectively by the circularly polarized light. In the previous study of the spin relaxation in the quantum wells, the pump-probe using the circular polarization where the wavelength of the pump and the prove are the same and resonant with the heavy hole exciton[1-4], or the time-resolved luminescence measurement where the heavy hole exciton is excited resonantly by the circularly polarized pulse, and the decay of the circular polarization in the luminescence is measured[5-9], have been employed. In both cases of the measurements for the undoped quantum wells, the spin relaxation of the heavy hole exciton is contributed to both the electron spin relaxation and the heavy hole spin relaxation, simultaneously. A possible way to isolate the electron spin relaxation from the heavy hole relaxation in GaAs/AlGaAs quantum wells, is to use the p-doped quantum wells for the electron spin relaxation and the n-doped one for the heavy hole spin relaxation[9]. However, the doped quantum wells may be quite different from undoped quantum wells in the spin relaxation mechanism such as carrier-impurity scattering, the Coulomb screening of the carriers and so on. Therefore, here, we present an approach to measure the electron spin relaxation separately from the heavy hole one in the undoped quantum wells by the measurement of the femtosecond time-resolved circular dichroic spectrum. The present pump-probe method has the unconventional configuration in the absorption saturation measured from unoccupied light hole (lh) spin state and occupied heavy hole (hh) spin state simultaneously using the circularly polarized probe pulse with the continuum spectrum. The sample used in our experiments is CdTe/Cd_0.6Mn_0.4Te quantum wells, where the sp-d exchange interaction between the carrier spin in the well and the Mn ion spin in the barrier can be controlled by the confinement degree of the carrier wave function and we can examine the effect of the sp-d exchange interaction on the carrier spin relaxation.

© 1996 Optical Society of America