  We present spin relaxation times of 2D holes obtained by means of spin
sensitive bleaching of the absorption of infrared radiation in p-type
GaAs/AlGaAs quantum wells (QWs). It is shown that the saturation of
inter-subband absorption of circularly polarized radiation is mainly controlled
by the spin relaxation time of the holes. The saturation behavior has been
determined for different QW widths and in a wide temperature range with the
result that the saturation intensity substantially decreases with narrowing of
the QWs. Spin relaxation times are derived from the measured saturation
intensities by making use of calculated (linear) absorption coefficients for
direct inter-subband transitions. It is shown that spin relaxation is due to
the D'yakonov-Perel' mechanism governed by hole-hole scattering. The problem of
selection rules is addressed.
