  Spin sensitive bleaching of the absorption of far-infrared radiation has been
observed in $p$-type GaAs/AlGaAs quantum well structures. The absorption of
circularly polarized radiation saturates at lower intensities than that of
linearly polarized light due to monopolar spin orientation in the first heavy
hole subband. Spin relaxation times of holes in $p$-type material in the range
of tens of ps were derived from the intensity dependence of the absorption.
