  By biasing a single barrier heterostructure with a 500nm-thick GaAs layer as
the absorption layer, the spin dynamics for both of the first and second
subband near the AlAs barrier are examined. We find that when simultaneously
scanning the photon energy of both the probe and pump beams, a sign reversal of
the Kerr rotation (KR) takes place as long as the probe photons break away the
first subband and probe the second subband. This novel feature, while stemming
from the exchange interaction, has been used to unambiguously distinguish the
different spin dynamics ($T_2^{1*}$ and $T_2^{2*}$) for the first and second
subbands under the different conditions by their KR signs (negative for
$1^{st}$ and positive for $2^{nd}$). In the zero magnetic field, by scanning
the wavelength towards the short wavelength, $T_2^{1*}$ decreases in accordance
with the D'yakonov-Perel' (DP) spin decoherence mechanism. At 803nm,
$T_2^{2*}$(450ps) becomes ten times longer than $T_2^{1*}$(50ps). However, the
value of $T_2^{2*}$ at 803nm is roughly the same as the value of $T_2^{1*}$ at
815nm. A new feature has been disclosed at the wavelength of 811nm under the
bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times
($T_2^{1*}$ and $T_2^{2*}$) and the effective $g^*$ factors ($|g^*(E1)|$ and
$|g^*(E2)|$) all display a sudden change, due to the "resonant" spin exchange
coupling between two spin opposite bands.
