  In the present work, we were able to identify and characterize a new source
of in-plane optical anisotropies (IOAs) occurring in asymmetric DQWs; namely a
reduction of the symmetry from $D_{2d}$ to $C_{2v}$ as imposed by asymmetry
along the growth direction. We report on reflectance anisotropy spectroscopy
(RAS) of double GaAs quantum wells (DQWs) structures coupled by a thin ($<2$
nm) tunneling barrier. Two groups of DQWs systems were studied: one where both
QWs have the same thickness (symmetric DQW) and another one where they have
different thicknesses (asymmetric DQW). RAS measures the IOAs arising from the
intermixing of the heavy- and light- holes in the valence band when the
symmetry of the DQW system is lowered from $D_{2d}$ to $C_{2v}$. If the DQW is
symmetric, residual IOAs stem from the asymmetry of the QW interfaces; for
instance, associated to Ga segregation into the AlGaAs layer during the
epitaxial growth process. In the case of an asymmetric DQW with QWs with
different thicknesses, the AlGaAs layers (that are sources of anisotropies) are
not distributed symmetrically at both sides of the tunneling barrier. Thus, the
system losses its inversion symmetry yielding an increase of the RAS strength.
The RAS line shapes were compared with reflectance spectra in order to assess
the heavy- and light- hole mixing induced by the symmetry breakdown. The
energies of the optical transitions were calculated by numerically solving the
one-dimensional Schr\"odinger equation using a finite-differences method. Our
results are useful for interpretation of the transitions occurring in both,
symmetric and asymmetric DQWs.
