  Second-harmonic generation (SHG) is a direct measure of the strength of
second-order nonlinear optical effects, which also include frequency mixing and
parametric oscillations. Natural and artificial materials with broken
center-of-inversion symmetry in their unit cell display high SHG efficiency,
however the silicon-foundry compatible group-IV semiconductors (Si, Ge) are
centrosymmetric, thereby preventing full integration of second-order
nonlinearity in silicon photonics platforms. Here we demonstrate strong SHG in
Ge-rich quantum wells grown on Si wafers. The symmetry breaking is artificially
realized with a pair of asymmetric coupled quantum wells (ACQW), in which three
of the quantum-confined states are equidistant in energy, resulting in a double
resonance for SHG. Laser spectroscopy experiments demonstrate a giant
second-order nonlinearity at mid-infrared pump wavelengths between 9 and 12
microns. Leveraging on the strong intersubband dipoles, the nonlinear
susceptibility almost reaches 10^5 pm/V
