  The Rashba effect in Ge/Si$_{0.15}$Ge$_{0.85}$ multiple quantum wells
embedded in a p-i-n diode is studied through polarization and time-resolved
photoluminescence. In addition to a sizeable redshift arising from the
quantum-confined Stark effect, a threefold enhancement of the circular
polarization degree of the direct transition is obtained by increasing the pump
power over a 2kW/cm$^2$ range. This marked variation reflects an efficient
modulation of the spin population and is further supported by dedicated
investigations of the indirect gap transition. This study demonstrates a viable
strategy to engineer the spin-orbit Hamiltonian through contactless optical
excitation and opens the way towards the electro-optical manipulation of spins
in quantum devices based on group-IV heterostructures.
