Uniaxial wurtzite group-III nitride heterostructures are subject to large p
olarization effects with significant consequences for device physics in opt
oelectronic and transport device applications. A central aspect for the pro
per implementation is the experimental quantification of polarization charg
es and associated fields. In modulated reflection spectroscopy of thin film
s and heterostructures of AlGaInN we observe pronounced Franz-Keldysh oscil
lations that allow direct and accurate readings of the Field strength induc
ed by polarization dipoles at the heterointerfaces. In piezoelectric GaInN/
GaN quantum wells this dipole is found to induce an asymmetry in barrier he
ights with a respective splitting of interband transitions. This splitting
energy appears to reflect in the transitions of spontaneous and stimulated
luminescence in the well. From these experiments the polarization dipole is
identified as controllable type-II staggered band offset between adjacent
barrier layers which can extend the flexibility in AlGaInN bandstructure de
sign. The derived field values can serve as important input parameters in t
he further interpretation of the entire system.