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The optical properties of hetero-polarization GaN/AlGaN (12% Al and 16.5% Al) are studied by photoluminescence (PL), photoreflectance (PR) and photoluminescence excitation (PLE) at low temperature. This material system is known to exhibit very strong internal electric fields and we show that PR spectroscopy is one of the best techniques to get a quantitative measurement of such fields, as it is a direct all-optical measurement. Electric field strength derived from FKO is approximately 120 kV/cm in AlGaN (12% Al) and 850 kV/cm in a 4.2 nm GaN QW. These values are in good agreement with other indirect determinations realized on the same samples by PL and PLE spectroscopy, but which involve computer simulations that need an accurate knowledge of geometrical and composition parameters of the samples. (C) 2003 Elsevier B.V. All rights reserved.