Photoreflectance (PR) has been widely used for the characterization of various semiconductors as well as their surface and interface properties due to its non-destructive and high sensitivity virtues. From the viewpoint of the employment of monochromator, the experimental setup may be classified into dark and bright configurations, which were applied to characterize the heterostructure of InP/In_0.52Ga_0.48As/InP grown by molecular beam epitaxy. It reveals that the front configuration well separates the luminescence from the modulation signal while the backside configuration benefits the extraction of weak modulation signals with the employment of high excitation power. Based on the backside configuration, we also observed a below band-gap excitation phenomenon, i.e. that the modulation signal of InP exhibits under the excitation of energetically low modulation light (1 064 nm laser). The result demonstrates that the backside configuration may be employed as a contactless electro-modulation technique for the characterization of wide band gap semiconductor heterostructures.