The charge modulation spectroscopy signal can be defined as the differential transmission divided by the total transmission . By modulating the mobile carriers, an increase transmission  and decrease transmission  features could be both observed. The former relates to the bleaching  and the latter to the charge absorption and electrically induced absorption (electro-absorption). The charge modulation spectroscopy spectra is an overlap of charge-induced and electro-absorption features. In transistors, the electro-absorption is more significant during the high voltage drop. There are several ways to identify the electro-absorption contribution, such as get the second harmonic  , or probe it at the depletion region.