Nature 451, 311–314 (2008)

The experiment measures the differential laser transmission through the quantum dot between the on- and off-exciton resonance condition. As a result, the origin in Fig. 2a–h corresponds to the zero of the measured differential transmission and does not exclude the existence of constant background absorption. It is therefore important to note that the undershoot in the Fano spectra does not correspond to a negative absorption (that is, an optical gain), but is consistent with the continuum broadband background absorption, as analysed in our observation of the nonlinear Fano effect. Similarly, the theoretical graphs in Fig. 2i–n are also given for the differential transmission.