Nature Communications 8:639 doi:10.1038/s41467-017-00691-5; Article published online: 21 September 2017

In the PDF version of this article, Equation 1 is missing an epsilon at the beginning. The correct version of Equation 1 is below. The HTML version of the paper was correct from the time of publication.

$$\epsilon \left( E \right) = \left( {n_{\mathrm{b}} + {\mathrm{i}}k_{\mathrm{b}}} \right)^2 + \frac{A}{{E_0^2 - E^2 - {\mathrm{i}}\gamma E}}.$$