Nature Photonics 9, 199–203 (2012); published online 16 February 2015; corrected after print 8 May 2015.

In the version of this Article originally published, in the expression for Leff on page 200 the exponential should have contained a minus sign and the expression should have read Leff = (1 − exp(−αL))/α. This has been corrected in the online versions.