Nature Photon. 7, 948–957 (2013); published online 28 November 2013; corrected after print 28 November 2013.

In the version of this Review published in print, “Si/Ge (ref. 23)” was listed on the eight line of the second column on the second page (page 949). This should have read “Ag/Ge (ref. 23)”. This error has been corrected in both the HTML and PDF versions of the Review.