Nature Photon. 7, 868–874 (2013); published online 20 October 2013; corrected after print 28 October 2013.

In the version of this Review Article originally published online and in print, the DOI was incorrectly specified as 10.1038/nphoton.2013.270. The correct DOI is 10.1038/nphoton.2013.280. This has now been corrected in both the HTML and PDF versions of the Review Article.