Nature Photonics 3, 220–224 (2009); published online 22 March 2009, corrected after print 24 March 2009.

In the version of this article originally published, in the caption to Fig. 4a the distance between the dashed lines was incorrectly given as 0.050 nm; this should have read 0.050 μm. The error has now been corrected in the HTML and PDF versions.