Nature Photonics 11, 159–162 (2017); published online 23 January 2017; corrected after print 1 August 2017.

In the version of this Letter originally published, in the Acknowledgements, the following information was unavailable at the time of publication: “B.E.L. was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, grant no. XDB24030000.” This information has now been added in the online versions of the Letter.