We thank S. C. Dhanabalan and J. S. Ponraj for their efforts in the early stages of this project. We thank M. H. Lu, L. Liu, C. W. Qiu and L. Wang for suggestions, and H. Yan and Q. Xing for their assistance with micro-FTIR measurements. We thank Quantum Design China (Beijing laboratory) for technical support of some s-SNOM measurements. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). We acknowledge support from the National Natural Science Foundation of China (grant numbers 51222208, 51290273, 51601131, 61604102, 51702219 and 91433107), the Youth 973 programme (2015CB932700), the National Key Research and Development Program (2016YFA0201900), ARC (DP140101501, IH150100006, FT150100450 and CE170100039), the Natural Science Foundation of Jiangsu Province (BK20150053), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Collaborative Innovation Center of Suzhou Nano Science and Technology, and the Spanish Ministry of Economy, Industry and Competitiveness (national projects MAT2015-65525-R, FIS2014-60195-JIN, MAT2017-88358-C3-3-R, MAT2014-53432-C5-4-R, and the project MDM-2016-0618 of the Maria de Maeztu Units of Excellence Programme). Q.B. acknowledges support from the Australian Research Council (ARC) Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET). P.A.-G. acknowledges support from the European Research Council under Starting Grant 715496, 2DNANOPTICA. J.M.-S. acknowledges support through the Clarín Programme from the Government of the Principality of Asturias and a Marie Curie-COFUND grant (PA-18-ACB17-29). P.L. acknowledges support from a Marie Sklodowska-Cuire individual fellowship (SGPCM-705960).

Reviewer information

Nature thanks A. Chaves and the other anonymous reviewer(s) for their contribution to the peer review of this work.