1. Beijing National Laboratory for Molecular SciencesCAS Research/Education Center for Excellence in Molecular SciencesLaboratory of Polymer Physics and ChemistryInstitute of ChemistryChinese Academy of Sciences Beijing 100190 P. R. China
2. University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. European Laboratory for Non‐Linear Spectroscopy (LENS)Università di Firenze Sesto Fiorentino FI 50019 Italy
4. Istituto Nazionale di Ricerca Metrologica (INRiM) Turin 10135 Italy
5. Department of PhysicsUniversità di Firenze Sesto Fiorentino FI 50019 Italy