Far-field nanoscale infrared spectroscopy of vibrational fingerprints of molecules with graphene plasmons

Infrared spectroscopy, especially for molecular vibrations in the fingerprint region between 600 and 1,500 cm(-1), is a powerful characterization method for bulk materials. However, molecular fingerprinting at the nanoscale level still remains a significant challenge, due to weak light-matter interaction between micron-wavelengthed infrared light and nano-sized molecules. Here we demonstrate molecular fingerprinting at the nanoscale level using our specially designed graphene plasmonic structure on CaF2 nanofilm. This structure not only avoids the plasmon-phonon hybridization, but also provides in situ electrically-tunable graphene plasmon covering the entire molecular fingerprint region, which was previously unattainable. In addition, undisturbed and highly confined graphene plasmon offers simultaneous detection of in-plane and out-of-plane vibrational modes with ultrahigh detection sensitivity down to the sub-monolayer level, significantly pushing the current detection limit of far-field mid-infrared spectroscopies. Our results provide a platform, fulfilling the long-awaited expectation of high sensitivity and selectivity far-field fingerprint detection of nano-scale molecules for numerous applications.National Basic Research Program of China [2015CB932400]; National Natural Science Foundation of China [51372045, 11504063, 11174252, 11474006, 51522201, 91433102]; International Science and Technology Cooperation Project (China) [2014DFR10780]; Academy of Finland [276376, 284548, 295777]; TEKES (OPEC); Nokia foundation; European Union [631610]SCI(E)ARTICLEkhliu@pku.edu.cn; zhipei.sun@aalto.fi; daiq@nanoctr.cn