Remotely excited Raman optical activity using chiral plasmon propagation in Ag nanowires

We experimentally investigated remotely excited Raman optical activity (ROA) using propagating surface plasmons in chiral Ag nanowires. Using chiral fmoc-glycyl-glycine-OH (FGGO) molecules, we first studied the local surface plasmon-enhanced ROA. We found that the Raman intensity can be excited by left- and right-circularly polarized lights and that the circular intensity difference (CID) can be significantly enhanced. Second, by selecting vibrational modes with large Raman and ROA intensities that are not influenced by chemical enhancements, we studied remotely excited ROA imaging and the CID of FGGO molecules by propagating a plasmonic waveguide using Ag chiral nanostructures. When laser light was radiated on one of the Ag terminals, the measured CID of the FGG at the other terminal showed little change compared to the local excited CID. Meanwhile, when the laser light was radiated on the Ag nanowires (not on the terminals) and was coupled to the nearby nanoantenna, the CID of the ROA could be manipulated by altering the coupling angle between the Ag nanowires. To directly demonstrate the propagation of ROA along the nanowire and its remote detection, we also measured the remotely excited ROA spectra. Our experimental method has the potential to remotely determine the chirality of molecular structures and the absolute configuration or conformation of a chiral live cell. Researchers in China have developed an optical technique for probing the chirality of molecular structures. Mengtao Sun and co-workers enhanced the strength of Raman measurements by using silver chiral nanowires as plasmonic waveguides. Such measurements, which determine the differences in Raman spectra excited by left- and right-handed circularly polarized light, can then be used to ascertain the chirality and conformation of the sample. A further benefit of this approach is that the use of nanowires allows the excitation and detection to be performed remotely. In their initial studies, the researchers studied a sample of chiral fmoc-glycyl-glycine-OH molecules, but say that the approach also suits use with living cells.