Abstract
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.
Highlights
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
Raman optical activity (ROA)[1,2,3] results in differences in the Raman spectra excited by right- and left-circularly polarized light; this activity is extremely sensitive to the chirality of the molecular structure[4,5,6] and can reveal the absolute molecular configuration[5] or conformation.[6]
We report an experimental realization of remotely excited ROA via chiral plasmons propagating on Ag nanowires
Summary
Raman optical activity (ROA)[1,2,3] results in differences in the Raman spectra excited by right- and left-circularly polarized light; this activity is extremely sensitive to the chirality of the molecular structure[4,5,6] and can reveal the absolute molecular configuration[5] or conformation.[6]. Chiral-sensitive vibrational spectroscopy has become increasingly popular in biological applications, this approach has an obvious flaw that is difficult to overcome: the ROA scattering intensities are 1023–1025 times the intensities of the parent Raman scattering. The intrinsic weakness of ROA can be resolved by utilizing surface plasmon enhancement.[10,11,12,13,14,15,16,17] The local electric field and field gradients generated by plasmon resonance can significantly enhance the ROA and CID.[9] Surface-enhanced ROA is attributed to the coupling between the electric dipole and the electric quadrupole terms in the process of Raman scattering.[16] The quadrupole transitions can only be significantly excited in an oriented molecule close to the metallic substrate; under far-field excitations, the transitions are weak
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