Plasmon-enhanced Raman optical activity in chiral tip-nanorod configuration

Nanostructures facilitate ultrasensitive chiral measurements by enabling tunable plasmon-enhanced Raman optical activity (PEROA) spectra. Using finite element analysis, we systematically studied how structural parameters govern PEROA in chiral tip-nanorod configurations. Multipolar decomposition was performed to analyze the contribution of scattering multipole moments to PEROA. Interesting patterns of PEROA enhancement indicate different active areas for the out-of-plane and in-plane molecular vibration modes. Nanoscale spatial variations drastically alter both the magnitude and the sign of PEROA enhancement. Both hotspot positions and the out-of-plane PEROA enhancement exhibit a strong dependence on excitation wavelengths. The new PEROA mode originates from the enhanced differential scattering between RCP and LCP illuminations at the scattering minimum. Coherent superposition between PSPs-dominant scattering by the nanorod and LSPR-dominant scattering by the tip leads to high-directivity radiation patterns. Angularly resolved PEROA distributions reveal highly directional emission into the prism with a divergence angle <5^∘. The directivity for the out-of-plane molecular modes is stronger than that for the in-plane modes. Our findings could advance high-sensitivity PEROA spectroscopy and guide the rational design of chiral spectra platforms.