Interference micro/nanolenses of salts for local modulation of Raman scattering

Abstract

Micro/nanolenses play a crucial role in optics and spectroscopy, but the effect of interference patterns within each lens has been largely unexplored. Herein, we investigate modulation of Raman scattering by the interference within a single micro/nanolens of a hygroscopic salt. Lenses having two different diameter (d) ranges, d > 2 μm and d ∼1 μm, are placed on a silicon substrate, followed by collection of a Raman intensity map of the silicon peak. Lenses with d > 2 μm show dark and bright circular fringes in the Raman map, resembling the Newton's rings formed by optical interference. In the smaller lenses (d ∼1 μm), the map yields only a single peak at the center, representing either an intensity maximum or minimum. In both diameter ranges, whether the Raman intensity is enhanced or suppressed is determined by interference conditions, such as wavelength of the excitation laser or thickness of the SiO₂ layer. The interference in salt micro/nanolenses finds applications in local modulation of Raman scattering of a nanoscale object, as demonstrated in individual single-walled carbon nanotubes decorated with the salt lenses.