Large-numerical-aperture optical platform for accurate scattering and far-field characterization of nanoplasmonic structures using supercontinuum laser illumination.

Nanoplasmonic structures have attracted significant interest due to their remarkable capability to confine and manipulate light at subwavelength scales. A critical limitation of conventional dark-field microscopy for characterizing these nanostructures arises from the inherently restricted numerical aperture (NA) of the objective lens, typically not larger than 0.8, while many nanostructures radiate beyond this angle range. In addition, the omnidirectional illumination scheme using a randomly polarized lamp and a ring-shaped condenser, as in conventional dark-field microscopes, results in complicated, non-uniform, and uncontrollable distributions of polarization states and light incident angles across the field-of-view, varying with position and wavelength. As a result, current dark-field microscopy techniques struggle to accurately and completely characterize the fine features of nanoplasmonic scattering patterns and spectra, which are closely correlated to the subtle details of the nanostructures. To address this challenge, here, we replace the omnidirectional illumination scheme with a nearly collimated and polarization-controlled supercontinuum laser source, which obliquely illuminates the sample through the objective's working distance. With this experiment platform, we achieved a uniform excitation of plasmonic nanoparticle-on-mirror structures, a high robustness against optical misalignment and aberration, and far-field imaging with a NA of up to 0.9.