Fourier Plane Tomographic Spectroscopy Reveals Orientation-Dependent Multipolar Plasmon Modes in Micrometer-Scale Janus Particles

Precise control of light-matter interactions is a cornerstone of next-generation technologies, from ultrasensitive biosensing and single-molecule tracking to the development of adaptive metamaterials. While small, symmetric nanostructures are well-understood, micrometer-scale plasmonic Janus particles (pJPs), comprising dielectric cores with thin metallic caps, exhibit complex optical properties due to their asymmetric structure. Despite widespread applications in active matter research, their orientation-dependent scattering properties remain largely unexplored. We introduce Fourier plane tomographic spectroscopy for simultaneous four-dimensional characterization of scattering from individual micrometer-scale particles across wavelength, incident angle, and scattering angle. Combining measurements with finite-element simulations, we identify discrete spectral markers in visible and near-infrared regions that evolve predictably with cap orientation. Spherical-harmonics decomposition reveals that these markers arise from three distinct multipolar modes up to fifth order: axial-propagating transverse-electric, transverse-propagating transverse-electric, and transverse-propagating axial-electric, with retardation-induced splitting. We observe progressive red-shifts and line width narrowing of higher-order resonances, demonstrating curvature's influence on mode dispersion. Orientation-specific scattering patterns exhibit polarization-dependent features enabling optical tracking of particle rotation. Beyond pJPs, this methodology establishes a general framework for characterizing asymmetric nanostructures of diverse material combinations and geometries, offering a toolkit for designing orientation-responsive nanoantennas, reconfigurable metasurfaces, active colloidal systems with tailored light-matter interactions, and high-precision optical tracking of particle rotation.