Research

Optics is generally interpreted as a world of electric fields and electric dipoles because magnetic-dipole transitions at optical (visible) frequencies are very weak. Can magnetic dipole resonance serve as active element in nano-optics similar to the electric dipole resonance? We take two different approaches to address this question. (i) We create light with spatially varying polarization properties (vector beams), and use this light to enhance and control magnetic dipole resonance in the optical frequencies. (ii) We design materials with structural symmetry, and excite them with cylindrical beam to enhance and control magnetic dipole transitions in the optical frequencies.

The magnetic dipole resonance can serve as probe of magnetic fields similar to the electric dipole resonances that are being used as probes to study electric light-matter interactions as well as realize novel optical phenomena, such as Invisibility Cloak, Unidirectional Scattering, Fano Resonances, Dark Mode, etc.

We collaborate with Prof. Hiroshi Sugimoto at the Mesocopic Materials Laboratory at the Kobe University, and Prof. Mahua Biswas at Illinois State University.

EXCITATION OF NONRADIATING ANAPOLES IN DIELECTRIC NANOSPHERES

Although the study of nonradiating anapoles has long been part of fundamental physics, the dynamic anapole at optical frequencies was only recently experimentally demonstrated in a specialized silicon nanodisk structure. We report excitation of the electrodynamic anapole state in isotropic silicon nanospheres using radially polarized beam illumination. The superposition of equal and out-of-phase amplitudes of the Cartesian electric and toroidal dipoles produces a pronounced dip in the scattering spectra with the scattering intensity almost reaching zero—a signature of anapole excitation. The total scattering intensity associated with the anapole excitation is found to be more than 10 times weaker for illumination with radially vs linearly polarized beams. Our approach provides a simple, straightforward alternative path to realizing nonradiating anapole states at the optical frequencies.

SELECTIVE INDUCTION OF OPTICAL MAGNETISM

An extension of the Maxwell–Faraday law of electromagnetic induction to optical frequencies requires spatially appropriate materials and optical beams to create resonances and excitations with curl. Here we employ cylindrical vector beams with azimuthal polarization to create electric fields that selectively drive magnetic responses in dielectric core–metal nanoparticle “satellite” nanostructures. These optical frequency magnetic resonances are induced in materials that do not possess spin or orbital angular momentum. Multipole expansion analysis of the scattered fields obtained from electrodynamics simulations show that the excitation with azimuthally polarized beams selectively enhances magnetic vs electric dipole resonances by nearly 100-fold in experiments. Multipolar resonances (e.g., quadrupole and octupole) are enhanced 5-fold by focused azimuthally versus linearly polarized beams. We also selectively excite electric multipolar resonances in the same identical nanostructures with radially polarized light. This work opens new opportunities for spectroscopic investigation and control of “dark modes”, Fano resonances, and magnetic modes in nanomaterials and engineered metamaterials.

This work was performed by Dr. Manna during his post-doctral tenure at the University of Chicago.