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EXPLORING LIGHT-MATTER INTERACTIONS AT THE NANOSCALE
When we think about light, most of us imagine how it interacts with materials — how it shines, reflects, or passes through materials. At the microscopic level, this usually comes down to electric dipoles — atoms or molecules that act like tiny electric antennas responding to light.
For more than a century, optics has mostly focused on these electric effects. That’s because, at visible light frequencies, magnetic interactions — the way light interacts with magnetic dipoles — are extremely weak.
But now, scientists are asking a bold new question: Can magnetic dipoles also play a powerful role in shaping and controlling light?
To explore this, we are taking two creative approaches:
Special kinds of light (vector beams): They make light whose polarization — the direction in which its electric field points — changes across the beam. These vector beams can be used to strengthen and control the usually weak magnetic dipole resonance in materials at optical frequencies.
Specially designed materials: By carefully shaping materials at the nanometer scale (a billionth of a meter!) and using light with cylindrical symmetry, we can boost and control magnetic dipole transitions.
Why is this important?
Because magnetic dipole resonances could help us sense and manipulate magnetic fields with light — in the same way that electric dipoles already help us explore electric light–matter interactions.
This could even lead to fascinating effects like: Invisibility cloaks (guiding light around objects), Unidirectional scattering (light going only one way), Fano resonances (sharp interference patterns of light), and Dark modes (hidden energy states that don’t radiate light).
In short, by learning how to control both the electric and magnetic sides of light, we can open new doors to the future of nano-optics and photonic technology.
Collaborators:
Prof. Hiroshi Sugimoto at Mesocopic Materials Laboratory at Kobe University
Prof. Mahua Biswas at Illinois State University
Dr. Jorge Olmos Trigo at Donostia International Physics Center (DIPC)
DETECTION OF HYBRID OPTICAL ANAPOLES IN DIELECTRIC MICROSPHERES
Nonradiating optical anapoles are special configurations of charge-current distributions that do not radiate. It was theoretically predicted that, for microspheres, electric and magnetic dipolar coefficients can simultaneously vanish by engineering the incident light, leading to the excitation of nonradiating hybrid optical anapoles. In this work, the experimental detection of hybrid optical anapoles in dielectric microspheres (TiO2) is reported using dual detection optical spectroscopy, developed to enable sequential measurement of forward and backward scattering under tightly-focused Gaussian beam (TFGB) illumination. The results show that the excitation of TiO2 microspheres (diameter, d ≈1 µm) under TFGB illumination leads to the appearance of scattering minima in both the forward and backward directions within specific wavelength ranges. These scattering minima are found to be due to vanishing electric and magnetic dipolar coefficients associated with hybrid optical anapoles. The ability to confine electromagnetic fields associated with hybrid optical anapoles can give rise to several novel optical phenomena and applications.
More details: Advanced Optical Materials, e01315, (2025).
NANOPHOTONIC PHENOMENA IN MID-INDEX MESOSCALE DIELECTRIC MATERIALS
Nanophotonic phenomena, such as zero optical back scattering, nonradiating anapole states, etc. are related to the excitation of single dipolar modes—hence so far, they have only been observed within a few relatively high-index dielectric materials (refractive index, n > 3.5) in the nanoscale regime at the optical frequencies. Here, dipolar regime is unraveled, close-to-zero backscattering is demonstrated, and optical anapoles are excited in mid-index dielectric spheres (titanium di-oxide, TiO2; n ≈ 2.6) at the mesoscale regime (particle diameter, d ≈ incident wavelength, λ) under illumination with tightly focused Gaussian beams (TFGBs). Successive scattering minima associated with dipolar excitation are observed satisfying the first Kerker condition in the scattering spectra of single TiO2 spheres with diameters in the micrometer range. Moreover, at specific wavelengths, the electric and magnetic dipolar scattering amplitudes of the dielectric microspheres simultaneously go close-to-zero, leading to the excitation of hybrid optical anapoles with a total scattering intensity ≈ 5 times weaker for TFGB illumination with numerical aperture, NA ≈ 0.95 compared to NA ≈ 0.1. The result pushes the boundary of the observation of nanophotonic phenomena to a new regime with regards to type and size of the materials.
More details: Advanced Optical Materials, 2202140, (2023).
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
More Details: Physical Review Letters, 124, 097402 (2020);
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. 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. This work opens new opportunities for spectroscopic investigation and control of “dark modes”, Fano resonances, and magnetic modes in nanomaterials and engineered metamaterials.
More Details: Nano Letters, 17 (12), 7196 (2017).
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