Unraveling Dipolar Regime and Kerker Conditions in Mid‐Index Mesoscale Dielectric Materials

• Published in: Advanced optical materials 2023-02, Vol.11 (3), p.n/a
• Main Authors: Coe, Brighton, Olmos‐Trigo, Jorge, Qualls, Dylan, Alexis, Minani, Szczerba, Michal, Abujetas, Diego R., Biswas, Mahua, Manna, Uttam
• Format: Article
• Language: English
• Subjects: Backscattering; Dielectrics; Excitation; Gaussian beams (optics); Illumination; Kerker condition; Materials science; mesoscale interactions; Mesoscale phenomena; Microspheres; mid‐index materials; nanophotonics; Numerical aperture; optical anapole; optical spectroscopy; Optics; Particle size; Refractivity; Scattering; Titanium dioxide; zero backscattering
• ISSN: 2195-1071; 2195-1071
• DOI: 10.1002/adom.202202140

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. In this manuscript, light‐matter interactions of relatively low‐index wavelength‐scaled dielectric particles are studied. More specifically, dipolar regime is unraveled, close‐to‐zero backscattering is demonstrated, and non‐radiating anapole states are excited in mid‐index dielectric spheres at the mesoscale regime. The results will open up enormous possibilities in terms of the availability of materials and their size parameters well beyond the current physical picture.