Extraordinary Multipole Modes and Ultra-Enhanced Optical Lateral Force by Chirality
Zhu T., Shi Y., Ding W., Tsai D. P., Cao T., Liu A. Q., Nieto-Vesperinas M., Sáenz J. J., Wu P. C., Qiu C
Strong mode coupling and Fano resonances arisen from exceptional interaction between resonant modes in single nanostructures have raised much attention for their advantages in nonlinear optics, sensing, etc. Individual electromagnetic multipole modes such as quadrupoles, octupoles, and their counterparts from mode coupling (toroidal dipole and nonradiating anapole mode) have been well investigated in isolated or coupled nanostructures with access to high Q factors in bound states in the continuum. Albeit the extensive study on ordinary dielectric particles, intriguing aspects of light-matter interactions in single chiral nanostructures is lacking. Here, we unveil that extraordinary multipoles can be simultaneously superpositioned in a chiral nanocylinder, such as two toroidal dipoles with opposite moments, and electric and magnetic sextupoles. The induced optical lateral forces and their scattering cross sections can thus be either significantly enhanced in the presence of those multipoles with high-Q factors, or suppressed by the bound states in the continuum. This work for the first time reveals the complex correlation between multipolar effects, chiral coupling, and optical lateral force, providing a distinct way for advanced optical manipulation.
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Characterization of the multipole modes supported by a dielectric chiral cylinder. (a) Spectra of scattering cross section and optical torques of the chiral nanocylinders near the BIC point. The solid lines and dashed lines represent the spectra and optical torques, respectively. The ϵ and κ of the nanocylinder are 80 and 0.4, respectively. (b) Comparison of optical forces in the x and y directions on the nanocylinders near the BIC point
