An exploration of lateral optical forces from a triangular periodic motif

TL;DR

This work addresses how geometric asymmetry in a periodic dielectric nanostructure induces lateral optical forces and how resonant light–matter interactions govern these forces. It deploys RCWA-based diffraction-efficiency calculations, Bayesian optimization to explore geometry, and spectral plus eigenfrequency analyses to link LOF behavior to Fano resonances. The study reveals two regimes—stable zones with robust LOF and switching bands with abrupt force changes—accompanied by dip-and-peak Fano-like spectra whose sharpness tracks eigenmode Q-factors. These insights offer practical guidance for designing optically driven devices with tailored LOF responses, leveraging geometry–resonance coupling in metastructures.

Abstract

This computational study investigates lateral optical forces in asymmetric dielectric nanostructures, focusing on their connection to resonant light-matter interactions. We examine isosceles triangular motifs that exhibit two distinct types of optical force response under plane wave illumination. Through parameter-space analysis, we identify stable zones where optical forces remain consistent and switching bands where forces change abruptly as parameters are altered. The observed force spectra show characteristic asymmetric lineshapes, suggesting Fano-resonance behavior. Eigenfrequency analysis confirms these effects arise from interference between discrete eigenmodes and continuum propagation states, with the eigenmode Q-factors correlating with transition sharpness. These findings provide insights into how structural geometry influences optical forces through resonant effects, offering guidance for designing optically-driven systems where controlled optical force responses are desired.

Figures (7)

• Figure 1: (a) A schematic view of the triangle model and coordinate systems. Periodic boundary conditions are applied along the x- and y-directions. The structure consists of three layers: the substrate layer, the middle motif layer, and the capping layer. The isosceles triangles in the motif layer are oriented to be symmetric in the y-direction. (b) A top-view presentation of the triangle shape in the motif layer. Definitions of major shape parameters are denoted in the figure. (c) A schematic view of the optical system in the X-Z plane. The incident light is a plane wave linearly polarized in the x-direction, propagating along the z-direction. The diffraction efficiencies (DEs) in both reflection and transmission sides, as well as the total optical force $\mathbf{F}_{\rm tot}$ and its x- and z-components, are presented.
• Figure 2: (a) A contour plot of LOF. The color at each point represents the value of ${\rm F_x}$ of the structure with corresponding ${\rm w_x}$ and ${\rm w_y}$. Red and blue indicate the direction of ${\rm F_x}$ being positive or negative, respectively. (b) A contour plot of the forward component of optical force, ${\rm F_z}$, for completeness. (c-f) Four structures along with their corresponding ${\rm w_x}$, ${\rm w_y}$ and ${\rm F_x}$ values are labeled aside as examples. The motif thickness in this figure is kept fixed at h=0.450. The six structures summarized in Table \ref{['tab:BO']} are also marked on the figure using the same labels.
• Figure 3: Diffraction efficiency contour plot of (a) $T^-$, (b) $T^0$, (c) $T^+$, (d) $R^-$, (e) $R_0$, and (f) $R^+$ diffraction channels, as defined in Figure \ref{['fig:scheme']}(c). The six structures summarized in Table \ref{['tab:BO']} are also marked on the figures for reference.
• Figure 4: Focusing on the structure with ${\rm w_x}=\qty{0.725}{\um}$, ${\rm w_y}=\qty{0.515}{\um}$, and triangle height ${\rm h}=\qty{0.45}{\um}$, the ${\rm F_x}$ responses are shown with respect to changes in (a) ${\rm w_x}$, (b) ${\rm w_y}$, and (c) $\rm h$. The wavelength dependence of (d-f) ${\rm F_x}$ and (g-i) ${\rm F_z}$ of the three series of structures with a gradually shifted color scheme are displayed, respectively. The vertical dashed lines represent the reference structure in (a-c) and the reference incident wavelength in (d-i). The horizontal dotted lines in (a-f) mark ${\rm F_x}=0$.
• Figure 5: A selected loop K-L-M-N-K in the ${\rm F_x}$ landscape, with segments crossing switching bands circled in black and denoted by $\alpha,\,\beta,\,\gamma,\,\delta,\,\zeta,\,~\text{and}~\xi$, respectively. The six structures summarized in Table \ref{['tab:BO']} are denoted by blue markers and labels for reference. Four switching bands are sketched and labeled with dashed lines for reference.