Lateral Optical Forces Achieve Stable Zones and Switching Bands in Nanostructures

Scientists are increasingly harnessing light to manipulate matter at the nanoscale, but controlling lateral optical forces remains a significant challenge. Bo Gao, Henkjan Gersen (iLof, Intelligent Lab on Fibre & H.H. Wills Physics Laboratory, University of Bristol) and Simon Hanna (H.H. Wills Physics Laboratory, University of Bristol) et al. have now explored how the geometry of nanoscale structures impacts these forces, specifically investigating triangular periodic motifs. Their computational study reveals that these structures exhibit distinct optical force responses linked to resonant light-matter interactions, identifying stable and switching behaviours dependent on structural parameters. This research is significant because it demonstrates a clear connection between structural design, resonant effects like Fano resonances, and the resulting optical forces , paving the way for the development of precisely controlled, optically-driven nanomechanical systems.

Their computational study reveals that these structures exhibit distinct optical force responses linked to resonant light-matter interactions, identifying stable and switching behaviours dependent on structural parameters. This research is significant because it demonstrates a clear connection between structural design, resonant effects like Fano resonances, and the resulting optical forces, paving the way for the development of precisely controlled, optically-driven nanomechanical systems.

Fano Resonance Drives Optical Force Switching in Microcavities

Scientists have demonstrated a computational study revealing the connection between lateral optical forces and resonant light-matter interactions in asymmetric dielectric nanostructures. Researchers examined isosceles triangular motifs, identifying two distinct types of optical force response when illuminated by a plane wave. Through detailed parameter-space analysis, the team identified stable zones exhibiting consistent optical forces, alongside switching bands where forces change abruptly with altered parameters. The observed force spectra displayed characteristic asymmetric lineshapes, strongly suggesting Fano-resonance behaviour, a phenomenon arising from interference between discrete and continuous light propagation states.
This breakthrough reveals how structural geometry influences optical forces through resonant effects, offering valuable guidance for designing optically-driven systems requiring controlled force responses. Experiments show that eigenfrequency analysis confirmed these effects originate from the interference between discrete eigenmodes and continuum propagation states, with the eigenmode Q-factors directly correlating with transition sharpness. The study unveils a ramp-and-step pattern in the optical force landscape, characteristic of Fano resonances, providing novel insights into potential applications and fabrication strategies for advanced optical devices. By analysing the shift in eigenfrequencies as a response to changing geometric parameters, scientists verified the association with Fano resonances and demonstrated the possibility of engineering the eigenmodes of the system to manipulate optical force for specific applications.

The research establishes a 3D model system suitable for fabrication, employing a computationally efficient method to simulate lateral optical forces for each structure. A periodic array of all-dielectric isosceles triangular motifs was used as a model, chosen for its single mirror symmetry and ease of parameterisation, described by only two geometric parameters, wx and wy, alongside a fixed motif thickness of 0.450μm. The team employed Bayesian optimization, a method renowned for efficiently finding near-global optima, to perform geometry optimisation and probe the maximum achievable lateral optical forces within the triangular motif system. Notably, Bayesian optimization revealed counterintuitive effects, including abrupt reversals in force direction from minimal geometric modifications and near-identical forces produced by visually distinct geometries. This work opens avenues for designing optical transducers and sensors, leveraging the precise control of optical forces through tailored nanostructure geometry and resonant effects, potentially impacting fields from nanoscale actuation to micro-robotics. The study’s findings provide a fundamental understanding of symmetry-breaking in light-matter interactions and its application in advanced optical manipulation techniques, such as particle sorting and self-stabilized levitation.

RCWA Simulation of Triangular Nanostructure Optical Forces reveals

Scientists investigated lateral optical forces within asymmetric dielectric nanostructures, specifically isosceles triangular motifs, to understand their relationship with resonant light-matter interactions. The study employed a three-dimensional model system designed for potential fabrication, utilising rigorous coupled-wave analysis (RCWA) to efficiently simulate optical forces for each structure. Researchers constructed a periodic motif with Λx = 0.950μm and Λy = 0.600μm periodicity, deliberately chosen to limit diffracted orders and optimise computational efficiency. The triangular motif, possessing a mirror plane perpendicular to the y-axis, was defined by two geometric parameters, wx and wy, representing the widths in the x- and y-directions, with corners rounded by a radius of R = 5nm to address fabrication constraints.

To accurately represent a realistic device, the team immersed the motif structure in water and layered it with 0.400μm of SiO2, a 0.450μm poly-Si isosceles triangle, and a final 0.400μm SiO2 encapsulating layer, resulting in a total device thickness of 1μm. Refractive indices of 1.33 for water, 1.45 for SiO2, and 3.45 for poly-Si were used at a wavelength of 1.064μm, corresponding to a frequency of 281.76THz. Illumination was achieved using a normally incident, x-polarized plane wave, ensuring y-directional symmetry and isolating the lateral optical force to the x-component (Fx). This configuration allowed the research to focus solely on the x-component of the total optical force, simplifying the analysis and enhancing precision.

The work pioneered an indirect method for calculating optical forces, deriving them from farfield diffraction efficiencies using a custom Python toolkit based on RCWA. This approach significantly reduced computational demands compared to direct Maxwell-stress-tensor methods, which require detailed nearfield knowledge. RCWA, a Fourier modal method approximating plane wave expansion, proved computationally efficient for calculating both optical force and diffraction efficiency in layered periodic structures. The team then converted the obtained diffraction efficiencies into optical forces, enabling a detailed investigation of the force landscape and the identification of ramp-and-step patterns characteristic of Fano resonances. Analysis of eigenfrequency shifts confirmed the association with these resonances, demonstrating the potential to engineer optical forces for applications like optical transducers or sensors.

Fano Resonance Drives Optical Force Switching in microcavities

Scientists have demonstrated a novel method for manipulating light using asymmetric dielectric nanostructures, revealing intricate connections between resonant light-matter interactions and lateral optical forces. The research focused on isosceles triangular motifs, exhibiting two distinct optical force responses under plane wave illumination, and identified stable zones alongside switching bands where forces change abruptly with varying parameters. Experiments revealed characteristic asymmetric lineshapes in the force spectra, strongly suggesting Fano-resonance behaviour, a phenomenon arising from interference between discrete eigenmodes and continuum propagation states. The team measured eigenmode -factors correlating with transitions, providing insights into how structural geometry influences optical forces through resonant effects.

Bayesian optimization was employed to probe the maximum achievable lateral optical forces (LOFs) within the triangular motif system, revealing counterintuitive effects: abrupt reversals in force direction from minimal geometric modifications, and near-identical forces produced by visually distinct geometries. This optimization process highlighted a ramp-and-step pattern in the LOF landscape, observed through systematic investigation of geometric parameters. Data shows this pattern is characteristic of Fano resonances, opening potential avenues for novel applications and fabrication strategies. Further analysis of eigenfrequency shifts in response to changing geometric parameters verified the association with Fano resonances, confirming the possibility of engineering eigenmodes to manipulate optical force for specific applications, such as optical transducers or sensors.

The model system comprised a 3D motif with periodicity of Λx = 0.950μm and Λy = 0.600μm, designed to limit diffraction orders. The isosceles triangular motif, rounded with a radius of R = 5nm to address fabrication concerns, is constructed from a 0.400μm thick layer of SiO2, a 0.450μm thick poly-Si motif layer, and a final SiO2 encapsulating layer, resulting in a total device thickness of 1μm. Rigorous coupled-wave analysis (RCWA) simulations were used to calculate optical forces indirectly from farfield diffraction efficiencies, significantly reducing computational cost. The simulations, conducted with a normally incident plane wave of λ = 1.064μm and frequency f = 281.76THz, revealed that only the 0 and ±1 orders of diffraction are allowed in the x-z plane, while higher orders are prohibited in the y-z plane. Optical forces were calculated in a dimensionless reduced unit, equivalent to physical forces divided by a factor of P/c, where P is the incident power and c is the speed of light, effectively isolating the light-device interaction. Tests prove the accuracy of this method, providing a robust framework for future investigations into optically-driven systems.

Fano Resonance Drives Nanostructure Optical Force Landscapes, enabling

Scientists have demonstrated that lateral optical forces in asymmetric dielectric nanostructures are strongly linked to resonant light-matter interactions. Through computational modelling of isosceles triangular motifs, researchers identified two distinct types of optical force response under plane wave illumination, revealing stable zones and switching bands within the parameter space. The observed force spectra exhibited asymmetric lineshapes characteristic of Fano resonance, indicating interference between discrete eigenmodes and continuum propagation states. This study mapped comprehensive landscapes of optical forces and diffraction efficiencies, revealing that geometrically dissimilar structures can produce nearly identical optical force responses, while minimal geometric modifications can cause dramatic force variations and reversals.

Detailed spectral analysis confirmed a recurring connection between switching bands and Fano-like responses, suggesting a universal mechanism for controlling optical forces through resonant effects. Eigenfrequency analysis definitively showed that these effects originate from Fano interference, with the Q-factors of eigenmodes correlating with transition rates. The authors acknowledge a limitation in focusing solely on isosceles triangular motifs, potentially restricting the generalizability of the findings to other geometries. Future research could explore the application of these principles to more complex nanostructures and investigate the potential for creating novel optical-driven devices through eigenmode engineering and incident frequency tuning.