Size-selective optical trapping of nanoparticles with bound states in the continuum

https://doi.org/10.1016/j.optlaseng.2023.107797

We consider a waveguide with a symmetrically integrated silicon cylinder. This design supports a symmetry protected bound state in the continuum (BIC) with Q-factor controlled by slight displacement of the cylinder. When excited by a TE₁₀ electromagnetic wave, the BIC leads to giant optical forces near the cylinder. These forces have a strong impact on nanoparticles being dragged by liquid flow over the waveguide as they approach the cylinder. At the same time, the nanoparticles perturb the resonant frequency of the BIC with a value proportional to their volume and proximity to the cylinder. Therefore, the interplay between the resonant width of the BIC and the nanoparticle frequency perturbation determines the positions of the nanoparticles trapped around the cylinder. This paradigm demonstrates resonant self-trapping and sorting of nanoparticles by size through BIC excitation. We highlight the extreme sensitivity of these effects to the frequency of the injected TE wave. Additionally, we show that these results remain valid when considering the finite conductivity of metal waveguides.