Sorting of extremely small nanoparticles by membranes supporting symmetry-protected bound states in the continuum

We consider double perforated slabs (membranes) that support a symmetry-protected bound state in the continuum (BIC). These slabs are immersed into a liquid at room temperature. Laser excitation of the BIC generates giant optical forces that strongly affect the Brownian motion of nanoparticles in a colloidal solution. By solving of the Fokker-Planck equation we show that a single membrane can localize only larger nanoparticles. However, system of two parallel membranes can trap extremely small nanometer-sized nanoparticles by a resonant excitation of quasi-BICs, with varying intensities in each membrane.