Reconfigurable label-free shape-sieving of submicron particles in paired chalcogenide waveguides

Up-to-date particle sieving schemes face formidable challenges for sieving label-free submicron molecules with similar sizes and dielectric constants but diverse shapes. Herein, optical sorting of polystyrene particles with various shapes is illustrated in optofluidic nanophotonic paired waveguide (ONPW) composed of chalcogenide semiconductor Sb₂Se₃. The Sb₂Se₃-ONPW creates the coupling length (C_L) between the neighboring hot spots that can be actively modulated via the transition of Sb₂Se₃ between amorphous (AM) and crystalline (CR) phases. Submicron particles interfere with the coupled hotspots, which can exert various optical torques on the particles according to their profiles. In the model system, spherical (diameter of 0.5 μm) and rod-shaped (diameter of 0.5 μm, length of 1.5 μm) polystyrene particles were employed to mimic two types of bacteria, namely, Staphylococcus aureus and rod-shaped Escherichia coli, respectively. For the AM state, the C_L value is ∼7.0 μm, enabling the structure to trap the sphere stably in the hot spots. For the CR state, the C_L value becomes ∼25 μm, leading to stable trapping of the rod-shaped particle. In this work, the working wavelength was fixed at 1.55 μm at which both AM- and CR-Sb₂Se₃ are transparent. Our scheme may offer a paradigm shift in shape-selective sieving of biomolecules and fulfill the requirements of the new-generation lab-on-chip techniques, where the integrated manipulation system must be much more multifunctional and flexible.