ИСТИНА

Bloch surface waves (BSWs) are propagating electromagnetic modes in photonic crystals which are known for their high-quality resonance and large local field enhancement. Unlike surface plasmon polaritons in metals, BSWs are excited in all-dielectric structures and not accompanied with local heating. At least partly for these reasons, BSWs can serve as a suitable biocompatible platform for optical manipulation and sorting of nano- and microobjects. In this work, the manipulation of particles with BSWs is demonstrated and BSW-induced optical forces are calculated and measured by photonic force microscopy (PFM). For measuring optical forces induced by BSWs, we use photonic force microscope based on conventional optical tweezers. The BSWs are excited at the interface between a dielectric multilayer and water. A polystyrene microparticle is localised in the optical tweezers trap near the multilayer surface and its displacement is measured when the BSW is excited. The particle is shown to be pulled to the surface by the gradient force and propelled in the BSW propagation direction by the scattering force. The dependence of the BSW-induced optical forces on the distance between the particle and the multilayer is obtained. At large distances, the forces decrease exponentially. However, at small particle/surface gaps there is a surprising diminution which is probably related to the presence of a surfactant in the suspension. Optical forces acting on a dielectric microparticle in the evanescent field of the BSW are estimated in the dipole approximation and calculated by integrating the Maxwell stress tensor in finite-difference time-domain (FDTD) simulation. The results show that a polystyrene particle with diameter of 1 micrometer does not perturb the BSW field significantly and can be used as a probe for imaging of BSWs. The manipulation of particles with BSWs is demonstrated. The motion of particles in the field of the BSW is observed. The scattering force is estimated using Stokes’ law using the propulsion velocity value. The obtained force is in agreement with the PFM results.