Giant and Unidiretional Recoil Optical Forces 

Light-induced forces have led to many exciting applications in nanotechnology and bioengineering by trapping, pushing, pulling, binding and manipulating nanoparticles and biological samples. The emergence of nano-optical plasmonic configurations has been exploited to boost the strength of optical forces at the nano-scale by exciting surface plasmon polaritons (SPPs), which are confined electromagnetic waves traveling along dielectric-metal interfaces. In fact, illuminating with light a nanoparticle located above a metallic surface induces optical forces on it that compensate the momentum of the directional SPPs excited in the scattering process. Such response has been further enhanced taking advantage of the quantum spin-Hall effect of light, exploiting the spin of circularly polarized incoming waves to control the excitation of directional plasmons and, in turn, the direction of the induced forces. Despite recent advances in this field, the exponential growth of nanophotonics and bioengineering is continuously imposing challenging demands to enhance the strength and control the direction of the forces induced on nanoparticles using low-intensity laser beams. 

In this research line, we investigate several mechanisms to dramatically enhance the strength and control the direction of the lateral optical forces induced on nanoparticles. For instance, we recently revealed that by simply replacing standard plasmonic surfaces with hyperbolic or extremely anisotropic metasurfaces enhances the strength of the lateral optical forces several orders of magnitude. The underlying physics are illustrated in Fig. 1. Upon adequate plane wave illumination, the particle behaves as an out-of-plane circularly polarized electric dipole that excites directional SPPs on the surface thanks to the photonic spin-Hall effect (see Fig. 1c). Due to momentum conservation, a lateral recoil force that strongly depends on the wavevector of the excited modes is induced on the particle. Since the SPPs supported by HMTSs possess very high wavenumbers compared to isotropic surfaces, the induced recoil forces over these structures are dramatically enhanced. Fig. 1b shows that such enhancement is maximum in the very near field of the metasurface and progressively lessens as the nanoparticle moves away from it. Fig. 1d-e show giant lateral optical forces over realistic HMTSs made of nanostructured silver, enabling a unique platform to route nanoparticles with low-intensity laser beams.

We are also exploring the use of Gaussian beams in combination with such surfaces to manipulate the recoil forces acting on the particles, so they can effectively be trapped and manipulated. In addition, we are investigating the use of non-reciprocal platforms to exert unidirectional and giant forces to the particles independently on the polarization and angle of direction of the incoming beams. Our works aims to enable new possibilities for the manipulation, trapping, assembly, and characterization of individual and multiple nanoparticles and structures, with strong implication in nanotechnology and biological systems.