UC Davis

Artificially engineered structures, namely metamaterials and their two-dimensional (2D) counterpart –metasurfaces, have been proven as promising platforms to realize unusual light-matter interactions. Such structures have enabled the efficient manipulation of electromagnetic waves in unprecedented ways that cannot be obtained using conventional materials and thus have triggered exciting applications such as hyperlensing, canalization of light, negative refraction, hyperbolic dispersion, cloaking, or the enhancement of the spontaneous emission rate of dipole emitters, amongst many others. Moreover, the discovery of graphene and other 2D materials, and their electrical tunability, have enabled the use of surface plasmon polaritons at terahertz and infrared frequencies in numerous nanophotonic applications.

In this thesis, I propose novel nano-optical plasmonic tweezers based on hyperbolic and nonreciprocalmetasurfaces to efficiently trap and manipulate nanoparticles in the near field with superior performance compared to the state of the art. To this purpose, I develop a rigorous theoretical framework able to compute optical forces on dipolar Rayleigh nanoparticles located near the metasurfaces. The theoretical model is based on Lorentz force within the dipole approximation combined with the scattered dyadic Green’s function of the system. Analytical expressions show that the force strength is directly proportional to the fourth power of wavenumber of the supported surface plasmons. This tells that the strength can be dramatically enhanced by the proper choice of metasurfaces that support ultra-confined surface plasmons with larger wavenumber. One potential candidate to achieve such response is the use of hyperbolic metasurface that supports surface plasmons with wavenumbers up to ~200 times larger than the ones supported in free space. My theoretical and numerical results using full wave simulations show that the use of hyperbolic metasurfaces enables unusual enhancement of the force strength (up to 3 orders of magnitude) in comparison to the one obtained above conventional isotropic media. Importantly, such response enables stable lateral trapping and efficient manipulation of nanoparticles using low-power laser beam thus reducing the photodamage threat. However, these general optical tweezers are static in the sense that their response cannot be dynamically controlled.

In this context, drift-biased nonreciprocal graphene has emerged as a promising platform to electrically tuneand manipulate the dispersion characteristic of the supported modes. I propose the use of this platform as a planar plasmonic hyperlens that provides ultra-subwavelength imaging with remarkable resolution over a broadband frequency range that cannot be obtained by other artificially engineered structure. In addition, drift-biased graphene can also readily be applied in the context of optical tweezers to provide novel responses: (i) particles can be manipulated unidirectionally independent to the direction of the incoming light, overcoming beam alignment challenges occurring in conventional optical tweezers; and (ii) the location of optical traps can efficiently be manipulated over a few microns range thanks to the electrically tunable response. In summary, I envisage that the proposed nano-optical hyperbolic and nonreciprocal plasmonic tweezers may open unprecedented venues for routing, trapping, and assembling nanoparticles and can effectively address some of the shortcomings of current techniques.