Phase Manipulation And Mode Coupling In Metasurface Nanostructures

Metasurfaces are two-dimensional artificial materials with thicknesses much smaller than incident light wavelength.Due to the strong ability to control the phase,amplitude,and polarization of light at subwavelength resolution,it has become a new research direction and hotspot in the field of metamaterials and nano science.Compared with bulk metamaterials,metasurfaces possess the merits of small thickness,low loss and easy fabrication and have potentials in a series of applications such as wave manipulation,optical antenna,planar optics,etc.Recent researches on low loss dielectric materials such as silicon,Ge,Ti O2 show that all dielectric meta-atoms can also have strong magnetic responses and open new doors for designing high efficiency and performance functional metasurface devices.This thesis mainly focus on two typical metasurfaces,Huygens' metasurface and geometric metasurface,and their applications in metasurface hologram,beam deflection,and planar lens;the near-field coupling between the meta-atoms is also investigated,which is essential for applications such as refractive sensing,optical filter,and optical modulator.Wave manipulations of silicon nanodisks based on the Huygens' principle are investigated.Silicon nanodisks with 7 different radii are chosen as the basic meta-atoms to realize phase modulation due to the low loss nature and simultaneous excitations of electric and magnetic dipole resonances in one element.The 7 level phase map is discretized according to the classical Gerchberg–Saxton algorithm.Experimental results agree very well with the calculations and demonstrate an optical efficiency of 23.6% and transmission efficiency up to 86%;a theoretical method using spatial multiplexing of dielectric metasurface sub-cells to realize colorful metasurface hologram is demonstrated.Silicon nanobars supporting narrow spectral response are chosen as the basic meta-atoms to imprint the geometric phase and response three primary colors to reconstruct arbitrary RGB images.The crosstalk between different colors,spatial overlap and alignment of the three image components,incident intensities for primary colors,and color match between computer display and hologram colorimetric system are demonstrated.A high-efficiency transmissive geometric metasurface combining the recently developed Huygens' surface is demonstrated.Silicon cross-shaped antenna supporting orthogonal electric and magnetic dipole resonances can be exquisitely constructed to fulfill the required surface polarizability tensor for a half-wave plate by which an incident circularly polarized light beam can be almost fully transformed to a beam of opposite helicity with an unprecedented efficiency up to 98%.The geometric phases covering from 0 to 2? are picked up by spatially rotating the antenna orientations while the transmission efficiencies vary slightly for all rotation angles,thus,providing the full control over the wavefront.Simulations using finite-difference-time-domain method show the great flexibility of such metasurface for anomalous refraction and beam focusing.The mode coupling through near-field interaction in a binary metal particle array is investigated.Two combined arrays hybridize to generate anti-phased lattice collective resonances and in-phased lattice collective resonances through lattice coupling effects.The Fano lineshape can be easily tailored by tuning the radius.Q factor and spectral contrast following different evolution tendencies for the variation of particle radii in the arrays can remain at high values simultaneously,which is very promising for a series of practical applications.Taking advantage of the high Q factor and high spectral contrast,the figure of merit(FOM)of the structure as a refractive sensor can be 1 or 2 orders higher than conventional plasmonic sensors.The mode coupling in an all dielectric binary particle array is investigated.Very different from that of meta-atoms made of metal,in such two sets of particle arrays made of silicon the trapped modes originate not only from the anti-phased electric dipole collective resonances,but also from the anti-phased magnetic dipole and quadrupole collective resonances.The structure has potentials in applications such as refractive sensor,optical filter,and optical modulator.As an optical filter,the silicon binary array offers an efficient way to tune the linewidth simply by changing the radius of the particles.Infiltrated with nematic liquid crystal,E7,the binary array can be used as an optical modulator with the transmission continuously tuned by an applied voltage.An unprecedented modulation depth up to 85% is achieved.