Metasurface-enabled Optical Waveplates Based On Surface Plasmons

Modern science and technology ask for more miniaturized and integrated optic devices,which stimulates the development of nanophotonics.Metasurface,which is an ultrathin artificial planar material that consists of subwavlength structures,is a new method for optical manipulation at nanoscale.Based on surface plasmons that are excited in metasurface,amplitude and phase of the transmitted or reflected light field can be easily and deeply modulated by engineering the geometry of the nanostructured metasurface.Due to its exceptional capabilities to tailor the optic response in an easy,light and integrated manner when compared with that of conventional bulk metamaterials,it has attracted considerable attentions.In this thesis,we investigate the theory and design of metasurface enabled waveplates based on surface plasmons.The main works are given as follows:1)An ultrathin plasmonic quarter waveplate using a subwavelength broken rectangular annulus arrays embedded in a silver film was proposed.We have demonstrated numerically that the elimination of the overlapping parts at four corners of a regular rectangular annulus increases significantly the phase anisotropy induced by localized surface plasmons,and thus decreases significantly the thickness of the metal film in order to achieve the required phase difference for a designed waveplate.Simulations show that an ultrathin quarter waveplate can be obtained with an only 10nm thickness of the metal at 1.55?m.We further put forward an ultrathin quarter waveplate based on subwavelength Au gratings on Si substrate with a SiO₂ anti-reflection layer.Grating structures have strong anisotropy,in which the height of the metallic grating can be reduced.By increasing the duty cycle of the grating,transmission of both TM waves and TE waves can be manipulated and function of waveplate can be implemented.An ultrathin near-perfect broadband 1/4 waveplate based on metallic grating was obtained for the first time,in which the thickness of the metallic grating is only 6nm,and the bandwidth of the waveplate can reach 340nm at central operational wavelength of 1.55?m.2)In order to broaden the bandwidth of the waveplate,a metasurface enabled quarter waveplate consisted of Ag fence-type gratings is proposed.Investigations show that by manipulating the dimensions of the fence-type gratings along two perpendicular directions in which two independent localized plasmonic resonances along the two directions dominate,the phase difference between the two orthogonal electric components can be flattened over an ultrabroad bandwidth.An ultrabroad quarter waveplate was realized at near-mid infrared region from 2000nm to 4500nm with the optimized parameters and the control of polarization orientation of a linearly polarized incident light.We further put forward a simplified ultrabroad quarter waveplate design based on subwavelength Ag gratings on SiO₂ substrate.Numerical simulation shows that period of the proposed grating determines the position of the plasmonic resonance,while duty cycle as well as the height of the grating controls the phase difference between two orthogonal electric components.Two optimized ultrabroad quarter waveplates based on the grating structure operating,respectively,at near and mid infrared region ranging from 760nm²000nm and2240nm⁵000nm were numerically demonstrated.The results of the design provide a novel method for designing ultrabroad nano optical devices.