Tuning The Electromagnetic Waves With Metallic Metastructures

The property of nature material is determined by the atoms and the way the atoms are assembled.In the traditional material science,we have already get the materials with superior mechanical,thermal or electro-magnetic property by changing the chemical composition of the material or improving the way to fabricate the material.Metamaterial offers a new way to control the material property.The metamaterial is constructed with the artificial structures that are far smaller than the wavelength.These artificial structures act as the atoms or molecule in the traditional material.By designing the shape of the structure,we can directly control the response of the atoms to the electromagnetic wave.Therefore,the material with fantastic electromagnetic property can be realized,such as the material with negative refractive index.Because these artificial material offers more freedom to control the propagation of light,the extraordinary optical phenomenon such as optical cloaking,superlens can be realized.On the nanometer scale,most available fabrication methods are two dimensional fabrication techniques,such as focused ion beam etching,the electron beam lithography.Therefore,the research on the two dimensional metallic nanostructure is of great practical value.On the other hand,the thickness of the two dimensional metallic nanostructure is far smaller than the wavelength.By designing the structures that can interact with light effectively,we can control the polarization,phase,amplitude and the propagation direction of light in a limited space.In this paper,we theoretically and experimentally studied the methods to control the polarization,phase,amplitude and propagation direction of light via two dimensional metallic nanostructure array.When the light illuminates the surface of the metallic nanostructure,the current on the surface of the structure is stimulated by the electric field or the magnetic field of the incident light.We can use the capacitance,resistance and inductance to describe the property of the nanostructure.The optical response of the structure can be simplified as a circuit function,which is called the equivalent circuit.Based on the theoretical research achievement,we designed and fabricated the optical devices to control the polarization,amplitude and propagation direction.1 We firstly controlled the amplitude ratio and the phase difference of the transmitted light simultaneously based on tuning the time delay of the surface current on the L shaped nanostructure.In the designed frequency range,the amplitude ratio of the transmitted light keeps unit,while the phase difference changes linearly with frequency.This work shows a new method to control the polarization state of light.2 The metamaterial interact with light with the current resonance on the surface of the structure.Since the surface current changes significantly around the resonance frequency,the bandwidth of metamaterial is usually very narrow.In the metal-interlayer-metal system,due to the existence of the metallic mirror,the reflection of the structure is not only related to the property of the metallic structure,but also depend on the distance between the structure and its mirror image.By tuning the thickness of the interlayer,the dispersion of the reflected light changes consequently.Our research firstly point out that the dispersion induced by the thickness of the interlayer can compensate the dispersion of the radiation of the nanostructure layer,therefore,the dispersion free optical device can be realized.Based on this method,we successfully controlled the polarization state of light in a limited space(far smaller than the wavelength)and fabricated the dispersion free quarter wave plate and half wave plate experimentally.3 Perfect optical absorber is an important research area of metamaterial.We theoretical studied the absorption of the traditional metal-interlayer-metal absorber with equivalent circuit method and designed a three dimensional fish spear structure to optimize the performance of the optical absorber.The good electric and thermal conductivity is introduced in the design of absorber.4 By generating the phase gradient with the nano structure,the propagation direction of light can be controlled.For the metasurface that generating the phase gradient by rotating the building block,we firstly proved that the polarization conversion ratio of the building block determines the light intensity of the anomalous beam.By using the nano structure with large polarization conversion ratio as the building block of the metasurface,we experimentally get the anomalous beam with the light intensity larger than 70%of incident light from the wavelength 1100nm to 1700nm.This metasurface can be used to control the propagation direction of circular polarized light:it can generate two beams of light with different chirality with linearly polarized incident light,or reflect the circular polarized light to different direction depending on its chirality.