Design Novel Subwavelength Structures And Functional Devices Based On The Principle Of Transformation Optics

In the past twenty years,the development of metamaterial has induced many novel electromagnetic properties,which cannot happen naturally in the traditional optical material,such as artificial magnetism,negative refraction and optical cloaking.Research on the topic of metamaterials will not only deepen our understanding of wave propagation in artificial structures,but open new perspective for manipulating electromagnetic waves and develop the all-light material and devices.Then,transformation optics as powerful method is proposed to design the novel optical devices.The basic idea lies in converting a geometric distortion of virtual space into a material distribution in real space to guide electromagnetic waves along a predesigned trajectory based on the form-invariant of Maxwell's equations under different coordinate transformations.Recently,the conformal transformation is also proposed and successfully applied to the research of optical properties of subwavelength plasmonic structures.With the parameter optimization method and further research,transformation optics gradually developed into a powerful and efficient method for designing novel optical devices and optimizing subwavelength plasmonic structures.In the view of development trend above,this dissertation mainly studies the optical properties of some subwavelength plasmonic structures and proposes some novel optical devices based on transformation optics.The main body and research results are listed below:First,we have theoretically studied and discovered that the nanotube system can achieve broadband optical absorption and nanofocusing.We have found that the nanofocusing property of the crescent nanotube not only relies on the geometric structure,but also the dielectric material around the structure.However,the broadband optical response only relies on the geometric structure.Thus,we propose a scheme for optimizing the property of nanofocusing and keeping the broadband optical response.When the system becomes nanotube dimers,we systematic study the optical response and optical field enhancement.It is found that the strength of coupling not only relies on the distance between two nanotube dimers,but also the inner radius of the single nanotube.Thus,the giant field enhancement can be obtained by increasing the inner radius or decreasing the distance between nanotubes.Second,we have discovered and theoretically demonstrated that the optical trapping force of the dielectric particle can be enhanced by designing the nanowire system.We analytic calculate the field distribution of the metal nanowire system based on the conformal transformation.Then,the optical force and optical potential well can be obtained by means of the calculative field distribution.Theoretical studies have confirmed that the optical force of the dielectric particle with several nanometers can occur in the gap between two nanowires,the optical force is enough strong to overcome the Brownian movement for optical trapping.Furthermore,the nanowire system can traps many dielectric particles to form one-dimension nanoparticle chain because the location of the field enhancement generates in a line.The research results may be potentially applied to the trapping and manipulating of the particle.Third,we have discovered and theoretically demonstrated that the optical longitudinal orbital angular momentum can be generated in the designed flat-plate type spiral phase plate based on the z axis coordinate transformation.The value of optical orbital angular momentum carried by the outgoing beam is tunable by changing the number of spiral phase plate.This flat-plate type spiral phase plate not only inherits the merit of high conversion efficiency and bearing high laser power of the conventional spiral phase plate,but also overcomes the defect of the conventional spiral phase plate that one spiral phase plate only produces beam with one orbital angular momentum.This technology may be potentially applied to produce the beam with arbitrary optical longitudinal orbital angular momentum.Fourth,we have discovered and theoretically demonstrated that the optical orbital transverse angular momentum can be generated in the designed metamaterials ring based on azimuth angle transformation.The research results show that the transverse optical orbital angular momentum not only depends on the frequency of the incident light,but also the azimuth angle transformation scale.Furthermore,the metamaterial ring can effectively confine light with a high quality factor and maintain steady modes with the orbital angular momentum,even if the dimension of the ring is much smaller than the wavelength of the incident light.This technique for exploiting the modes with optical transverse orbital angular momentum may provide a unique platform for applications related to micromanipulation.Fifth,we have proposed and theoretically demonstrated that the designing metamaterial can achieves the function of optical reproduction.We design the metamaterial counterpart by assembling the compressing core part and the predesigned metamaterial shell part.The counterpart with small scale can reconstructs the image of actual object for the same incident light,i.e.,realizing the purpose of optical reproduction.Furthermore,the predesigned metamaterial shell can be reusable,the images of different actual objects can be reconstructed by assembling different core parts and the same metamaterial shell.The results show that the near-field distribution and far-field signature of an actual object can be completely reconstructed by the counterpart under the illumination of the same incident light.The result may provide a unique way for optical image.In summary,we have theoretically realised the broadband optical absorption and nanofocusing in metal nanotube system based on transformation optics.We haveshown that the enhancement effect of optical trapping force in the metal nanowire system.We have designed the metamaterial to produce the optical orbital angular momentum and realize the manipulation of the light field.These investigations have broadened the application of transformation optics,and deepen our understanding for the optical property in the nano-micro structure.The results provide the scientific approach for designing the novel subwavelength functional plasmonic structures and developing the novel optical functional materials and devices.