Jianjia Yi

Illusion optics: Optically transforming the nature and the location of electromagnetic emissions

Complex electromagnetic structures can be designed by using the powerful concept of transformation electromagnetics. In this study, we define a spatial coordinate transformation that shows the possibility of designing a device capable of producing an illusion on an antenna radiation pattern. Indeed, by compressing the space containing a radiating element, we show that it is able to change the radiation pattern and to make the radiation location appear outside the latter space. Both continuous and discretized models with calculated electromagnetic parameter values are presented. A reduction of the electromagnetic material parameters is also proposed for a possible physical fabrication of the device with achievable values of permittivity and permeability that can be obtained from existing well-known metamaterials. Following that, the design of the proposed antenna using a layered metamaterial is presented. Full wave numerical simulations using Finite Element Method are performed to demonstrate the performances of such a device.

Electromagnetic field tapering using all-dielectric gradient index materials.

The concept of transformation optics (TO) is applied to control the flow of electromagnetic fields between two sections of different dimensions through a tapering device. The broadband performance of the field taper is numerically and experimentally validated. The taper device presents a graded permittivity profile and is fabricated through three-dimensional (3D) polyjet printing technology using low-cost all-dielectric materials. Calculated and measured near-field mappings are presented in order to validate the proposed taper. A good qualitative agreement is obtained between full-wave simulations and experimental tests. Such all-dielectric taper paves the way to novel types of microwave devices that can be easily fabricated through low-cost additive manufacturing processes.

Realizable design of field taper via coordinate transformation

Complex electromagnetic structures can be designed by exploiting the concept of spatial coordinate transformations. In this paper, we define a coordinate transformation scheme that enables one to taper the electric field between two waveguides of different cross-sections. The electromagnetic field launched from the wide input waveguide is compressed in the proposed field tapering device and guided into the narrow output waveguide. In closed rectangular waveguide configurations, the taper can further play the role of a mode selector due to the output waveguides cut-off frequency. Realizable permittivity and permeability values that can be achieved with common existing metamaterials are determined from the transformation equations and simplified by a proposed parameter reduction method. Both a 2D continuous design model and a potential 3D discretized realization model are presented at microwave frequencies and the performances of the tapering devices are verified by full-wave finite element numerical simulations. Finally, near-field distributions are shown to demonstrate the field tapering functionality.

3D field-shaping lens using all-dielectric gradient refractive index materials

A novel three-dimensional (3D) optical lens structure for electromagnetic field shaping based on spatial light transformation method is proposed at microwave frequencies. The lens is capable of transforming cylindrical wavefronts into planar ones, and generating a directive emission. Such manipulation is simulated and analysed by solving Laplace’s equation, and the deformation of the medium during the transformation is theoretically described in detail. The two-dimensional (2D) design method producing quasi-isotropic parameters is further extended to a potential 3D realization with all-dielectric gradient refractive index metamaterials. Numerical full-wave simulations are performed on both 2D and 3D models to verify the functionality and broadband characteristics of the calculated lens. Far-field radiation patterns and near-field distributions demonstrate a highly radiated directive beam when the lens is applied to a conical horn antenna.

Transformation Optics Concept: Definition and Tools

The exciting field of metamaterials has attracted the interest of researchers from all across the world over the past few years. With roots in the foundations of optics and electromagnetics, researchers are exploring the underlying physics of metamaterials. Electromagnetic metamaterials are artificially structured materials composed of periodic arrays of sub-wavelength resonating structures, scatterers or apertures, whose electric or magnetic response provides the ability to engineer dielectric or magnetic properties that do not exist in naturally occurring materials. The dielectric or magnetic properties can be tailored by changing the geometrical parameters of the constituent structure of the metamaterial.

Space Transformation Concept: Controlling the Path of Electromagnetic Waves

In this chapter, we will deal with devices designed using space transformation concept. Quasi-conformal transformation optics (QCTO) is applied for the design of electromagnetic devices at microwave frequencies. Antenna applications are targeted for focusing and collimating properties. Two lenses are studied and designed by solving the Laplace’s equation that describes the deformation of a medium in the space transformation. The material parameters of the lenses are derived from the analytical solutions of the Laplace’s equation. The first lens is conceived to produce an overall directive in-phase emission from an array of sources conformed on a cylindrical structure. The second lens allows steering a directive beam to an off-normal direction. Theoretical formulations are given to describe the transformations and full-wave simulations are performed to verify the functionality of the calculated lenses. Theoretical designs are adapted judiciously to be compatible with the constraints of realization technologies so as to have the best agreement between numerical calculations and experimental tests. Prototypes presenting a graded refractive index are fabricated through three-dimensional (3D) polyjet printing using all-dielectric materials and experimental measurements are carried out to validate the proposed lenses. Such easily realizable designs open the way to lowcost all-dielectric broadband microwave lenses for beam forming and collimation.