Novel Integrated Optical Waveguide Devices Based On Transformation Methods

Over the last decade,transformation optics has become one of the most active research topics in the field of optics.Transformation optics provides a powerful method to manipulate light propagation by mapping the distributions of permittivity and permeability to design novel transformation media based on the spatial coordinate transformation.At present,transformation optics is mainly used to design metamaterials and metasurfaces to realize applications such as invisibility cloaks,superlens and so on,with relatively few studies on integrated optical chips.In this thesis,we design some novel waveguide devices on integrated optical chips using transformation optics,and explore the corresponding interesting physical effects,including surface plasmon polaritons waveguide,nonlinear optical waveguide and topological optical waveguide.The main content include:1.We utilize transformation optics to design a one-dimensional surface plasmon polaritons “photon black hole”and demonstrate experimentally the achievement of nanofocusing of surface plasmon polaritons.By placing a silver microwire on the surface of a silver film,we fabricated a gradually-varying Metal-Insulator-Metal waveguide.The effective refractive index of the waveguide increases as the thickness of air decreases.It is subtle that this waveguide naturally establishes a precise inverse square law refractive index profile which corresponds to a one-dimensional metric of a gravitational object.Both analytical theory and full-wave numerical simulations demonstrate a good performance of nanofocusing.In our experiment,an obvious local field enhancement was observed by fluorescence imaging.In the simulations,we also obtain an enhancement of nonlinear optical process if filling the dielectric layer with nonlinear optical materials.2.We apply conformal mapping to design gradually-varying nonlinear optical waveguide and realize broadband four-wave mixing process.We transform the curved silicon-on-insulator(SOI)waveguide in the real physical space into a straight waveguide in the virtual space through a logarithmic conformal transformation,so that the dispersion properties of the curved waveguide can be analyzed more conveniently.In the simulations,we find that SOI waveguides with different bending radii support different propagation constants and altering bending curvature can change the group velocity dispersion of the waveguide.We apply it to control the bandwidth of the fourwave mixing and design SOI waveguides with gradually-varying bending radii.A gradual acceleration is defined.It is theoretically found that the greater the acceleration,the broader the bandwidth of the four-wave mixing.Therefore,we can arbitrarily manipulate the bandwidth of four-wave mixing by changing the gradual acceleration of the curved waveguide,which is promise for on-chip supercontinuum generation and integrated quantum optical chips.3.We use a rotation transformation to design topological interface state(TIS).We propose quaternary waveguide arrays(QWAs)on lithium niobate-on-insulator(LNOI)chips.By incorporating two geometric parameters of QWAs with the Bloch wave vector,we construct a three-dimensional parameter space in which Weyl points are obtained.Moreover,by a rotation transformation of the parameter spaces,we construct arbitrary interface between two independent Weyl structures for the first time.The interface can either host gapless TISs or not,depending on the relative rotational directions of the two Weyl structures.If the two Weyl structures rotate in the opposite directions,two types of topological interface states arise.Otherwise,if the two Weyl structures rotate in the same direction,only trivial interface state can exist.In experiments,we designed and fabricated spliced QWAs on an LNOI chip,and observed the TISs through linear and nonlinear optical measurements.