Manipulating Evanescent Waves in a Gradient Waveguide(PR Applied 2020, in collaboration with Assi. Prof. Longfang Ye)
In this paper, manipulation of evanescent waves in a waveguide with gradient-index metamaterials is analyzed. It is shown that evanescent waves excited by a cutoff waveguide can be efficiently transmitted over a long distance by a mode-conversion mechanism. The experimental results are in good agreement with simulated results, and the manipulation is of broadband frequencies.
Photonic hyperinterfaces for light manipulations (Optica 2020, in collaboration with Asso. Prof. Yadong Xu, Prof. Xin Ou)
In this paper, we theoretically and experimentally study a new type of photonic interface (namely a hyperinterface) inside an optical metamaterial made of a zigzag alternating multilayer structure [namely structuredmetamaterials (SMMs)] in the deep-subwavelength regime. It is found that the subwavelength hyperinterfaces play a great role in the optical properties of such SMMs, and the electromagnetic properties of the hyperinterfaces can be effectively manipulated in a feasible way.
Conformal Landscape of a Two-Dimensional Gradient Refractive-Index Profile for Geometrical Optics (PR Applied 2020, in collaboration with Assi. Prof. Lin Xu, Prof. Hui Liu)
In this paper, we demonstrate that a flat gradient index optical thin-film waveguide is equivalent to a curved homogenous optical thin film waveguide for light rays by utilizing conformal equivalence. Such a relationship provides us an alternative method to design optical devices with either an inhomogeneous medium or equivalently curved surfaces made from only a homogeneous material, leading to applications on on-chip optical devices.
Bioinspired Conformal Transformation Acoustics (PR Applied 2020)
In this paper, we propose a bioinspired semianalytical conformal acoustics that can predict the evolution of acoustic functions strictly. Based on this method, we design a series of acoustic steering and collimation models. The proposed bioinspired conformal acoustics may bridge the gap between an animal’s biosonar and artificial materials, which show potential application value in underwater acoustics, medical ultrasonography, and other related applications.
Transformation Caustics (PR Applied 2019)
In this article, we propose transformation caustics by applying coordinate transformation to the caustic effect. The limit boundaries of caustics can be used to design devices with asymmetric light propagation or light confinement, thus providing an alternative degree of freedom for metamaterials.
Light rays and waves on geodesic lenses (Photonics Research 2019, in collaboration with Dr. Lin Xu, Profs. Tomas Tyc, Hui Liu and Shining Zhu)
We demonstrate that light rays confined on geodesic lenses form closed trajectories, and that for optical waves, the spectrum of a geodesic lens is (at least approximately) degenerate and equidistant with the numerical method. Moreover, we fabricate two geodesic lenses in micrometer and millimeter scale and observe curved light rays along geodesics.
Broadband cloaks for Water Waves (PRL 2019, On the cover, Editors' Suggestion, and Featured in Physics, in collaboration with Prof. Zhenyu Wang from Zhejiang University)
Inspired by electromagnetic waveguide cloaks with gradient index metamaterials, we fabricated a broadband cloak with simply a gradient depth profile on the bottom and without any other structures on the top to confine water waves in a certain area for cloaking regions. Being easy to construct, this design is potentially of significance for port applications.
Inverse transformation optics design (PR Applied 2019, in collaboration with Profs. Hui Liu and Shining Zhu from Nanjing University)
In this article, we proposed the "inverse transformation optics", and found that a conformal singularity of a refractive-index profile, with either zero or infinity resulting from a power mapping w=z^a, is equivalent to a topological defect with positive charge or negative charge. We fabricated a "hat" shape device with such a topological defect with positive charge in experiments and its related light-bending functionality with laser beams.
Universal multimode waveguide crossing (Optica 2018, in collaboration with Prof. Dingshan Gao from Huazhong University of Science and Technology)
We demonstrate for the first time a universal multimode waveguide star crossing based on transformation optics, which can handle in principle any number of waveguide modes and any number of crossing channels as well. The structure is transformed from a Maxwell’s fisheye, which could realize aberration-free imaging for each waveguide mode. A grayscale E-beam lithography is adopted to fabricate it on commercial silicon-on-insulator wafer. The proposed multimode waveguide star crossing has little loss and low crosstalk throughout an ultra-broad wavelength range of ~400 nm. Our study paves the way for realizing highly integrated and large capacity on-chip multimode routing and communication systems.
Imaging along conformal curves(PRA 2018)
In the letter, we have proposed a method to construct new types of absolute instruments by combining the Mikaelian lens together with conformal mappings. There will be self-imaging and imaging effect along closed conformal curves. If the scaling parameter in Mikaelian lens is an irrational number, the imaging effect will be maintained along the curves forever while there will be no self-imaging effect. If we choose other orthogonal directions for the Mikaelian lens, e.g., the radial directions, the hyperbolic directions, or the parabolic directions from parabolic coordinates, the imaging effect will precisely happen along those predesigned open curves. Our method is very general and could be used on special waveguide design, microcavity design, and even cloaking designs in future.
Concentrators for Water Waves (PRL Editors' Suggestion, Featured in Physics, and Highlighted by Nature 2018, in collaboration with Prof. Zhenyu Wang from Zhejiang University)
By introducing concepts from transformation optics to the manipulation of water waves, we design and experimentally demonstrate two annular devices for concentrating waves, which employ gradient depth profiles based on Fabry-Pérot resonances. Our measurements and numerical simulations confirm the concentrating effect of the annular devices and show that they are effectively invisible to the water waves. We show that transformation optics is thus an effective framework for designing devices to improve the efficiency of wave energy collection, and we expect potential applications in coastline ocean engineering.
Self-Focusing and the Talbot Effect in Conformal Transformation Optics (PRL Editors' Suggestion 2017, in collaboration with Profs. Hui Liu and Shining Zhu from Nanjing University)
Transformation optics has been used to propose various novel optical devices. With the help of metamaterials, several intriguing designs, such as invisibility cloaks, have been implemented. However, as the basic units should be much smaller than the working wavelengths to achieve the effective material parameters, and the sizes of devices should be much larger than the wavelengths of illumination to work within the light-ray approximation, it is a big challenge to implement an experimental system that works simultaneously for both geometric optics and wave optics. In this Letter, by using a gradient-index microstructured optical waveguide, we realize a device of conformal transformation optics (CTO) and demonstrate its self-focusing property for geometry optics and the Talbot effect for wave optics. In addition, the Talbot effect in such a system has a potential application to transfer digital information without diffraction. Our findings demonstrate the photon controlling ability of CTO in a feasible experiment system.
Fano resonances from GIM (Scientific Reports 2016)
In this work, a waveguide structure is designed by employing gradient-index metamaterials, supporting strong Fano resonances with extremely sharp spectra. As the changes in the transmission spectrum originate from the interaction of guided modes from different channels, instead of resonance structures or metamolecules, the Fano resonances can be observed for both transverse electric and transverse magnetic polarizations.
Waveguide cloak (Scientific Reports 2015)
In this work, we demonstrate an one-dimensional cloak consisting of parallel-plated waveguide with two slabs of gradient index metamaterials attached to its metallic walls. In it objects are hidden without limitation of polarizations, and good performance is observed for a broadband of frequencies. The experiments at microwave frequencies are carried out, supporting the theoretical results very well. The essential principle behind the proposed cloaking device is based on mode conversion, which provides a new strategy to manipulate wave propagation.
Transformation optics with Fabry-Pérot resonances (Scientific Reports Top 100 cited in 2015)
In this article, Fabry-Pe´rot resonances in materials of extreme anisotropy are used to design various transformation optical devices that are not only easy to realize but also work well for a set of resonant frequencies (multiple frequencies). As an example, a prototype of a cylindrical concentrator is fabricated for microwaves.
Playing the tricks of numbers of light sources (NJP Highlights of 2013)
We design a new class of gradient index lenses from multivalued optical conformal mapping. Such lenses can make one active source appear omnidirectionally as two (or many) in-phase sources, each interfering with others. As a self-interference phenomenon, this has not been discussed before. Meanwhile, they can transform multiple in-phase sources into one.
Cloak and imaging at the same time (EPL Issue Cover Image)
We propose a conceptual device to perform good imaging with positive refraction. At the same time, this device is an isotropic omnidirectional cloak with a perfect electric conductor hiding region and shows versatile illusion optical effects. Numerical simulations are performed to verify the functionalities.
Asymmetry waveguiding (Nature Communications)
We employ the recent concept of gradient index metamaterials to demonstrate a waveguide with asymmetric propagation of light, independent of polarization.
The device blocks both transverse electric and magnetic polarized modes in one direction but transmits them in the other for a broadband spectrum. Unlike previous works using chiral properties of metamaterials, our device is based on the principle of momentum symmetry breaking at interfaces with phase discontinuities. Experiments in the microwave region verify our findings, which may pave the way to feasible passive optical diodes.
Inside-out Eaton lens (APL)
We report an experiment on an inside-out Eaton lens fabricated using H-fractal metamaterials, copper printed on printed circuit boards in H-fractal patterns. The lens can perform good imaging functionality with both sources and images inside the vacuum core. The H-fractal metamaterials also provide design technique to achieve refractive index ranging in [0,1] with little loss in microwave spectrum. Excellent agreements between numerical and experimental results have been demonstrated.
First illusion optics experiment (PRL)
We experimentally demonstrate the first metamaterial "illusion optics" device—an "invisible gateway" by using a transmission-line medium. The device contains an open channel that can block waves at a particular frequency range. We also demonstrate that such a device can work in a broad frequency range.
Field rotator for liquid surface wave (EPL)
We extend the transformation media concept to the linear liquid surface waves. A mapping is introduced to generate an anisotropic depth parameter that corresponds to the anisotropic permittivity tensor in electromagnetic waves. A device that can rotate the liquid wave front is introduced, which is an analog of the metamaterial electromagnetic wave field rotator. The structure is based on a layered design. Simulation results are compared with experimental measurements.
Electromagnetic field rotator (PRL)
We designed a metamaterial field rotator that can rotate electromagnetic wave fronts. Our starting point was the transformation-media concept. Effective medium theories and full simulations facilitated the actual design process. We created at a very simple structure comprising of an array of identical aluminum metal plates. We made and measured a sample and we experimentally demonstrated the field rotation effect as well as the broadband functionality at microwave frequencies.