Arbitrary waveguide connector based on embedded optical transformation

Optical meta-waveguides for integrated photonics and beyond

The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous meta-waveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

Design of an ultra-short coupler in an asymmetric twin-waveguide structure using transformation optics

An integrated vertical coupler that transfers light from the lower passive waveguide to the upper active waveguide in an asymmetric twin-waveguide integration structure (in InGaAsP/InP material system) is designed using transformation optics (TO). The length of the coupler is as short as 3 μm, which is two orders of magnitude shorter than that of traditional tapered couplers. According to three-dimensional full-wave simulations, the designed optimized coupler has a high coupling efficiency of 94.9%, and a low reflection at the wavelength of 1.55 μm. Subsequently, quasi-conformal mapping is employed to reduce the material complexity and to make it possible to realize the coupler by purely using an isotropic dielectric material. Applying TO to integrated photonic devices may motivate new applications, and improve integration density on the InP platform.

Optimization for nonmagnetic concentrator with minimized scattering

Based on the generalized transformation for a nonmagnetic cylindrical concentrator, we present nonlinear coordinate transformations to realize excellent concentrating performance with minimized scattering. By matching the impedance at both the inner and outer boundaries and utilizing nonlinear optimization techniques, the best parameters of the nonlinear transformation can be determined. Results show that the concentrating power is sensitive to the impedance at the inner interface. In addition, an alternating layered system consisting of nonmagnetic isotropic materials is applied to experimentally mimic the concentrator, and excellent concentrating performances have been observed.

Controlling electromagnetic scattering of a cavity by transformation media

Based on the transformation media theory, we proposed a way to control the scattering of a cavity, or trench, located on a metallic plane. Specifically, we show how is possible to design transformation medium to fill up a cavity with arbitrary cross section, which is capable of enhancing the specularly reflection wave. As the inverse problem, we also address the design of transformation medium coating, which is laid on the metallic plane, to mimic the scattering of the cavity. Based on the effective medium theory, the transformation medium for the case of a polygonal cavity can be realized by oblique layered structures, and each layered structure is consisting of two kinds of isotropic dielectrics, thus leading an ease of practical fabrication.

Waveguide devices with homogeneous complementary media

We report the design of microwave waveguide devices based on complementary media. Various kinds of waveguide devices that possess nearly 100% transmission efficiency are proposed, such as waveguide bends, splitters, connectors, and shifters. Compared with previous work on waveguide devices of low reflection and minimized distortion based on transformation optics, our transform media are homogeneous metamaterials. Electromagnetic simulations by a finite-element method on detailed examples have been performed to validate the designs and these functionalities can be close to the practical.

Homogeneous and isotropic bends to tunnel waves through multiple different/equal waveguides along arbitrary directions

We propose a novel optical transformation to design homogeneous isotropic bends connecting multiple waveguides of different cross sections which can ideally tunnel the wave along any directions through multiple waveguides. First, the general expressions of homogeneous and anisotropic parameters in the bend region are derived. Second, the anisotropic material can be replaced by only two kinds of isotropic materials and they can be easily arranged in planarly stratified configuration. Finally, an arbitrary bender with homogeneous and isotropic materials is constructed, which can bend electromagnetic wave to any desired directions. To achieve the utmost aim, an advanced method is proposed to design nonmagnetic, isotropic and homogeneous bends that can bend waves along arbitrary directions. More importantly, all of the proposed bender has compact shape due to all flat boundaries, while the wave can still be perfectly tunneled without mode distortion. Numerical results validate these functionalities, which make the bend much easier in fabrication and application.