Theoretical analysis of mode conversion by refractive-index perturbation based on a single tilted slot on a silicon waveguide

Design and Test of Embedded Reconfigurable Mode Converter Based on Spontaneous Deformable Materials

The mode converter, as a passive mode conversion device in transmission lines, is well-investigated and widely implemented in various electromagnetic systems. However, most traditional mode converters can only realize a single conversion mode. Thus, a mode converter achieving multiple controllable output modes is urgently needed. In this paper, a reconfigurable mode converter operating in the microwave range is achieved by embedding a deformable all-dielectric material with quadrilateral shape into a rectangular waveguide based on coupled-mode theory. It can achieve different target modes with controllable output for the same input by exciting the deformable all-dielectric material. The design principle of the mode converter is expounded concretely and simulation is carried out using HFSS software 2022 R2. Experimental results, consisting of the simulation results, demonstrate that the proposed mode converter can achieve various mode conversions with mode purity higher than 95%. This article innovatively applies deformable materials to waveguide mode conversion, expanding the application of deformable memory materials in electromagnetic devices.

Experimental demonstration of a flexible and high-performance mode-order converter using subwavelength grating metamaterials

Mode-order converters, transforming a given mode into the desired mode, have an important implication for the multimode division multiplexing technology. Considerable mode-order conversion schemes have been reported on the silicon-on-insulator platform. However, most of them can only convert the fundamental mode to one or two specific higher-order modes with low scalability and flexibility, and the mode conversion between higher-order modes cannot be achieved unless a total redesign or a cascade is carried out. Here, a universal and scalable mode-order converting scheme is proposed by using subwavelength grating metamaterials (SWGMs) sandwiched by tapered-down input and tapered-up output tapers. In this scheme, the SWGMs region can convert, TEp mode guided from a tapered-down taper, into a TE₀-like-mode-field (TLMF) and vice versa. Thereupon, a TEp-to-TEq mode conversion can be realized by a two-step process of TEp-to-TLMF and then TLMF-to-TEq, where input tapers, output tapers, and SWGMs are carefully engineered. As examples, the TE₀-to-TE₁, TE₀-to-TE₂, TE₀-to-TE₃, TE₁-to-TE₂, and TE₁-to-TE₃ converters, with ultracompact lengths of 3.436-7.71 µm, are reported and experimentally demonstrated. Measurements exhibit low insertion losses of < 1.8 dB and reasonable crosstalks of < -15 dB over 100-nm, 38-nm, 25-nm, 45-nm, and 24-nm working bandwidths. The proposed mode-order converting scheme shows great universality/scalability for on-chip flexible mode-order conversions, which holds great promise for optical multimode based technologies.

Compact and broadband silicon mode-order converter using bricked subwavelength gratings

A compact and broadband silicon mode-order converter (MOC) scheme by employing reciprocal mode evolution between asymmetric input/output taper and bricked subwavelength gratings (BSWG) is proposed. In the proposed MOC, a quasi-TE₀ mode is generated in the BSWG region, which can be regarded as an effective bridge between the two TE modes to be converted. Flexible mode conversion can be realized by only choosing appropriate structure parameters for specific mode transitions between input/output modes and the quasi-TE₀ mode. By combing 3D finite difference time domain (FDTD) and particle swarm optimization (PSO) method, TE₀-TE₁ and TE₀-TE₂ MOCs are optimal designed, which can efficiently convert TE₀ mode to TE₁ and TE₂ modes with lengths of 9.39 µm and 11.27 µm, respectively. Results show that the insertion losses of <1 dB and crosstalk of <-15 dB are achieved for both TE₀-TE₁ and TE₀-TE₂ MOCs, the corresponding working bandwidth are 128 nm (1511∼1639 nm) and 126 nm (1527∼1653 nm), respectively. Additionally, the MOCs can be fabricated with only single etch step with minimum feature size of 145 nm.

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.