Directional and contra-directional coupling in Huygens' metawaveguide microring resonators

TL;DR

This work introduces and experimentally validates integrated Huygens’ metawaveguide microring resonators and both directional and contra-directional couplers operating at $\lambda = 1550$ nm. By exploiting resonant Huygens’ waveguides, the authors achieve efficient evanescent coupling to high-$Q$ resonators with negative group index $n_{\mathrm{g}}$ and near-zero dispersion $D_{\lambda}$, enabling compact add-drop filtering. A key advance is the hybrid subwavelength grating–Huygens’ contra-directional coupler, which enables backward coupling between resonant and non-resonant metawaveguides and supports broad spectral rejection bandwidth, enabling FSR-free racetrack resonators. Collectively, these results advance the integration of resonant metamaterials into scalable photonic platforms with potential impact on optical communications, quantum photonics, and sensing.

Abstract

Huygens' metawaveguides represent a transformative concept in photonic device engineering, enabling unprecedented control over light propagation. This study presents, for the first time, integrated Huygens'-based microring resonators and directional and contra-directional couplers, specifically designed for operation at the 1550 nm telecommunication wavelength. By leveraging the unique properties of resonant Huygens' waveguides, we demonstrate efficient evanescent directional coupling with high-Q resonators, characterized by negative group index and near-zero dispersion, which are critical for enhancing performance in compact, high-performance add-drop filters. The research further explores the implications of these novel structures on group index and group velocity dispersion, providing insights into their potential applications in nonlinear optics and quantum information technologies. Notably, the introduction of a hybrid subwavelength grating-Huygens' contra-directional coupler facilitates backward coupling between resonant and non-resonant metawaveguides, achieving a broad spectral rejection bandwidth. Our findings advance the integration of resonant metamaterials into scalable photonic platforms, laying the groundwork for innovative applications in optical communications, quantum photonics and sensing systems.

Figures (6)

• Figure 1: (a) Overlap between Mie resonant electric and magnetic dipoles (ED and MD) in an individual dielectric nanoantenna (i) enables forward Huygens' scattering (ii). (b) An array of Huygens’ antennas facilitates forward light propagation. (c) Dispersion band diagram of a Huygens’ metawaveguide shows the Huygens’ band around 1550 nm wavelength just below the light line (yellow line). (d) Simulated electric field profile and (e) a scanning electron micrograph (SEM) of a Huygens’ metawaveguide directional coupler with coupling gap, $g$ = 150 nm. (f) Individual antennas’ electric and magnetic dipole (ED and MD) scattering cross-section in the coupled waveguide with coupling gap of 200 nm. (g) The magnitude of the decomposed ED and MD as a function of the coupling gap. (h) Measured (at 1530 nm) self and cross-coupling efficiency as the function of coupling gap. (i) Comparison between the cross-coupled backward propagation in Huygens’, SWG and conventional directional couplers as the function of coupling gap.
• Figure 2: Simulated light propagation (a) and a scanning electron microscopy (SEM) image (b) of a straight-bend Huygens’ metawaveguide directional coupler with a coupling gap, $g$ = 150 nm and a bend radius, $r$ = 9 $\mu$m. (c) Measured self and cross-coupling efficiency as the function of coupling gap. (d) An SEM image of a Huygens ring resonator. The inset shows the simulated mode profile in the ring resonator. (e) The transmission spectra at through port of the Huygens' ring resonator with a coupling gap of 200 nm and a curved radius of 9 $\mu$m. (f) Group index, $n_{\mathrm{g}}$ and group velocity dispersion parameter, $D_{\mathrm{\lambda}}$ of the Huygens’ metawaveguides extracted from the measured resonance free-spectral range (FSR) of the ring resonator in (d) (presented in Fig. S3 in Supporting Information Section III).
• Figure 3: Scanning electron micrographs (SEM) of the Huygens’ four-port (a) ring and (b) racetrack resonator add-drop filters. Measured transmission spectra (c-e) corresponding to (a) and (f-h) corresponding to (b) for the coupling gap of 150 nm, 200 nm and 250 nm, respectively. Measured $Q$-factor at 3-dB bandwidth of the resonances, (i) corresponding to (c-e) and (j) corresponding to (f-h).
• Figure 4: (a) Schematical 3D view of the SWG-assisted Huygens’ contra-directional coupler (CDC). (b) SEM image of the SWG-assisted Huygens’ CDC. The period of the lateral SWG waveguide is exactly half the period of the Huygens’ waveguide. Modulating the width of the SWG waveguide segments enables the backward coupling of light from the Huygens’ waveguide. (c) Band diagram corresponding to the SEM image in (b). The SWG waveguide was tuned to enable energy transfer to the Huygens’ waveguide at the center of its operating bandwidth ($\lambda$ = 1550 nm). (d) Outer segment width ($w_\mathrm{o}$) as a function of inner segment width ($w_\mathrm{i}$) of the lateral SWG waveguide, which together modify the bandgap width while keeping the center wavelength constant at 1550 nm (blue data). Bandgap as a function of inner segment width ($w_\mathrm{i}$) of the SWG waveguide (red data). Markers represent simulation results from 3D band diagram calculations, while solid curves are second-degree polynomial fits to the simulation data.
• Figure 5: (a) Schematic of the apodized SWG-assisted Huygens’ CDC. Light is injected at the input port (left) and optically measured at the through, add and drop ports (right). The total length of the device consists of the length of the CDC ($L$), the length of the SWG waveguide to solid waveguide transitions ($L_\mathrm{trans}$), and the length of the solid waveguide bends ($L_\mathrm{bend}$). (b) Target Gaussian apodization of the CDC bandgap (orange curve). The edges of the apodization function ($\sim$1 nm) are set to 10 $\%$ of the peak bandgap ($\Delta\lambda_\mathrm{peak}$ = 10 nm). Inner and outer segment widths (blue curves) are mapped through the inverse of the function $\Delta\lambda$($w_\mathrm{i}$) and the function $w_\mathrm{o}$($w_\mathrm{i}$), respectively. (c) Experimental optical measurements of the through, add and drop ports when light is injected from a tunable laser to the input port of the device. The target peak bandwidth closely matched the band diagram calculations, but the center wavelength experienced a blue-shift due to fabrication-induced feature deviations.