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Electromagnetic coupling via an evanescent field or radiative wave is a primary characteristic of light, allowing optical signal/power transfer in a photonic circuit but limiting integration density. A leaky mode, which combines both evanescent field and radiative wave, causes stronger coupling and is thus considered not ideal for dense integration. Here we show that a leaky oscillation with anisotropic perturbation rather can achieve completely zero crosstalk realized by subwavelength grating (SWG) metamaterials. The oscillating fields in the SWGs enable coupling coefficients in each direction to counteract each other, resulting in completely zero crosstalk. We experimentally demonstrate such an extraordinarily low coupling between closely spaced identical leaky SWG waveguides, suppressing the crosstalk by ≈40 dB compared to conventional strip waveguides, corresponding to ≈100 times longer coupling length. This leaky-SWG suppresses the crosstalk of transverse–magnetic (TM) mode, which is challenging due to its low confinement, and marks a novel approach in electromagnetic coupling applicable to other spectral regimes and generic devices.

Optical delay lines control the flow of light in time, introducing phase and group delays for engineering interferences and ultrashort pulses. Photonic integration of such optical delay lines is essential for chip-scale lightwave signal processing and pulse control. However, typical photonic delay lines based on long spiral waveguides require extensively large chip footprints, ranging from mm²to cm²scales. Here we present a scalable, high-density integrated delay line using a skin-depth engineered subwavelength grating waveguide, i.e., an extreme skin-depth (eskid) waveguide. The eskid waveguide suppresses the crosstalk between closely spaced waveguides, significantly saving the chip footprint area. Our eskid-based photonic delay line is easily scalable by increasing the number of turns and should improve the photonic chip integration density.

A photonic Bragg grating is a fundamental building block that reflects the direction of wave propagation through spatial phase modulation and can be implemented using sidewall corrugation. However, due to the asymmetric aspect ratio of a waveguide cross section, typical Bragg gratings exhibit a strong polarization sensitivity. Here, we show that photonic Bragg gratings with cladding asymmetry can enable polarization-independent notch filters by rotating input polarizations. Such Bragg gratings strongly couple transverse electric (TE) and transverse magnetic (TM) modes propagating in opposite directions, filtering the input signal and reflecting the rotated mode. We analyzed this polarization-rotating Bragg grating using the coupled-mode theory and experimentally demonstrated it on a silicon-on-insulator platform. Our device concept is simple to implement and compatible with other platforms, readily available as polarization transparent Bragg components.

We present a broadband integrated photonic polarization splitter and rotator (PSR) using adiabatically tapered coupled waveguides with subwavelength grating (SWG) claddings. The PSR adiabatically rotates and splits the fundamental transverse-magnetic (TM₀) input to the fundamental transverse-electric (TE₀) mode in the coupler waveguide, while passing the TE₀input through the same waveguide. The SWGs work as an anisotropic metamaterial and facilitate modal conversions, making the PSR efficient and broadband. We rigorously present our design approaches in each section and show the SWG effect by comparing with and without the SWG claddings. The coupling coefficients in each segment explicitly show a stronger coupling effect when the SWGs are included, confirmed by the coupled-mode theory simulations. The full numerical simulation shows that the SWG-PSR operates at 1500–1750 nm (≈250 nm) wavelengths with an extinction ratio larger than 20 dB, confirmed by the experiment for the 1490–1590 nm range. The insertion losses are below 1.3 dB. Since our PSR is designed based on adiabatical mode evolution, the proposed PSR is expected to be tolerant to fabrication variations and should be broadly applicable to polarization management in photonic integrated circuits.

We show that a TM leaky mode realized by subwavelength gratings (SWGs) can eliminate crosstalk between proximate waveguides. Oscillating fields i n t he SWGs allow coupling coefficient components to counteract each other, resulting in zero crosstalk.

We present an ultra-broadband silicon photonic polarization beam splitter (PBS) using adiabatically tapered extreme skin-depth (eskid) waveguides. Highly anisotropic metamaterial claddings of the eskid waveguides suppress the crosstalk of transverse-electric (TE) mode, while the large birefringence of the eskid waveguide efficiently cross-couples the transverse-magnetic (TM) mode. Two eskid waveguides are adiabatically tapered to smoothly translate TM mode to the coupled port via mode evolution while keeping the TE mode in the through port. The tapered cross-section of the eskid PBS was designed numerically, achieving a large bandwidth at 1400–1650 nm with extinction ratios$><\#comment/>20\mathrm{d}\mathrm{B}$. We experimentally demonstrated the tapered-eskid PBS and confirmed its broad bandwidth at 1490–1640 nm, limited by laser bandwidth. With its mode evolution, the tapered-eskid PBS is tolerant to fabrication imperfections and should be crucial for controlling polarizations in photonic circuits.

In this Letter, we present a high extinction ratio and compact on-chip polarization beam splitter (PBS), based on an extreme skin-depth (eskid) waveguide. Subwavelength-scale gratings form an effectively anisotropic metamaterial cladding and introduce a large birefringence. The anisotropic dielectric perturbation of the metamaterial cladding suppresses the TE polarization extinction via exceptional coupling, while the large birefringence efficiently cross-couples the TM mode, thus reducing the coupling length. We demonstrated the eskid-PBS on a silicon-on-insulator platform and achieved an ultra-high extinction ratio PBS ($\approx <\#comment/>60\mathrm{d}\mathrm{B}$for TE and$\approx <\#comment/>48\mathrm{d}\mathrm{B}$for TM) with a compact coupling length ($\approx <\#comment/>14.5\text{µ<\#comment/>}\mathrm{m}$). The insertion loss is also negligible ($<<\#comment/>0.6\mathrm{d}\mathrm{B}$). The bandwidth is$><\#comment/>80$(30) nm for the TE (TM) extinction ratio$><\#comment/>20\mathrm{d}\mathrm{B}$. Our ultra-high extinction ratio PBS is crucial in implementing efficient polarization diversity circuits, especially where a high degree of polarization distinguishability is necessary, such as photonic quantum information processing.

We present an ultra-high extinction-ratio silicon photonic polarization beam splitter with anisotropic metamaterial claddings. Exceptional coupling with anisotropic metamaterial achieved 58- 60 dB (46-48 dB) extinction-ratios for TE (TM) mode and the large birefringence resulted in a short coupling length (14.5 µm).