The unexpected coupling with leaky modes introduces a new pathway for dense photonic integration

July 13, 2023

(Nanowerk News) Defying conventional wisdom, researchers have devised a new coupling mechanism involving leaky modes, which were previously considered unsuitable for high-density integration in photonic circuits. This unexpected finding opens up new possibilities for dense photonic integration, revolutionizing the scalability and application of photonic chips in optical computing, quantum communications, light detection and range (LiDAR), optical metrology, and biochemical sensing.

Recently Light Science & Applications publication (“Interferences such as anisotropic leakage with subwavelength gratings allow zero crosstalk”), Sangsik Kim, professor of electrical engineering at the Korea Advanced Institute of Science and Technology (KAIST), and his students at Texas Tech University demonstrated that an anisotropic leakage wave can achieve zero crosstalk between closely spaced identical waveguides using subwavelength grating (SWG) metamaterials. This counter-intuitive finding drastically increases the length of the transverse-magnetic (TM) mode coupling, which has historically posed a challenge due to its low confinement. Illustration depicting the propagation of light without crosstalk in the waveguide array of the developed metamaterial-based optical semiconductor. (Image: KAIST)

This study builds on their previous studies of SWG metamaterials for reducing optical crosstalk, including control of evanescent wave skin depth (General Nat 9, 1893 (2018)) and excellent coupling in anisotropic guided mode (OPTICAL 7, 881-887 (2020)). SWG has recently made significant advances in photonics, enabling a variety of high-performance PIC components. However, integration density challenges remain for the TM mode, which exhibits approximately 100 times greater crosstalk than the mains-transverse (TE) mode, hindering high-density chip integration.

“Our group has explored SWG for dense photonic integration, achieving significant improvements. However, the previous approach was limited to TE polarization only. In photonic chips, there are other orthogonal polarization TMs, which can double the capacity of the chip and are sometimes more desirable than TEs, such as in evanescent field sensing. TMs are more difficult to integrate densely than TEs because they are usually less constrained by their low width-to-height waveguide aspect ratio,” explained Kim.

Initially, the team believed it would be impossible to reduce crosstalk using SWG, as they expected the leak mode to improve coupling between waveguides. However, they focused on the potential for anisotropic interference with leaky modes, hypothesizing that cross cancellation might be achievable.

Applying pairwise mode analysis to the modal properties of the leaky SWG modes, they found unique anisotropic interference with leak-like modes, resulting in zero crosstalk between closely spaced identical SWG waveguides. Utilizing Floquet limit simulations, they designed a SWG waveguide that can be practically implemented on standard silicon-on-insulator (SOI) platforms already available in the industry, demonstrating outstanding crosstalk suppression and a more than two-fold increase in coupling length compared to conventional guides. striped wave.

This breakthrough also significantly reduces noise levels in PICs, with potential impact on quantum communications and computing, optical metrology, and biochemical sensing. The researchers further emphasized the wider implications of their work, noting that this new coupling mechanism can be extended to other integrated photonic platforms and wavelength regimes across the visible light, mid-infrared, and terahertz outside of the telecommunication bands.

This surprising coupling mechanism has expanded the potential of dense photonic integration, defying conventional wisdom and pushing field boundaries. As research continues, the photonics industry will likely see a shift towards denser, lower noise, and more efficient PIC technologies.

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