Going against conventional wisdom, scientists have discovered a new coupling mechanism involving leakage modes that was previously considered unsuitable for high-density integration of photonic circuits. This amazing discovery paves the way for high-density photonic integration, changing the potential and scalability of photonic chips in areas such as optical computing, quantum communications, light detection and ranging (LiDAR), optical metrology, and biochemical sensing.
In a recent issue of the journal Light Science & Application, Sangsik Kim, associate professor in the Department of Electrical Engineering at the Korea Advanced Institute of Science and Technology (KAIST), and his students at Texas Tech University demonstrated that anisotropic leaky waves can achieve zero crosstalk between closely spaced identical waveguides using subwavelength grating (SWG) metamaterials. This counterintuitive discovery greatly increases the coupling length of transverse magnetic (TM) modes, which have been a challenge due to their low confinement.
This research builds on their previous work on the use of SWG metamaterials to reduce optical crosstalk, including controlling the skin depth of evaporation waves and special coupling in anisotropic guided wave modes. Recently, SWG has made significant progress in the field of photonics, enabling a variety of high-performance PIC components. However, the integration density of TM mode still faces challenges, and its crosstalk is approximately 100 times that of lateral electrical (TE) mode, hindering high-density chip integration.
"Our research group has been exploring SWGs for dense photonic integration and has achieved significant improvements. However, previous methods were limited to TE polarization. In photonic chips, there is another orthogonal polarization TM, which can double the chip capacity and is sometimes more popular than TE, such as in gradient field sensing." Kim explained: "TM is more difficult to densely integrate than TE because its waveguide aspect ratio is generally lower and less restrictive."
Initially, the team thought it would be impossible to reduce crosstalk using SWGs because they expected leaky modes to enhance coupling between waveguides. However, they focused on the potential of anisotropic perturbations with leakage modes and assumed that cross-cancellation could be achieved.
By performing coupled-mode analysis of the modal properties of leaky SWG modes, they discovered unique anisotropic perturbations with similar leakage modes, enabling zero crosstalk between closely spaced identical SWG waveguides. Using Floquet boundary simulation, they designed a feasible SWG waveguide on the industry's standard silicon-on-insulator (SOI) platform. Compared with strip waveguides, its crosstalk suppression effect is significant and the coupling length is increased by more than two orders of magnitude.
This breakthrough also significantly reduces noise levels within PICs, with potential implications for quantum communications and computing, optical metrology, and biochemical sensing. The researchers further emphasized the broad implications of their work, noting that this novel coupling mechanism could be extended to other integrated photonics platforms and wavelength ranges, including visible, mid-infrared, and terahertz beyond telecommunications bands.
This amazing coupling mechanism expands the potential of dense photonic integration, breaks conventional wisdom, and advances the field. As research continues, the photonics industry is likely to move toward denser, lower-noise, and more efficient integrated circuit technologies.