Transformation optics is a powerful technique that simplifies the modelling of optical devices by suitably modifying the coordinate system. Thanks to the form-invariance of Maxwell equations, the transformed device in the “virtual space” behaves as original one in the “physical space,” but its design can be optimized more easily. This technique has been applied to the design of metamaterials, metasurfaces, and plasmonic structures for advanced manipulation of light beams. But the potential of transformation optics is indeed much wider.

In this work, Li and coauthors have exploited transformation optics to optimize the profile of a bent silicon waveguide supporting the propagation of several optical modes. Multimode waveguides are of large interest for the realization of high-density mode-division multiplexing systems on chip. However, an issue to face when using multimode waveguides is that waveguide bending may introduce high loss and crosstalk between the different guided modes unless the bending radius is kept large (> 500 µm). Tightening the bend is the challenge addressed here with transformation optics. The shape of the waveguide is optimized in virtual space in such a way that the scattering loss and the inter-mode crosstalk are suppressed. Mapping back to the physical space, the result is a compact (17 μm radius) four-mode bend with low loss (< 0.1 dB) and low inter-modal crosstalk (< -20 dB), which can also be fabricated easily in one-step lithography.

Not only does this study add a fundamental building block for the realization of high capacity on-chip interconnects, but the proposed approach can also be combined with advanced optimization methods, such as genetic algorithms or topology optimization, for the realization of complex, yet easy to fabricate, photonic integrated devices.

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