Terahertz All‐Dielectric Maxwell Fisheye Lens Waveguide Crossings via Transformation Optics

Abstract Terahertz waveguide crossings are critical for compact, integrated signal routing in monolithic platforms, but simple waveguide intersections suffer from high losses and crosstalk due to mode mismatch in the regions where the waveguide channels overlap. The Maxwell fisheye lens with its inherent imaging properties is an excellent solution for multichannel intersections, however, its circular shape is not easily integrated with common planar input/output waveguides. Here, all‐silicon waveguide crossings are introduced based on Maxwell fisheye lenses reshaped via conformal transformation optics for improved planar waveguide integration in the terahertz range. Using effective medium techniques with subwavelength air inclusions, and crossings operating over the 220–330 GHz frequency band are designed and fabricated. The transformed lenses enable aberration‐free imaging without mode mismatch, implemented through a single deep reactive ion etching step. Experimental characterization reveals average insertion loss of 1.2 dB and crosstalk below –50 dB for the fundamental quasi‐transverse electric (TE) mode, with a 40% bandwidth across the entire 220–330 GHz band, while the quasi‐transverse magnetic (TM) mode is also supported for dual‐polarization applications. The transformed lenses have a diameter of just 4 mm ( at 275 GHz), while the total device footprint including input and output tapers is . This approach is scalable to waveguide crossings, providing a broadband and compact solution for low‐loss terahertz integrated optics.