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A compact waveguide crossing structure with low transmission losses and negligible crosstalk is demonstrated for silicon-on-insulator circuits. The crossing structure is based on a mode expander optimized by means of a genetic algorithm leading to transmission losses lower than 0.2 dB and crosstalk and reflection losses below 40 dB in a broad bandwidth of 20 nm. Furthermore, the resulting crossing structure has a footprint of only 6x6 microm(2) and does not require any additional fabrication steps.

Cuesta, F. S.; Faniayeu, I. A.; Asadchy, V. S.; Tretyakov, S. A. Planar broadband Huygens' metasurfaces for wave manipulationsAbstract-Electrically thin and effectively two-dimensional material composites, metasurfaces, have been widely exploited for manipulation of electromagnetic waves. For many applications it is desired to transform incident waves of a specific frequency range keeping the metasurface invisible at other frequencies.Such frequency-selective response can be achieved based on subwavelength Huygens' inclusions. However, their fabrication requires sophisticated processes due to the three-dimensional geometry. Here, we propose a planar Huygens' meta-atom with the goal to open a way to realize broadband invisible metasurfaces with topologies suitable for the conventional printed circuit board fabrication technology. We synthesize and analyse, both numerically and experimentally, three different metasurfaces capable of polarization and amplitude transformations of incident waves.

We propose a structure that allows the splitting of electromagnetic waves with a phase shift of 180° between output signals based on photonic crystals. The structure consists of two parallel coupled-cavity waveguides placed in proximity. The performance of the splitting structure is theoretically discussed, evaluated by means of finite-difference time-domain method simulations and experimentally demonstrated at microwave frequencies. As both output paths have the same physical length, the two output signals are synchronized, which is very attractive for splitting high-speed optical signals in photonic-crystal-based integrated circuits.

A detailed analysis of adiabatic coupling between conventional photonic crystal single-line-defect and coupled-resonator optical waveguides is reported. Adiabatic coupling by progressive variation of the radii of the spacing defects between cavities is investigated. Flat transmission spectra with coupling efficiencies greater than 90% are achieved in a broad frequency range with short coupling lengths. Moreover, we find that flat transmission at low frequencies requires longer coupling lengths partly because the requirements imposed for adiabatic transmission in photonic crystals are violated.