Si photonic waveguides with broken symmetries: applications from modulators to quantum simulations

Symmetries of waveguides determine fundamental properties of photons such as mode profiles, polarisation, and effective refractive indexes as well as practical properties affecting the propagation loss. Here, we review our recent progress on manipulating symmetries of silicon (Si) photonic waveguides. Starting from the strategic choice of Si-on-insulator (SOI) wafer specifications, we established the process technologies to fabricate Si wire and slot waveguides with atomically-flat interfaces, defined by Si (111) planes. These waveguides have relatively low propagation loss (∼1 dB cm−1), even though they were fabricated in a university line. By combining patterning and re-growth of deposited amorphous Si, we also fabricated an Si slot waveguide with a nano-meter-scale vertical oxide layer, which is useful for optical modulators and various sensing applications. We also fabricated a horizontal slot waveguide using our manually bonded double-SOI substrate. The self-limited alkali-wet-etching allowed us to pattern the bottom SOI layer on top of the top SOI layer, by properly designing the mask to align along the mirror asymmetric Si (110) surface, allowing to access to top and bottom SOI layers individually through connected multiple-fin arrays. The patterning technique can be readily applicable to the other platform such as Si/LiNbO3-hybrid wafers, and we discuss our design of electro-optic modulator towards zero power consumptions. We also investigate photonic crystal waveguides with broken mirror symmetries. By manipulating the mismatch between adjacent photonic crystals across the waveguide made of line defects, we could continuously control the band-gap of the photonic crystals. Moreover, the phase profiles of modes exhibited photonic graphene and poly-acetylene shapes, made of optical vortices with optical orbital angular momentum. This shows that the most energetically favourable configuration of a photonic material under the triangular lattice is topologically equivalent to an organic material. We discuss the potential for the photonic organic chemistry and possible applications in quantum technologies.