When talking about photonic integrated circuits, one of the first questions we are asked about is their polarization sensitivity. The main reason is that it is extremely challenging to realize photonic devices that are polarization insensitive, especially when we use high-index-contrast platforms like silicon photonics. On the other hand, it is also true that making a device with an ultra-high polarization dependence can be equally difficult. Polarizers should be as transparent as possible to one polarization state while being as lossy as possible to the orthogonal state. However, it’s still a challenge to produce integrated polarizers with a small size, low loss, high polarization selectivity, and operating on a broad band.
In this work, Xu and coworkers demonstrate that it is actually possible to face all these issues in a rather simple way. They conceived a compact silicon device consisting of a sharply bent waveguide, which is assisted by an anisotropic sub-wavelength grating (SWG), that is a series of dielectric nano-stripes with deep sub-wavelength scale. This SWG structure provides enhanced polarization sensitivity, because only the TE polarization can propagate through the bend, while the TM polarization is radiated away regardless of the wavelength. As a result, high polarization extinction ratio (>20 dB) is achieved over a record bandwidth >415 nm (from 1.26 μm to 1.675 μm) in a less than 100 μm2 footprint device.
This polarization sensitive "guardrail" is a new tool for on-chip management of polarization and could find useful applications in integrated devices for optical communication systems, sensing, and imaging.
