Two-dimensional extreme skin depth engineering for CMOS photonics.

Extreme skin depth engineering (e-skid) can be applied to integrated photonics to manipulate the evanescent field of a waveguide. Here we demonstrate that e-skid can be implemented in two directions in order to deterministically engineer the evanescent wave allowing for dense integration with enhanced functionalities. In particular, by increasing the skin depth, we create a large gap, bendless directional coupler (gap of 1.44 $\mu$m, length of 23.5 $\mu$m) that has significantly broader operational bandwidth ($\geq$ 40 nm) than traditional strip waveguide directional couplers. Lastly, our experimental results validate e-skid for integrated photonics in a CMOS photonics foundry.