  The wavelength selective linear absorption in communication C-band is
investigated in CMOS-processed PECVD silicon nitride rings. In the overcoupled
region, the linear absorption loss lowers the on-resonance transmission of a
ring resonator and increases its overall quality factor. Both the linear
absorption and ring quality factor are maximized near 1520 nm. The direct
heating by phonon absorption leads to thermal optical bistable switching in
PECVD silicon nitride based microring resonators. We calibrate the linear
absorption rate in the microring resonator by measuring its transmission
lineshape at different laser power levels, consistent with coupled mode theory
calculations.
