  In this paper, we demonstrate strip-loaded guiding optical components
realized on a 27 nm ultra-thin SOI platform. The absence of physically etched
boundaries within the guiding core suppresses majorly the scattering loss, as
shown by us previously for a silicon nitride (Si$_3$N$_4$) platform [Stefan
\textit{et. al.}, OL 40, 3316 (2015)]. Unexpectedly, the freshly fabricated Si
devices showed large losses of 5 dB/cm, originating from absorption by free
carriers, accumulated under the positively charged Si$_3$N$_4$ loading layer.
We use 254 nm ultraviolet (UV) light exposures to neutralize progressively and
permanently silicon nitride's bulk charge associated with diamagnetic
K+defects. This in turn leads to a net decrease of electron concentration in
the SOI layer, reducing thus the propagation loss down to 0.9 dB/cm. Detailed
MOS-capacitance measurements on test samples were performed to monitor the
UV-induced modification of the electronic properties of the system. The
evolution of loss mitigation was directly monitored both by Beer-Lambert
approach in waveguide transmission experiments, as well as through more
accurate cavity linewidth measurements. In the last case, we demonstrate how
intrinsic cavity $Q$'s boost from 60,0000 to up to 500,000 after UV treatment.
Our results may open routes towards engineering of new functionalities in
photonic devices employing UV-modification of space charges and associated
local electric fields, unveil the origin of induced optical nonlinearities in
Si$_3$N$_4$/Si micro-photonic systems, as well as envisage possible integration
of these with ultra-thin SOI electronics.
