  Photonic technologies lack non-invasive monitoring tools to inspect the light
inside optical waveguides. This is one of the main barriers to large scale of
integration, even though photonic platforms are potentially ready to host
several thousands of elements on a single chip. Here, we demonstrate
non-invasive light observation in silicon photonics devices by exploiting
photon interaction with intra-gap energy states localized at the waveguide
surface. Light intensity is measured through a ContactLess Integrated Photonic
Probe (CLIPP) that introduces no measurable extra-photon absorption and a phase
perturbation as low as 0.2 mrad, comparable to thermal fluctuations of less
than 3 mK. Multipoint light monitoring is demonstrated with a sensitivity of
-30 dBm and a dynamic range of 40 dB. CLIPP technology is simple, inherently
CMOS compatible, and scalable to hundreds of probing points per chip. This
concept provides a viable way to real-time conditioning and feedback control of
densely-integrated photonic systems.
