  We study an integrated silicon photonic chip, composed of several
sub-wavelength ridge waveguides, and immersed in a micro-cell with rubidium
vapor. Employing two-photon excitation, including a telecom wavelength, we
observe that the waveguide transmission spectrum gets modified when the
photonic mode is coupled to rubidium atoms through its evanescent tail. Due to
the enhanced electric field in the waveguide cladding, the atomic transition
can be saturated at a photon number $\approx$ 80 times less than a
free-propagating beam case. The non-linearity of the atom-clad Si-waveguide is
about 4 orders of magnitude larger than maximum achievable value in doped Si
photonics. The measured spectra corroborate well with a generalized effective
susceptibility model that includes the Casimir-Polder potentials, due to the
dielectric surface, and the transient interaction between flying atoms and the
evanescent waveguide mode. This work paves the way towards a miniaturized,
low-power, and integrated hybrid atomic-photonic system compatible with CMOS
technologies.
