  We demonstrate the post-induction of high-quality microcavity on silicon
photonic crystal (PC) waveguide by integrating few-layer GaSe crystal, which
promises highly efficient on-chip optical frequency conversions. The
integration of GaSe shifts the dispersion bands of the PC waveguide mode into
the bandgap, resulting in localized modes confined by the bare PC waveguides.
Thanks to the small contrast of refractive index at the boundaries of
microcavity, it is reliably to obtain quality (Q) factors exceeding 10^4. With
the enhanced light-GaSe interaction by the microcavity modes and high
second-order nonlinearity of GaSe, remarkable second-harmonic generation (SHG)
and sum-frequency generation (SFG) are achieved. A record-high on-chip SHG
conversion efficiency of 131100% W^-1 is obtained, enabling the clear SHG
imaging of the resonant modes with the pump of sub-milliwatts continuous-wave
(CW) laser. Driven by a pump of on-resonance CW laser, strong SFGs are
successfully carried out with the other pump of a CW laser spanning over the
broad telecom-band. Broadband frequency conversion of an incoherent
superluminescent light-emitting diode with low spectral power density is also
realized in the integrated GaSe-PC waveguide. Our results are expected to
provide new strategies for high-efficiency light-matter interactions, nonlinear
photonics and light source generation in silicon photonic integrated circuits.
