  Second-harmonic generation allows for coherently bridging distant regions of
the optical spectrum, with applications ranging from laser technology to
self-referencing of frequency combs. However, accessing the nonlinear response
of a medium typically requires high-power bulk sources, specific nonlinear
crystals, and complex optical setups, hindering the path toward large-scale
integration. Here we address all of these issues by engineering a chip-scale
second-harmonic (SH) source based on the frequency doubling of a semiconductor
laser self-injection-locked to a silicon nitride microresonator. The
injection-locking mechanism, combined with a high-Q microresonator, results in
an ultra-narrow intrinsic linewidth at the fundamental harmonic frequency as
small as 57 Hz. Owing to the extreme resonant field enhancement,
quasi-phase-matched second-order nonlinearity is photoinduced through the
coherent photogalvanic effect and the high coherence is mapped on the generated
SH field. We show how such optical poling technique can be engineered to
provide efficient SH generation across the whole C and L telecom bands, in a
reconfigurable fashion, overcoming the need for poling electrodes. Our device
operates with milliwatt-level pumping and outputs SH power exceeding 2 mW, for
an efficiency as high as 280%/W under electrical driving. Our findings suggest
that standalone, highly-coherent, and efficient SH sources can be integrated in
current silicon nitride photonics, unlocking the potential of $\chi^{(2)}$
processes in the next generation of integrated photonic devices.
