  Highly-tunable coherent light generation is crucial for many important
photonic applications. Second-harmonic generation (SHG) is a dominant approach
for this purpose, which, however, exhibits a trade-off between the conversion
efficiency and the wavelength tunability in a conventional nonlinear platform.
Recent development of the integrated lithium niobate (LN) technology makes it
possible to achieve a large wavelength tuning while maintaining a high
conversion efficiency. Here we report on-chip SHG that simultaneously achieves
a large tunability and a high conversion efficiency inside a single device. We
utilize the unique strong thermo-optic birefringence of LN to achieve flexible
temperature tuning of type-I inter-modal phase matching. We experimentally
demonstrate spectral tuning with a tuning slope of 0.84 nm/K for a telecom-band
pump, and a nonlinear conversion efficiency of 4.7% W$^{-1}$, in a LN
nanophotonic waveguide only 8~mm long. Our device shows great promise for
efficient on-chip wavelength conversion to produce highly-tunable coherent
visible light for broad applications, while taking advantage of the mature and
cost-effective telecom laser technology.
