  Thin film lithium niobate (LN) has recently emerged as a playground for
chip-scale nonlinear optics and leads to highly efficient frequency conversions
from near infrared to near-visible bands. For many nonlinear and quantum
photonics applications, it is desirable to operate deep into the visible band
within LN's transparency window. However, the strong material dispersion at
short wavelengths makes phase-matching difficult, necessitating sub-micron
scale control of domain structures for efficient quasi-phase-matching (QPM).
Here we report the operation of thin film LN in the blue wavelength and high
fidelity poling of thin-film LN waveguide to this regime. As a result,
quasi-phase matching is realized between IR (871nm) and blue (435.5nm)
wavelengths in a straight waveguide and prompts strong blue light generation
with a conversion efficiency $2900\pm400\%W^{-1}cm^{-2}$
