  Dynamically reconfigurable metasurfaces promise compact and lightweight
spatial light modulation for many applications, including LiDAR, AR/VR, and
LiFi systems. Here, we design and computationally investigate high quality
factor silicon-on-lithium niobate metasurfaces with electrically-driven,
independent control of its constituent nanobars for full phase tunability with
high tuning efficiency. Free-space light couples to guided modes within each
nanobar via periodic perturbations, generating quality factors exceeding
30,000, while maintaining bar spacing <$\lambda$/1.5. We achieve nearly 2$\pi$
phase variation with an applied bias not exceeding $\pm$ 25 V, maintaining
reflection efficiency above 91%. Using full-field simulations, we demonstrate a
high angle, 51\deg, switchable beamsplitter with a diffracted efficiency of
93%, and an angle-tunable beamsteerer, spanning 18-31\deg, with up to 86%
efficiency, all using the same metasurface device. Our platform provides a
foundation for highly efficient wavefront shaping devices with a wide dynamic
tuning range capable of generating nearly any transfer function.
