  Active metasurfaces, which are arrays of actively tunable resonant elements,
can dynamically control the wavefront of the scattered light at a subwavelength
scale. To date, most active metasurfaces that enable dynamic wavefront shaping
operate in reflection. On the other hand, active metasurfaces operating in
transmission are of considerable interest as they can readily be integrated
with chip-scale light sources, yielding ultra-compact wavefront shaping
devices. Here, we report designs for all-dielectric low-loss active
metasurfaces which can dynamically manipulate the transmitted light wavefront
in the near-infrared wavelength range. Our active metasurfaces feature an array
of amorphous silicon (a-Si) pillars on a silica substate, which support
resonances with quality factors (Q-factors) as high as 9800, as well as other
lower-Q resonances. First, we demonstrate that high-Q resonance dips observed
in transmission can be transformed into a transmission resonance peak by
positioning a-Si pillar resonators at a prescribed distance from a crystalline
Si substrate, defined by a silica spacer layer. Next, we report the design of
metasurface geometry with realistic interconnect architectures that enable
thermo-optic dynamic beam switching with switching times as low as 7.3 {\mu}s.
Beam switching is observed for refractive index differences between neighboring
metasurface elements as low as 0.0026. Finally, we demonstrate that metasurface
structures with both high-Q and lower-Q modes and realistic interconnect
architectures can be used for dynamic beam steering.
