  Recently, moir\'e engineering has been extensively employed for creating and
studying novel electronic materials in two dimensions. However, its application
in nanophotonic systems has not been widely explored so far. Here, we
demonstrate that twisted bilayer photonic crystals provide a new photonic
platform with twist-angle-tunable optical dispersion. Compared to twisted
two-dimensional materials, twisted bilayer photonic crystals host a rich set of
physics and provide a much larger number of degrees of freedom choice of
material, lattice symmetry, feature size, twist angle, and interlayer gap,
which promises an unprecedented toolbox for tailoring optical properties. We
directly visualize the dispersion throughout the optical frequency range and
show that the measured optical response is in good quantitative agreement with
numerical and analytical results. Our results reveal a highly tunable band
structure of twisted bilayer photonic crystals due to moir\'e scattering. This
work opens the door to exploring unconventional physics and novel applications
in photonics.
