  In the last decade, symmetry-protected bound states in the continuum (BICs)
have proven to be an important design principle for creating and enhancing
devices reliant upon states with high quality (Q) factors, such as sensors,
lasers, and those for harmonic generation. However, as we show, current
implementations of symmetry-protected BICs in photonic crystal slabs can only
be found at the center of the Brillouin zone and below the Bragg-diffraction
limit, which fundamentally restricts their use to single-frequency
applications. By 3D-micro printing a photonic crystal structure using
two-photon polymerization, we demonstrate that this limitation can be overcome
by altering the radiative environment surrounding the slab to be a
three-dimensional photonic crystal. This allows for the protection of a line of
BICs by embedding it in a symmetry bandgap of the crystal. Moreover, we
experimentally verify that just a single layer of this photonic crystal
environment is sufficient. This concept significantly expands the design
freedom available for developing next-generation devices with high-Q states.
