  Optical bound states in the continuum (BICs) provide a unique mechanism of
light confinement that holds great potential for fundamental and applied
research in optics and photonics. Of particular interest are merging BICs,
realized in planar periodic structures by coalescence of accidental and
symmetry-protected BICs. Topological nature of merging BICs renders their $Q$
factors exceptionally high and robust. However, this cannot be achieved if the
structure lacks the up-down mirror symmetry, which is the case for all
substrate-supported structures. Here, we show that the detrimental effect of
the symmetry breaking can be perfectly compensated for by an additional thin
layer of a dielectric material on one side of the structure. Our approach can
enable flexible design, feasible realization, and extended applications of
various topologically robust systems.
