  In this work, we theoretically propose and experimentally demonstrate the
formation of a super bound state in a continuum (BIC) on a photonic crystal
flat band. This unique state simultaneously exhibits an enhanced quality factor
and near-zero group velocity across an extended region of the Brillouin zone.
It is achieved at the topological transition when a symmetry-protected BIC
pinned at $k=0$ merges with two Friedrich-Wintgen quasi-BICs, which arise from
destructive interference between lossy photonic modes of opposite symmetries.
As a proof-of-concept, we employ the super flat BIC to demonstrate
three-dimensional optical trapping of individual particles. Our findings
present a novel approach to engineering both the real and imaginary components
of photonic states on a subwavelength scale for innovative optoelectronic
devices.
