Left: experimental images of A-SubSy-preserving lattice with B–B coupling ( d ) and A-SubSy-breaking lattice with A–A coupling ( e ). The white arrows indicate that solely the left-most waveguide (A sublattice site) is excited at z = 0. Middle: output intensity patterns of the probe beam after propagating 20 mm through corresponding lattices in the left panels, respectively. The intensity profile in the A-SubSy-preserving lattice resides solely on the A sublattice ( d ), whereas the intensity profile in the A-SubSy-breaking lattice is present on both A and B sublattices ( e ), which demonstrates SubSy protection of the left topological edge mode. The lattice constant (size of the unit cell) is 40 μm, and the twist angle is 57°. Right: numerical simulations in the middle panels agree with the related measurements.
