Fig. 3. Experimental realization and probing of linear 2D photonic SSH lattices.

a1–c1 Laser-written 2D SSH lattices tuned to nontrivial, square, and trivial regimes at different dimerizations, where the dashed circles indicate the lattice sites for corner, edge, and bulk excitations, and a and b mark the waveguide distances for the weak and strong coupling bonds. a2–c2 Experimental results for a linear output corresponding to single-site excitations in a1–c1, where the corner excitation in a1 leads to a localized BIC with evident topological features: no light distribution appears in the nearest neighboring sites, but we observe a non-zero intensity in the NNN sites along two edges. Discrete diffraction is observed for all other excitations. a3–c3, a4–c4 Numerical results corresponding to corner excitations in a2–c2 obtained using the continuum model, where the propagation distance is a3–c3 20 mm corresponding to the length of the crystal used in the experiments and a4–c4 120 mm corresponding to a longer propagation, for direct comparison. Experimental parameters: a = 31 μm, b = 23 μm; the bias field during the writing process is E₀ = 130 kV m⁻¹