Fig. 4. Modal robustness of a topological edge state (TES).

a Schematics of two coupled cavities (top) and a topologically coupled cavity array (or Type-1, bottom). b Electromagnetic energy fraction stored in the first cavity on the left (marked by coloured circles in a) as a function of the disorder factor for the coupled two-cavity system (blue squares) and Type-1 coupled cavity array (red circles). c Edge-state laser-emission spectra (left) and corresponding near-field scanning optical microscope (NSOM) images (right) acquired from four nominally identical but practically different Type-1 devices. During the measurements, the excitation spot was intentionally shifted to the left to trigger lasing at a TES. d Bulk-state laser-emission spectra (left) and corresponding NSOM images (right) from the four Type-1 devices shown in c recorded with the excitation spot aimed at the centre of the device to trigger bulk-state lasing