Extended Data Fig. 8: FDTD simulation with dipole sources aligned with the main component of the FW quasi-BIC field.

The dipoles cover a circular area of several microns squared (same size as the input spot focused with an objective lens) at the boundary of the PCNS with the uniform slab. Their emission is pumped by the maxima of the local optical field (shown in the experimental near-field map in Supplementary Fig. 3), but their fields also collectively add up coherently emitting radiation in the plane of the slab as triggered by the patterned geometry affecting their spatially correlated emission. a, Field propagates along the direction (+1, 0) when the right boundary is excited with intensity enhancement as large as 1.5 × 10⁴ (normalized to the number of emitters) at the quasi FW-BIC wavelength (810 nm). b, At shorter wavelengths (540 nm), such as those produced with the experimental UCNP emitters, the spatially correlated fields produce narrow emission (calculated along 1 mm of propagation from the edge) and with beam divergence even of 0.02° as shown in c. d, The analysis over the whole visible and near-infrared spectrum revealed that the typical value of divergence was below 0.5°. e, The full width at half maximum of the beam shown in b changes along the propagation, obeying a mechanism of self-healing, which compensates the diffraction.