Near-field probing of the local density of optical states enhanced by bound states in the continuum in nonlocal metasurfaces.

Bound states in the continuum (BICs) are optical modes that remain decoupled from free-space radiation. Symmetry-protected BICs in metasurfaces offer powerful means to control light-matter interactions. A key property governing these interactions is the partial local density of optical states (PLDOS), which describes the number of electromagnetic modes available for a photon to occupy at a specific position, frequency and polarization. Here, we employ a terahertz near-field microscope with dual local probes to directly excite and detect quasi-BICs in finite metasurfaces that possess inversion symmetry, corresponding to a symmetry-protected BIC in their infinite counterpart. We observe a strong enhancement of the PLDOS associated with these modes. As the metasurface size increases, the quasi-BIC evolves into a true BIC, with the quality factor diverge while the PLDOS saturates. This establishes an upper limit for enhanced light-matter interaction by BICs. Our findings pave the way for on-chip metasurfaces with maximum light-matter interaction strengths.