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 exotic optical modes that remain decoupled from free-space radiation. Symmetry-protected BICs in infinite metasurfaces provide unique opportunities to tailor light–matter interactions in extended photonic structures. One of the most important properties of these structures 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. It quantifies how the surrounding photonic environment modifies the ability of light to couple with matter at a given location. Here, we employ a terahertz near-field microscope with dual local probes to directly excite and detect quasi-BICs in finite metasurfaces with inversion symmetry, which support a symmetry-protected BIC in their infinite counterpart. Our results provide the first direct evidence of a large PLDOS enhancement associated with BICs and quasi-BICs in finite metasurfaces. As the metasurface size increases, the quasi-BIC evolves into the BIC, and the quality factor (Q) diverges while the PLDOS saturates at a finite value. This result reflects an upper limit for enhanced light-matter interaction by BICs. Our findings pave the way for next-generation on-chip metasurfaces with maximum light-matter interaction strengths.