Merging bound states in the continuum and unidirectional guided resonances in ultrathin photonic-crystal slabs enabled by TM polarization

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Bound states in the continuum (BICs) and unidirectional guided resonances (UGRs) offer exceptional light-confinement capabilities, yet their realization in ultrathin subwavelength regimes remains constrained. Here, we reveal a polarization-dependent symmetry selection rule governing accidental BICs in photonic-crystal (PhC) slabs: fundamental transverse magnetic (TM 0 ) guided resonance selectively couples with odd-order Fabry-Pérot (FP) modes, whereas fundamental transverse electric (TE 0 ) guided resonance couples exclusively with even-order FP modes. In ultrathin PhC slabs, blue-shifted FP modes leave only the fundamental FP mode within the sub-diffraction region, rendering accidental BICs uniquely accessible to TM polarization. Utilizing this TM-driven mechanism, we demonstrate merging BICs at an ultrathin thickness, as well as momentum-tunable UGRs via out-of-plane mirror symmetry breaking. Our work establishes a symmetry-guided framework for topological light manipulation in ultrathin nanophotonic platforms.