Revealing the emissivity of a photonic crystal slab with Ge(Si) nanoislands beyond the Г-point of the Brillouin zone

The paper presents an experimental method for analyzing the dispersion dependences of the emissivity of photonic crystal slabs, which allows one to reconstruct the dispersion of photonic crystal modes directly from photoluminescence measurements and study their features in a wide range of wave vectors, beyond the Г-point of the first Brillouin zone. As an example, we analyzed the dispersion dependences of the emissivity of a photonic crystal slab with Ge(Si) nanoislands. The study revealed a number of features associated with mode interaction phenomena such as mode crossings and anti-crossings, as well as the emergence of bound states in the continuum (BIC). Bound states in the continuum with quality factors Q exceeding 103 were detected both at the Г-point of the Brillouin zone (symmetrically protected BICs) and beyond it. We show that the bound states in the continuum observed beyond the Г-point are Friedrich–Wintgen states arising as a result of destructive interference of modes and accompanied by the modes avoided crossing. The experimental data presented in this paper are discussed together with the results of theoretical analysis, in particular, calculations of the far-field emissivity of the photonic crystal slab, analysis of the field patterns and symmetry of the selected modes, as well as calculations of the emissivity of the photonic crystal slab in s and p polarizations.