Borrmann effect in Laue diffraction in one-dimensional photonic crystals under a topological phase transition

摘要

We present experimental and theoretical studies of the anomalous high transmission of light (the Borrmann effect) under the Laue diffraction in a one-dimensional photonic crystal (PhC) characterized by spatial modulation of both refractive index and absorption. We show that a strong modulation of the refractive index along with the large PhC period provide new features of the Borrmann effect as compared to the well-known x-ray Borrmann effect in crystals appearing in PhC wavelength-angular transmission spectra. Namely, the maximal transmission is attained at the Bragg angles of incidence and corresponds alternatively to even or odd orders of the Bragg angles depending on the light wavelength. Second, a dramatic decrease of the angular width of the high transmission areas in the spectrum appear near the diabolic points. According to our description, this effect can be treated as a result of the topological phase transition accompanied by exchange of the parity of spatial distribution of the electromagnetic field of the two eigenmodes experiencing degeneracy. We demonstrate that these peculiarities are inherent to the PhC with the optical losses located in layers with higher refractive index, and disappear if the losses are specific for the PhC layers with lower refractive index. The suggested underlying mechanism involves the contribution of the waveguide PhC modes to the PhC transmission spectra.