Robustness of bound states in the continuum in metasurface based on Ge${}_2$Sb${}_2$Te${}_5$ versus structural imperfections

Abstract

We study the impact of lithography imperfections on quasi-bound states in the continuum (quasi-BICs) supported by a one-dimensional metasurface of Ge${}_2$Sb${}_2$Te${}_5$ (GST) bars with trapezoidal deviations from rectangular cross-sections. Several mechanisms of quality ($Q$) factor scaling, including the impact of material losses, dispersion, and geometric imperfections are established. We demonstrate that transition to identical isosceles trapezoids, despite preserving the required $C_2$ symmetry, reduces the $Q$ factor in the amorphous phase due to absorption changes accompanying the resonance shift. Further, the $Q$ factor remains robust for both GST phases under random element-to-element variations of the trapezoid angle, while analytical and numerical estimations in the absence of material losses show inverse-quadratic scaling of the Q factor with the disorder amplitude. We reveal that in the GST-based metasurface, the $Q$ factor is tolerant to geometric imperfections for insignificant dispersion near the BIC wavelength, but changes in case of substantial dispersion. The phase shifting and established robustness of BICs in GST can be useful for applications where stable moderate $Q$ factors are essential.

7 pages, 6 figures