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Recent advances in photonic phase change materials (PCMs) brought about a new alloy, GSS4T1,-a fully dielectric PCM for free-space wavelengths above 5 microns in both its amorphous and crystalline phases. The refractive index of GSST4T1 changes by about Dn=~1.4, when transitioning from the amorphous to the crystalline phase. Hence, GSS4T1 has opened new powerful capabilities for reconfigurable beam manipulation with all-dielectric platforms [2]. In particular, a binary metagrating comprising amorphous GSS4T1 and GaAs behaves as a homogenous film within wide spectral regions because of the similar refractive index of amorphous GSS4T1 and GaAs. On the other hand, the same binary metagrating with GSS4T1 switched to its crystalline phase, exhibits a rich spectral response, favoring many Fano resonances in its transmission spectrum. These emanate from the underlying leaky Floquet-Bloch modes supported by the metagrating. Depending on the structural parameters the emerging Fano spectral features can be very different, ranging from highly asymmetric to quasi-symmetric. Here, we discuss how the characteristics of these Fano features relate to the performance of the beam steering and splitting capability in the crystalline GSS4T1 metagrating. Specifically, our results support that an efficient re-configuration of the light beam from a straight to a back-bent path can occur only when the crystalline GSS4T1 metagrating supports quasi-symmetric Fano resonances [2, 3]. [1] Y. Zhang et al., Broadband transparent optical phase change materials for high-performance nonvolatile photonics, Nature Comm. 10, 4279 (2019). [2] N. L. Tsitsas and S. Foteinopoulou, Non-volatile MWIR/LWIR beam reconfigurability with all-dielectric metagratings comprising phase-change materials with a high-refractive-index shift, Opt. Mat. Express 12, pp. 3187-3212 (2022). [3] N. L. Tsitsas and S. Foteinopoulou, Semi-Analytical approach for the design of reconfigurable all-dielectric metagratings with phase-change material constituents, paper accepted in EuCAP 2023, to appear in IEEExplore.
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