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Keynote Speaker

Dr. Shirly Espinoza

Department of Structural Dynamics, ELI BEAMLINES, Czech Republic

Seeing Phase-Change Materials Locally: Imaging Ellipsometry from Anisotropy to Reversible Optical Switching

 

Low-loss phase-change chalcogenides such as Sb₂S₃ and Sb₂Se₃ are emerging as key materials for reconfigurable photonics because they combine large refractive-index tunability with negligible absorption across the visible and near-infrared. Yet their performance is not determined only by the average dielectric function. At the device scale, polycrystallinity, grain orientation, and local optical anisotropy can strongly affect phase accumulation, switching thresholds, and reproducibility.
 

In this keynote, I will discuss how imaging spectroscopic ellipsometry, imaging Mueller matrix polarimetry, and spatially resolved optical modeling can uncover the local dielectric response of these materials beyond conventional area-averaged measurements. Using Sb₂Se₃ thin films as a first example, I will show how differently oriented crystalline domains produce strong birefringence and dichroism and yield measurable refractive-index variations at telecommunication wavelengths. I will then extend this picture to Sb₂S₃, where anisotropy and polycrystalllinity have direct consequences for reversible laser-induced phase switching, including variations in amorphization thresholds and access to partial recrystallization states.
 

Together, these results show that controlling crystallographic texture is as important as minimizing optical loss. Imaging ellipsometry therefore offers a powerful bridge between local structure, dielectric function, and functional performance in next-generation programmable photonic devices

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