Realization of 4-Bit Multilevel Optical Switching in Ge₂Sb₂Te₅ and Ag₅In₅Sb₆₀Te₃₀ Phase-Change Materials Enabled in the Visible Region

Non-volatile phase-change photonic memory devices are of particular interest in recent years due to their high-density storage with excellent scalability features. The optical chip employed in on-chip photonic memory devices has displayed outstanding performance operating at infrared wavelengths; however, realizing multilevel switching in the visible region is a key challenge owing to the limitations of a high absorption coefficient and the undesirable volume changes. In this study, 4-bit multilevel switching operation (16 levels) with a uniform reflectivity contrast (∼1.5% per level) in Ge2Sb2Te5 and Ag5In5Sb60Te30 (AIST) films operating at the visible wavelength (532 nm) is demonstrated by optimizing the pump beam (PB) diameter. The optimization of the PB diameter has illustrated its larger influence in determining the substantial reflectivity contrast, especially in growth-dominated AIST. Additionally, the role of PB diameter is corroborated through the calculation of crystal growth velocity in AIST. The simulation reveals the higher growth velocity of 110 m/s for smaller PB diameter (0.4 mm), whereas 84.16 and 7.94 m/s are obtained for diameters of 0.6 and 0.7 mm, respectively. Furthermore, the vibrational modes of individual optical levels have been systematically explored using Raman spectroscopy, and the underlying mechanism behind multilevel switching has been validated in technologically important nucleation and growth-dominated phase-change materials. The present experimental findings demonstrating the feasibility of 16 multilevel states in the visible region would be promising for designing future photonic memory devices.