Herein, eight uniform optical states (3 bit) are demonstrated by irradiating nanosecond laser pulses on thin In3SbTe2 films having high stability (260 °C), revealing at least 1% reflectivity contrast between any two consecutive states with strikingly low noise variation of 0.18% at each level, which is almost a 50% lower value compared to Ge2Sb2Te5 and AgInSbTe materials, revealing the two times enhanced signal‐to‐noise ratio of the In3SbTe2 material. Furthermore, a systematic structural evolution during multilevel switching is investigated using confocal Raman spectroscopic studies. The experimental findings demonstrate low‐noise yet highly stable multilevel switching toward the development of reliable phase change photonic memory devices.

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Non-volatile phase-change photonic memory devices are of particular interest in recent years due to their highdensity 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 Ge 2 Sb 2 Te 5 and Ag 5 In 5 Sb 60 Te 30 (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.

Phase change materials including GeSbTe and AgInSbTe have successfully demonstrated multilevel switching capabilities, yet achieving precise controllability and reproducibility are crucial towards technological applications. In this study, we demonstrate nine distinct optical levels in Ge2Sb2Te5 (GST225) and Ag5In5Sb60Te30 (AIST) phase-change materials using pump-probe experiments under identical device conditions and the role of the crystallization mechanism is examined for realization of reliable multi-level programming. Nucleation-dominated GST225 material corroborates improved performance characteristics of low threshold fluence (6 mJ cm−2), least optical variation (±0.25%), and high reflectivity contrast (∼2.5%) between any two consecutive levels as compared to growth-dominated AIST material. Furthermore, the opto-thermal simulations depict a gradual change in the crystalline fraction in GST225 and an abrupt change in AIST, which further confirms the improved controllability in nucleation-dominated crystallization. Hence, these identical measurements along with the opto-thermal simulations elucidate that the role and nature of crystallization play a critical role in precise control of variation of reflectivity in multi-level states of GST225 and AIST, respectively. These findings will be useful towards the development of reliable multi-bit phase-change photonic memory devices.

Multilevel storage in chalcogenide‐based phase‐change materials is one of the desired characteristics to design neuromorphic and in‐memory computing applications. However, precisely controlling the crystalline and amorphous fraction to achieve reliable multilevel states is one of the key challenges in multilevel switching. Herein, multilevel switching is focused on the aspect of optical domain, where it enjoys the benefits of higher bandwidth with low delay connectivity suitable for non‐von Neumann architecture. The essential requirements for multilevel optical switching are discussed in terms of programming techniques, novel device structures, and emerging materials for its better optimization. Furthermore, the impact of nature of crystallization mechanism on the multilevel switching for different families of phase‐change materials is reviewed. In addition, the multilevel switching for neuromorphic engineering and in‐memory computation based on the integrated photonic memory devices are assessed. Finally, several challenges and different strategies to improve the performance of multilevel switching in phase‐change materials are discussed and thereby signify its importance for the design of future on‐chip phase‐change photonic memory devices.