consumption. Routinely, the recording (amorphization) process is obtained by applying an intense and short pulse, either a laser or a current pulse, which induces melting and fast quenching, while a moderately intense and long laser/current pulse is used to thermally anneal the material above the crystallization temperature within a sufficient amount of time in the erasing (crystallization) process. With the application of multilevel storage, each appropriate excitation with a pulse contributes to achieving a partial crystallization/ amorphization and this allows multiple bits for recording/erasing of information in each memory cell, based on the respective mode of energy accumulation. [2,5,11] Consequently, the energy consumption per cell can be reduced.Phase-change material (PCM), like Ge-Sb-Te-based alloys, exhibit remarkable material properties: significantly fast crystallization and amorphization process (order of nanoseconds), high data retention at room temperature (over more than ten years), large cyclability (>10 12 times), scalability, and an excellent contrast in terms of optical reflectivity and electrical resistivity between the amorphous and crystalline states that allow to populate the intermediate states. [12][13][14][15][16][17] Currently, chalcogenide-based PCMs are already being applied in commercial optical storage media such as CDs, DVDs, and Blu-ray disks. [16] At the same time, the reversible electrical resistivity change of PCMs can be applied in nonvolatile electronic memory, which is considered to succeed FLASH memory technology and is becoming the most promising candidate for future data storage technology. [17] Realization of multilevel states in Ge-Sb-Te-based materials is a promising response to the performance improvements in commercialization of PCMs regarding above optical and solid state memory devices, but it is also central for applications in reconfigurable photonic devices, arithmetic processes, logic and neuromorphic computing. [5,9,[18][19][20][21] In order to realize multilevel states in Ge-Sb-Te-based materials, various approaches have been explored, including either engineering the cell structure/stacking multilayers architecture, or iterative programming techniques. [6][7][8][22][23][24][25] These previous reports on realization of multilevels strategies, however, were mainly focused on the control of the Joule heating dissipation profile in the films which required complex device fabrication processes or extra circuits for varying the amplitude of applied pulses. But due to the fast phase transition in the materials by Multilevel storage techniques are promising for increasing storage density and for reducing energy consumption in the application of phase-change materials based memory devices. However, accurately controlling the phase transitions as well as understanding the underlying switching mechanisms are still under investigation. In this study, nonvolatile optical multilevel switching in single-layered GeTe phase-change films prepared by laser ablation is demons...
