Summary. The optical switch from crystalline to amorphous state of GST225 nanaparticles with size <10nm is obtained via single shot laser pulses with very low laser fluence.
Ultrafast active materials with tunable properties are currently investigated for producing successful memory and data-processing devices. Among others, Phase-Change Materials (PCMs) are eligible for this purpose. They can reversibly switch between a high-conductive crystalline state (SET) and a low-conductive amorphous state (RESET), defining a binary code. The transformation is triggered by an electrical or optical pulse of different intensity and time duration. 3D Ge-Sb-Te based alloys, of different stoichiometry, are already employed in DVDs or Blu-Ray Disks, but they are expected to function in non-volatile memories and RAM. The challenge is to demonstrate that the scalability to 2D, 1D up to 0D of the GST alloys improves the phase-change properties in terms of lower power threshold and faster switching time. Nowadays, GST thin films and nanoparticles have been synthetized and have beenshown to function with competitive results.[3, 4]
The present work aims at showing the very low power limit of an optical pulse needed to amorphyze crystalline GST225 nanoparticles. We considered particles of 7.8nm and 10.4nm diameter size deposited on Mica and capped with ~100nm of PMMA. A table-top Ti:Sapphire regenerative amplified systemproduced pump laser pulses at 400nm, of ~80fs and with a repetition rate from 1kHz to single shot. We created a grid of single shot laser spots with increasing average fluence on the GST NPs samples. According to the pump average fluence applied, we measured -with an overlapping probe beam at 800nm- a difference in optical transmittivity before and after the laser pump treatment. Optical microscope images revealed how the laser spotted areas have a brighter contrast with respect to the “virgin” ones. Then, by annealing the sample for 20 min at 150°C (crystallization temperature) the spots disappeared. All these evidences lead to the conclusion that the single shot pulses, above a specific average fluence, amorphyze the crystalline GST NPs. By measuring the amorphyzed areas, we extrapolated these switching threshold fluences for both the different-size NPs. The results are reported and discussed.
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