Ultrafast temporal-spatial dynamics of amorphous-to-crystalline phase transition in Ge₂Sb₂Te₅ thin film triggered by multiple femtosecond laser pulses irradiation

Abstract

Studies have shown that the crystallization phase state of Ge 2 Sb 2 Te 5 (GST) can be reversibly modulated by femtosecond (fs) laser multiple pulses, which have excellent applications in reconfigurable multi-level operation fields. In this study, the temporal-spatial crystalline evolution dynamics of amorphous GST film is investigated during two fs laser pulses excitation through a pump-probe shadowgraph imaging technique. A quasi-amorphous phase state, which is different from that in the initial as-deposited amorphous GST, is emerged through the first fs laser pulse excitation with a pulse energy lower than crystallization threshold. The experimental results reveal that a crystallization enhancement effect can be induced through the second pulse excitation based on this quasi-amorphous surface structure. The stimulative cluster generated in the quasi-amorphous reduces the amorphous-to-crystalline phase transition threshold for the second fs laser pulse irradiation. The spatially-resolved phase-transition threshold extension effect in a horizontal direction is proposed with the increasing pulse number to summarize the mechanism of the crystallization enhancement effect. The specific-grain-appearance (coarse grains and fine grains representing different phase transition approach) distributed area induced by single and double fs laser pulses irradiation are experimentally demonstrated corresponding to threshold extension theory.

Ultrafast Dynamics of Different Phase States Ge2Sb2Te5 Film Induced by a Femtosecond Laser Pulse Irradiation

“…The samples annealed at different temperatures were also measured using the Raman system, as shown in Figure 3 . According to the characteristic peaks of GST [ 26 , 29 , 31 , 32 , 33 ], it can be seen that a broad peak at about 150 cm −1 (Peak A) results from the stretching vibrations of Te-Te bonds Sb m Te 3 (m = 1, 2) pyramidal units [ 31 ], and another peak at about 110 cm −1 (Peak B) is caused by the heteropolar bonds of GeTe 4 or GeTe 3 Ge [ 26 ] for the as-grown GST and films annealed at 150 °C for 30 min. The relative intensity of Peak A to Peak B is about 1.4 for the amorphous GST, while it decreases to around 1.31 for the films annealed at 150 °C for 30 min.…”

“…Zhang and Gan [ 25 ] found that the carriers could be excited within 1 ps and recovered less than 3 ns. In addition, the ultrafast dynamics of amorphous and quasi-amorphous GST films were compared in pump–probe experiments, and the mechanism of the crystallization enhancement effect was proposed [ 26 ]. However, there were the amorphous, crystalline, and intermediate phase states in reversible phase transitions, which had multiple applications e.g., non-volatile memory, because the characteristics of these phase states were different [ 27 , 28 , 29 ].…”

Investigation of the Crystallization Characteristics of Intermediate States in Ge2Sb2Te5 Thin Films Induced by Nanosecond Multi-Pulsed Laser Irradiation

“…On the contrary, as the laser fluence was gradually increased, the dominated peak shifted to ~105 cm −1 with a weaker one at ~135 cm −1 . Interestingly, the peak at ~105 cm −1 was considered as a characteristic peak of crystallization [ 42 ]. Among these intermediate states, the peak intensity at ~105 cm −1 was gradually increased with the increase of laser energy.…”

Optical diffraction from Ge2Sb2Te5 fishnet metasurfaces

“…In GST -coated opaline photonic crystals, light control mediated by interplay of Wood anomalies and 3D Bragg diffraction resonances was demonstrated [20,21]. Raman spectroscopy measurements were performed to reveal the detailed phase state of GST [23]. We conclude our short review by mentioning that in phase-change materials based on GST, it is possible to produce laser-induced periodic ripples which are formed by interference of the incident laser beam with the surface electromagnetic wave excited by this beam [22,24].…”

“…The optical properties of both three-dimensional metamaterials and metasurfaces based on GST, including transmission, reflection, absorption and Raman spectra, have been actively studied [12][13][14][15][16][17][18][19][20][21][22][23][24]. Transmission and absorption studies have focused on the mid-infrared region, where strong spectral changes were observed as a result of the phase transition between the amorphous and crystalline phases, which can be used for practical applications.…”