Dynamics of Ultrafast Phase Changes in Amorphous GeSb Films

Abstract

Time resolved imaging has been used to analyze structural transformations induced by intense 100 fs laser pulses in amorphous GeSb films. Above a threshold of 19 mJ͞cm 2 the data show the formation of a transient nonequilibrium state of the excited material within 300 fs. The results are consistent with an electronically induced, amorphous-to-crystalline phase transition. [S0031-9007(98) PACS numbers: 61.80. Ba, 61.43.Dq, 68.35.Bs, 68.35.Rh The existence of nonthermal, ultrafast phase transitions after strong femtosecond laser excitation has been demonstrated in several materials such as silicon [1][2][3], gallium arsenide [3-6], indium antimonide [7], and carbon [8]. It is accepted that such transitions are induced by a softening of the lattice structure due to the generation of a very high density electron-hole plasma, as first proposed by Van Vechten [9]. Most of the experimental and theoretical work [10][11][12][13] has concentrated on the solid-to-liquid phase transformation of the above mentioned materials. Some authors [2,11] have also claimed the existence of an ultrafast solid-to-solid phase transition preceding nonthermal melting, although there is experimental evidence [3,7] refusing this hypothesis.In this Letter we present a time resolved study on the dynamics of femtosecond laser induced structural transformations in amorphous GeSb. We describe experimental evidence for the subpicosecond formation of a transient phase of the material and interpret our results as first indication for an ultrafast, nonthermal amorphousto-crystalline phase transition. Sb-rich amorphous GeSb films can be crystallized upon irradiation with ultrashort laser pulses [14] and show a large optical contrast upon transformation [15]. This effect makes GeSb a promising material for the development of rewritable optical memories driven by ultrashort laser pulses [16]. Recently, it has been demonstrated [17] that the energy fluence necessary to induce crystallization in this material decreases significantly for pulses shorter than a picosecond, suggesting the relevance of electronic processes in the amorphousto-crystalline transition.Amorphous Ge 0.06 Sb 0.94 , 50-nm-thick films were grown on glass substrates at room temperature in a multitarget dc magnetron sputtering system from pure (99.999%) Ge and Sb targets. The films were irradiated with 100 fs laser pulses at 620 nm delivered by a 10-Hz amplified colliding-pulse mode-locked (rhodamine 6G͞DODCI) dye laser. The evolution of the reflectivity of the irradiated surface was monitored with both femtosecond time and micrometer spatial resolution by means of ultrafast time-resolved microscopy [18]. A first laser pulse excites the sample. A second time-delayed probe pulse replaces the standard illumination of an optical microscope and provides snapshot pictures of the excited surface with 100 fs time resolution. The optical micrographs are recorded with the help of a charged coupled device detector in conjunction with a computer controlled frame-grabber. Since a single laser pu...

Ultrafast heating and resolution of recorded crystalline marks in phase-change media

“…͑17͒ and ͑20͒ also occur within a narrow range that approaches the melting temperature of the phasechange medium under consideration, even for modest crystalline fractions s . This could explain the published results of fast laser heating experiments of amorphous phase-change material which either lead to melting 9,27 or resulted in burned-out holes in the amorphous material.…”

Influence of pulse duration on the amorphization of GeSb thin films under ultrashort laser pulses

“…In contrast, studies performed on 50-nm-thick films under 100 fs pulses showed an a-to-c transformation time of ϳ30 ns. 19 Globally, all these results indicate that phase reversibility and the transformation time under ultrashort pulses are essentially controlled by the heat flow dynamics of the system under each pulse duration. The aim of this work is to determine the influence of pulse duration on the dynamics of the c-to-a phase transformation in GeSb films, covering a span of pulse durations of five orders of magnitude, from the ns to the fs regime.…”

“…19,21 However, solidification is still essentially a thermal process. 24,25 The primary effect of reducing the pulse duration below 2 ps is to produce a strong increase in both the initial thermal gradient across the melt volume ͑the film is again completely melted͒ and the initial supercooling that leads to a sharp increase in the nucleation rate of the solid phase.…”

Development of an electron-temperature-dependent interatomic potential for molecular dynamics simulation of tungsten under electronic excitation

“…[4][5][6][7] These experiments point toward changes occurring a short time after irradiation with intense pulses at high fluence. This is different from thermal melting of the material that can be induced at lower fluence, occurring on longer time scales.…”