Time-resolved structural dynamics of thin metal films heated with femtosecond optical pulses

Abstract

We utilize 100 fs optical pulses to induce ultrafast disorder of 35-to 150-nm thick single Au(111) crystals and observe the subsequent structural evolution using 0.6-ps, 8.04-keV X-ray pulses. Monitoring the picosecond time-dependent modulation of the X-ray diffraction intensity, width, and shift, we have measured directly electron/phonon coupling, phonon/lattice interaction, and a histogram of the lattice disorder evolution, such as lattice breath due to a pressure wave propagating at sonic velocity, lattice melting, and recrystallization, including mosaic formation. Results of theoretical simulations agree and support the experimental data of the lattice/liquid phase transition process. These time-resolved X-ray diffraction data provide a detailed description of all the significant processes induced by ultrafast laser pulses impinging on thin metallic single crystals.ultrafast X-ray diffraction | laser-induced phase transition | coherent phonon | annealing | grain growth L attice disorder and phase transition in metal, semiconductor, and alloy are of vital importance in both fundamental science and technology. Irradiation with femtosecond laser pulses induces lattice disorder (1), which could be (i) a purely electronic nonthermal order-disorder process by injecting carriers into bulk semiconductor crystals, such as Si (2, 3) and GaAs (4, 5) within 1 ps; (ii) a purely thermal-disorder thermal process that depends on electron/phonon, phonon/phonon, and phonon/lattice interaction time in metal films, especially in noble metals, such as Au (6, 7); (iii) a combination of thermal-and nonthermal-disorder with the ratio depending on the laser fluence (8). When the laser fluence is sufficient, the phase transition from solid to liquid will occur and the melting process is considered to involve both thermal and nonthermal processes, whereas in gold (9-11) and aluminum films, which is the dominant process is still under debate (12-14). The laser heating and melting of the gold film studies presented in this paper were induced by low excitation levels, therefore the electronic effects on the stability of the lattice are not expected to occur (15), which is in contrast to the high excitation levels used for electronic hardening in gold (10).In this paper, we describe experiments that utilize subpicosecond time-resolved X-ray diffraction (XRD) to measure directly the transient changes in the crystal structure induced by 400-nm, 100-fs laser pulses impinging upon the surface of 35-, 70-, and 150-nm-thick Au(111) single-crystal films. The experiments presented here may be assigned to a weak excitation region, at fluence lower than 15 mJ∕cm 2 , where the blast wave and coherent phonons were observed, and a strong excitation region, at fluencies of 37 mJ∕cm 2 and above, where melting occurs. In addition, a pressure blast wave, formed immediately upon excitation, induced lattice contraction (16) that propagated through the bulk of the crystal at sonic velocity, generating stress and the wellknown sonic wave. We will show that, i...

Structural dynamics of laser-irradiated gold nanofilms

“…5,7 In particular, the femtosecond structural evolution of photoexcited gold and other metals has been studied recently both experimentally and theoretically. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] Different regimes of excitation have been identified, depending on the energy deposited by the laser pulse in a gold nanofilm: a low-fluence regime in which sample does not melt, where coherent acoustic phonon generation is observed; 13 a medium-fluence regime in which the sample undergoes melting, where a competition between homogeneous and heterogeneous melting is identified; 9 a high-fluence regime in which electronic effects are expected to affect the melting process. 10,11,22 The initial stages of the dynamics are extremely difficult to unravel and recent results still stir a lot of controversy 10,11,14,15 even for relatively simple gold nanofilms.…”

Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

“…The small size of the laser-modified zone and the ultrafast rates of processes induced by short pulse laser irradiation present a challenge for experimental probing [15][16][17][18] and, at the same time, make atomistic modeling a suitable tool for revealing the mechanisms and pathways of laser-induced structural and phase transformations. The atomistic molecular dynamics (MD) simulation technique has indeed been instrumental in providing valuable information on the microscopic mechanisms of laser melting [19][20][21][22], photomechanical spallation and ablation [19,[23][24][25][26][27][28][29].…”

Ultrafast time resolved x-ray diffraction, extended x-ray absorption fine structure and x-ray absorption near edge structure

“…The lattice near the surface is first compressed by the blast force wave, then expansion is launched owing to the larger amplitude acoustic wave. 24 The periods of the coherent lattice vibration for 150 nm Au(111) are estimated to be 98 ps, which is slightly shorter than the period measured experimentally, owing to the damping of the coherent acoustic oscillations.…”

“…In contrast, the generation of the sound wave is slower, requires 10-20 ps to develop, and therefore, it is separated enough from the blast wave to be detected at even longer delay times. The blast wave is considered to be a compressive wave that acts on the crystal before the acoustic wave, 24 then lattice expansion is observed due to the launch of the larger amplitude acoustic wave. The influence of the blast wave is relatively small compared to the acoustic wave, consequently it may be masked and appear as the onset of the expansion.…”

“…In addition, several electron and x-ray ultrafast time resolved experiments have reported on the transient changes in the lattice structure of single crystals after excitation with an fs optical pulse. 3,[19][20][21][22][23][24][25][26][27][28][29][30][31] In this paper, we present the experimental data for three different processes that illustrate how these methods make it possible to determine, in real time, transient structures and very short lifetimes phenomena that in conjunction with density functional theory (DFT) calculation determine the operating mechanism of many basic processes and reactions. The experiments described were performed by using a tabletop laser system that generated femtosecond optical pump pulses and subpicosecond hard 6-22 keV x-ray probe pulses.…”