Dynamics of femtosecond laser-induced melting of silver

Wai-Lun Chan, Robert S. Averback, David G. Cahill, and Alexei Lagoutchev

Phys. Rev. B 78, 214107 – Published 17 December 2008

Abstract

We use optical third-harmonic generation to measure the melting dynamics of silver following femtosecond laser excitation. The dynamics reveals an unusual two-step process that is associated with the extreme electronic temperatures and very short time and length scales. In the first, which lasts a few picoseconds, the electron and phonon systems begin to equilibrate, and a thin surface layer undergoes melting. Heat conduction during this period is strongly suppressed by electron scattering from $d$ -band excitations. In the second stage, the surface region remains above the melting temperature for a surprisingly long time, 20–30 ps, with the melt front propagating into the bulk at a velocity of $\approx 350 m s^{- 1}$ . In this stage, the electron and phonon systems again fall out of equilibrium and conduction of heat away from the surface region is now limited by the weak electron-phonon $(e − p)$ coupling. From our model calculation, we propose that the melt depths in noble metals irradiated by femtosecond lasers are limited to thicknesses on the order of two to three times of the optical-absorption depth of the light.

Received 4 August 2008
Corrected 19 December 2008

DOI:https://doi.org/10.1103/PhysRevB.78.214107

Corrections

19 December 2008

Erratum

Publisher's Note: Dynamics of femtosecond laser-induced melting of silver [Phys. Rev. B 78, 214107 (2008)]

Wai-Lun Chan, Robert S. Averback, David G. Cahill, and Alexei Lagoutchev

Phys. Rev. B 78, 219901 (2008)

Authors & Affiliations

Wai-Lun Chan^1,*, Robert S. Averback¹, David G. Cahill¹, and Alexei Lagoutchev²

¹Department of Materials Science and Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801, USA
²School of Chemical Science, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801, USA

^*wlchan@uiuc.edu

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Vol. 78, Iss. 21 — 1 December 2008

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Images

Figure 1
Normalized TH generation as a function of delay time for a 200 nm Ag film. The axis on the right shows the corresponding melt depth. The melting can be divided into two stages: (I) initial melting due to transfer of energy from electron to phonon and (II) propagation of the melting front into the superheat solid. The dotted line represents a melting speed of $350 m s^{- 1}$ . All the experimental data are taken by a single shot. Each data point represents the average of at least three independent measurements. The error bar represents the standard deviation of these measurements.Reuse & Permissions
Figure 2
(a) The decrease in TH signal as a function of the depth of the Fe layer for two probe fluences. The slope in the figure represents the extinction depth for the TH generation. The inset is a schematic of the samples used in the calibration experiment. (b) The extinction depth for TH generation as a function of probe fluence.Reuse & Permissions
Figure 3
(Color online) TH generation as a function of pump fluence for different film thicknesses. The delay time is fixed at 25 ps.Reuse & Permissions
Figure 4
(Color online) (a) Calculated phonon temperature from the TTM at different delay times (only the top 100 nm is shown). The film thickness is 200 nm and the absorbed fluence is $0.025 J {cm}^{- 2}$ . Additional $e − e$ scattering by $d$ -band holes, modeled by Eq. (3), is included in the TTM. (b) An effective interface impedance ( $G = 8 GW m^{- 2} K^{- 1}$ , see the text) is added to the calculation in (a) in order to account for the nonlocal effect of the $d$ -band holes.Reuse & Permissions
Figure 5
(Color online) Melt depth as a function of film thickness at delay time equals to 25 ps. The experimental points (black solid circles) are determined from Fig. 3, at $fluence = 0.45 J {cm}^{- 2}$ (the vertical dashed line in Fig. 3). For the TTM calculation, the absorbed fluence is fixed at $0.025 J {cm}^{- 2}$ . The modeling parameters used are green line with squares—no enhancement in $e − e$ scattering rate from $d$ -band holes [ $C = 0$ in Eq. (3)]; red line with crosses—enhancement in $e − e$ scattering modeled by Eq. (3), with $C = 150$ ; orange line with open circles— $C = 1500$ ; and blue lines with triangles— $C = 150$ , with the addition of an effective interface impedance $G$ (see the text). Only $G = 5$ and $8 GW m^{- 2} K^{- 1}$ are shown.Reuse & Permissions
Figure 6
Constant temperature contours at 1234 and 1620 K calculated by TTM. The parameters used are same as in Fig. 4b. In stage I melting, the solid melt homogenously along the $T = 1620 K$ contour. In stage II, the melt front propagates within these two contours. The dashed line represents a melting speed of $350 m s^{- 1}$ .Reuse & Permissions

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