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Research Papers: Micro/Nanoscale Heat Transfer

Ultrafast Spectroscopy of Electron-Phonon Coupling in Gold

[+] Author and Article Information
Liang Guo

School of Mechanical Engineering and Birck
Nanotechnology Center,
Purdue University,
West Lafayette, IN 47907

Xianfan Xu

School of Mechanical Engineering and Birck
Nanotechnology Center,
Purdue University,
West Lafayette, IN 47907
e-mail: xxu@purdue.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received June 26, 2014; final manuscript received September 4, 2014; published online September 30, 2014. Assoc. Editor: Robert D. Tzou.

J. Heat Transfer 136(12), 122401 (Sep 30, 2014) (6 pages) Paper No: HT-14-1429; doi: 10.1115/1.4028543 History: Received June 26, 2014; Revised September 04, 2014

Transient reflectance of gold was measured using ultrafast spectroscopy by varying the wavelength of the probe laser beam in the visible range. Based on the band structure of gold, the influence of the probe beam wavelength on the signal trend is analyzed in terms of sensitivity, effect of nonthermalized electrons, and relaxation rate. It is found that probing around 490 nm renders the best sensitivity and a simple linear relation between the transient reflectance and the electron temperature. The two-temperature model (TTM) is applied to calculate the electron-phonon coupling factor by fitting the transient reflectance signal. This work clarifies the ultrafast energy transfer dynamics in gold and the importance of using proper probe laser wavelength for modeling the transient heat transfer process in metal.

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Figures

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Fig. 1

Qualitative illustration of the DOS of gold: the light and the dark blue regions indicate the s/p and the d bands, respectively, which overlap partially in energy; the light and the dark arrows indicate transition from the d band to states below and above EF

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Fig. 2

(a) Transient reflectance signals of gold by 800 nm pump and varying probe wavelength. (b) Dependence of the amplitude of the transient reflectance signal on the probe photon energy (the horizontal dashed line marks ΔR/R = 0 and the vertical dashed line marks the energy for ITT).

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Fig. 3

(a) Fermi–Dirac distribution function at varying temperature and (b) difference in Fermi–Dirac distribution function between Te = EF/50kB and Te = EF/20kB

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Fig. 4

Dependence of amplitude of the transient reflectance signal on ΔTe probed by 490 nm with linear fitting indicated by the black line

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Fig. 5

Comparison between the change of the electron distribution due to the nonthermalized electrons based on Eq. (1) and due to the thermalized electrons (the vertical dashed line indicates the states probed by 800 nm)

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Fig. 6

Transient reflectance signals of gold films of varying thickness excited by 1300 nm and probed by 800 nm

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Fig. 7

Dependence of the electron relaxation time on the probe wavelength: (a) probe photon energy below ITT; (b) probe photon energy above ITT (470 nm and 460 nm are generated from third harmonic of the OPA output so the noise level is larger due to more nonlinear processes involved)

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Fig. 8

Comparison between the transient reflectance signals of bulk gold with the ΔTe calculated by the TTM for pump fluence (a) 1.76 mJ/cm2; (b) 3.52 mJ/cm2; (c) 5.28 mJ/cm2; and (d) 7.04 mJ/cm2

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