Thin Solid Films 529 (2013) 209–216

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Thin Solid Films

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Ultrafast investigation of electron dynamics in the gold-coated two-layer metal films

Anmin Chen a, Laizhi Sui a, Ying Shi a, Yuanfei Jiang a, Dapeng Yang b,⁎, Hang Liu a,Mingxing Jin a,⁎, Dajun Ding a

a Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, People's Republic of Chinab Key Laboratory of Geo-exploration Instrumentation Ministry of Education, College of Instrumentation & Electrical Engineering, Jilin University, Changchun 130012, People's Republic of China

⁎ Corresponding authors.E-mail addresses: ydp@jlu.edu.cn (D. Yang), mxjin@jl

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.06.027

a b s t r a c t

a r t i c l e i n f o

Available online 12 June 2012

Keywords:Electron dynamicsTransient reflectivityTwo-layer filmTwo-temperature modelDamage threshold

The gold-coated metal thin films are widely used in modern engineering applications. In this paper, the ultra-fast electron dynamics of gold-coated two-layer thin films has been investigated by ultrafast time-resolvedpump–probe experiment. The dependence of the surface electron temperature on the film structure was con-sidered based on the two-temperature model at the different two-layer film structure. The effect of laserfluence (3, 6 and 17 mJ/cm2), and two-layer film thickness (the thickness of 50 nm and 100 nm gold layer)is considered. The theoretical predictions are compared with experimental data, which agree well withboth thermal model and transient reflectivity.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

With the development of ultrashort laser based on chirped-pulseamplification [1], it is possible to carry out powerful femtosecondlaser systems. The femtosecond pulsed lasers are widely used in a va-riety of fields including material processing [2], pulsed laser deposi-tion [3], molecular spectroscopy [4,5], ionization and dissociation ofpolyatomic molecules [6,7], and so on. For the sake of increasing theoutput power of such femtosecond laser systems, an important limitingfactor in the high power operation of lasers is the damage threshold ofthe optical components of the laser system. Hence, comparative damagethreshold measurements on laser optical components are essential forthe evaluation of different materials as well as different depositiontechniques in respect to their applicability in high-power femtosecondlaser systems. Due to the high reflectivity of gold surface in the infraredbeyond 0.7 μm (an averaged reflectance is above 98%), gold coating op-tical components (mirrors and gratings, etc.) are widely used in femto-second pulsed laser systems (for example, Ti:Sapphire laser system)and infrared optical systems (for example, Terahertz system [8]). The in-teraction of femtosecond laser and gold film has been a challenging re-search topic. During pulsed laser irradiation of gold film, the electron–electron interaction time is very short, on the order of femtoseconds,compared with electron–lattice interaction time, which is on the orderof picoseconds. It has been assumed that the incident photon energy of

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the laser beam is absorbed instantaneously by the free electrons of themetal and is confined close to the surface. Hence, a strongnonequilibriumis created between the electrons and lattices. The thermal energy pos-sessed by these “hot” electrons diffuses deeper into the film.

Optical pump–probe measurement using femtosecond laser hasbeen proven to be a very sensitive tool for the investigation of electrondynamics in metals. They have been used to study many phenomena offundamental and applied interests such as optical orientation of spin,ultrafast demagnetization, and ultrafast excitation of coherent lattices.Their potential for material characterization is illustrated by measure-ments of the electron–lattice coupling constant in metals [9], hot elec-tron linear and angular momentum relaxation times and nonlinearsusceptibility tensor components in metals, and the spin wave modespectrum of nanomagnets. For the interaction of the laser and metal,the investigation of ultrafast electron dynamics investigation has beenreported using the pump–probe measurement by many researchers[10–13]. In these electron dynamics studies, the single-layer metalshave been used. The electron dynamics of the multi-layer metal hasbeen investigated by Ibrahim et al. [14]. The Au/Cr two-layer film hadbeen experimentally studied by Qiu and Tien [15]. And the Au/glasstwo-layer film had been experimentally studied by Wang and Ma etal. [16]. However, the researches of the ultrafast electron dynamics arestill lacking for the multi-layer metal films.

In this paper, we report the experimental results of the transient re-flectivity of the gold-coated two-layer film using the femtosecondpump–probe technique for three different pump powers. Experimentalresults show that the reflectivity change increaseswith the power of thepump laser. Numerical solutions of the two-temperature model (TTM)are compared with experimental results. The distributions of electrontemperature and lattice temperature are considered. The results showthat the substrate layer chrome film can influence the variation ofgold film temperature.

210 A. Chen et al. / Thin Solid Films 529 (2013) 209–216

2. Mathematical model

The theoretical method to investigate the ultrashort laser–matterinteraction is the well-known two-temperature model [17]. Laserlight is absorbed in metals by the conduction-band electrons withina few femtoseconds. After the fast thermalization of the laser energyin the conduction band, electrons may quickly diffuse and therebytransport their energy deep into the internal target (within a fewfemtoseconds). At the same time, the electrons transfer their energyto the lattice. The TTM describes the evolution of the temperature in-crease due to the absorption of a laser pulse within the solid and isapplied to model physical phenomena like the energy transfer be-tween electrons and lattice occurring during the target–laser interac-tion [18]. The one-dimension two-temperature equation is givenbelow [19,20]:

Ce∂Te

∂t ¼ ∂∂x ke

∂Te

∂x

� �−G Te−Tlð Þ þ S ð1Þ

Cl∂Tl

∂t ¼ ∂∂x kl

∂Te

∂x

� �þ G Te−Tlð Þ ð2Þ

Where t is the time, x is the depth, Ce is the electron heat capacity,Cl is the lattice heat capacity, ke is the electron thermal conductivity,Te is the electron temperature, Tl is the lattice temperature, G is theelectron–lattice coupling factor [21], and S is the laser heat source.The heat source S can be modeled with a Gaussian temporal profile[22]:

S ¼ffiffiffiffiβπ

r1−Rð ÞαI

tpexp −xα−β

t−2tptp

!2" #ð3Þ

Where R=0.369 (the wavelength of the pump beam is 400 nm) isthe target reflection coefficient, tp is the full-width at the half maxi-mum (FWHM) with the linear polarization, α is the absorption coef-ficient and I is the incident energy, β=4 ln(2).

The reflectivity (R) of metal is mainly due to the Drude free electronmodel. The electrical permittivity ε (dielectric function) of metalsmodeled as a plasma, is expressed as [23]

ε ¼ ε1 þ iε2 ð4Þ

ε1 ¼ 1−ω2

pτ2

1þω2τ2ð5Þ

ε2 ¼ ω2pτ

2

ω 1þω2τ2� � ð6Þ

ω is laser frequency, ωp ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffinee

2= meε0ð Þq

is the plasma frequency, ne isthe density of the free electrons,me is the mass of electron and ε0 is theelectrical permittivity of free space. τ is the electron relaxation time. Ingeneral, for good conductors, the e–e collision rate may be determinedby υe–e=ATe

2 whereas the e–ph collision rate is independent of Te, butproportional to Tl, namely, υe–ph=BTl. Here A and B are constants, andboth contribute to the electron collision frequency υ. A relationship be-tween the electron relaxation time τ and the e–e and e−ph collisionrates for electron temperatures below the Fermi temperature is givenby

1τ¼ υ ¼ υe−e þ υe−ph ¼ AT2

e þ BTl: ð7Þ

We then work out n and κ, the real and imaginary parts of thecomplex refractive index. These are:

ε1 ¼ n2−κ2 ð8Þ

ε2 ¼ 2nκ ð9Þ

and

n ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiε1 þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiε21 þ ε22

q2

vuutð10Þ

κ ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi−ε1 þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiε21 þ ε22

q2

vuut: ð11Þ

The reflectivity depends on both n and κ and is given by

R ¼ n−1ð Þ2 þ κ2

nþ 1ð Þ2 þ κ2 : ð12Þ

The absorption coefficient is determined by κ, and is given by

α ¼ 2κωc

: ð13Þ

The electron heat capacity is proportional to the electron tempera-ture when the electron temperature is less than the Fermi temperatureas Ce=γTe [24] and γ=π2nekB/2TF. ne is the density of the free elec-trons, kB is the Boltzmann's constant and TF is Fermi temperature. Thelattice heat capacity is set as a constant because of its relatively smallvariation as the temperature changes. The electron heat conductivitycan be expressed as ke=ke0BTe/(ATe2+BTl) [25], where ke0, A and B arethe constants. Many of the ultrafast laser heating analyses have beencarried out with a constant electron–lattice coupling factor G. However,due to the significant changes in the electron and lattice temperaturescaused by high-power laser heating, G should be temperature depen-dent (G=G0(A(Te+Tl)/B+1), where G0 is the coupling factor at roomtemperature) [26]. The lattice thermal conductivity is therefore takenas 1% of the thermal conductivity of bulk metal since the mechanismof heat conduction in metal is mainly due to electrons [27]. As the tem-perature changes, the variety of the lattice heat conductivity is relativelysmall and it is assumed a constant.

Considering a one-dimensional two-layered thin film, Fig. 1 showsthe schematic view of the laser heating, which indicates a two-layermetal film with an interface at x= l. For the two-layer thin film, thetwo-temperature equation (Eqs. (1) and (2)) for studying thermal be-havior in the thin film can be expressed as

CeI ∂Te

I

∂t ¼ ∂∂x ke

I ∂TeI

∂x

!−G Te

I−TlI

� �þ SI ð14Þ

ClI ∂Tl

I

∂t ¼ ∂∂x kl

I ∂TlI

∂x

!þ G Te

I−TlI

� �ð15Þ

CeII ∂Te

II

∂t ¼ ∂∂x ke

II ∂TeII

∂x

!−G Te

II−TlII

� �ð16Þ

ClII ∂Tl

II

∂t ¼ ∂∂x kl

II ∂TlII

∂x

!þ G Te

II−TlII

� �: ð17Þ

To solve Eqs. (4)–(7), the following initial and boundary condi-tions must be used. Before irradiated by the laser pulse, the electronand lattice sub-systems are considered to be at the same initial tem-perature (T0=300 K)

TeI x;0ð Þ ¼ Tl

I x;0ð Þ ¼ T0 ð18Þ

Fig. 1. The schematic of a two-layer metal film. The thickness of Au is l. The thickness ofCr is 200 nm.

211A. Chen et al. / Thin Solid Films 529 (2013) 209–216

TeII x;0ð Þ ¼ Tl

II x;0ð Þ ¼ T0: ð19Þ

The energy of the convective and radiative losses from the frontand back surfaces of the two-layer film, in addition, is negligible duringthe femtosecond transient. The boundary conditions are formulated, asfollows

∂TeI

∂x jx¼0¼ ∂Te

II

∂x jx¼L¼ 0 ð20Þ

∂TlI

∂x jx¼0¼ ∂Tl

II

∂x jx¼L¼ 0: ð21Þ

At the interface of the film (x= l), the two-layer thin film is in per-fect thermal contact. Therefore we set the boundary conditions of theinterface, as follows

TeIj

x¼l¼ Te

IIjx¼l

ð22Þ

TlIj

x¼l¼ Tl

IIjx¼l

ð23Þ

keI ∂Te

I

∂x jx¼l¼ ke

II ∂TeII

∂x jx¼lð24Þ

klI ∂Tl

I

∂x jx¼l¼ kl

II ∂TlII

∂x jx¼l: ð25Þ

3. Experimental details

A schematic diagram of the standard pump–probe experimentalsetup is shown in Fig. 2. The laser systemwas a regenerative amplified

Fig. 2. The schematic drawing of the apparatus. Components include beam splitter (BS),

Ti:Sapphire laser (Spectra Physics, Tsunami oscillator and Spitfire am-plifier), which provides 90 fs pulses at 800 nm with 0.6 mJ per pulseand a repetition rate of 1 kHz. The laser beamwas then split to generatepump and probe parts. The pump beam (400 nm) was obtained bysecond harmonic generation in a 0.5 mm thick β-BaB2O4 (BBO) crys-tal. The intensity of the pump beam can be controlled by a λ/2 platein combination with a Glan prism. The residual 800 nm laser beamacts as a probe beam. In our experiment the intensity ratio of pumpto probe beam is about 80:1. A computer-controlled translationstage (Physik instrumente, M-505) was used to control the timedelay between the pump and the probe pulses with a resolution of1 μm. The delay time may be changed from −100 to 900 ps. By acombination of a Glan laser polarizer and a half-wave plate, the ener-gy of each pulse can be attenuated to the desired value. The pumpand probe pulses were directed by two concavemirrors (Focal lengthis 25 cm) and a beam splitter onto the sample with the focal diame-ter spots of 300 μmand 200 μm. The spots were carefully overlapped,while being viewed with a CCD camera. The metal target wasmounted on a computer-controlled X–Y–Z stage, which can guaran-tee the sample location. All experiments are performed in air at at-mospheric pressure. The angles of incidence of the pump and probebeams at the sample were very small. The pump and probe beamswere reflected from the sample and filtered by a filter so that the800 nm probe beam could be detected by a photodiode. Differentialdetection (ThorLabs PDB150A-AC) was used so as to cancel out thefluctuations of the laser output, leaving only the signal caused bythe chopped pump beam. Thus, the lock-in amplifier (StanfordSR830) detects modulation in the received probe intensity that wascaused only by the effect of the pump on the sample. The samplesused in the experiments were double-layer gold and chrome thinfilms deposited on K9 glass. The two-layermetallic filmswere preparedusing a physical vapor deposition method. The sample structure andgeneral optical layout were shown in Fig. 1. The thickness of chromefilm was 200 nm. The thickness of gold layer was 50 nm, and 100 nm,respectively. We substituted a BBO crystal (0.2 mm) for the sample togenerate a sum-frequency pulse (267 nm) of pump and probe pulses.Through detecting the correlation signal, the time zero-point and thetime resolution of our system could be evaluated.

4. Results

In this experiment, the time dependence of transient relativereflectivities for 400 nm pump light and 800 nm probe light are dis-played in Figs. 3 and 4 for both gold-coated two-layer films. Thethree different pump laser powers are 2 mW, 4 mW, and 12 mW, re-spectively. Fig. 3 (a and b) shows the normalized ΔR/R for the same

concave mirror (CM), filter (F), Glan laser polarizer (G) and half-wave plate (HWP).

Fig. 3. The normalized transient reflectivity data for both films recorded with three dif-ferent laser powers: (a) the film is Au/Cr of 50 nm and 200 nm and (b) the film is Au/Crof 100 nm and 200 nm.

Fig. 4. The normalized transient reflectivity data for three laser powers recorded withboth different two-layer films. The pump laser powers: (a) 2 mW, (b) 4 mW, and(c) 12 mW.

212 A. Chen et al. / Thin Solid Films 529 (2013) 209–216

gold film thickness. The normalized curves are calculated by dividingthe minimum value of three curves. As shown in these figures, the re-flectivity change process consists of three different time stages. First-ly, the transient relative reflectivity decreases rapidly for the delaytime up to a few hundred femtoseconds and reaches the minimumpoint. Next, the increasing reflectivity occurs for the delay time of2 ps. Finally, the reflectivity after delay time of 2 ps nears a constant.Fig. 4 (a–c) shows the normalized ΔR/R for the 50 nm and 100 nmAu/Cr two-layer films at the same laser power. These data are normal-ized at the peak reflectance to observe the difference between the elec-tron cooling profiles at three different laser fluences. For the differentthickness of gold layer, the surface electron temperatures rose rapidlywith the maximum temperatures. Next, surface electron temperaturesdecreasedwith time. In the range of 0–2 ps, the variations of the surfaceelectron temperature were almost same for the different thickness ofgold layer [28]. The variation of the surface electron temperature is pro-portional to the variation of the reflectivity. It is the shape of the coolingprofile after the maximum electron temperature that is related to therate of electron–lattice equilibration; therefore, the normalization ofthe data is for clear comparison between the two data sets. Themajorityof the electron energy loss to the lattices occurswithin 1–2 ps after laserheating. Differences in the data are seenwithin 2 ps after themaximumreflectance, indicating a difference in the cooling rate of the electronsystem during electron–lattice equilibration.

Two major processes of electron–electron collision and scatteringprocess and electron–lattice coupling process occur in the interactionbetween the femtosecond laser and the thin film. The nonequilibriumtemperature induced in the experiments can be predicted with thetwo temperature model. The mathematical model that is calculatedby the finite difference method, given by Eqs. (1) and (2), describes

Table 1Thermal and optical physical parameters for gold and chrome.

Au Cr

Electron–lattice coupling coefficient G0

[1017 J m−3 s−1 K−1]0.22 4.2

Electron heat capacity coefficient γ[J m−3 K−2] 68 194Electron thermal conductivity coefficient ke0[J m−1 s−1 K−1] 315 95Lattice heat capacity Cl[106 J m−3 K−1] 2.5 3.3Penetration depth α[10−9 m] 17.7A [107 s−1 K−2] 1.18 7.9B[1011 s−1 K−1] 1.25 13.4Melting temperature Tm[103 K] 1.337 2.180

213A. Chen et al. / Thin Solid Films 529 (2013) 209–216

the rate of energy exchange between the electrons and lattices in ametallic film. The temporal distributions of the electron are presentedin Fig. 5 for both thin films. The laser fluences are 3 mJ/cm2, 6 mJ/cm2,and 17 mJ/cm2, respectively. Table 1 [29–33] lists the values of ther-mal physical parameters of the two noble metals used in these calcu-lations. One can see from Fig. 5 that the surface electron temperaturesrose rapidly with maximum temperatures in the range of a few hun-dred femtoseconds. Consequently, surface electron temperature de-creases with time due to the heat diffusion effect in the electrongas, at a short time delay (about 2 ps). The distributions of the surfaceelectron temperature for three different laser fluences are noticeablydifferent. The decay rate of electron temperature of the low laserfluence is less than that of the high laser fluence, and the thermalequilibrium time is extended.

5. Discussion

Generally, the lattice cannot be affected by the absorption of apump light, but a nonequilibrium process of hot electron and coldlattice will be created. The scattering among the hot electrons aswell as from the lattices, defects, etc., results in the creation of the

Fig. 5. The variation of surface electron temperature with the delay time for the differ-ent laser fluences by TTM predictions. The thickness of Au/Cr films: (a) 50 nm and200 nm; (b) 100 nm and 200 nm.

equilibrium electron distribution described by the Fermi–Dirac func-tion [34,35]

f FD E; Teð Þ ¼ 1

exp E−EF Teð ÞkBTe

� �þ 1

ð26Þ

where, E is the electron energy, EF is the Fermi energy, and kB is theBoltzmann constant. At moderate temperatures, the Fermi energyvaries with temperature as

EF Teð Þ ¼ EF0 1− π2

12kBTe

EF0

� �2" #

: ð27Þ

Where, EF0 is the Fermi energy at absolute zero temperature. Thetransient reflectivities are not affected by holes in the d-band whichare initially created by optical excitation since the lifetime of theseholes is vanishingly small [36].

When the electrons have been heated, the electron temperaturecan still be higher than that of the lattice. The main mechanism of theTTM is that the electron and lattice are well characterized by their re-spective temperatures which equalize with rate proportional to theelectron–lattice coupling constant. The process is a very complex [37],since the specific heat capacity of electron is low. Irradiated by a laserbeam, the electron temperature increases rapidly in an extremelyshort time, producing a great temperature difference between electronand lattice. The nonequilibrium energy transport, which is due to theelectron–lattice coupling mechanism [38], will take place. The excessenergy of the thermalized electron subsystem is equal to the productof the excess electron temperature ΔTe and the electron heat capacityCe, which is in turn proportional to the electron temperature Te=Te+ΔTe. In the transient thermoreflectance experiments, it is the change inreflectivity ΔR resulting from a change in temperature in the samplethat is measured. The change in reflectance of a metal can be related tothe change in temperature through the change in the complex dielectricfunction Δε=Δε1+ iΔε2, here Δε1 and Δε2 are the real and imaginarypart changes of the complex dielectric function. When the changes ofΔTe and ΔTl are small, Δεcan be expressed by ΔTe and ΔTl

Δε ¼ Δε1 þ iΔε2 ¼ ∂ε1∂Te

ΔTe þ∂ε1∂Tl

ΔTl þ i∂ε2∂Te

ΔTe þ∂ε2∂Tl

ΔT� �

: ð28Þ

The changed reflectivity can be expressed by the dielectric func-tions

ΔRR

¼ 1R

∂R∂ε1

Δε1 þ∂R∂ε2

Δε2

: ð29Þ

Combining Eqs. (28) and (29), the change of the reflectivity can berewritten as [39,40]

ΔRR

¼ aΔTe þ bΔTl ð30Þ

214 A. Chen et al. / Thin Solid Films 529 (2013) 209–216

where a∝∂R/∂Te and b∝∂R/∂Tl. The electron and lattice tempera-tures directly relate to the change of the reflectivity. Compared withthe change of the electron temperature, the change of the lattice tem-perature is very small and can be ignored. Eq. (30) can be simplifiedas

ΔRR

¼ aΔTe: ð31Þ

The original measured signals and theoretical results are shown inFigs. 3 and 4, and Fig. 5, respectively. As mentioned above, ΔTe/Te isproportional to ΔR/R. As a result, the transient reflectivity signalscan be represented by the variation of the electron temperaturewith time. The electron temperature starts to rise when the pumppulse irradiates the surface of the target, and then the electron tem-perature decreases because of the non-equilibrium heat transportfrom the hot electrons to the cold lattices. In order to analyze thedata, the electron temperature is normalized. In Fig. 6, we comparethe calculations with experimental data. Agreement with experimentis rather satisfactory. However, it should be noted that the measureddata are in fact a function not only of film thickness [41,42] (asexpected from Fig. 4) but also of film morphology [43,44]. The mor-phology dependence is a subject of continuing study. In Fig. 6 (a),there is some disagreement between calculated and experimentaldata. This is due to electron–lattice coupling factor that governs the

Fig. 6. The comparison of theoretical and experimental results: (a) fixed film thicknesswith the different laser energy and (b) fixed laser energywith the different film thickness.

rate of energy transfer to the lattice from the hot electrons. In here,we use theoretical predictions that differ with the actual value.

The two-layer film structure can change the damage threshold ofthe gold surface [45]. As shown in Fig. 4, for the fixed laser power,the ΔR/R of the thinner gold layer is less than that of the thickergold layer at the thermal equilibrium. In contrast with the experi-mental results of Hohlfeld et al. [46], it is just contrary. In theHohlfeld's experiment, the gold films were directly deposited on op-tical fused silica plates. However, we introduced the chrome film asthe substrate layer in this experiment. The substrate layer will act asa heat sink absorbing the thermal energy transmitted through the in-terface and then coupling that energy to the lattice away from theheat affected area [47]. According to the previous theory, ΔTe/Te isproportional to ΔR/R, Fig. 4 (a–c) illustrates that the surface of thethicker gold layer will obtain higher temperature compared to thatof the thinner gold layer. Subsequently, we calculate the variation ofthe lattice temperature with the depth of the two-layer film.

Fig. 7 shows the distribution of the lattice temperature for boththin films at three different fluences. We notice that the lattice tem-perature distribution has big ups and downs in the interface regionof the gold-layer and chrome-layer. This is due to the fact that theelectron–lattice coupling factor is considerably higher or lower forthe substrate layer film than that for the top layer. This results inthe redistribution of the deposited laser energy from the gold film

Fig. 7. The distribution of the lattice temperature with the different laser fluences forboth two-layer films. The thickness of Au/Cr films: (a) 50 nm and 200 nm; (b) 100 nmand 200 nm.

215A. Chen et al. / Thin Solid Films 529 (2013) 209–216

layer to the substrate layer, where the energy of the excited electronscouples more effectively with the lattice vibrations, leading to thepreferential or disadvantageous lattice heating in the substratelayer. Fig. 8 clearly shows the distribution of the lattice temperature

Fig. 8. The comparison of the lattice temperature distribution for the single-layer Aufilms and the two-layer Au/Cr films at the different laser fluences. The fluence:(a) 3 mJ/cm2, (b) 6 mJ/cm2, and (c) 17 mJ/cm2.

for the different thick gold layer. At the surface, the lattice tempera-ture of 50 nm gold layer is lower than that of 100 nm for the two-layer films, the lattice of single-layer films are higher than that ofthe two-layer films. The results provide way for the improvement ofgold surface damage threshold.

6. Conclusion

In summary, the transient reflectivity of the gold-coated two-layermetal film is investigated by femtosecond time-resolved pump–probetechnique. The two-layer structure is the 50 nmand 100 nmgold layerspadding on the 200 nm chromes. Experiments are performed for threedifferent pump powers. Experimental results show that the reflectivitychange increases with the power of the pump laser. The experimentalresults are analyzed within the framework of the two-temperaturemodel, which describes the energy relaxation in ultrafast heating. Acomparison between the experimental results for both two-layer filmsrevealed the difference of the thermoreflectivity signal at the thermalequilibrium. By the theoretical analysis, the introduced substrate layerwill signify a reduction in the surface lattice temperature. Taking advan-tage of the two-layer structure, it is believed that the damage thresholdof the gold film can be improved.

Acknowledgment

This project is supported by the Chinese National Fusion Projectfor ITER (Grant no. 2010GB104003) and the National Natural ScienceFoundation of China (Grant nos. 10974069, 11034003).

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