OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 8 — Apr. 9, 2012
  • pp: 8629–8640
« Show journal navigation

Linear and nonlinear optical properties of Ag/Au bilayer thin films

James Hsu, Canek Fuentes-Hernandez, Alfred R. Ernst, Joel M. Hales, Joseph W. Perry, and Bernard Kippelen  »View Author Affiliations


Optics Express, Vol. 20, Issue 8, pp. 8629-8640 (2012)
http://dx.doi.org/10.1364/OE.20.008629


View Full Text Article

Acrobat PDF (1164 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

The linear and nonlinear optical properties of Ag/Au bilayer metallic thin films with a total thickness of around 20 nm and with different Ag/Au mass-thickness ratios were studied. This study shows that the spectral dispersion of the effective refractive index of bilayer films can be tuned by controlling the mass-thickness ratio between Au and Ag. Improvement of the figure-of-merit for potential plasmonic applications and linear optical filters in the visible spectral range are reported and discussed. The nonlinear optical properties of bilayer metal films studied using femtosecond white-light continuum pump-probe experiments are also shown to be tunable with this ratio. The nonlinear change of optical path length is extracted from the pump-probe data and agrees with simulated values derived from a combination of the two-temperature model, describing the ultrafast electron heating dynamics, and a physical model that describes the dielectric permittivity of Au as a function of electron and lattice temperature.

© 2012 OSA

1. Introduction

Noble metals, such as gold and silver, have a wide range of applications in photonics and electro-optics. Within the visible and infrared spectral regions, gold and silver can have a negative permittivity that allows for the excitation of surface plasmon polaritons (SPPs), which have been actively exploited in recent years for sub-wavelength photonic circuits [1

1. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]

]. This negative permittivity also enables artificial metamaterial structures which allow remarkable control over the dispersion and sign of the refractive index of a material and consequently over the flow of electromagnetic energy throughout its structure [2

2. H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices,” Nature 444(7119), 597–600 (2006). [CrossRef] [PubMed]

]. In addition, highly transparent metallic structures [3

3. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “Nonlinear optical properties of induced transmission filters,” Opt. Express 18(18), 19101–19113 (2010). [CrossRef] [PubMed]

] have been reported and are attractive as optical filters with high out-of-band rejection [4

4. M. J. Bloemer and M. Scalora, “Transmissive properties of Ag/MgF2 photonic band gaps,” Appl. Phys. Lett. 72(14), 1676–1678 (1998). [CrossRef]

] and thin film metal electrodes in organic light-emitting diodes [5

5. S. Y. Ryu, C. H. Lee, I. S. Oh, S. Y. Song, K. H. Hwang, H. S. Hwang, M. H. Han, B. H. Hwang, H. K. Baik, Y. S. Kim, and J. Y. Lee, “Efficient inverted top-emitting organic light emitting diodes with transparent and surface-modified multilayer anodes,” Electrochem. Solid-State Lett. 13(5), J43–J46 (2010). [CrossRef]

]. Noble metals have also attracted great attention because their extremely large and ultrafast non-linear optical (NLO) response [6

6. N. Rotenberg, A. D. Bristow, M. Pfeiffer, M. Betz, and H. M. van Driel, “Nonlinear absorption in Au films: Role of thermal effects,” Phys. Rev. B 75(15), 155426 (2007). [CrossRef]

,7

7. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys. 107(12), 123114 (2010). [CrossRef]

] can be exploited to achieve all-optical control of metallic nanostructures [8

8. M. A. Swillam, N. Rotenberg, and H. M. van Driel, “All-optical ultrafast control of beaming through a single sub-wavelength aperture in a metal film,” Opt. Express 19(8), 7856–7864 (2011). [CrossRef] [PubMed]

11

11. J. Y. Zhang, L. Wang, S. Krishna, M. Sheik-Bahae, and S. R. J. Brueck, “Saturation of the second harmonic generation from GaAs-filled metallic hole arrays by nonlinear absorption,” Phys. Rev. B 83(16), 165438 (2011). [CrossRef]

]. This NLO response is described as a χ(1) process caused by electron and lattice heating [6

6. N. Rotenberg, A. D. Bristow, M. Pfeiffer, M. Betz, and H. M. van Driel, “Nonlinear absorption in Au films: Role of thermal effects,” Phys. Rev. B 75(15), 155426 (2007). [CrossRef]

,7

7. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys. 107(12), 123114 (2010). [CrossRef]

,12

12. S. I. Anisimov, B. L. Kapeliovich, and T. L Perel'Man, “Electron emission from metal surfaces exposed to ultrashort laser pulses,” Sov. Phys. JETP 39, 375–377 (1974).

].

The linear and nonlinear optical responses of a noble metal and their potential applications are therefore determined by the inherent electronic response which gives rise to their dielectric permittivity. The electronic response of a noble metal in the visible spectral region can be divided into two separate mechanisms: interband and intraband electronic transitions [13

13. C. Voisin, N. Del Fatti, D. Christofilos, and F. Vallee, “Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles,” J. Phys. Chem. B 105(12), 2264–2280 (2001). [CrossRef]

,14

14. P. E. Hopkins, “Influence of Inter- and Intraband Transitions to Electron Temperature Decay in Noble Metals After Short-Pulsed Laser Heating,” J. Heat Transfer 132(12), 122402 (2010). [CrossRef]

]. Electronic interband transitions in the visible or ultraviolet (UV) spectral region arise from bound electrons excited from fully occupied electronic states within the d-band, below the Fermi energy level, to the half filled s-p electronic bands in the conduction band. In this spectral region, metals are opaque because optical fields are strongly absorbed. At lower energies, electronic intraband transitions occur from free electrons stimulated within the conduction band. In this spectral region, metals are opaque mainly because the optical fields are reflected off its surface, rather than absorbed in the bulk.

When a metal is excited with an ultrafast optical pulse, the energy absorbed by the electron gas raises its temperature and smears the electronic distribution around the Fermi energy (Fermi-smearing), causing a very strong change of the dielectric permittivity of the metal around the onset of interband region [13

13. C. Voisin, N. Del Fatti, D. Christofilos, and F. Vallee, “Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles,” J. Phys. Chem. B 105(12), 2264–2280 (2001). [CrossRef]

]. In the intraband region, an increased electron temperature increases electron scattering processes which change the dielectric permittivity, albeit these changes are of smaller magnitude than the ones produced in the onset of the interband region [7

7. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys. 107(12), 123114 (2010). [CrossRef]

]. Therefore, gold and silver have very different linear and nonlinear optical properties not only because of inherent differences in their electronic configurations, but also because these differences cause the onset of interband transitions to lie in very different spectral regions (in the visible for gold and in the UV for silver).

Although silver and gold are commonly used as single component materials, configurations of Ag/Au bilayer thin films have also been used to solve multiple engineering problems. For instance, high chemical stability and high sensor sensitivity have been achieved by using Ag/Au bilayer films in different surface plasmon resonance (SPR) sensor designs [15

15. B. H. Ong, X. C. Yuan, and S. C. Tjin, “Bimetallic silver-gold film waveguide surface plasmon resonance sensor,” Fiber Integr. Opt. 26(4), 229–240 (2007). [CrossRef]

,16

16. H. Tóháti, A. Sipos, G. Szekeres, A. Mathesz, A. Szalai, P. Jojart, J. Budai, C. Vass, A. Kohazi-Kis, M. Csete, and Z. Bor, “Surface plasmon scattering on polymer-bimetal layer covered fused silica gratings generated by laser induced backside wet etching,” Appl. Surf. Sci. 255(10), 5130–5137 (2009). [CrossRef]

]. Al/Ag bilayer films have also been used as transparent electrodes in top-emitting organic light-emitting diodes [17

17. M. Thomschke, S. Hofmann, S. Olthof, M. Anderson, H. Kleemann, M. Schober, B. Lussem, and K. Leo, “Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing,” Appl. Phys. Lett. 98(8), 083304 (2011). [CrossRef]

]. Additionally, the interactions between multi-layer metal films and femtosecond laser pulses have been studied in terms of increasing the damage threshold of laser mirrors [18

18. T. Q. Qiu, T. Juhasz, C. Suarez, W. E. Bron, and C. L. Tien, “Femtosecond laser heating of multi-layer metals—II. Experiments,” Int. J. Heat Mass Transfer 37(17), 2799–2808 (1994). [CrossRef]

,19

19. T. Q. Qiu and C. L. Tien, “Femtosecond laser heating of multi-layer metals—I. Analysis,” Int. J. Heat Mass Transfer 37(17), 2789–2797 (1994). [CrossRef]

]. However, little attention has been directed at understanding the linear and nonlinear optical properties of Ag/Au bilayer thin films by controlling the thickness ratio between the two metals.

In this paper, the linear and nonlinear optical properties of Ag/Au bilayer metallic thin films with a total thickness of around 20 nm and with different Ag/Au mass-thickness ratios were studied. This study shows that the spectral dispersion of the effective refractive index of bilayer films preserves the general dispersion features found in their component materials, namely distinct interband-like and intraband-like transition regions. In addition, it is also shown that the effective refractive index can be tuned spectrally by controlling the mass-thickness ratio between gold and silver. As a consequence, it is shown that the magnitude and spectral dispersion for the NLO response of the bilayer films can also be tuned. These changes are modeled using the two-temperature model and a physical model that describes the dielectric permittivity in terms of their interband and intraband transition terms. Potential linear and nonlinear optical applications of these metallic bilayers are also discussed.

2. Experimental method

2.1 Fabrication and characterization

All thin films were deposited on Ted Pella Micro Cover Glass substrates with a Kurt. J Lesker Axxis electron beam deposition system. Substrates were cleaned ultrasonically in deionized water, acetone and isopropanol for 15 min each. The films were deposited under vacuum at a pressure of 8.5 × 10−7 Torr (1.1 × 10−4 Pa) with a rotating sample holder that actively cools the substrates to room temperature. The Ag and Au were deposited at a rate of 0.1 nm/s and SiO2 was deposited at a rate of 0.5 nm/s, monitored by crystal sensors.

Ag/Au bilayer thin films were fabricated by deposition of the Ag layer onto a glass substrate followed by the deposition of the Au layer. Such bilayer films were sandwiched by SiO2 thin films as protection layers. Bilayers with three thickness ratios of Au and Ag with a total thickness of 20 nm were fabricated. Single layers of Au and Ag were fabricated as reference samples with the same total thickness. Single layer and bilayer metallic samples with the following geometry were deposited:
  • R1: Glass/Au (23 nm).
  • S1: Glass/SiO2(64 nm)/ M1/SiO2(98 nm).
  • S2: Glass/SiO2(64 nm)/ M2/SiO2(98 nm).
  • S3: Glass/SiO2(64 nm)/ M3/SiO2(98 nm).
  • R2: Glass/Ag (20 nm).
where Au and Ag corresponds to a 23 nm and 20 nm thick Au and Ag film, respectively, and M1 corresponds to Ag(4 nm)/Au(14 nm), M2 to Ag(10 nm)/ Au(10 nm) and M3 to Ag(15 nm)/ Au(6 nm) bilayers. The layer thicknesses (shown inside the parentheses) were individually estimated by matching transfer matrix method simulations with measured values of the transmittance (T), reflectance (R), and absorptance (A) spectra taken by a Shimadzu UV-Vis-NIR scanning spectrophotometer. Refractive index values of deposited Au and Ag films were obtained by modeling spectroscopic ellipsometric (SE) data (J.A. Woollam M-2000UI), taken on individual films, as a perfectly flat continuous layer. The effective refractive indices (Neff = neff + ikeff) of the bilayer films were calculated from SE data imposing Kramers-Kronig consistency to the calculated values.

The nonlinear optical properties (NLO) of these films were characterized by a commercially available white-light continuum (WLC) pump-probe spectroscopy system (Helios, ultrafast system). The pump pulse obtained from an optical parametric amplifier (TOPAS-C, Spectra-Physics) was tuned to a wavelength of 560 nm. A laser beam from a Ti:Sapphire regenerative amplifier (Spitfire, Spectra-Physics) operating at 800 nm pumped the TOPAS-C, while a small portion of this beam generated the WLC (420 - 950 nm) probe pulse. The WLC probe pulse measured 60 μm half-width-1/e (HW 1/e) at the sample position using a knife-edge scan, and the pump pulse was 285 μm (HW 1/e). Because the probe size is significantly smaller than the pump, it is assumed that the probe overlaps with a region of approximately constant peak fluence from the pump. The pump has a pulse width of 60 fs (HW 1/e) and the total instrument response time is 150 fs full-width-half-maximum (FWHM). The pump beam was chopped at 500 Hz with a 50% duty cycle to obtain pumped (signal) and non-pumped (reference) probe spectra sequentially. After averaging over one thousand probe pulses at each time delay, the change in optical density (ΔOD(λ, t)) was recorded as a function of wavelength (λ) and delay time (t). The fluence of the probe pulses for all samples was confirmed to be low enough to produce no observable NLO response. Scattered pump light was subtracted from the data based on measurements at negative delay times. A temporal correction factor was applied to all data sets to provide equivalent zero delay onsets for all probe wavelengths; this correction factor was determined by measuring the chirp of the WLC probe passing through the glass substrate. The transmittance spectra change (ΔT(λ, t)) and reflectance spectra change (ΔR(λ, t)) of the WLC probe pulses were calculated from measured ΔOD(λ, t) as a function of delay time for a variety of pump fluences by ΔT(λ,t)=ln(10)T(ΔOD(λ,t)) and ΔR(λ,t)=ln(10)R(ΔOD(λ,t)), where T and R are the linear transmittance and reflectance spectra, respectively. This formula is derived from a Taylor series expansion and therefore is only valid for small values of ΔOD(λ, t).

3. Results and discussion

3.1. Linear optical properties

Figure 1
Fig. 1 Comparison of measured (symbols) and simulated (linear model 1: thin lines, linear model 2: thick lines) transmittance (T) (blue), reflectance (R) (green) and absorptance (A) (red) spectra in the visible range for fabricated samples R1, S1, S2, S3 and R2.
, shows a comparison of the measured T, R and A spectra (symbols) on all samples and the simulated spectra following two different approaches. In the first approach, referred to as linear model 1 (thin lines), the optical properties of the Ag/Au bilayers were modeled as two continuous layers with uniform thickness, each with refractive index values obtained from the analysis of SE data on samples R1 and R2. The mean square error (MSE) values of the fits to the SE data are 5.7 and 4.7 for samples R1 and R2, respectively. Following this approach a fair description of the optical properties of the bi-metal layers can be obtained. The discrepancy between experimental and simulated spectra is worst for sample S1, likely because the bottom 4 nm Ag film is expected to be non-continuous, resulting in a very rough top (14 nm) Au layer and differing significantly from the assumptions made in the model. In the second approach, referred to as linear model 2 (thick lines), each Neff and thickness (shown inside the parenthesis) of bilayers for samples S1 (M1 24 nm), S2 (M2 20 nm), and S3 (M3 24 nm) were extracted from SE data using a single layer model to describe the bi-metal (without surface roughness). The MSE values of the fits to the SE data are 5.2, 5.5 and 5.0 for samples S1, S2 and S3, respectively. These Neff values were then used to simulate the T, R and A spectra. This single layer model yields better agreement between simulated and measured values in sample S1, since it bypasses the morphological details of the Ag/Au interface.

Figures 2(a)
Fig. 2 (a) Real and (b) imaginary effective refractive index values of bilayer Ag/Au metal thin films: M1, M2 and M3; Au and Ag are shown as reference. (c) Quality factor spectra for localized surface plasmon and (d) quality factor spectra for surface plasmon polariton. (e) Simulated maximum potential transmittance spectra.
and 2(b) show the Neff values of bilayers M1, M2 and M3, as well as the refractive index values of single layer Au and Ag. For Au and Ag, the index values are close to literature values, with the interband transition onset of bulk Au [20

20. W. J. Scouler, “Temperature-modulated reflectance of gold from 2 to 10 ev,” Phys. Rev. Lett. 18(12), 445–448 (1967). [CrossRef]

] located at 520 nm and the interband transition onset of bulk Ag [13

13. C. Voisin, N. Del Fatti, D. Christofilos, and F. Vallee, “Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles,” J. Phys. Chem. B 105(12), 2264–2280 (2001). [CrossRef]

] at 313 nm. At these wavelengths, inflection points are present in the real part of refractive index. In contrast, for M1, M2 and M3 the inflection point in Neff in the visible range moves away from Au and towards Ag as the thickness ratio of Au to Ag decreases, resulting in an apparent blue shift of the onset of interband transitions (Fig. 2(a)). In other words, by controlling the Ag/Au mass thickness ratio in these bilayers metallic films, the apparent onset of interband transitions can be spectrally tuned. This tunability, as will be described next, can be attractive for a variety of linear and nonlinear applications.

For applications of Au/Ag bilayer films in linear optical filters, such as band pass filters (metal-dielectric band gap structures, induced transmission filters, etc.) or as transparent electrodes in electro-optic devices, the concept of maximum potential transmittance is useful since it provides an upper limit to the transmittance of an absorbing film, after all reflectance losses are suppressed., The maximum potential transmittance, Ψ, for a single metallic film is defined by the following equation [22

22. H. A. Macleod, “The induced-transmission filter,” in Thin-Film Optical Filters, 3rd ed. (Institute of Physics Publishing, London, 2001).

]
ψ=((n2k22nk(Z/X))(n2+k2)(sin2αcosh2β+cos2αsinh2β)+(cos2αcosh2β+sin2αsinh2β)+1X(nsinhβcoshβ+kcosαsinα)+X2+Z2X(n2+k2)(nsinhβcoshβkcosαsinα))1withparametersα=2πnd/λandβ=2πkd/λ.XandZaredefinedasX=[(n2+k2)(nsinhβcoshβ+ksinαcosα)(nsinhβcoshβksinαcosα)n2k2(sin2αcosh2β+cos2αsinh2β)2(nsinhβcoshβksinαcosα)2]12Z=nk(sin2αcosh2β+cos2αsinh2β)(nsinhβcoshβksinαcosα)
(1)
where d is the layer thickness, λ is the free-space wavelength, and n and k are the real and imaginary parts of the refractive index, respectively.

In Fig. 2(e), the values of the maximum potential transmittance for Ag, Au, and bilayer films are compared by using the Neff values previously derived and assuming films of equal thickness, 20 nm, to provide a fair comparison. Note that as a general statement, Au always displays lower values of Ψ than Ag. For wavelengths 350-500 nm, strong absorptive losses in Au due to interband transitions limits the values of Ψ compared to Ag, which has an interband transition region in the UV range. As expected from the apparent tuning of the onset of interband transitions, decreasing the ratio of Au to Ag leads to larger values of Ψ in all Ag/Au bilayer films compared to Au, in the 350-500 nm wavelength range. For instance, at wavelength 400 nm, Ψ improves from 70% for Au to 77% for M1, 79% for M2, and 84% for M3, as the thickness ratio of Au to Ag decreases (Fig. 2(e)). In the 500-950 nm range, bilayer films do not display improved values of Ψ but remain still higher than 90%. Once more, although Ag offers better linear optical properties across the visible spectrum, its more reactive nature to the environment and its much smaller NLO response make it less attractive for applications where improved environmental stability and higher NLO response are required.

3.2. Nonlinear optical properties

Potential applications for all-optical control that use the very large magnitude of the NLO response of noble metals, Au and Cu in particular, make it interesting to explore the NLO response of these bilayer films. The largest NLO response in a thin film of a noble metal arises due to the so-called Fermi smearing process. This process is driven by the rapid heating of electrons upon the absorption of energy from an ultrafast optical pulse. The rise of electronic temperature broadens the electronic distribution around the Fermi energy, at the onset of interband transitions, causing a drastic change of the dielectric permittivity of the metal [13

13. C. Voisin, N. Del Fatti, D. Christofilos, and F. Vallee, “Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles,” J. Phys. Chem. B 105(12), 2264–2280 (2001). [CrossRef]

].

Figures 3(a)
Fig. 3 (a), (b) Spectral dependence of transmittance and reflectance changes (ΔT(λ, tpeak) and ΔR(λ, tpeak)) measured from WLC pump probe experiment (solid line) and simulation by two-temperature model (dashed line) of samples R1, S1, S2 and S3 with a pump fluence of 25 J/m2; (c), (d) experimental (solid line) and simulated (dashed line) temporal dependence of the normalized absolute transmittance and reflectance changes probed at 520 nm. The excitation wavelength in all cases was 560 nm.
and 3(b) show the values of ΔT(λ, tpeak) and ΔR(λ, tpeak) measured in pump-probe experiments in samples R1, S1, S2 and S3 for a pump fluence of 25 J/m2. Figures 3(c) and 3(d) shows the temporal evolution of ΔT(λpeak = 520 nm, t) and ΔR(λpeak = 520 nm, t) measured on the same samples. Here, the subscript peak denotes the maximum value of the NLO response in the spectral and temporal ranges studied. For a single Ag layer, the maximum |ΔT(λ, tpeak) | and |ΔR(λ, tpeak) | are at least an order of magnitude smaller (<0.5% for a pump fluence of 50 J/m2) than those found in bilayer or single Au layer films [7

7. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys. 107(12), 123114 (2010). [CrossRef]

], so their values are not included in Fig. 3. For bilayer films, S1, S2, and S3, the ΔT(λ, tpeak) and ΔR(λ, tpeak) spectra display similar dispersion characteristics found in a single Au layer (R1). Under the same incidence pump fluence, the peak-to-valley magnitude of ΔT(λ, tpeak) and ΔR(λ, tpeak) gradually reduces as the mass thickness ratio of Au to Ag decreases from R1, S1, S2 to S3 (Figs. 3(a) and 3(b)). Interestingly, there is a consistent blue-shift of the wavelength at which maximum values of ΔT(λpeak, tpeak) and ΔR(λpeak, tpeak) are observed: by decreasing the mass thickness ratio of Au to Ag, the |ΔT(λpeak, tpeak)|, peak wavelength and magnitude, changes from |ΔT(513 nm, tpeak)| = 7.8% for R1 to |ΔT(498 nm, tpeak)| = 4.8% for S1, |ΔT(496 nm, tpeak)| = 2.5% for S2, and |ΔT(490 nm, tpeak)| = 0.7% for S3. A similar trend is found for ΔR as shown in Fig. 3(b). The trend of these shifts is consistent with the shifts in the onset of interband transitions shown in Fig. 2(a). This correspondence shows that both linear and NLO properties can be tuned by controlling the mass-thickness ratio between Ag and Au in bilayer films, albeit the magnitude of the NLO changes appears to be reduced as the Ag content increases. This apparent reduction, as will be later described, arises primarily from differences in the absorbed power within the Au layer.

In order to gain a better understanding of the NLO properties of these bi-metal layers, the two-temperature model was introduced (Eq. (2)) to describe the heating of electrons and the lattice when optically pumped [7

7. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys. 107(12), 123114 (2010). [CrossRef]

,13

13. C. Voisin, N. Del Fatti, D. Christofilos, and F. Vallee, “Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles,” J. Phys. Chem. B 105(12), 2264–2280 (2001). [CrossRef]

].

Ce(Te)dTedt=G(TeTl)+P(t)CldTldt=G(TeTl)
(2)

The changes in electron and lattice temperatures calculated through the two temperature model depend upon intrinsic material properties such as the electron and lattice specific heats, Ce and Cl, respectively, and the electron phonon coupling constant, G, and are driven by the density of absorbed power within each individual layer P(t). In our simulation Ce(Te) = (62 ± 5) × Te [J/m3K], Cl = (3.2 ± 0.4) × Te [J/m3K], G = (1.8 ± 0.1) × 1016 [W/m3K] and the density of absorbed power is estimated as P(t)=I(t)A/d[W/m3], where I(t) is the pump irradiance, here assumed to have a Gaussian temporal profile with a pulse width of 60 fs (HW 1/e); A is the linear absorptance in the metal layer, calculated using the transfer matrix method; and d is the thickness of the metal layer. These values were obtained by fitting ΔT(λpeak, t) and ΔR(λpeak, t), measured for an incident pump fluence of 8 J/m2, with the physical model described in following paragraphs and show magnitudes that are comparable to literature values [6

6. N. Rotenberg, A. D. Bristow, M. Pfeiffer, M. Betz, and H. M. van Driel, “Nonlinear absorption in Au films: Role of thermal effects,” Phys. Rev. B 75(15), 155426 (2007). [CrossRef]

]. Here, we note that the linear approximation of Ce (Te) and the temperature independent value of G are valid only in the low-fluence regime; this is, for electron temperatures smaller than around 3000 K for Au [23

23. Z. Lin, L. V. Zhigilei, and V. Celli, “Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium,” Phys. Rev. B 77(7), 075133 (2008). [CrossRef]

]. At the highest pump fluence of 25 J/m2 used in this work, the maximum electron temperature in all samples is calculated to be between 1300 – 1500 K.

The electron and lattice temperatures derived from the two temperature model are used in a temperature-dependent physical model of the dielectric permittivity to calculate the complex refractive index changes Δnsim(λ, t) and Δksim(λ, t).

For simplicity, the model implemented here to describe the NLO response of bilayers only accounts for the contribution of the Au component. This is motivated by the fact that only very minor adjustments were found when the contribution of Ag, as described in [7

7. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys. 107(12), 123114 (2010). [CrossRef]

] was included in the calculations. The dielectric permittivity of Au, was modeled as a set of explicit equations (Eqs. (3)-(5)), as functions of frequency, temperature and time, via the superposition of a Drude-like intraband transition term (εintra) and an interband transition term (εinter) following Bigot, et al. [24

24. J. Y. Bigot, J. Y. Merle, O. Cregut, and A. Daunois, “Electron dynamics in copper metallic nanoparticles probed with femtosecond optical pulses,” Phys. Rev. Lett. 75(25), 4702–4705 (1995). [CrossRef] [PubMed]

], as follows

ε(ω,Tl(t),Te(t))=εintra(ω,Tl(t),Te(t))+εinter(ω,Te(t))
(3)

Hereafter, ω is the free space optical frequency, t is the time, and Tl(t) and Te(t) are dependent lattice and electron temperatures, respectively.

For the first term in Eq. (3),
εintra(ω,Tl(t),Te(t))=1+εbωp2ω2+iγωwithparameters,γ[Tl(t),Te(t)]=γ0+γ1×Tl(t)+γ2×Te2(t)+γ3×ω2
(4)
where, εb is the background dielectric constant, γ is the damping constant, ωp is the bulk plasma frequency, and γ0, γ1, γ2, and γ3 are constant coefficients [7

7. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys. 107(12), 123114 (2010). [CrossRef]

].

The value of the constant coefficients used in the Au permittivity model are listed in Tables 1

Table 1. Constant Coefficients of a Au Dielectric Permittivity Model for Intraband Transition Term (Eq. (4))

table-icon
View This Table
| View All Tables
and 2

Table 2. Constant Coefficients of a Au Dielectric Permittivity Model of the First and Second Interband Transition Term (Eq. (5)), Respectively

table-icon
View This Table
| View All Tables
. These values were obtained by fitting the measured steady-state permittivity at room temperature [25

25. P. G. Etchegoin, E. C. Le Ru, and M. Meyer, “An analytic model for the optical properties of gold,” J. Chem. Phys. 125(16), 164705 (2006). [CrossRef] [PubMed]

], Tl(t) = Te(t) = 300 K, with Eqs. (3)-(5). Figure 4
Fig. 4 Comparison of measured (symbols) and simulated (solid lines) complex dielectric permittivity spectra in visible range for fabricated samples R1. The dielectric permittivity of Au was modeled as a set of explicit equations (Eqs. (3)-(5)) and compared with measured ellipsometric data.
shows the comparison between simulated Au permittivity and that obtained by spectroscopic ellipsometry.

Simulated ΔTsim(λ, t) and ΔRsim(λ, t) are obtained by replacing NAu with NAu(λ, t) = NAu + ΔNsim(λ, t) (where ΔNsim(λ, t) = Δnsim(λ, t) + iΔksim(λ, t)) in our bilayer linear model 1 (which considers two independent layers). Using this approach and without any further adjustment of the parameters in the model, ΔTsim(λ, t) and ΔRsim(λ, t) closely match with the experimental values. This comparison is shown in Figs. 3(c) and 3(d), where a similar ultrafast temporal evolution is found for all samples. Although better fits could be obtained if small modifications to the value of the electron-phonon coupling rates are introduced to account for its temperature dependence and differences in sample morphology, the model here proposed is sufficient to describe the basic features of the NLO response measured in the bilayers.

In the proposed model, ΔNsim(λ, t) is only ascribed to the Au layer. However, for potential applications in all-optical photonic devices it is useful to derive ΔNeff = Δneff + iΔkeff values which could be ascribed to the entire bilayer. Using a first order approximation, a Taylor expansion of ΔT and ΔR as fuctions of ΔNeff yields the following system of linear equations
ΔT=TneffΔneff+TkeffΔkeffΔR=RneffΔneff+RkeffΔkeff
(6)
In solving this system, ΔNeff is extracted from the experimental data. The partial derivatives, ∂T/∂n, ∂T/∂k, ∂R/∂n and ∂R/∂k were approximated by their differentials (ΔT/Δneff, ΔT/Δkeff, etc.) by introducing a small perturbation to Neff in the linear model 2.

3. Conclusion

The optical properties of Ag/Au bilayer metallic thin films with a total thickness of approximately 20 nm and with different Ag/Au mass-thickness ratios were studied. The effective refractive index values were found to be spectrally tunable by controlling the mass-thickness ratio between Au and Ag. Hence, the optical loss introduced by interband transitions in Au layers can be reduced. As a consequence, improvement of the quality factors (QLSP and QSPP) derived for plasmonic applications and the potential transmittance (Ψ) for optical filter applications are calculated within the visible range. These spectral shifts also lead to similar spectral shifts on the NLO response of the bilayers. The NLO response is shown to be ultrafast and comparable in origin and magnitude to that observed in single Au films. The NLO response in the bilayer films is dominated by the ultrafast dynamics of the thermal exchange between the absorbed optical field and the electron cloud and the lattice in the Au layer. The combined properties of these bilayers could therefore be attractive for a variety of linear and nonlinear photonic applications.

Acknowledgments

This work was partially funded by NSF through STC-DMR-0120967, by ARO through contract/ grant 50372-CH-MUR, by AFOSR (BIONIC Center grant No. FA9550-09-1-0162), and AFOSR (grant No. FA9550-09-1-0418).

References and links

1.

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]

2.

H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices,” Nature 444(7119), 597–600 (2006). [CrossRef] [PubMed]

3.

D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “Nonlinear optical properties of induced transmission filters,” Opt. Express 18(18), 19101–19113 (2010). [CrossRef] [PubMed]

4.

M. J. Bloemer and M. Scalora, “Transmissive properties of Ag/MgF2 photonic band gaps,” Appl. Phys. Lett. 72(14), 1676–1678 (1998). [CrossRef]

5.

S. Y. Ryu, C. H. Lee, I. S. Oh, S. Y. Song, K. H. Hwang, H. S. Hwang, M. H. Han, B. H. Hwang, H. K. Baik, Y. S. Kim, and J. Y. Lee, “Efficient inverted top-emitting organic light emitting diodes with transparent and surface-modified multilayer anodes,” Electrochem. Solid-State Lett. 13(5), J43–J46 (2010). [CrossRef]

6.

N. Rotenberg, A. D. Bristow, M. Pfeiffer, M. Betz, and H. M. van Driel, “Nonlinear absorption in Au films: Role of thermal effects,” Phys. Rev. B 75(15), 155426 (2007). [CrossRef]

7.

D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys. 107(12), 123114 (2010). [CrossRef]

8.

M. A. Swillam, N. Rotenberg, and H. M. van Driel, “All-optical ultrafast control of beaming through a single sub-wavelength aperture in a metal film,” Opt. Express 19(8), 7856–7864 (2011). [CrossRef] [PubMed]

9.

G. A. Wurtz, R. Pollard, W. Hendren, G. P. Wiederrecht, D. J. Gosztola, V. A. Podolskiy, and A. V. Zayats, “Designed ultrafast optical nonlinearity in a plasmonic nanorod metamaterial enhanced by nonlocality,” Nat. Nanotechnol. 6(2), 107–111 (2011). [CrossRef] [PubMed]

10.

N. Rotenberg, M. Betz, and H. M. van Driel, “Ultrafast all-optical coupling of light to surface plasmon polaritons on plain metal surfaces,” Phys. Rev. Lett. 105(1), 017402 (2010). [CrossRef] [PubMed]

11.

J. Y. Zhang, L. Wang, S. Krishna, M. Sheik-Bahae, and S. R. J. Brueck, “Saturation of the second harmonic generation from GaAs-filled metallic hole arrays by nonlinear absorption,” Phys. Rev. B 83(16), 165438 (2011). [CrossRef]

12.

S. I. Anisimov, B. L. Kapeliovich, and T. L Perel'Man, “Electron emission from metal surfaces exposed to ultrashort laser pulses,” Sov. Phys. JETP 39, 375–377 (1974).

13.

C. Voisin, N. Del Fatti, D. Christofilos, and F. Vallee, “Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles,” J. Phys. Chem. B 105(12), 2264–2280 (2001). [CrossRef]

14.

P. E. Hopkins, “Influence of Inter- and Intraband Transitions to Electron Temperature Decay in Noble Metals After Short-Pulsed Laser Heating,” J. Heat Transfer 132(12), 122402 (2010). [CrossRef]

15.

B. H. Ong, X. C. Yuan, and S. C. Tjin, “Bimetallic silver-gold film waveguide surface plasmon resonance sensor,” Fiber Integr. Opt. 26(4), 229–240 (2007). [CrossRef]

16.

H. Tóháti, A. Sipos, G. Szekeres, A. Mathesz, A. Szalai, P. Jojart, J. Budai, C. Vass, A. Kohazi-Kis, M. Csete, and Z. Bor, “Surface plasmon scattering on polymer-bimetal layer covered fused silica gratings generated by laser induced backside wet etching,” Appl. Surf. Sci. 255(10), 5130–5137 (2009). [CrossRef]

17.

M. Thomschke, S. Hofmann, S. Olthof, M. Anderson, H. Kleemann, M. Schober, B. Lussem, and K. Leo, “Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing,” Appl. Phys. Lett. 98(8), 083304 (2011). [CrossRef]

18.

T. Q. Qiu, T. Juhasz, C. Suarez, W. E. Bron, and C. L. Tien, “Femtosecond laser heating of multi-layer metals—II. Experiments,” Int. J. Heat Mass Transfer 37(17), 2799–2808 (1994). [CrossRef]

19.

T. Q. Qiu and C. L. Tien, “Femtosecond laser heating of multi-layer metals—I. Analysis,” Int. J. Heat Mass Transfer 37(17), 2789–2797 (1994). [CrossRef]

20.

W. J. Scouler, “Temperature-modulated reflectance of gold from 2 to 10 ev,” Phys. Rev. Lett. 18(12), 445–448 (1967). [CrossRef]

21.

M. G. Blaber, M. D. Arnold, and M. J. Ford, “A review of the optical properties of alloys and intermetallics for plasmonics,” J. Phys. Condens. Matter 22(14), 143201 (2010). [CrossRef] [PubMed]

22.

H. A. Macleod, “The induced-transmission filter,” in Thin-Film Optical Filters, 3rd ed. (Institute of Physics Publishing, London, 2001).

23.

Z. Lin, L. V. Zhigilei, and V. Celli, “Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium,” Phys. Rev. B 77(7), 075133 (2008). [CrossRef]

24.

J. Y. Bigot, J. Y. Merle, O. Cregut, and A. Daunois, “Electron dynamics in copper metallic nanoparticles probed with femtosecond optical pulses,” Phys. Rev. Lett. 75(25), 4702–4705 (1995). [CrossRef] [PubMed]

25.

P. G. Etchegoin, E. C. Le Ru, and M. Meyer, “An analytic model for the optical properties of gold,” J. Chem. Phys. 125(16), 164705 (2006). [CrossRef] [PubMed]

OCIS Codes
(160.3900) Materials : Metals
(240.6680) Optics at surfaces : Surface plasmons
(300.6500) Spectroscopy : Spectroscopy, time-resolved
(310.6860) Thin films : Thin films, optical properties

ToC Category:
Thin Films

History
Original Manuscript: February 22, 2012
Revised Manuscript: March 16, 2012
Manuscript Accepted: March 19, 2012
Published: March 28, 2012

Citation
James Hsu, Canek Fuentes-Hernandez, Alfred R. Ernst, Joel M. Hales, Joseph W. Perry, and Bernard Kippelen, "Linear and nonlinear optical properties of Ag/Au bilayer thin films," Opt. Express 20, 8629-8640 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-8-8629


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature424(6950), 824–830 (2003). [CrossRef] [PubMed]
  2. H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices,” Nature444(7119), 597–600 (2006). [CrossRef] [PubMed]
  3. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “Nonlinear optical properties of induced transmission filters,” Opt. Express18(18), 19101–19113 (2010). [CrossRef] [PubMed]
  4. M. J. Bloemer and M. Scalora, “Transmissive properties of Ag/MgF2 photonic band gaps,” Appl. Phys. Lett.72(14), 1676–1678 (1998). [CrossRef]
  5. S. Y. Ryu, C. H. Lee, I. S. Oh, S. Y. Song, K. H. Hwang, H. S. Hwang, M. H. Han, B. H. Hwang, H. K. Baik, Y. S. Kim, and J. Y. Lee, “Efficient inverted top-emitting organic light emitting diodes with transparent and surface-modified multilayer anodes,” Electrochem. Solid-State Lett.13(5), J43–J46 (2010). [CrossRef]
  6. N. Rotenberg, A. D. Bristow, M. Pfeiffer, M. Betz, and H. M. van Driel, “Nonlinear absorption in Au films: Role of thermal effects,” Phys. Rev. B75(15), 155426 (2007). [CrossRef]
  7. D. T. Owens, C. Fuentes-Hernandez, J. M. Hales, J. W. Perry, and B. Kippelen, “A comprehensive analysis of the contributions to the nonlinear optical properties of thin Ag films,” J. Appl. Phys.107(12), 123114 (2010). [CrossRef]
  8. M. A. Swillam, N. Rotenberg, and H. M. van Driel, “All-optical ultrafast control of beaming through a single sub-wavelength aperture in a metal film,” Opt. Express19(8), 7856–7864 (2011). [CrossRef] [PubMed]
  9. G. A. Wurtz, R. Pollard, W. Hendren, G. P. Wiederrecht, D. J. Gosztola, V. A. Podolskiy, and A. V. Zayats, “Designed ultrafast optical nonlinearity in a plasmonic nanorod metamaterial enhanced by nonlocality,” Nat. Nanotechnol.6(2), 107–111 (2011). [CrossRef] [PubMed]
  10. N. Rotenberg, M. Betz, and H. M. van Driel, “Ultrafast all-optical coupling of light to surface plasmon polaritons on plain metal surfaces,” Phys. Rev. Lett.105(1), 017402 (2010). [CrossRef] [PubMed]
  11. J. Y. Zhang, L. Wang, S. Krishna, M. Sheik-Bahae, and S. R. J. Brueck, “Saturation of the second harmonic generation from GaAs-filled metallic hole arrays by nonlinear absorption,” Phys. Rev. B83(16), 165438 (2011). [CrossRef]
  12. S. I. Anisimov, B. L. Kapeliovich, and T. L Perel'Man, “Electron emission from metal surfaces exposed to ultrashort laser pulses,” Sov. Phys. JETP39, 375–377 (1974).
  13. C. Voisin, N. Del Fatti, D. Christofilos, and F. Vallee, “Ultrafast electron dynamics and optical nonlinearities in metal nanoparticles,” J. Phys. Chem. B105(12), 2264–2280 (2001). [CrossRef]
  14. P. E. Hopkins, “Influence of Inter- and Intraband Transitions to Electron Temperature Decay in Noble Metals After Short-Pulsed Laser Heating,” J. Heat Transfer132(12), 122402 (2010). [CrossRef]
  15. B. H. Ong, X. C. Yuan, and S. C. Tjin, “Bimetallic silver-gold film waveguide surface plasmon resonance sensor,” Fiber Integr. Opt.26(4), 229–240 (2007). [CrossRef]
  16. H. Tóháti, A. Sipos, G. Szekeres, A. Mathesz, A. Szalai, P. Jojart, J. Budai, C. Vass, A. Kohazi-Kis, M. Csete, and Z. Bor, “Surface plasmon scattering on polymer-bimetal layer covered fused silica gratings generated by laser induced backside wet etching,” Appl. Surf. Sci.255(10), 5130–5137 (2009). [CrossRef]
  17. M. Thomschke, S. Hofmann, S. Olthof, M. Anderson, H. Kleemann, M. Schober, B. Lussem, and K. Leo, “Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing,” Appl. Phys. Lett.98(8), 083304 (2011). [CrossRef]
  18. T. Q. Qiu, T. Juhasz, C. Suarez, W. E. Bron, and C. L. Tien, “Femtosecond laser heating of multi-layer metals—II. Experiments,” Int. J. Heat Mass Transfer37(17), 2799–2808 (1994). [CrossRef]
  19. T. Q. Qiu and C. L. Tien, “Femtosecond laser heating of multi-layer metals—I. Analysis,” Int. J. Heat Mass Transfer37(17), 2789–2797 (1994). [CrossRef]
  20. W. J. Scouler, “Temperature-modulated reflectance of gold from 2 to 10 ev,” Phys. Rev. Lett.18(12), 445–448 (1967). [CrossRef]
  21. M. G. Blaber, M. D. Arnold, and M. J. Ford, “A review of the optical properties of alloys and intermetallics for plasmonics,” J. Phys. Condens. Matter22(14), 143201 (2010). [CrossRef] [PubMed]
  22. H. A. Macleod, “The induced-transmission filter,” in Thin-Film Optical Filters, 3rd ed. (Institute of Physics Publishing, London, 2001).
  23. Z. Lin, L. V. Zhigilei, and V. Celli, “Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium,” Phys. Rev. B77(7), 075133 (2008). [CrossRef]
  24. J. Y. Bigot, J. Y. Merle, O. Cregut, and A. Daunois, “Electron dynamics in copper metallic nanoparticles probed with femtosecond optical pulses,” Phys. Rev. Lett.75(25), 4702–4705 (1995). [CrossRef] [PubMed]
  25. P. G. Etchegoin, E. C. Le Ru, and M. Meyer, “An analytic model for the optical properties of gold,” J. Chem. Phys.125(16), 164705 (2006). [CrossRef] [PubMed]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1 Fig. 2 Fig. 3
 
Fig. 4 Fig. 5
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited