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We report strong optical nonlinearity of glasses embedded with copper and silver nanoparticles. In pump-probe experiments with copper-doped glasses, the observed absorption bleaching with picosecond relaxation time is as high as 22%. Transmission femtosecond measurements reveal the reverse saturable absorption with nonlinear absorption coefficient of 10-10 cm/W in both copper- and silver-doped nanocomposites.
©2007 Optical Society of America
1. Introduction
Materials embedded with metal nanoparticles [1] have recently attracted a considerable attention because these materials constitute a unique playground to study temporal evolution of the strongly correlated electron system [2]. In these materials, excitation and thermalization processes are strongly influenced by the scattering of electrons on nanoparticle surface and excitation of surface plasmon modes that mediate the energy transfer from electron to lattice subsystems [3, 4]. When the size of a metal particle is smaller than the electron mean free path, the surface scattering dominates relaxation process because excited electrons move ballistically inside the particle. Since this mechanism is less effective than generation of phonons in bulk metals, the achievable electron temperature can be considerably higher than that of the lattice [2].
The optical and electronic properties of materials embedded with metal nanoparticles can be tailored by varying size, shape and packing density of the particles and the particle-matrix interface [5]. Although most experiments on nonlinear optical properties of such materials were performed with liquid suspensions of metal nanoparticles [3, 6, 7], glass matrix for holding metal, dielectric or semiconductor nanoparticles is much more attractive for nonlinear-optical and optoelectronic applications [8]. Metallic inclusions in bulk glasses can be made in the course of glass synthesis or the synthesis followed by secondary thermal treatment of the glass [9], and ion exchange [10] or ionic implantation [11] with subsequent treatment in reducing atmosphere can also be used for their formation in the subsurface layer of glasses.
The linear and nonlinear optical properties of the glass-metal nanocomposites show pronounced spectral features in the vicinity of the surface plasmon resonance (SPR) [2–4, 12]. In this spectral range, relatively slow nonlinear response is often associated with interfacial and thermal effects [9], while ultrafast light-induced absorption change is manly due to dynamics of collective electronic excitations, which have been studied by Z-scan [11, 14] and pump-probe [2–4, 12, 13, 15] techniques. However, until now, light-induced change of the absorption coefficient in metal-glass nanocomposites [2–4, 12] has been found to be much lower than that in liquid suspensions of metal nanoparticles [14]. In this paper, we report results of the time-resolved absorption and self-induced nonlinear transmission measurements in glasses embedded with silver and copper nanoparticles in the subpico- and picosecond time scale. In particular, in the vicinity of the surface plasmon resonance, we observe up to 22% light-induced change of the absorption coefficient of copper-glass nanocomposite.
2. Experiment
In the experiment, we studied three different glass-metal nanocomposites. The first one was created by melting boron-silicate glass (composition in wt.%: SiO2 – 76, B2O3 – 1, Na2O – 16, CaO- 7) in slightly reduced atmosphere using the batch containing 0.03 wt.% of copper oxide and 0.3 wt.% of tin oxide. Since neutral copper is hardly solvable in a glass matrix, annealing of the glass results in phase decomposition and formation of homogeneously distributed copper nanoparticles. The obtained red-colored glassy composite (so-called copper ruby glass) will be referred to as Cu-bulk.
The second glassy composite was produced using copper to sodium and potassium ion exchange in a boron-silicate glass. A polished 1 mm thick glass sample initially containing 16.2 wt.% of alkaline oxides (glass composition in wt.%: SiO2 – 69,8, B2O3 – 11, Na2O – 10.2, K2O- 6, BaO- 3) was immersed into the eutectic melt of copper and sodium sulfates and subjected to ion exchange for 30 minutes at 540C. After thermal treatment for one hour in hydrogen atmosphere at the temperature of 450C the reactive diffusion of hydrogen [17] resulted in the formation of copper nanoparticles in the 35 micrometers thick surface layer. The obtained glassy composite will be referred to as Cu-surf.
The third nano-composite was prepared using silver-to-sodium ion exchange in an alkaline-niobate-silicate glass containing 13 wt.% of alkaline oxides oxide (glass composition in wt.%: SiO2 – 17, B2O3 – 1, Li2O – 4.6, K2O- 8.4, CdO- 2, Nb2O5 – 67) at 240C. The bath of 5 mol% solution of silver nitrate melt in sodium nitrate was used for one hour ion exchange processing with following thermal treatment hydrogen atmosphere at 340C for 15-30 min. Corresponding thickness of the layer embedded with silver nanoparticles was ∼7 microns. This glassy composite will be referred to as Ag-surf.
The metal nanoparticles in glass matrix were identified and characterized using X-ray diffraction measurements. Obtained diffractograms show peaks that correspond to crystalline planes of copper (see inset to Fig. 1) and silver. The widths of diffraction peaks allowed us to evaluate the average radius of the nanoparticles as 10 nm (at the particle density ∼1013 cm-3), 8 nm (5∙1014 cm-3) and 15 nm (~1016 cm-3) for Cu-bulk, Cu-surf, and Ag-surf, respectively.
Fig. 1. Optical density of Cu-bulk (a), Cu-surf (b), and Ag-surf (c) materials. Insert shows X-ray diffraction pattern of the Cu-surf glassy nanocomposite. The diffraction maxima correspond to position of copper crystalline planes.
The optical densities of the nanocomposites in the spectral range of 300-800 nm are shown in Fig. 1. In Ag-surf, the absorption is dominated by the SPR at 410 nm, which is slightly blue-shifted with respect to the position calculated using permittivity data [10]. This shift is due to contamination of the subsurface glass layer by ionic silver. One can observe from Fig.1 that in Cu-bulk and Cu-surf composites, the absorption resonance is located at 560 nm, i.e. it well corresponds to the SPR for copper nanoparticles in glass matrix [2, 12]. The enhanced absorption at the “blue” side of the plasmon resonance in Cu-bulk and Cu-surf originates from the interband d → p transition at 571 nm [2]. By using concentration of the nano-particles and their distribution within the sample, it can be shown that number of Cu nano-particles per cross-section of the laser beam in Cu-surf composite is two times higher than that in Cu-bulk. This finding well corresponds to both measured optical density spectra presented in Fig. 1 and also to those calculated using Lorentz-Drude model.
Pump-probe and non-linear transmission measurements were performed using experimental setups shown in Figs 2(a) and 2(b), respectively. In the pump-probe transmission measurements, we employed an ExciPro femtosecond system (CDP Corp). In this system - (see Fig 2(a) - the pump at 400 nm (pulse duration 50 fs, energy up to 0.1 mJ) excites the sample, while the change of the optical absorption is probed by femtosecond continuum. The pump-probe delay can be varied using 2.0-ns optical delay line. The temporal and spectral profile of the absorption change in the spectral range of 450-650 nm is visualized using a two-channel imaging spectrometer.
The nonlinear transmission measurements - see Fig 2(b) - were performed with 50 fs long optical pulses (repetition rate 50 Hz) at 400 nm and 570 nm obtained using frequency doubled beams of the amplified Ti:Sapphire oscillator and optical parametric amplifier, respectively. The beam diameter (at 50% intensity level) was about 1 mm.
3. Results and discussion
Results of the pump-probe measurements for Cu-bulk and Cu-surf composites are presented in Figs. 3(a) and 3(b), respectively. One can observe that in both materials, the excitation at 400 nm with intensity of about 1011 W/cm2 results in an instantaneous decrease of transmission at the blue side of the SPR. However, in few picoseconds, we observed a strong increase of the transmission in the vicinity 560 nm, i.e. close to the SPR and interband absorption edge. In such a situation, the broadening of the resonance is a dominant process, while the spectral position of the transmission maximum does not change [2]. The broadening of the resonance is caused by the Coulomb interaction [4], which results in decrease in the electron scattering time and in the enhanced surface plasmon damping [12].
In the Cu-bulk and Cu-surf glass-copper nanocomposites, we observed 22% and 15% increase in transmission, respectively, at the pump intensity of about 1011 W/cm2. In Cu-surf, the copper nanoparticles at high density are concentrated in a 35 microns thick sub-surface layer, while in the Cu-bulk, and they are homogeneously distributed at much lower density over the 1mm thick glass plate. Since the number of nano-particles interacting with light is about the same in both samples, the femtosecond pump produces comparable changes in absorption coefficient. For the best of our knowledge, it is the strongest light-induced transmittance change in the glassy composites reported by now.
Although Cu-bulk and Cu-surf nano-composites were produced using the similar boron-silicate matrix, the measured relaxation times of the nonlinear transmission at λ = 560 nm differ considerably (about 5.3 ps and 8.7 ps, respectively). Such a difference in the relaxation time probably originates from different interface properties of copper nanoparticles in Cu-bulk and Cu-surf composites, which were manufactured using the phase decomposition of glass doped with copper in glass-melting process and ion exchange followed by reactive diffusion [17] techniques, respectively. The difference in the interface properties is probably responsible also for the different widths of SPR peaks observed in these composites. The effect of the glass matrix on the relaxation time one can observe by comparing Fig. 3 with results reported in [2, 4, 12]. Specifically, the different thermal conductivity of the glass matrix results in shorter (about 2.5 ps) relaxation time of the light-induced transmission in [2, 4, 12]. However, both our experimental findings and those reported in [4, 12] show that that relaxation of the optical excitation in copper composites takes place faster out of the plasmon resonance.
In Ag-surf nanocomposite, the threshold of the d → p interband transition in silver (310 nm) is shifted considerably from the SPR (400 nm). In such a condition the Coulomb interaction between hot electrons induces the shift of the SPR through the modification of the real part of silver permittivity [2]. However, in the Ag-surf, the obtained transmission change was lower than that demonstrated (see Fig. 3) at similar pump fluence for Cu-based nanocomposites. In Fig. 4, one can observe that spectral shift of the transmission minimum is accompanied with its broadening. The relaxation time, which is mainly determined by the electron-lattice interaction, was found to be about 1.3 ps. This value well corresponds to 1.4 ps reported in Ref. [2], however is somehow longer than that 0.8 ps obtained in Ref. [16].
Fig. 3. Temporal evolution of the nonlinear transmission spectra for Cu-bulk (a) and Cu-surf (b) nanocomposites at pump intensity of about 10 11 W/cm2.
Fig. 4. Temporal evolution of the nonlinear transmission spectra for Ag-surf nanocomposite at pump intensity of 1011 W/cm2.
Pump-probe measurements revealed that in the femtosecond time scale, the excitation of the conduction electrons in Cu- and Ag-based composites is accompanied with instantaneous decrease in the transmission (see also [4, 12–14]). This indicates that in these materials, the phenomenon of the reverse saturable absorption occurs, i. e. the higher intensity of the femtosecond pulses, the lower transmittance of the material. To investigate this phenomenon quantitatively, we performed transmission measurements in the Cu-bulk and Ag-surf nano-composites at 565 nm and 400 nm, respectively. The experimental setup is shown in Fig. 2(b).
Fig. 5. Transmitted fluence as a function of incident one for (a) Cu-bulk at λ = 565 nm and (b) Ag-surf at λ = 400 nm nanocomposites. Dashed lines correspond to the linear transmission.
From Fig. 5 one can observe that transmitted intensity is a nonlinear function of the intensity of the incident femtosecond pulse for both Cu-bulk and Ag-surf nanocomposites. Assuming that absorption coefficient α is a linear function of the light intensity I [18], α = α 0 + α 2 I, where α 0 and α 2 describe linear and nonlinear absorption, respectively, we obtained α 2≈ 1.2∙10-10 cm/W and α 2≈ 1.7∙10-10 cm/W for Ag-surf and Cu-bulk, respectively. This indicates that in the vicinity of plasmon resonance, instantaneous change of the absorption coefficient is of the same order in both Ag- and Cu-based nanocomposites. The obtained estimation is comparable with that observed in the pump-probe experiments at the same levels of light intensity.
It is important to note that in the femtosecond time scale, we observed only reverse saturable absorption. Our pump-probe measurements indicate that saturation of the optical absorption in glass-metal nanocomposites can be observed with longer optical pulses [8] when the decrease of the absorption coefficient due to thermal effects or internal electron emission dominates.
4. Conclusion
We demonstrate the record bleaching of the SPR absorption in glasses embedded with copper nanoparticles. Both bulk copper-doped glasses (copper ruby) and ion-exchange surface doped glasses showed about 20% increase of the relative transmission in the vicinity of the SPR in the pico-second time scale. In the femtosecond time scale, glasses doped with copper and silver nanoparticles shows reverse saturable absorption with nonlinear absorption coefficient of about α2∼10-10 cm/W. We expect that the developed technique will allow us to create in future other glass-metal nano-composites with strong and fast optical nonlinearity that will be prospective for non-linear optical and optoelectronic applications.
Acknowledgments
This work was supported by TEKES FinNano technology Program, Finish Academy (grants # 214678, 115781) and by RFBR (grant #06-02-81009). The authors thank Mr. D. A. Lyashenko for assistance in pump-probe measurements.
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