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Ultrafast Dynamics of Femtosecond Laser-Induced Shape Transformation of Silver Nanoparticles Embedded in Glass Armin Warth, Jens Lange, Heinrich Graener,† and Gerhard Seifert* Institut f€ur Physik, Martin-Luther-Universit€at, Hoher Weg 8, D-06120 Halle, Germany ABSTRACT: Metallic nanoparticles in a dielectric matrix can be transformed from spherical to nonspherical shapes by irradiation with intense femtosecond laser pulses. In particular, it has been previously shown that several hundred linearly polarized pulses can successively transform initially spherical Ag nanoparticles in glass into prolate spheroids with symmetry axes along the laser polarization. Using pulses of ∼150 fs duration at a wavelength of 400 nm, which is close to the surface plasmon resonance of the nanoparticles, this process works well for peak intensities in the range of ∼0.5 to 1.5 TW/cm2. We have now studied the ultrafast dynamics of this reshaping process in situ for each individual laser shot by help of an optimized femtosecond pump supercontinuum probe setup. This unique setup allows us to trace the transient spectral changes up to ∼1 ns after each pulse as well as the step-by-step evolution of the persisting changes of the extinction spectrum in the focal volume. The results of our investigation provide experimental evidence that the directed emission of electrons from the nanoparticles and their trapping in the glass matrix within the first few picoseconds after the 400 nm pump pulse are the key to explain the anisotropy of the reshaping process occurring on considerably slower time scales. In addition, the very prominent role of the first of several hundred laser shots is discussed.
1. INTRODUCTION Nanocomposites consisting of metallic nanoparticles (NPs) embedded in a dielectric matrix experience considerable interest in the scientific community because of their characteristic linear and nonlinear optical properties, which can be tailored in a wide range of parameters by gaining control over spatial distribution, size, and, in particular, shape of the metal inclusions.1,2 A very special and flexible way to manipulate the shape of particles in a well-defined area is using ultrashort laser pulses. This has been previously shown on the example of Ag nanoparticles in glass: upon irradiation with intense femtosecond pulses at wavelengths close to their surface plasmon resonance (SPR), initially spherical nanoparticles can be persistently transformed to spheroids.3,4 Controlling polarization state, peak intensity, and number of laser pulses applied to one spot, it is possible to prepare prolate as well as oblate spheroids with tailored aspect ratio and deliberately chosen, uniform orientation of the particles’ symmetry axes.5,6 Whereas much of the information about the ultrafast dynamics of the processes responsible for reshaping has been obtained during the last years by several ex situ experimental techniques,7,8 in situ studies in the femto- to nanosecond time range are still lacking. Femtosecond pump probe spectroscopy, particularly when spectrally very broad, so-called supercontinuum probe pulses are utilized,9 11 appears to be very well suited to close this gap. However, these experiments are usually conducted in the weak excitation limit; that is, reversible processes are investigated.12,13 There, averaging over many individual laser shots enables high-precision measurements. State of the art in this field is the readout of complete probe spectra at 1 kHz r 2011 American Chemical Society
repetition rate combined with a carefully designed optical layout, giving rise to an experimental accuracy of ∼2 10 5 for determination of changes in the optical density (OD).14 For irreversible processes like the shot-by-shot reshaping of nanoparticles under discussion here, a large amount of averaging is discouraged because every pump pulse changes the irradiated spot of the sample permanently. Other than in liquids, which can be circulated through a flow cell providing a “fresh” sample volume for each pump probe pair,15 this requires a new sample position for each individual laser shot. In this work, we report on an investigation of the in situ ultrafast dynamics of laser-induced reshaping of Ag nanoparticles in glass. For this purpose, we used a femtosecond pump supercontinuum probe setup similar to the one described, for example, in ref 9, which has been optimized to provide the necessary single-shot precision. We have studied in particular the multishot irradiation regime, where 500 1000 pulses on one spot yield prolate spheroids with their symmetry axis being oriented along the (linear) laser polarization. The multishot regime allows generation of relatively large aspect ratios and is of more relevance for applications. We were able to derive the transient (time range up to ∼1 ns after pump pulse) as well as the permanent (10 ms after pump pulse) spectral changes individually for each of the up to 1000 transforming pulses. This extensive data material is discussed on the basis of an empirical evaluation Received: September 21, 2011 Revised: October 19, 2011 Published: October 19, 2011 23329
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The Journal of Physical Chemistry C of the ultrafast dynamics observed in the transient spectral changes. This paper is organized as follows: Chapter 2 describes sample preparation, pump probe setup and the complex data acquisition procedure. After a brief overview of the conclusions from previous (ex situ) studies in Section 3, we present a selection of novel in situ results in the order: persistent spectral changes (Section 4.1), transient changes after first laser shot (Section 4.2), and evolution of transient changes during the reshaping procedure (Section 4.3). In the following discussion section (Section 5) we merge the previous state of knowledge with the new results to point out the special role of the first laser shot as well as the remarkably delayed buildup of anisotropy. The work is completed by a brief conclusion.
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Figure 1. Schematic sketch of pump-supercontinuum probe setup; P: polarizer, OC: optical chopper.
2. EXPERIMENTAL SECTION 2.1. Preparation of Ag-Glass Nanocomposites. The samples used in this study were prepared from soda-lime float glass (72.5 SiO2, 14.4 Na2 O, 0.7 K2O, 6.1 CaO, 4.0 MgO, 1.5 Al2O3, 0.1 Fe2O3, 0.1 MnO, and 0.4 SO3 in wt %), which was subjected to an Ag+ Na+ ion-exchange process in a mixed melt of AgNO3 and KNO3 at 400 °C. Concentration and distribution of Ag+ ions in the glass depend on the time of ion-exchange and the weight concentration of AgNO3 in the melt. Subsequent thermal annealing of the ion-exchanged glass in a H2 atmosphere, typically at 400 450 °C, then resulted in reduction of all implanted Ag+ ions, followed by formation of spherical silver nanoparticles; parameters were chosen such that nanoparticles with a gradient of sizes and concentration precipitated in thin surface layers of ∼6 μm thickness of the glass samples (total thickness of each glass plate was 1.2 mm). Finally, the glasses were etched in a 12% HF acid. Etching was pursued for relatively short time (∼60 s) on one side of the sample to remove the uppermost particle layer containing comparably high concentration of smaller NPs (∼10 nm diameter); the backside of each sample was etched for much longer time to remove all NPs at this side. This procedure results in single-sided samples containing Ag NPs of 30 40 nm mean diameter in the uppermost 1 to 2 μm below the surface at a concentration (volume fill factor) of ∼10 3. The pertinent peak extinction at the surface Plasmon resonance (∼410 nm wavelength) lies between OD = 1 and 2, which is an important prerequisite to conduct the pump probe experiments with satisfying signal-to-noise ratio. First, this causes rather large, well-detectable spectral changes and sufficiently large probe transmission also in the SPR spectral region; second, the unavoidable strong scattering of metallic NPs is kept at an acceptable level by this choice. 2.2. Pump Probe Experimental Setup. The setup for the femtosecond pump supercontinuum probe experiments is based on a Ti/sapphire laser delivering 150 fs pulses at up to 1 kHz repetition rate; a schematic overview is given in Figure 1: The frequency doubled output at λ = 400 nm with up to 300 μJ pulse energy is split by 80:20. The larger part is used as pump pulse, whereas the smaller part is focused into a 3 mm sapphire plate to generate the spectrally very broad (λ ≈ 300 600 nm) probe pulse. The pump pulse is attenuated by a half-wave plate/ polarizer combination, and then focused to the sample with an f = 300 mm lens. For the experiments in this work, the peak energy density was adjusted to ∼0.1 J/cm2 (multishot regime of NP shape transformation). The probe pulses, which can be delayed up to ∼1 ns after the pump, are split into two equal parts.
Figure 2. Transient spectral changes ΔOD of Ag NPs in glass, at delay time Δt = 1.4 ps; red, dashed curve: original data of a single shot; black solid curve: average of 20 laser shots, corrected for scattering.
The spectral intensities of both parts are monitored individually by help of two identical grating spectrometers with 512-element Si detector arrays (corrected for dark current). The beam coupled out before the sample represents the reference intensity Iref, whereas the other part passes the sample and provides the signal intensity ISig. Therefore, a complete transmission spectrum T(λ) = ISig(λ)/Iref(λ) is obtained for each individual laser shot regarding a baseline correction given by the beam splitter characteristics. Using a synchronized optical chopper and a computer-controlled shutter, only individual pump pulses are admitted to hit the sample. The ratio of transmission of two selected, successive laser shots with and without pump pulse then yields the change of optical density of the sample at a well-defined delay time Δt: ΔOD(λ,Δt) = log{T0(λ)/TPump(λ,Δt)}. All obtained ultrafast spectral data have to be corrected for the chirp of the broadband pulses to ensure a reliable zero delay time for all spectral positions. This has been done using the ultrafast nonlinear response of methanol, as described in detail in ref 9. A further correction of the transient spectra is required owing to the strong scattering intensity around λ ≈ 400 nm, which has typically the same order of magnitude as the wanted transient signals. (For an example, see Figure 2.) This correction is derived from ΔOD measured at negative delay times, where the probe pulse actually monitors the sample spectrum before pump interaction. Because of the slow detection system, however, this signal contains the scattered pump intensity, too. It should be mentioned that all data, unless specified separately, have been recorded with parallel polarization of pump and probe pulses. 2.3. Data Acquisition Procedure. The typical measuring sequence for multishot NP shape transformation by n (we studied here up to n = 1000) pump pulses goes as follows: an initial pair 23330
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The Journal of Physical Chemistry C of laser shots first without, then with pump gives ΔOD(k=1) (λ;Δt); n such pairs are registered continuously, providing the transient ΔOD(k) at the selected delay time for each individual intense pump pulse (multipulse shape transformation at half the laser repetition rate) at one sample position. The baseline T0(k)(λ) of every shot k = 2...n represents a very long delay time g1 ms after the preceding pump. At this time, the focal volume is in its new equilibrium state (apart from a very small signal that lasts for days due to color centers), so that the quasi-persisting optical changes induced by pump pulse (k 1) can be determined: ΔODP(k 1) = log{T0(k 1)/T0(k)}. Adding these persisting changes, the shot-to-shot evolution of the sample extinction spectra can be followed. When this evolution is analyzed, it has to be regarded that due to the Gaussian shape of the pump beam the probe averages over varying pump intensities; that is, the measured changes are usually considerably smaller than expected in the center of the irradiated spot. The whole cycle is then repeated at a different, “fresh” sample spot addressed by moving the sample on a precise motorized translation stage within the focal plane; each of usually 50 100 different delay times is repeated 10 20 times to allow for some averaging. Therefore, a typical data set requires ∼1000 spots with ideally identical initial conditions regarding sample parameters, focal position, and pump probe overlap. The sample positions are chosen statistically to average out the residual sample inhomogeneity. The resulting data have a typical noise of ΔOD e 3 10 3. (The accuracy depends on λ due to spectral characteristics of the generated supercontinuum.) The effect of averaging and scattering correction is demonstrated by the data presented in Figure 2: whereas the red, dashed curve shows a transient spectrum of ΔOD (Δt = 1.4 ps) without scattering correction determined from a single pump probe pair, the black solid line gives results for the same delay time after averaging over 20 shots and after subtraction of the scattering spectrum obtained as average of the registered negative delay times at Δt < 1 ps. The benefit of the described sophisticated procedure is a large amount of dynamical information obtained in situ during the NP reshaping procedure: (i) first, the transient spectral changes up to 1 ns after the excitation pulse are monitored individually for each of up to 1000 successive laser shots; (ii) second, the shot-by-shot development of the extinction spectrum is measured; (iii) third, burt not least, the modification of the scattering intensity from shot to shot can be traced by analyzing the spectra for considerably negative delay times.
3. SUMMARY OF PREVIOUS EX SITU STUDIES Before we present new results, it appears necessary to give a brief summary of the conclusions from our previous work on the cascade of ultrafast processes involved in the femtosecond laserinduced nanoparticle reshaping mechanism. As detailed in our recent review chapter,16 the fastest process, being also decisive for the final anisotropy, is directional photoionization of the Ag nanoparticles along the laser polarization. This electron emission is assisted by SPR field enhancement at the NP surface and the very high electron temperature of ∼104 K and more, which is created already within the pulse duration. The next slower processes (time scale: several picoseconds) are (i) trapping of part of the electrons in the glass matrix and (ii) heating of the Ag lattice by electron phonon coupling, followed by thermal (isotropic) emission of Ag+ ions (Coulomb explosion). The latter provides a very effective heat transfer to the first matrix shell
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Figure 3. Persisting spectral changes during in total 1000 laser shots. (a) Relative changes of OD at long delay times between individual shots, as specified in legend, and (b) absolute change of OD after n shots, with respect to initial spectrum.
(thickness of few nanometers) but additionally leads to recombination of electrons and ions to Ag atoms, favorably in the positions to which more electrons had been emitted. The very hot NP (∼2000 K) and the surrounding hot shell (up to 1000 K) then start to cool by heat conduction into the glass on a much slower time scale (up to nanoseconds). This sets a time window of several hundred picoseconds, in which Ag atoms can diffuse to and precipitate at the poles of the Ag NPs (symmetry axis defined by laser polarization), whereas Ag+ ions around the equator are partially remaining in the matrix. Repeating this procedure several hundred times finally creates the elongated, spheroidal shapes of the NPs and a “halo” of very small particles (2 to 3 nm) distributed in the matrix, in the discussed situation mainly around the NP equator. Because of the created, uniformly oriented silver nanospheroids, the extinction spectra have become dichroic after irradiation: with light polarized parallel to the NP symmetry axis, one observes a broad red-shifted band (typically at λpeak = 450 500 nm) associated with the SPR of the longer particle axis, whereas for the perpendicular polarization a slightly blue-shifted band (λpeak = 390 400 nm) due to the short particle axis occurs. Also, the original, isotropic SPR band at ∼410 nm shows still considerable extinction indicating a substantial percentage of unchanged NPs, the actual amount of which depends on irradiation parameters as well as individual sample composition. In our previous ex situ studies, we annealed the samples after irradiation, typically at 150 °C for 1 h. This removes color centers and other laser-related defects and, apparently by removing local charges from the NP surroundings, slightly modifies the newly occurred SPR bands of the spheroidal NPs.7 During irradiation by n pulses, however, the situation was absolutely identical to the experiments reported in this work.
4. RESULTS 4.1. Persisting Spectral Changes as a Function of Shot Number n. We start the presentation of new results with the
spectral changes obtained for very long delay times (10 ms), showing the evolution of laser-induced persistent changes from 23331
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Figure 4. Relative scattering intensity at λ = 400 nm as a function of laser shot number n; initial value (n = 1) given also as dashed-dotted horizontal line.
shot to shot. This is a prerequisite for discussion of the short-time dynamics because the changes ΔOD(λ) apparently have to be correlated with the pertinent initial extinction spectrum OD(λ) of each individual shot n. The development of the persistent OD changes (ΔODP) of a sample induced by 1000 successive femtosecond laser pulses is illustrated by some selected examples in Figure 3: the upper panel (Figure 3a) shows the relative changes induced by the very first pulse (n = 1) and three examples for the further development up to n = 10. Quite obviously the first pulse causes a significant change of the SPR at 412 nm (black solid curve), which can be recognized as a decrease in extinction accompanied by broadening in the wings. In contrast, the following nine pulses leave spectrally different changes (only extinction increase around 425 nm) with amplitude more than an order of magnitude smaller than the first one. The complete shot-to-shot evolution, now shown in Figure 3b as total difference to the initial spectrum, makes clear that the effect of the first laser pulse is in fact a very special one: in the further course of the irradiation, typically 50 100 pulses are required to achieve similarly large changes to OD as observed for n = 1 alone. Apart from this peculiarity, the multishot behavior matches the expectation that Ag NPs are transformed to prolate spheroidal shape: the extinction of the original SPR band at 412 nm decreases, whereas the SPR of the long particle axes causes the slowly increasing extinction at longer wavelengths, ending up after 1000 pulses at a peak position of ∼470 nm. This position suggests an average aspect ratio of c/a ≈ 1.5 of the transformed NPs (estimated by Mie theory for spheroids, see ref 16). The main changes are already achieved after 500 pulses; obviously the deformation process comes to a more-or-less saturated state later on. This saturation does, however, not mean that all NPs have been transformed: a spectrum from the center of the irradiated area (not shown) yielded a residual SPR band at 412 nm with peak extinction of OD ≈ 0.5, whereas the peak value at 470 nm is OD ≈ 0.3. Finally, control experiments with orthogonal polarization showed that the changes for n = 1 are isotropic within experimental accuracy, whereas only later on dichroism step-by-step arises. Interestingly, we observed a similarly prominent role of the first shot also in the measured scattering intensities (the peak value of which is located at the pump wavelength of λ = 400 nm) traced as a function of pulse number, as shown in a normalized representation in Figure 4: from the initial value of the pristine sample (solid circle at n = 0, and dashed-dotted horizontal line) the scattered intensity jumps to a considerably larger value after the first pump pulse. From then on, the scattering step-by-step
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Figure 5. Ultrafast dynamics (short delay time range) of ΔOD at λ = 447 nm after an intense pump pulse at λPu = 400 nm, for a typical sample containing Ag nanoparticles (black squares) and a comparable glass substrate without particles (red circles).
decreases again, crossing the initial value at n ≈ 60, and then reaches a fairly stable state after ∼200 pulses, clearly below the initial value. The slow decrease from n = 1 on is easily understandable because it mainly mirrors the changes of OD presented in Figure 3: it is well known, for example, from Mie’s theory, that metallic nanoparticles cause a strong resonant scattering related to their SPR; because we observed a decrease in the SPR peaked at 412 nm within increasing number of laser shots, it is obvious that the reshaping of the NPs to spheroids and the concomitant shift of their (long-axis) SPR to longer wavelengths is the reason for the observed decrease in scattered light at λ = 400 nm. The explanation for the large increase found after the first laser shot, however, remains a point for the discussion section, as well as the outstanding effect of the first pulse on the extinction spectra. It is apparent, though, that these effects are caused by matrix rather than NP modification because their following shape transformation occurs in much smaller steps. 4.2. Transient Spectral Changes: First Laser Shot. Accommodating to the fact that only the initial excitation pulse interacts with a pristine sample, we will now discuss the transient ultrafast dynamics measured by help of the very first pump probe pulse pair. A first important point regarding the information content of time traces derived from transient data is seen in Figure 5: the black solid squares represent a time trace for a fixed wavelength at very early delay times from a typical sample with Ag NPs, whereas the red open circles refer to the same spectral position obtained from a pure glass sheet of the same type and thickness. Clearly the signals around zero delay (from Δt ≈ 200 to +200 fs) in pure glass represent a coherent (electronic) ultrafast response that is present only for the time of pump probe temporal overlap. Apparently the same signal is present in the ΔOD obtained on the sample containing nanoparticles, overlapping with the actual signal from the particle layer; the blue solid line, referring to an interpolated difference of the two data sets, shows the tentative “real” time trace for ΔOD, corrected for the glass background. Such a correction might be erroneous due to the well-known large χ(3) of metal-glass nanocomposites around its SPR; therefore, in this work we will not discuss in detail the delay time range where the pump probe correlation is not negligible. Fortunately, as will be seen below, the observable ultrafast ΔOD contributions related to the NP shape transformation are occurring on the picosecond rather than femtosecond time scale so that the exclusion of the time interval around zero does not mean a loss of information. 23332
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Figure 7. Time dependence of spectral changes, given in parametrized form: (a) band areas of bleaching of surface plasmon resonance (lower panel, red circles) and red-shifted absorption increase (upper panel, black solid squares) and (b) peak position of absorption increase A450. (See the text for details.)
Figure 6. Ultrafast dynamics of ΔOD after the first pump pulse (n = 1). (a) Overview of transient spectral changes during the first 30 ps. (b) Full delay time range of transients at two selected spectral positions, short time range of the same data given as inset.
An overview of the spectral changes occurring within the first 30 ps after excitation is given in the 3D plot in Figure 6a. The most prominent effects are (i) a rather fast bleaching of the SPR at 412 nm with a peak at Δt ≈ 0.5 ps, which decays afterward on a slower time scale of several picoseconds, and (ii) an increased optical density at the long-wavelength side of the SPR, which reaches a maximum considerably later and then also starts to decay. Also clearly recognizable is an induced extinction at ∼340 nm showing lower peak amplitude and a time evolution similar to the SPR bleaching. Finally, albeit hardly visible in the smoothed 3D plot, the strong bleaching hosts a narrow positive contribution on its short-wavelength edge at λ = ∼403 nm. In Figure 6b, two selected time traces (referring to the largest extinction increases on short- and long-wavelength sides of the SPR, at λ = 340 and 460 nm) are shown on an extended delay time scale up to 700 ps to indicate exemplarily the full temporal evolution of ΔOD: whereas the first few picoseconds (given as inset) exhibit individual differences regarding signal rise time and peak position, the two shown curves (and similarly any other position with considerable ΔOD) are decaying after their maxima first very quickly until, around Δt = 15 20 ps, a much slower decay on a time scale of g100 ps starts, which is not yet finished even after 700 ps. Such transients at a fixed spectral position may directly be used for further discussion of the dynamics behind the pump probe data in simple cases, where only a single physical process contributes at the respective wavelength. In the present case, however, one has to find a way to disentangle several potentially overlapping effects because it is well known from similar experiments at considerably lower excitation intensity that the
increased electron temperature induced by the pump pulses causes dynamical bleaching, red shift, and broadening of the SPR.17 19 The time evolution of such effects is apparently not reproduced correctly by the measured transients at a single wavelength because, for instance, red shift and bleaching might compensate each other. Therefore, we decided to parametrize the data empirically by performing least-mean-squares fits using a sum of in total four spectral bands so that the dynamics can then be discussed more reliably by means of the parameters of these bands. The idea is that these four bands (three of Lorentzian and one of Gaussian shape) are related to the four above-mentioned signal components, and each of them represents a single physical process. Gaussian shape was providing better fit results for the band at λ = 340 nm, whereas the others, as expected for surface plasmon related bands, can be explained very well by Lorentzian bands. We abbreviate the four components by one capital letter specifying if it is absorption increase (“A”) or bleaching (“B”) and an index giving the approximate wavelength position (in nanometers) of the individual band at early delay times (∼3 ps). In that nomenclature, the transient spectra consist essentially of B412, A340, and A450. The weak and narrow A403 will not be discussed here in detail because it might be an artifact caused by an imperfect correction of the much larger and probably transiently modified scattering signal at this spectral position; nonetheless, the band was included in the fitting routine to stabilize the resulting fit parameters. Also, fit results of A340 will not be shown separately because they fully confirm the behavior at 340 nm already presented in Figure 6b. For the two SPRrelated bands, B412 and A450, the band areas (product of amplitude and bandwidth) are compared in Figure 7a. In addition, for A450, also the central wavelength is given (Figure 7b) because it is strongly varying with time, whereas in the case of B412 it remains constant within experimental accuracy. We start to assess the results with a coarse analysis of the ultrafast dynamics of the three major spectral components: 23333
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Figure 8. Evolution of transient spectra at a delay time of Δt = 5 ps as function of shot number n (as specified in the Figure); the two vertical dashed-dotted lines mark the wavelength positions of the dichroism data presented below (in Figure 9).
(i) The UV absorption A340 rises very fast around zero delay time, reaching a maximum slightly delayed at Δt ≈ 400 fs; the following two-step decay can with satisfying accuracy be explained by a biexponential decay with the time constants τ1 = 1.8 ( 0.5 ps and τ2 = 150 ( 20 ps. (ii) The bleaching of the original SPR (B412) rises in the same way as A340, but the subsequent decay is not simply biexponential; nonetheless, biexponential fits were performed to get an estimate for the typical decay times, which turned out to be ∼7 ps and 150 200 ps for fast and slow component, respectively. (iii) Roughly the same decay behavior is found for A450, but this extinction band rises considerably slower, exhibiting its maximum around Δt = 2.5 ps. The information is completed by the band position of A450: the wavelength λpeak (Figure 7b) reaches its largest value slightly above 450 nm already after 0.4 ps, remains at this plateau until Δt = 2 ps, and then shifts back within ∼20 ps to ∼425 nm. Overall, the behavior of B412 and A450 comprises the transient changes of the SPR of the Ag NPs in the sample in reaction to the very intense excitation. It should be mentioned that we also tried to explain the spectral changes due to the nanoparticles’ SPR by bleaching, red shift, and broadening in the form of just one Lorentzian band with transiently changing parameters. This approach failed, indicating that not all NPs throughout the depth of the particle layer are reacting similarly; apparently the strong extinction of the uppermost NPs is partially screening those ones located deeper below the surface from the laser pulse intensity so that the probe pulses average over various levels of excitation down to the weak excitation limit, and part of the NPs do not experience observable SPR changes at all. This question will be resumed in the discussion section. 4.3. Transient Spectral Changes: Evolution with Laser Shot Number n. To complete the overview of results, we will now look briefly into the dynamical results obtained for all of the following laser shots. In view of the vast amount of data, one has to select very few but significant examples to characterize the most prominent changes occurring in the course of several hundred pulses, which step by step cause the reshaping of the Ag NPs. As we have seen in the previous section, the UV absorption A340 and the SPR bleaching A412 are found at a fixed wavelength position and can easily be separated. In contrast, A450 shows a dynamically moving position, and the shot by shot growing extinction at ∼470 nm (compare Figure 3) allows one to expect further effects on the long wavelength side of the initial
Figure 9. Shot-to-shot evolution of the transient extinction changes observed at a delay time of Δt = 5 ps at wavelengths of (a) λ = 391 nm and (b) λ = 427 nm; blue squares (red circles) refer to parallel (perpendicular) polarization of pump and probe pulses.
SPR. Therefore, we chose to present the shot-to-shot evolution for the delay time of Δt = 5 ps, where ΔOD in the wavelength range λ g ∼425 nm shows on average its largest amplitudes, whereas the other effects at shorter wavelengths are still clearly visible. A selection of characteristic transient spectra at this delay time up to n = 500 laser shots is given in Figure 8. The first important finding is that the prominent persisting changes due to the first laser shot are also reflected in the transient behavior: whereas the amplitude of bleaching (B412) reduces from ΔOD ≈ 0.32 at n = 1 to ΔOD ≈ 0.28 at n = 2, it is afterward changing much more slowly (e.g., still ΔOD ≈ 0.28 at n = 10), ending up at ΔOD ≈ 0.13 at n = 500. A similar evolution is seen for the long-wavelength extinction increase (A450), which experiences a considerable decrease from first to second shot and then very slowly becomes flatter and slightly broader during the first 100 pulses; later on, as clearly visible at n = 200, a valley located at λ = 470 nm occurs, separating finally (at n = 500) two regions of positive ΔOD at λ ≈ 440 and 525 nm. Taking into account the “long-time” spectra (Figure 3), we can understand the valley as bleaching of the long axis SPR band of the newly created elongated Ag NPs and the extinction increase around λ ≈ 525 nm (by analogy to the initial situation at n = 1) as being due to the transient changes of this SPR in reaction to the strong pump pulse. In contrast with these SPR-related changes, the transient short-wavelength extinction increase A340 does actually not change at all during the 500 laser shots. This statement holds not only for the amplitude after 5 ps but also for its temporal decay (as shown in Figure 6b for n = 1). The slightly smaller amplitude visible at n = 500 (Figure 8) is probably caused by the strongly reduced, overlapping SPR changes. In general, we may conclude that the UV extinction increase is an effect that is not noticeably modified by the NP shape change. That might be different for the small bleaching peak around λ = 393 nm, where a contribution is expected from the short axis SPR of the NPs being reshaped to spheroids shot-by-shot. To get more details about this, we have conducted the experiment again, only now with crossed polarization of pump and probe. The comparison of the two measurements then provides the development 23334
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The Journal of Physical Chemistry C of dichroism during the whole irradiation time, as shown in Figure 9 for two selected wavelengths (marked in Figure 8 by vertical, dashed-dotted lines): in Figure 9a, the result at λ = 391 nm is presented, whereas Figure 9b (referring to λ = 427 nm) details the effects on the long-wavelength side, which is related to the long axis SPR. In both cases, blue squares (red circles) symbolize the data obtained with parallel (perpendicular) polarization of probe and pump pulses. An in-depth discussion of the very instructive data appears reasonable only at the end of the next section, when all arguments are prepared. For the moment, we notice that the transient dynamics starts isotropic within experimental accuracy, and a considerable dichroism is only developed from n g 2 on. It is also distinctive that the dichroism observed at λ = 430 nm is reversed compared with the situation at λ = 390 nm. This corresponds qualitatively well to the fact that the two selected wavelengths represent positions, where effects from either long or short axis of reshaped NPs are being monitored, which are apparently sensitive to the appropriate linear polarization only.
5. INTERPRETATION AND DISCUSSION We start the interpretation of this in situ investigation of femtosecond laser-induced Ag NP shape transformation with the important novel finding of a special role of the very first laser shot, which could not be obtained from all previous ex situ studies. As clearly indicated by the above-shown results, in particular, Figures 3 and 4, charged particles (electrons and Ag+ ions) emitted in the matrix and remaining there for at least a few milliseconds (until the next pulse hits the sample) make the difference after the first shot. If the concomitant extinction and scattering changes would be due to a significant size or shape change of the NPs, then there is no argument why the second and following shots should not create modifications of comparable magnitude. Therefore, it is believed that the inclusion of charged particles into and the ultrafast heating and cooling of the first matrix shell around the NPs changes the optical properties of the nanocomposite in the observed way. Possible mechanisms are a change of the refractive index and a modification of the electric field enhancement at the NP glass interface, probably in combination of both effects. Having this in mind, we can now discuss the ultrafast dynamics monitored in the pump probe experiments. We start with the transient absorption centered around ∼340 nm, which shows, actually independent of the shot number, a fast increase, followed by fast (∼2 ps) and slow (∼150 ps) decay. The comparative experiment on pure glass yielded in this spectral region also only a coherent signal (similar to the one shown in Figure 5) but within experimental accuracy no extinction changes at later delay times Δt > 300 fs. Thus the signal is definitely correlated to the presence of metal nanoparticles. Two possible interpretations are suggested by the spectral position: (i) It has been observed previously that intense femtosecond irradiation of glasses can create color centers, in particular, electron traps, which cause long-lived absorption in the spectral region between 300 and 400 nm.20 22 Although we are using lower peak intensity in our case (no macroscopic changes observed on pure glass), the field enhancement close to the NPs or the electrons emitted from the NPs might transiently create comparable color centers in their immediate vicinity; because of the very small volume fraction of this region (few nanometer shell around NPs at 10 3 volume fraction in a 1 to 2 μm layer) the metastable part of this electron
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trap will not lead to a measurable extinction but could be the reason for the changes of the SPR band. (ii) The other possible explanation is a transient modification of the interband transitions in the Ag NPs, which show a small but distinct contribution around ∼3.5 eV (350 nm):23,24 because electrons are ejected from the NP and the electron temperature in the NP becomes very high, an increased extinction by way of either depleted upper states of interband transitions or an increased transition dipole moment are plausible explanations for the transient increase and decay of the extinction around λ ≈ 340 nm. Even the much stronger main interband transition at Ωib ≈ 4 eV (∼310 nm) might be the source of the changes because it has been shown that already at much lower intensities the change of the electron distribution leads to a transient decrease in transmission on the low-frequency side of Ωib.17 So we probably observe the longwavelength wing of the pertinent effect, cut off from further visibility toward short wavelengths by the much stronger glass absorption. Tentatively, we are favoring the latter interpretation, but of course the data of the current study alone do not provide clear evidence of this point. Next, the transient changes to the SPR have to be discussed, which show a considerable development with increasing shot number. The ultrafast dynamics is, in general, compatible to what has been known for more than 10 years about the ultrafast changes to SPR bands of Ag NPs after comparably weak excitation by resonant femtoseconds pulses:17 19 the absorbed laser energy is converted from a nonequilibrium to a thermalized, hot electron system within a few hundred femtoseconds, observable as a red shift and broadening of the SPR extinction. As long as the increase in electron temperatures remains below ΔTe ≈ 300 K, the optical changes decay by electron phonon coupling within a few picoseconds back to the equilibrium. With increasing laser intensity, the buildup of a hot Fermi distribution becomes even faster, whereas the cooling is slowed down.17 When after electron phonon equilibration the NP as a whole is considerably heated, an even slower decay of the optical changes limited by the heat conduction into the dielectric matrix is observed. We were able to show that for intensities like the ones used in the present study electron temperatures of Te ≈ 104 K are reached, equilibrating with the lattice within 20 40 ps at a lattice temperature of Tl ≈ 2000 K; further cooling, starting efficiently with emission of Ag+ ions, then takes typically 1 ns to arrive at a temperature of 600 K for the NP and the surrounding matrix shell (∼5 nm extension).25,26 These findings correspond well with the observations (Figures 6 and 7): bleaching of the original SPR (B412) as well as the red-shifted increase in OD (A450) are decaying on a several picoseconds time scale first, then around Δt = 15 20 ps enter into a much slower decay, which is by no means finished after 700 ps. The delayed increase in A450 as compared with B412 indicates additional processes beyond the effects of hot electrons in our high-intensity case. The two most straightforward ideas to explain the slower buildup of A450 are: (i) heating of the lattice (within 2 to 3 ps a temperature of Tl ≈ 1000 K can be reached, which apparently may influence the SPR band via strongly increased electron phonon scattering) and (ii) charges (electrons) emitted from the NP, which may affect the SPR by their electric field once they are trapped in the matrix. The evolution of all of these effects as a function of shot number n, as shown exemplarily for Δt = 5 ps in Figure 8, mostly follows the expectation of step-by-step NP shape transformation: With the decrease in extinction at 412 nm, the concomitant transient effects lose amplitude, whereas after ∼200 pulses a 23335
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The Journal of Physical Chemistry C similar, red-shifted pair of ΔOD effects due to the SPR of the long axis of the already elongated NPs comes up. The latter exhibit much smaller amplitude, an obvious consequence of the fact that the pump wavelength 400 nm is getting more and more out of resonance of the long axis SPR with increasing shot number n. The dynamics (temporal variation of ΔOD) of all SPR-related effects is similar and fairly independent of n. As the comparison to the permanent optical changes (“long-time” ΔOD; Figure 3) shows, the persisting effects are, in general, much smaller than the amplitudes of ΔOD after Δt = 700 ps; that is, the decay to this state takes at least several nanoseconds. It is also apparent that the amplitude decrease in the transient changes as well leads to a decrease in the permanent changes with growing n. In particular, one observes a saturation behavior, that is, almost no further persisting modification of OD between n = 500 and 1000, which leaves a remarkably high percentage (30 40%) of the NPs obviously unchanged. This may be partially due to geometrical averaging because also peripheral regions of the Gaussian pump beam are being monitored by the probe, where the peak intensity is insufficient for permanent changes. However, it has been shown already in previous studies that in this type of samples not all NPs can be shape-transformed, and even longer irradiation with, for instance, n > 2000 pulses does only lead to partial destruction of NPs.6 Possible reasons for this behavior are either a considerable part of relatively small NPs in the size distribution, which need higher peak intensities to be transformed, or the strong absorption and scattering of the uppermost NP layer, which “screens” the particles located further away from the sample surface from the peak intensity required for reshaping. Finally, we must discuss the dichroism of the transient dynamical OD changes as a function of n, as shown exemplarily in Figure 9. To understand the shot-to-shot evolution visible in Figure 9b, we must recall that at 427 nm we observe for the first shot a transient extinction increase due to red shift and broadening of the original SPR band of spherical Ag NPs. With increasing shot number n, NPs are successively reshaped to spheroids with growing aspect ratio, oriented uniformly along the pump polarization; this will create an additional SPR with growing peak wavelength, which can be transiently bleached by the pump and which is only seen by the probe pulses when they are polarized parallel to the pump. In this picture, we can understand the observation: along with the SPR of reshaped NPs, which moves shot-by-shot to longer wavelengths, its transient bleaching can increasingly (over)compensate the initial extinction increase; that is why the signal for parallel polarization drops to zero at n = 30 and reaches a negative maximum between n = 100 and 150. The partial recovery of ΔOD back to zero later on indicates that the SPR of reshaped particles on average moves further to the red; that is, the NPs are still being elongated. In contrast, the signal measured for perpendicular polarization is not at all sensitive to the excited long axis of the shapetransformed NPs and therefore remains owing to the chosen wavelength position more or less constant throughout the whole processing time. The situation is somehow different at 391 nm (Figure 9a), where initially the wing of SPR bleaching of the pristine sample is observed. The amplitude of this bleaching is considerably increased at n = 2 in full accordance with the prominent changes to OD by the first laser shot. From then on, dichroism occurs: for parallel polarization, the growing red shift of the long-axis SPR of successively reshaped particles apparently removes their contribution step-by-step from the
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bleaching signal at 391 nm. In contrast, the bleaching observed for perpendicular probe polarization even grows in amplitude. This indicates that the short-axis SPR of the elongating NPs contributes more and more to the signal at 391 nm; although the long-axis SPR is excited by the pump, the concomitantly higher electron temperature will of course also affect (bleach) the shortaxis SPR. To finish up this point, we want to stress that the dynamical changes of the SPR do not show any anisotropy within experimental accuracy for the first two laser shots. So it is apparently not the response of the surface plasmon itself that leads to anisotropy and following shape change of NPs, but in fact the directional emission and trapping of electrons into the glass matrix initiates the shape transformation developing during the further irradiation steps.
6. CONCLUSIONS In summary, we have investigated in situ the persisting shape transformation of initially spherical Ag nanoparticles in glass to uniformly oriented spheroids, which is induced by 1000 successive, intense femtosecond laser pulses, with a femtosecond pump supercontinuum probe setup providing the ultrafast dynamics for each individual laser shot. The results show clearly that the very first laser pulse (n = 1) applied for this multishot processing creates the strongest but within experimental accuracy still isotropic optical changes due to emission of electrons and Ag ions from the NPs into the glass matrix. The pertinent changes of extinction and scattering are followed for n g 2 by much smaller stepwise permanent modifications leading to dichroism and spectral shifts due to the step-by-step formation of nanospheroids via emission of electrons and Ag+ ions, their recombination and precipitation at the NP poles. The dynamics (first nanosecond after excitation) spectrally reflects the shot-toshot development of the persisting spectral changes, but its time dependence does not change noticeably during the whole processing time: all transient changes related to SPR (observed at wavelengths above λ ≈ 400 nm) reach their maximum within 2.5 ps or less, then decay with a fast component (related to electron and ion emission as well as electron phonon coupling) until, after 15 20 ps, a much slower decay on the time scale of several hundred picoseconds remains, which is apparently related to heat conduction and the actual shape transformation processes like recombination and diffusion. An additional, previously unknown transient extinction increase around λ = 340 nm exhibits an even faster initial decay within a few picoseconds, which then also passes over into a slow decrease lasting for several hundred picoseconds at least. That effect is assigned to the emission of electrons from Ag NPs, optically active either via short-lived color centers or indirectly by pertinent modification of the nanoparticle interband transitions. As a whole, the in situ results presented here fully confirm and in several aspects complete the previous picture (obtained by ex situ studies) of Ag NP shape transformation induced by femtosecond laser irradiation. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Universit€at Hamburg, D-20146 Hamburg, Germany
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The Journal of Physical Chemistry C
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’ ACKNOWLEDGMENT We are very grateful to N.P. Ernsting for the good cooperation concerning the further development of the femtoseconds pump supercontinuum probe setup used in this study. We also thank CODIXX AG in Barleben (Germany) for providing the base material for the samples used in this study. ’ REFERENCES (1) Gonella, F.; Mazzoldi, P. In Handbook of Nanostructured Materials and Nanotechnology; Nalwa, H. S., Ed.; Academic Press: San Diego, 2000. (2) Gan, F.; Xu, L. Photonic Glasses; World Scientific: Singapore, 2006. (3) Kaempfe, M.; Rainer, T.; Berg, K.-J.; Seifert, G.; Graener, H. Appl. Phys. Lett. 1999, 74, 1200–1202. (4) Stalmashonak, A.; Podlipensky, A.; Seifert, G.; Graener, H. Appl. Phys. B: Laser Opt. 2009, 94, 459–465. (5) Stalmashonak, A.; Seifert, G.; Graener, H. Opt. Lett. 2007, 32, 3215–3217. (6) Stalmashonak, A.; Podlipensky, A.; Seifert, G.; Graener, H. Appl. Phys. B: Laser Opt. 2009, 94, 459–465. (7) Podlipensky, A.; Grebenev, V.; Seifert, G.; Graener, H. J. Lumin. 2004, 109, 135–142. € nal, A. A.; Stalmashonak, A.; Seifert, G.; Graener, H. Phys. Rev. (8) U B 2009, 79, 115411. (9) Kovalenko, S. A.; Dobryakov, A. L.; Ruthmann, J.; Ernsting, N. P. Phys. Rev. A 1999, 59, 2369–2384. (10) Megerle, U.; Pugliesi, I.; Schriever, C.; Sailer, C. F.; Riedle, E. Appl. Phys. B: Laser Opt. 2009, 96, 215–231. (11) Schriever, C.; Pugliesi, I.; Riedle, E. Appl. Phys. B: Laser Opt. 2009, 96, 247–250. (12) Kovalenko, S. A.; Schanz, R.; Hennig, H.; Ernsting, N. P. J. Chem. Phys. 2001, 115, 3256–3273. (13) Klopf, J. M.; Norris, P. Appl. Surf. Sci. 2007, 253, 6305–6309. (14) Dobryakov, A. L.; Kovalenko, S. A.; Weigerl, A.; Perez-Lustres, J. L.; Lange, J.; M€uller, A.; Ernsting, N. P. Rev. Sci. Instrum. 2010, 81, 113106. (15) Werner, D.; Furube, A.; Okamoto, T.; Hashimoto, S. J. Phys. Chem. C 2011, 115, 8503–8512. (16) Stalmashonak, A.; Graener, H.; Seifert, G. In Silver Nanoparticles: Properties, Characterization and Applications; Welles, A. E., Ed.; Nova Science Publishers: Hauppauge NY, 2010; pp 116 168. (17) Del Fatti, N.; Vallee, F. Appl. Phys. B: Laser Opt. 2001, 73, 383–390. (18) Bigot, J.-Y.; Halte, V.; Merle, J.-C.; Daunois, A. Chem. Phys. 2000, 251, 181–203. (19) Hodak, J. H.; Martini, I.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 6958–6967. (20) Hertwig, A.; Martin, S.; Kr€uger, J.; Kautek, W. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 1075–1077. (21) Lonzaga, J. B.; Avanesyan, S. M.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 2003, 94, 4332–4340. (22) Efimov, O. M.; Glebov, L. B.; Grantham, S.; Richardson, M. J. Non-Cryst. Solids 1999, 253, 58–67. (23) Christensen, N. E. Phys. Status Solidi B 1972, 54, 551–563. (24) Pinchuk, A.; von Plessen, G.; Kreibig, U. J. Phys. D: Appl. Phys. 2004, 37, 3133–3139. € nal, A. A.; Stalmashonak, A.; Graener, H.; Seifert, G. Phys. Rev. (25) U B 2009, 80, 115415. € nal, A. A.; Graener, H.; Seifert, G. J. Phys. (26) Stalmashonak, A.; U Chem. C 2009, 113, 12028–12032.
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