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Shape-dependent Nonlinear Optical Properties of Anisotropic Gold Nanoparticles Yi Hua, Kavita Chandra, Duncan Hieu M. Dam, Gary P. Wiederrecht, and Teri W. Odom J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02263 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 25, 2015

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The Journal of Physical Chemistry Letters

Shape-dependent Nonlinear Optical Properties of Anisotropic Gold Nanoparticles Yi Hua,† Kavita Chandra,† Duncan Hieu M. Dam,‡ Gary P. Wiederrecht§ and Teri W. Odom†,‡ †

Department of Materials Science and Engineering, and ‡Department of Chemistry, Northwestern University, Evanston, Illinois, 60208, United States

§

Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, 60439, United States

ABSTRACT This paper reports the shape-dependent third-order nonlinear optical properties of anisotropic gold nanoparticles. We characterized the nonlinear absorption coefficients of nanorods, nanostars, and nanoshells using femtosecond Z-scan measurements. By comparing nanoparticle solutions with a similar linear extinction at the laser excitation wavelength, we separated shape effects from that of the localized surface plasmon wavelength. We found that the nonlinear response depended on particle shape. Using pump-probe spectroscopy, we measured the ultrafast transient response of nanoparticles, which supported the strong saturable absorption observed in nanorods and weak nonlinear response in nanoshells. We found that the magnitude of saturable absorption as well as the ultrafast spectral responses of nanoparticles were affected by the linear absorption of the nanoparticles.

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TOC

KEYWORDS Saturable absorption, anisotropic metal nanoparticles, Z-scan, nonlinear, hot electrons, gold nanostars


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Metallic nanoparticles (NPs) support collective oscillations of conducting electrons where the local fields can be greatly enhanced at the localized surface plasmon (LSP) resonance.1 Since LSPs can be tuned over a broad range of wavelengths by controlling the shape, size, and materials properties of NPs, their linear properties can be exploited in applications from surface enhanced spectroscopies to fluorescence enhancement to chemical and biological sensing.2 Unlike the linear behavior, the nonlinear properties of metallic nanostructures are not well understood. Under high intensity illumination, metallic nanostructures can show a wide range of nonlinear responses, including photothermal reshaping,3-4 second harmonic generation,5 and enhanced optical Kerr effects.6 An understanding of how the nonlinear optical properties of NPs are related to linear properties is important. The nonlinear optical properties of NPs in solution are commonly described by the effective medium model,7 where the third-order nonlinearity scales as the fourth power of the local electric field enhancement. In the quasi-static limit, under which the model can be applied, the particle diameter (d) is much smaller than the wavelength of light (λ), and NP shape does not need to be considered. The local field enhancement around large NPs (d > λ/10), however, depends on the shape and size of NPs.8 Previous work found that shape affected the nonlinear absorption in large NPs, 9-13 but changes in particle shape were always accompanied by a change in the LSP. Thus, the difference in nonlinear response could not be attributed solely to a change in NP shape or shifting of the LSP wavelength. Therefore, to differentiate shape from LSP effects, particles with the same LSP wavelength and different shapes should be compared. The open-aperture (OA) Z-scan technique is commonly used to characterize the third-order nonlinear coefficient χ(3) of materials.14 Metallic NPs can display a range of nonlinear behavior in Z-scan including two-photon absorption, saturable absorption, and reverse saturable



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absorption.15-17 Even though Z-scan can characterize the nonlinearity of NPs, the technique by itself cannot differentiate among the different processes contributing to nonlinear absorption.18 Thus, additional measurements, such as pump-probe experiments, are needed to understand electron excitation and relaxation in NPs. Ultrafast pulse absorption by metal nanoparticles leads to the excitation of hot electrons which causes modification of the dielectric function19 and can contribute to third-order nonlinear properties of the nanoparticles. The interplay between hot electron effects and local field enhancement can alter transmission through the sample.6, 20-21 Here we report how colloidal Au NPs with different shapes and similar LSP resonances exhibit different nonlinear absorption properties. We selected particles with LSPs around 800 nm and kept the linear extinction of the solutions the same so that contributions from small changes in LSP would be minimized. Using open-aperture (OA) Z-scan at the LSP resonance, we found that the saturable absorption signal depended on Au NP shape (nanorods, nanostars, and nanoshells). To explain the different nonlinear behaviors, we correlated the shape-dependent Zscan signals with the transient optical responses of the anisotropic Au NPs obtained from spectrally resolved pump-probe measurements. Measured under the same experimental conditions, nanorods showed the strongest saturable absorption in Z-scan and longest electronphonon relaxation time, which corresponded to high electron temperatures. In contrast, nanoshells showed weak nonlinear signals because of the smaller perturbation of electron distribution around the Fermi level due to low absorption at the laser excitation wavelength.


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Figure 1. Solutions of AuNP with different shapes showed different nonlinear absorption when excited at their LSP wavelength. (a) TEM images of Au nanorods, Au nanostars, and Au nanoshells. (b) Linear extinction of NPs measured using a UV-Vis spectometer before (solid curves) and after (dashed curves) taking the Z-scan measurement. (c) OA Z-scan of NPs with incident pulse power of 150 nJ (21.8 GW/cm2). The solid data points represent scaning from negative z to positive z, and the open data points represent scanning from positive z to negative z. Over five measurements were taken on each scan and the average is presented. The sample was shaken between the measurmenets to ensure homogeneity of the solution. The OA transmission is normalized to the linear transmission of each sample at 796 nm measured by UV-Vis. We performed OA Z-scan on nanorods, nanostars, and nanoshells (Figure 1a), where samples were moved translationally along one direction (z) across the focus of the lens (z=0). The power density in the sample was highest at the focus of the lens and less away from the lens (Supporting Information). The transmission through the sample depended on power density, and nanoparticles that showed saturable absorption displayed peak-shaped curves. Samples that 5 ACS Paragon Plus Environment


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exhibited nonlinear absorption showed valley-shaped responses in OA Z-scan. We used low repetition rate (1 kHz) femtosecond pulses in the Z-scan measurement to eliminate undesirable cumulative thermal effects that could often occur in picosecond or nanosecond experiments with high repetition rate (MHz).22-23 Anisotropic NPs are of interest because their shapes support distinct electric field enhancements. To minimize any variation in linear optical properties, we controlled the concentration of the three NP solutions to have the same linear extinction at the excitation wavelength (796 nm) (Figure 1b). Nanorods and nanostars also supported transverse modes around 520 nm. The interband transition and transverse LSP around 520 nm did not contribute to the nonlinear signal measured by Z-scan (Figure S1). In OA Z-scan, the three different shapes of Au NPs showed different nonlinear behavior (Figure 1c). Nanostars and nanorods showed a peak-shaped curve indicating saturable absorption, where transmission increased with the rise of power density. In contrast, nanoshells showed a very weak valley-shape in the OA measurement where transmission slightly decreased with increasing power density. We used the conventional Z-scan analysis (Sheik-Bahae model)14 to fit the data (Figure S2): ௗூ ௗ௭

≈ −ߙ‫ ܫ‬− ߚ‫ ܫ‬ଶ

where α is the linear extinction coefficient, and β is the nonlinear coefficient of the NP. Fitted β values were -0.307 ± 0.027 cm/GW, -0.14 ± 0.004 cm/GW, 0.016 ± 0.048 cm/GW for rods, stars and shells, respectively. Negative β represents saturable absorption and is strongest in nanorods, while nanoshells showed a small positive β (and large standard deviation), corresponding to weak nonlinearity. Since our measurements are performed on colloidal nanoparticles in solution, the nonlinear response represents an average from particles with different orientations.


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In nanorods and nanostars, an asymmetric line shape was observed with a sharper slope on the rising side of the saturable absorption signal compared to the descending side. This asymmetry is preserved upon reversing the scan direction and is likely related to local particle reshaping under high intensity fs illumination.24-25 Since the linear extinction spectra of the NP solutions after the Z-scan (dashed curves, Figure 1b) overlapped with the spectra measured before, the majority of the NP shapes, however, remained unchanged. Unlike small NPs,7, 26 large NPs have significant scattering at the 796-nm LSP that cannot be neglected.13 We performed FDTD simulations (Supporting Information) to calculate the scattering and absorption cross-sections of single Au NPs (Figure 2), and the simulated linear extinction agreed well with measurements. All particles showed an extinction peak around 800 nm, where nanoshells had the broadest resonance with an additional peak around 630 nm. The absorption of Au nanoshells at 800 nm is very weak, and hence the extinction peak was dominated by scattering. The 630-nm shoulder in extinction corresponds to an absorption peak in

Figure 2. FDTD simulated normalized linear absorption and extinction cross section of Au nanorods, Au nanostars, and Au nanoshells. For easy comparison with the experiments, the calculated cross section are all normalized by the extinction cross-section at 796 nm so that all particles have the same extinction cross section. Nanorods and nanostars both showed strong absorption peak at 800 nm. The absorption of nanoshells is very weak at 800 nm where the peak in extinction has dominant contribution from scattering.



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nanoshells. Both nanorods and nanostars, however, showed strong absorption at 796 nm. Thus, most of the 796-nm photons were scattered by the Au nanoshells, which explains the absence of saturable absorption. Since the OA signals of nanoshells showed little variation (Figure 1c), nonlinear scattering in Au nanoshells was negligible at 796 nm and can be ignored. In Z-scan measurements, the optical pulse plays the role of both the pump and the probe.20 Electron dynamics in Au NPs were investigated using a spectrally resolved pump-probe setup to better understand the shape-dependent saturable absorption in OA Z-scan (Supporting Information). For consistency with Z-scan, the pump beam was at 796 nm and a white light continuum was used as the probe. A notch filter with a cut-off window from 790 nm to 820 nm

Figure 3. NPs displayed shape-dependent nonlinear spectral responses. Transient extinction map (left) and transient extinction spectra at various time delays (right) measured for (a) nanorods, (b) nanostars, and (c) nanoshells. Nanorods and nanostars both showed strong saturable absorption around surface plasmon resonance at 800 nm and a dip in ∆OD around 520 nm. (c) Nanoshells displayed a saturable absorption peak blue shifted to c.a. 700 8 nm, an additional saturable absorption peakPlus at Environment 600 nm, and a nonlinear absorption peak ACS Paragon around 520 nm.

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was placed in front of the detector to avoid oversaturation from the pump signal. The change in the optical density ∆OD = - log10(TON/ TOFF) was measured, where TON and TOFF are transmission through the sample with and without the pump beam. Even though ∆OD at the pump excitation wavelength cannot be measured directly, the line shape trend suggests that both nanorods (Figure 3a) and nanostars (Figure 3b) had negative ∆OD maximized around 796 nm. At t = 0.12 ps, the negative ∆OD peak had a full width half maximum (FWHM) of 120 nm in nanorods and 166 nm in nanostars. The peak became narrower as time increased, and at 4.26 ps, the FWHM became 92 nm and 147 nm, respectively. The negative ∆OD indicates higher transmission due to pump excitation (TON > TOFF) and is consistent with the saturable absorption observed in Z-scan where the transmission increased with power density. In nanoshells (Figure 3c), a negative ∆OD peak was measured around 700 nm instead; and the value of ∆OD at 796 nm was close to 0. The FWHM of the 700-nm peak was 220 nm at t = 0.12 ps and increased to 245 nm at t = 4.26 ps. The 700-nm negative ∆OD peak was considerably blue-shifted compared to the linear scattering peak around 800 nm, and further work is needed to understand the correlation between the scattering peak and the feature in the ultrafast response. Based on the estimated ∆OD at the pump excitation wavelength (796 nm), nanorods had the highest saturable absorption while nanoshells showed only a very weak nonlinear response, consistent with the OA Z-scan measurement. Interestingly, the transient extinction spectra also showed spectral differences at wavelengths below the LSP (800 nm). Below 700 nm, nanorods and nanostars both showed positive ∆OD while nanoshells displayed an additional negative ∆OD peak at 600 nm (Figure 3c). The spectral position of this additional saturable absorption peak is similar to the linear absorption peak in the nanoshells. Around 520 nm, a positive ∆OD peak was observed in nanoshells while both



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nanorods and nanostars showed a dip in ∆OD. The positive ∆OD peak indicates increased absorption of the probe beam due to pump excitation (TON < TOFF), most likely from the interband transition in gold. Due to absorption of the pump beam, the electron population below the Fermi level decreased19 and resulted in increased excitation from the d-band to sp-band. The ∆OD dip in nanorods and nanostars can be explained by the presence of their transverse plasmon resonances. ∆OD is caused by the variation in dielectric function ∆ε(ω), and this change in

Figure 4. NPs with different shapes shows different lifetime of the excited state. Transient extinction measurements show two relaxation processes. The electron-phonon relaxation process is 5.7 ps for Au nanorods, 3.1 ps for Au nanostars and 1.8 ps for Au nanoshells respectively. The phonon-phonon relaxation process is on the time scale of 100 ps. optical density can be amplified at the plasmon resonance.19 ∆ε(ω) is caused by excitation of hot electrons due to absorption of the pump beam, and the magnitude of change is sensitive to the hot electron temperature.19-20 The strong field enhancement at LSP leads to a local minimum in ∆OD at LSP resonance,19,

21

and pump-probe measurements on nanospheres (Figure S3)

confirmed that the 520 nm LSP would induce a minimum in ∆OD. Thus, the ∆OD peak in nanoshells was from increased interband transitions, whereas the dip for nanorods and nanostars around 520 nm was related to enhancement at the transverse LSP.


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Since the lifetime of hot electrons depends on initial hot electron temperature and electron specific heat capacity,20 we examined the electron relaxation time in different shaped NPs. For the three particle shapes, NPs showed two relaxation processes (Figure S4): electron-phonon (eph) and phonon-phonon (ph-ph) relaxation.27 Electron-electron (e-e) relaxation is on the order of 100 fs35 and so we were unable to resolve in the transient extinction. The fitted e-ph coupling time (τ) showed that nanorods had a longer τ (5.4 ps) compared to nanostars (3.0 ps) and nanoshells (1.8 ps) (Figure 4). Longer relaxation times correspond to higher initial temperatures of hot electrons28 and stronger saturable absorption. Therefore, the long e-ph relaxation time in nanorods was consistent with strong saturable absorption and high ∆OD in transient extinction. The differences in electron temperature reflect the change of electron distribution around the Fermi level and which was found to depend on particle shape. Since this change depends on pulse absorption,19 the relatively weak absorption in Au nanoshells at the laser excitation wavelength is likely responsible for the smaller perturbation and weaker nonlinear signals compared to nanorods and nanostars. Since perturbation depends on absorbed power, the nonlinear properties of NPs also show power-dependent responses.24, 29 We also found that the transmission at the focus of the OA Z-scan increased with power for all NPs (Figure S5), which indicated power-dependent nonlinear behavior. In summary, we observed that NP shape significantly affected nonlinear optical properties. Of the three particle shapes studied, Au nanorods showed the strongest saturable absorption, while Au nanoshells showed only a weak nonlinear response. FDTD simulations confirmed that the low linear absorption in Au nanoshells at the laser wavelength was responsible for the weak nonlinear signal. Transient extinction measurements showed that the change in optical density around the laser wavelength had the same shape-dependence as the nonlinear response measured



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by Z-scan. The different electron relaxation times indicated a variation in electron temperature reached in the NPs with different shapes. Shape-dependent nonlinear behavior of plasmonic NPs excited at their LSP could facilitate applications of metallic nanostructures at high power densities, such as laser-triggered drug release,30 plasmonic lasers31 and ultrafast nonlinear metamaterials.32

ASSOCIATED CONTENT Supporting information. Experimental methods including sample preparation and characterization, Z-scan Measurements, pump-probe measurement and FDTD simulations. OA Z-scan characterization of Au nanospheres. OA Z-scan results of NPs from multiple measurements. Transient extinction responses of Au nanospheres. Relaxation processes in NPs. Power-dependent nonlinear responses in NPs. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported by National Science Foundation NSF grants CHE-1058501 and CHE-1507790 and a National Institutes of Health (NIH) Director’s Pioneer Award (DP1 EB016540). This work made use of the NU Keck Biophysics Facility supported by the Cancer Center Support Grant, Biological Imaging Facility and the Center for Nanoscale Materials that was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.


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Shape-Dependent Nonlinear Optical Properties ... - ACS Publications

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