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Plasmon Dephasing in Single Gold Nanorods Observed by Ultrafast Time-resolved Near-field Optical Microscopy Yoshio Nishiyama, Keisuke Imaeda, Kohei Imura, and Hiromi Okamoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03341 • Publication Date (Web): 22 Jun 2015 Downloaded from http://pubs.acs.org on July 3, 2015

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The Journal of Physical Chemistry

Plasmon Dephasing in Single Gold Nanorods Observed By Ultrafast Time-Resolved Near-Field Optical Microscopy Yoshio Nishiyama,† Keisuke Imaeda,§ Kohei Imura,§* Hiromi Okamoto†‡* † Institute for Molecular Science, Myodaiji, 38 Nishigonaka, Okazaki, Aichi 444-8585, Japan ‡ The Graduate University for Advanced Studies, Myodaiji, 38 Nishigonaka, Okazaki, Aichi 444-8585, Japan § School of Advanced Science and Engineering, Waseda University, Okubo, Shinjuku, Tokyo 169-8555, Japan * Email: [email protected] (KI); [email protected] (HO).

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ABSTRACT

We applied time-resolved near-field optical microscopic measurements with ultrashort light pulses of ~16 fs duration to observe plasmon dephasing processes in single gold nanorods. The correlation widths of the time-resolved signals obtained at each position on the nanorods were broadened compared with the auto-correlation width of the pulse because of the plasmon lifetime. The correlation width maps of the rods showed spatially oscillating patterns that look similar to the plasmon mode structures observed in the static near-field optical images. The spatial variation of the correlation widths was explained as arising from the position dependent contribution of the resonant plasmon excitation in the time-resolved signals relative to that of the non-resonant excitation. This finding indicates that the dephasing times of the resonant plasmon modes were constant regardless of the excitation position. This result is understood to be a consequence of the spatial coherence of the plasmon mode that causes the local excitation to be immediately delocalized across the rod after irradiation. A comparison between the timeresolved signals of the inner parts and the outer parts of the nanorods suggests that the nonresonant contribution to the time-resolved signals may be driven by the lower-order plasmon modes having resonances in a much longer wavelength region.


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I. Introduction Surface plasmons in noble metal nanostructures have attracted much attention in various research fields. Light can be highly confined in nanospace, and the optical field is locally enhanced when plasmons are excited on the nanostructures. Such field enhancement causes unique optical phenomena, such as surface enhanced Raman scattering1 and surface enhanced infrared absorption,2 or it enables multiphoton-induced chemical reactions3 and photoelectron emission4 at a much lower light intensity than usual. In addition, the strongly confined optical field is well suited to potential applications in molecular sensors, nanophotonic devices and photovoltaic devices.5 The properties of the local optical fields near the metal nanostructures are strongly influenced by the spatial and temporal characteristics of plasmons, and thus, the direct observation of the spatio-temporal behavior of plasmons is of fundamental importance. The spatial scale of plasmons in noble metal nanostructures is essentially smaller than the diffraction limit of light (a few hundred nanometers), and the timescale of plasmon dynamics is faster than the rapid dephasing process in the range of several fs to 20 fs in the case of gold nanostructures.68

Therefore, we need to simultaneously realize high spatial and temporal resolutions to deeply

understand the fundamental properties of plasmons. Recently, advances in microscopic techniques with nanometric spatial resolution, such as scanning near-field optical microscopy (SNOM),9-15 photoelectron emission microscopy (PEEM),16,17 and electron microscopy with cathodoluminescence detection18 or with electron energy loss spectroscopy,19-22 have allowed us to visualize the spatial structures of plasmon modes or the local optical fields around the metal nanostructures with various shapes and sizes. However, techniques with space-time resolution capable of the direct observation of plasmon dynamics are limited. Time-resolved PEEM, a combined method of PEEM and ultrafast time-


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resolved spectroscopy, was applied to the visualization of surface plasmons propagating on a silver film23 and to the investigation of the dephasing processes of plasmons localized on a rough silver surface.24-26 Although a combination of SNOM with ultrafast spectroscopy also has the potential to realize a nanometer and femtosecond spatio-temporal resolution, the technique at present remains in the development stage. The optical components that compose the SNOM apparatus cause significant pulse broadening due to the dispersion effects of the materials, which broadening prevents the achievement of a high temporal resolution. In the case of an aperturetype SNOM, an optical fiber of the probe broadens a 10-fs pulse duration to several picoseconds. To recover the original pulse width, it is necessary to compensate for the material dispersion not only in the 1st and 2nd orders but also up to higher orders, which is beyond the capacity of conventional passive compensation techniques that use a pair of diffraction gratings or prisms. Recently, Ropers et al. developed a new type of scattering-type near-field probe which utilized nanofocusing of surface plasmons on a tapered gold tip.27 With this probe suppressing the pulse broadening, they achieved 16 fs duration at a 10 nm spot size.28 Furusawa et al. compensated for the material dispersion of optical components by a pulse shaping technique in a scattering-type SNOM and realized a near-field pulse with a 10 fs duration.29 Our group achieved a 17-fs pulse duration at an aperture-type SNOM probe tip by a combination of passive compensation devices and a pulse shaping technique and succeeded to make a real-time observation of a plasmon response of less than 10 fs for a gold nanostructure.30 These ultrafast SNOM methods enable real-time observations of plasmon dynamics in various noble metal nanostructures at nanoscale dimensions. There have been a number of reports of the visualization of plasmon modes in noble metal nanostructures in the visible to near-infrared wavelength region based on SNOM. The spatial structure of the longitudinal plasmon modes in


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gold nanorods has been investigated most systematically, wherein the plasmon wavefunctions that oscillate spatially along the rod axis were visualized by near-field imaging for various eigenmodes from the lowest dipolar mode to higher-order modes with several nodes.9,10-12,15 In the previous study,6 plasmon dephasing of nanorods was discussed for the dipolar modes based on an analysis of the spectral widths obtained by far-field measurements of single nanorods. The spatially-resolved observation of plasmon dynamics in nanorods by ultrafast SNOM may provide further details of the spatio-temporal behavior and an understanding of the basic principles needed to control and utilize the dynamic properties of plasmons. In this study, we apply ultrafast near-field optical microscopy to the observation of plasmon dynamics in single gold nanorods after local excitation and analyze the spatial characteristics of the dephasing dynamics of multipolar plasmon modes. II. Experimental Section The ultrafast time-resolved SNOM system used in the present study is an improved version of that reported previously.30 Briefly, the system consists of a Ti:Sapphire laser (~800 nm, 12 fs, 80 MHz), a Michelson interferometer, dispersion compensation optics and an aperture-type SNOM. The SNOM was equipped with an optical fiber probe with an aperture of 100 nm diameter for near-field illumination. To obtain near-field images, the distance between the tip apex and the sample surface was regulated to be ~10 nm by the shear-force feedback mechanism. As described previously, an optical fiber of the probe significantly broadens the pulse duration. We thus precisely compensated for the dispersion effects in the fiber by employing three devices, i.e., a grating pair, chirped mirrors, and a pulse shaping system with either a deformable mirror (DFM) or a spatial light modulator (SLM). The pulse shaping system enables the compensation of higher order dispersions. To optimize the pulse shape, we set a β-barium borate (BBO) crystal


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at the sample stage, which was irradiated by the near-field optical pulses from the probe tip to generate the second harmonics, and the surface shape of the DFM or the phase retardation of the SLM was controlled to maximize the second harmonic intensity. With this procedure, the pulse duration at the probe tip was minimized. The pulse duration after optimization was evaluated by second harmonic generation (SHG) auto-correlation measurements. From the correlation trace shown in Fig. 1(a), the pulse duration was found to be 16 fs at the probe tip. Using the near-field ultrashort pulses, we performed time-resolved near-field measurements of gold nanorods based on a pump-probe experimental scheme. We detected two-photon-induced photoluminescence (TPI-PL, in the wavelength range of 500-650 nm) as the optical signal. The strong scattering of the incident radiation was eliminated with a short-pass filter. The plasmon resonance of the nanorod was an intermediate state involved in the two-photon excitation of the TPI-PL.12 By scanning the delay time between the pump and probe pulses at each position on the rod while detecting the TPI-PL intensity, we can observe the plasmon dynamics (dephasing and other processes) at each position. To clarify the static characteristics of the plasmon modes, i.e., the resonant wavelengths and spatial structures, we performed static near-field transmission and two-photon excitation imaging measurements. In the static transmission measurement, we used a Xe discharge lamp or a halogen lamp as a light source and analyzed the transmitted light using a spectrometer. Static two-photon excitation images were obtained by detecting the TPI-PL during irradiation by narrow spectral-band femtosecond pulses from a wavelength-tunable Ti:Sapphire laser (wavelength range 680-1080 nm, spectral width 1000 nm) and is sufficiently distant from the wavelength of irradiation, and thus, the excitation dynamics of the dipolar mode can be treated as those of a non-resonant one. At the center of the rod, the non-resonant dipolar mode is excited more efficiently than at either side and may contribute dominantly to the time-resolved TPI-PL signals, which results in the narrower correlation width at the center. We estimate the relative contributions of the resonant 3rd mode and the non-resonant dipolar mode to the time-resolved signals at several positions in nanorod A. We assume the following form for the plasmon response function, wherein the non-resonant contribution is considered as an impulse response. χ (t ) = Ap1 exp(−t / τ dp ) cos(ω p t ) + Ap2 δ (t )

(5)

The first and second terms represent the resonant and the non-resonant components, respectively. Here, we fix the value of τdp at 15 fs, as determined by the fitting at the edge of the rod, and the two amplitudes Ap1 and Ap2 are considered as fitting parameters. As shown in Fig. S2(j), the value of Rnr = Ap2/Ap1 varies with the position of observation. The Rnr value decreases as the position moves closer to the edge and at center of the rod where the resonant component is considered to be dominant. This result also explains why the correlation times were found to be short for nanorods B and C relative to shorter nanorod A, which could not be understood in terms of the radiation damping effects. For nanorods B and C, the plasmon bands near the incident pump wavelength (Fig. 3(c))


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were not as distinct as that for nanorod A (Fig. 1(b)) in the extinction spectra at the edges. In other words, the relative contribution of resonant excitation for nanorods B and C is less significant than that for nanorod A. It is highly probable that the non-resonant component makes a substantial contribution even at the edges in nanorods B and C, which rationalizes the apparently shorter correlation times. It seems that consideration of the non-resonant contribution to the TPI-PL signal explains all the time-resolved data that we obtained in this study. However, at the present stage, we cannot reject other possible mechanisms for acceleration of the plasmon dephasing, such as coupling with radiative plasmon modes leading to the Fano-interference. The dephasing processes in gold nanorods with lengths of 500 nm or longer, where coupling among plasmon modes might be stronger than shorter rods because of higher density of modes, have not been studied well, and their nature remains an open issue for future study. A higher time resolution is essential to the clear separation of the resonant and non-resonant processes in timeresolved signals. VI. Conclusion In this study, we applied time-resolved near-field microscopic measurements with ultrashort pulses of ~16 fs duration to observe plasmon dephasing processes in single gold nanorods. We prepared nanorods with lengths ranging from 400 to 770 nm and measured time-resolved TPI-PL signals at each position on the nanorods. The correlation widths of the time-resolved TPI-PL signals were broadened compared with the auto-correlation width of the pulse due to the plasmon lifetime. The time profile was reproduced well by a simulation based on a damped harmonic oscillator model, and the plasmon dephasing time was determined to be as long as ~15 fs, which is close to the prediction of the Drude model. This result indicates that the surface roughness found in gold nanorods fabricated with electron beam lithography technique is of little effect on


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dephasing of plasmons excited. The maps of the correlation width showed spatially oscillating patterns which look very similar to the plasmon mode structures observed in the static near-field two-photon excitation images. The spatial variation of the correlation widths was explained as arising from the position dependent relative weight of the contribution of the resonant plasmon excitation in the time-resolved TPI-PL signals compared to that of the non-resonant excitation. The dephasing times of the resonant plasmon modes were considered to be constant regardless of the excitation position. This result is consistent with the spatial coherence of the longitudinal plasmon mode that makes the local excitation delocalized across the rod immediately after irradiation. We also found that the correlation widths at the antinodes of the plasmon modes tend to be narrower at the inner parts of the rods than at the outer parts. This result suggests that the non-resonant excitation becomes more significant as the position of excitation moves towards the center of the rod. From the near-field extinction spectra and transmission images, it was suggested that the strong non-resonant excitation was driven by the lower-order plasmon mode (e.g., the dipolar plasmon mode) having a resonance in a much longer wavelength region. The time-resolved signals observed at various positions on the rod were reproduced well by a model incorporating the non-resonant excitation process of TPI-PL in addition to the resonant plasmonmode excitation that dephases with a finite lifetime. The ultrafast time-resolved near-field microscopic technique adopted in this study is applicable to the observation of the dynamics after multimode plasmon excitation. We can suppose that a coherent superposition of simultaneously excited multiple plasmon modes is generated, in which the excitation amplitude should be initially localized and propagates through the metal nanostructure as a wave packet. This technique also has the potential to tune the plasmonic properties of metal nanostructures through active control of the phases of the spectral


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components. We are now attempting to observe the plasmon dynamics following multi-mode excitation through time-resolved measurements for nanorods with a length of 1000 nm or longer that have high densities of resonant plasmon modes. Preliminary results clearly include position dependent time-resolved signals, indicating that the spatial features of the plasmonic excitation are time dependent. The details will be reported elsewhere. Acknowledgement The authors thank Dr. T. Narushima for his assistance in developing the ultrafast SNOM system and Ms. A. Ishikawa (IMS) for the nanostructured sample fabrication. This work was supported by Grants-in-Aid for Scientific Research (Grant Nos. 22225002, 25810013, 24655020, 24350014, 25109713, 26620018) from the Japan Society for the Promotion of Science and that for Scientific Research on Innovative Areas “Photosynergetics” (Area No. 2606, Grant No. 26107003) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. Corresponding Author *E-mail: [email protected]. Fax: +81-564-54-2254. Tel.: +81-564-55-7320. Supporting Information Available: Near-field extinction spectrum and transmission image of a chemically synthesized nanorod D and calculations for time-resolved TPI-PL signals at each position of nanorod A. This material is available free of charge via the Internet at http://pubs.acs.org

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Figure Captions Figure 1. (a) Second harmonic generation (SHG) auto-correlation trace measured at the nearfield probe tip after optimization of pulse shape. (b) Near-field extinction spectrum of gold nanorod A (solid green curve) and spectrum of near-field ultrashort pulses (broken black curve). (c) Near-field transmission image of nanorod A at 840 nm. The dotted line represents the approximate shape of the nanorod. The inset shows line profile along the dotted red line. The scale bar is 100 nm. (d) Near-field two-photon excitation image of nanorod A at an excitation wavelength of 840 nm. The time-resolved two-photon induced photoluminescence (TPI-PL) signals in Figs. 2(a) and (b) were measured at the marked positions 1 and 4, respectively. The extinction spectra in Fig. 6(a) were measured at the marked positions 1 to 3. Figure 2. (a) Time-resolved TPI-PL signal (dotted red curve) measured at the antinode of the resonant plasmon mode in nanorod A (the marked position 1 in Fig. 1(d)) and the SHG autocorrelation trace of the near-field pulses (solid blue curve). (b) Time-resolved TPI-PL signal measured at the node of the resonant plasmon mode (the marked position 4 in Fig. 1(d)). (c) Time-resolved TPI-PL signal in the extended time region. (d) Time-resolved TPI-PL signal after frequency filtering (dotted red curve) and the signal simulated with eqs. (1) to (4) (solid blue curve). Figure 3. (a) and (b) Time-resolved TPI-PL signals measured for nanorod B and nanorod C, respectively. (c) Near-field extinction spectra of nanorod B (dotted red curve) and nanorod C (solid blue curve). (d) and (e) Near-field two-photon excitation images of nanorod B and nanorod C, respectively. The excitation wavelengths were 870 nm and 860 nm for (d) and (e), respectively. Time-resolved TPI-PL signals in (a) and (b) were measured at the marked positions


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in (d) and (e), respectively. The dotted lines represent the approximate shapes of the nanorods. The scale bars are 100 nm. Figure 4. (a), (b), and (c) Correlation width (full-width at half maximum) maps obtained for time-resolved signals of nanorods A, B, and C, respectively. The black parts outside of the nanorods are the areas where the correlation widths were not evaluated because of low signal intensities. The extinction spectra in Fig. 6(a) were measured at the marked positions in panel (a). The dotted lines represent the approximate shapes of the nanorods. The scale bars are 100 nm. Figure 5. (a) Near-field two-photon excitation image of gold nanorod D. The time-resolved TPIPL signals in (b) were measured at the marked positions. The dotted lines represent the approximate shape of the nanorod. The scale bar is 100 nm. (b) Time-resolved TPI-PL signal (dotted red curve) measured at the antinode of the resonant plasmon mode (the marked position in (a)) and the SHG auto-correlation trace of the near-field pulses (solid blue curve). (c) Correlation width map obtained for time-resolved signals of the nanorod D. The black areas in the image indicate areas where the correlation widths were not evaluated because of low signal intensity. The scale bar is 100 nm. Figure 6. (a) Near-field extinction spectra of gold nanorod A. The blue (dotted), green (broken), and red (solid) spectra were measured at positions 1, 2, and 3 in Fig. 4(a) (or in Fig. 1(d)), respectively. (b) Near-field transmission image of nanorod A observed at 940 nm. The dotted line represents the approximate shape of the nanorod. The inset shows line profile along the dotted red line. The scale bar is 100 nm.


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Table 1. Dimensions of the gold nanorods measured.

nanorod

length / nm

width / nm

A

405±10

55±10

B

615±10

55±10

C

770±10

55±10

D

240±30

40±10#

# The diameter of the rod is listed.


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Figure 1.


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Figure 2.


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Figure 3.


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Figure 4.


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Figure 5.


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Figure 6.


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TOC GRAPHICS


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164x191mm (300 x 300 DPI)


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153x230mm (300 x 300 DPI)



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152x234mm (300 x 300 DPI)


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165x147mm (300 x 300 DPI)



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104x213mm (300 x 300 DPI)


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158x137mm (300 x 300 DPI)


Plasmon Dephasing in Single Gold Nanorods ... - ACS Publications

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