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We developed an advanced near-field optical method by combining an ultrafast near-field optical microscope with a prism-based pulse shaping system. We used this apparatus to visualize plasmonic optical fields and to measure the lifetime of plasmons excited on a rough gold film. We also studied the influence of the phase-modulation of the excitation pulse on the spatial distribution of the optical fields. We found that the spatial distribution of the optical fields can be controlled by a negatively chirped pulse.
© 2013 Optical Society of America
1. Introduction
Metal nanostructures exhibit unique optical properties due to the collective oscillations of free electrons, known as surface plasmon resonances [1–3]. Plasmon confines optical fields into sub-wavelength volumes and results in enhanced optical fields in the vicinity of the nanostructures [4,5]. Enhanced optical fields have been utilized for various applications, such as surface enhanced fluorescence [6–8] and Raman scattering [9–12], photochemical reaction fields [13,14], and nonlinear spectroscopy [15]. Spatial control of plasmonic optical fields is a promising technique for improving the potential of plasmon-based applications. Coherent control of plasmons is one of the approaches to achieve.
In molecular systems, the coherent control technique is well established and has been applied to various chemical reactions [16–18]. For example, excitation of isolated diatomic molecules with femtosecond pulses generated a coherent superposition of vibrational states, namely wave packets in a molecule. The wave packet generated by the first pulse was actively controlled by interference with another wave packet generated by the second pulse. The dephasing time of the vibrational state in the molecule is relatively long, and probing of the wave packet evolution up to several tens of picoseconds has been reported [19].
To achieve coherent control of plasmons, ultrafast pulse excitation, which is shorter than or close to the plasmon lifetime, is indispensable. The dephasing time of the plasmon is in the range of 2~20 fs [20], and thus, establishing coherent control of the plasmons is very challenging. Several theoretical studies have suggested that amplitude-, phase-, and polarization-modulated femtosecond pulses are useful for realizing coherent control of plasmons [21–26]. In a plasmonic system, visualization of the optical fields is essential for probing the influence of plasmons on the ultrashort pulse excitations, and therefore, an imaging apparatus with high temporal and spatial resolutions is of great use. Photoelectron emission microscopy (PEEM) is an experimental method used to achieve high spatio-temporal resolution. The use of PEEM has been demonstrated for the coherent control of plasmonic optical fields on gold nanostructures and rough silver films [27,28]. Scanning near-field optical microscopes are also able to achieve a high spatial resolution beyond the diffraction limit and are useful for visualizing plasmon modes and optical fields with a wider spectral range [29]. However, optical control of plasmons with a near-field optical microscope has not been demonstrated due to the poor temporal resolution that is limited by a light source or the group velocity dispersion arising from the optical component in the microscope. Very recently, the time resolution of the near-field optical microscope was improved to shorter than 20 fs [30], and thus, a combination of the ultrafast near-field method with a pulse shaping technique enables the demonstration of optical control of plasmons.
In this study, we developed an apparatus by combining an ultrafast near-field microscope with a prism-based pulse shaper, and used it to study the lifetime distribution of plasmons excited on a rough gold film and to demonstrate optical control of plasmons. We found that the dephasing time distribution of the sample scatters ranged from several fs to 15 fs, which is consistent with that estimated from spectroscopic measurements. We also found that the spatial distribution of the plasmonic optical fields, which were visualized using the two-photon luminescence (TPL) from gold, varies with the phase-modulation of the excitation pulse used.
2. Experimental method
A rough gold film (thickness 20 nm, root mean square roughness ~7 nm) was prepared on a cover slip using the sputtering method. The morphology of the sample surface was characterized using an atomic force microscope (AFM) and a near-field optical microscope. Figure 1 shows a schematic diagram of the experimental setup. A home-built aperture-type scanning near-field optical microscope was operated under ambient conditions. A chemically etched near-field optical fiber probe [31] was used for the near-field excitation. The typical aperture size of the near-field probe was approximately 50-100 nm. In this study, the sample was locally illuminated through the near-field probe aperture, and the luminescence or transmitted light from the sample was collected by an objective lens (OB) beneath the glass substrate (GS) and delivered to photodetectors. A Xe lamp and a mode-locked Ti:sapphire (TSL) laser (λ = 780 ~870 nm, pulse width ~10 fs, repetition rate 80 MHz) were used as the light sources for transmission and TPL measurements, respectively. For excitation of the TPL, typical incident power before coupled to the optical fiber (OF) was ca. 1 mW. TPL from the sample was detected by either an avalanche photodiode (APD) or a charge coupled device (CCD) detector-equipped monochromator.
Fig. 1 Schematic diagram of the experimental set-up. TSL: mode-locked Ti:s laser, RM: reflecting mirror, BS: beam splitter, PR: prism, SLM: spatial light modulator, XL: Xe discharge lamp, FC: fiber coupler, OF: optical fiber, OB: objective lens, FL: optical filter, CCD: charge coupled devise detector, APD: avalanche photodiode, RGF: rough gold film, GS: glass substrate.
For excitation of the TPL, the excitation pulse at the near-field should be as short as possible. As the short pulse propagates in the optical medium, the pulse width becomes broader because of the group velocity dispersion (GVD). A prism pair was used for pre-compensating the GVD arising from the optical fiber. A spatial light modulator (SLM) was used for optical control measurements to modulate the phases of the individual spectral components in the excitation pulse. This device was also used for the compensation of the higher-order GVD. For the time-resolved pump-probe autocorrelation measurements, output from the Ti:sapphire laser was split into two-beams. After reflection by mirrors (RM), the two beams are collinearly coupled to the optical fiber (OF). To control the delay time between the two beams, one of the reflectors was mounted on a piezo-driven movable stage.
3. Results and discussion
Figure 2(a) shows the typical topography of the sample. The roughness of the sample surface is due to the protrusions. A line profile of the image, shown in Fig. 2(b), indicates that the height of the protrusions is approximately 10-30 nm. A near-field transmission spectrum collected at the protrusion of the sample in Fig. 2(c) exhibits broad extinction bands near the infrared region (700 – 900 nm). We conducted the electromagnetic calculation to simulate the absorption spectrum of the structure, where the protrusion was approximated by a spherical nanoparticle (diameter 20 nm) on the film [32–34]. Figure 2(d) shows the calculated absorption spectrum. The spectral features in Fig. 2(b) are roughly reproduced by the simulations, and thus are assigned to the plasmon resonances. Because the plasmon resonance wavelength is within the spectral range of the Ti:sa laser, TPL from the sample is resonantly enhanced by the laser pulse excitation.
Fig. 2 (a) Topography of the rough gold film. (b) Line profile of the dotted line in (a). (c) Typical near-field transmission spectrum observed at the protrusion on the film. (d) Simulated absorption spectrum of the protrusion. Simulation model is described in the text. (e) Near-field two-photon luminescence from the gold film. (f) Near-field two-photon excitation image of the film.
Figure 2(e) shows a near-field TPL spectrum collected at the rough gold film. The spectrum shows a shoulder near 550 nm and a tail of the intense band from 600 nm to 720 nm. In the TPL, the process is initiated by two successive one-photon absorptions [35,36]. Namely, the first photon excites an electron in the sp conduction band below the Fermi surface to the sp conduction band above the Fermi surface, and creates a hole in the sp conducting band below the Fermi surface. The second photon excites an electron in the d band to the sp conduction band, where the hole was created by the first photon. As a consequence, an electron and hole pair is created in the d and sp bands. PL is radiated when the electron-hole pair recombines near the L and X symmetry points of the Brilloiun zone [37]. According to the calculated band structure of gold [38] and also the PL mechanism described above, PL should be observed near 520 and 630 nm. The observed spectral features in Fig. 2(e) are consistent with the PL mechanism, and thus, the PL from the sample can be attributed to the radiative recombination of an electron-hole pair created in the sp and d bands. One of the authors previously reported that TPL is of great use for the visualization of plasmonic optical fields in the vicinity of single gold nanostructures [35,39]. Figure 2(f) shows a TPL excitation image of the sample. Note that the topography and optical images shown in Figs. 2(a) and 2(f) were observed simultaneously, and therefore, a direct comparison between them is feasible. From the comparison, as is highlighted by the arrows, we found that the TPL excitation probability (and hence the optical field strength) was locally enhanced at the protrusions on the sample. This result is consistent with the predictions from the classical electromagnetic field calculations [40–42].
3.1 Pulse compression
For optical control of the plasmons, excitation with the ultrashort pulse is essential. The second-order GVD arising from the optical medium can be compensated by using the prism pairs, and a pulse width near several tens of femtoseconds can be achieved. To recover the original pulse width at the near-field aperture, the higher-order GVD must be compensated. In recent years, there have been a number of reports of pulse compression by combining genetic algorithms with the adaptive pulse shaping technique based on the SLM [43–52]. In this study, the pulse shaping system with the prism pair was constructed as shown in Fig. 1. The pulse was compressed by the system using second harmonic (SH) light generated from the sample surface under the aperture of the near-field probe. The SH intensity increases with the inverse of the excitation pulse width, and the signal can thus be used as a feedback signal (fitness value) in the genetic algorithm. We used 640 vectors as parameters for one generation at each pixel in the SLM, and more than 100 pixels were used for the phase control. Each pixel covered approximately 15-20 nm bandwidth.
Figure 3(a) shows the SH intensity evolution as a function of the generation. The SH intensity increases until generation 500, and after that the intensity reaches its maximum. Figures 3(b) and 3(c) show the SH autocorrelation traces observed before and after the compression, respectively. The envelope of the trace was fitted by a Gaussian function, and from the analysis, we found that the correlation width was compressed from 55 fs to 15 fs. The correlation width is approximately 1.4 times wider than that of the original pulse width, and thus, the pulse width after compression is approximately 11 fs. As demonstrated here, the prism based pulse shaper is capable of compressing the pulse width, as is similar to the standard 4-f pulse shaper consisting of a pair of diffraction gratings and lenses. In practical operation, the achievable pulse width depends on factors such as the fiber length of the near-field fiber probe and temporal characteristics of the light source, and consequently, the typical pulse width after optimization is between 15 and 25 fs.
Fig. 3 (a) Evolution of the SH intensity as a function of the generation during the operation of the genetic algorithm. (b, c) SH autocorrelation traces observed before and after the pulse compression, respectively.
3.2 Visualization of plasmonic optical fields and plasmon lifetime measurements
To obtain the information on the lifetime of the plasmon, fringe-resolved autocorrelation measurements were performed by monitoring the TPL signal at every point in a scan area of the sample. More than 400 autocorrelation traces were measured in a single image (3 × 3 μm). By fitting the envelope of each trace with a Gaussian function, the correlation width at each point was determined. Figure 4(a) shows the histogram of the observed correlation widths. In this measurement, the typical correlation width was 28 ± 3 fs. As seen from the figure, some of the observed data exhibit longer correlation widths compared to that of the original one. The difference between the longer component and the original one ranges from several to 15 fs, and is close to the dephasing time of plasmons reported [20]. Dephasing time of the plasmons can be obtained by de-convoluting the autocorrelation trace [30,53,54]. Near-field spectral transmission measurements of the sample also provide the spectral bandwidth of plasmon resonance, and thus the dephasing time of the plasmon. Figure 4(b) shows the histogram of the dephasing times determined from the near-field transmission measurements. The result is consistent with the one obtained by the autocorrelation measurements, and we thus concluded that the distributions observed in Figs. 4(a) and 4(b) reflect the dephasing time of the plasmon excited on the sample.
Fig. 4 (a) Histogram of the correlation widths obtained from Gaussian fitting of the autocorrelation traces. (b) Histogram of the dephasing times of the plasmons determined from the near-field transmission measurements.
3.3 Influence of the excitation pulse characteristics upon the optical field distribution
The optical field distribution excited in the nanostructure depends on the excitation wavelength [35,55,56]. To examine the influence of the excitation wavelength on the optical field generated in the rough gold film, two-photon excitation images were taken at various excitation wavelengths. Figures 5(b) and 5(c) show near-field TPL excitation images taken with excitation pulses of spectral bandwidth 785-810 nm and 810-870 nm, respectively. Fourier-transform-limited temporal widths for the excitation pulses are 22 fs and 45 fs, respectively. We found that the spatial characteristics of the two images are very similar to each other. Furthermore, a near-field TPL excitation image was observed by a temporally shorter pulse (20 fs) of spectral bandwidth extending from 780 to 880 nm, as shown in Fig. 5(d). The spatial character of the image in Fig. 5(d) looks similar to those observed in Figs. 5(b) and 5(c). However, close inspection of the images revealed clear discrepancies, which are highlighted by the arrows in the figure. This finding indicates that the excitations of various plasmon modes are involved in the observed images. From a comparison of the images in Figs. 5(c) and 5(d), we also found that in most locations in Fig. 5(d), the signal intensity is higher than that in Fig. 5(c). This observation can be understood by considering the excitation pulse width because the signal intensity is inversely proportional to the excitation pulse width. The pulse width in Fig. 5(d) is shorter than that in Fig. 5(c), and therefore, the intensity in Fig. 5(d) is in general higher than that in Fig. 5(c).
Fig. 5 (a) Spectral characteristics of the excitation pulses (B, C, D) used in the experiment. (b, c, d) Near-field two-photon luminescence images observed with excitation pulses B, C, D, respectively. Scale bars: 200 nm.
To examine the influence of the phase-modulation of the excitation pulse on the optical field distribution, near-field TPL excitation images were taken with two excitation conditions: one with a nearly transform-limited pulse, and the other one with a phase-modulated pulse. In the modulated pulse, the phase of the spectral components in the region of 785-840 nm was linearly shifted, with respect to the original one, from −70° to −10° as the increase of the wavelength. It is noted that in this study all the spectral components were not equally phase-modulated because of the spectral resolution of the pulse shaper. As a consequence of the modulation, the autocorrelation width was varied from approximately 26 fs to approximately 30 fs. The spectral characteristics of the excitation pulse used for the measurements are shown in Fig. 6(a). Figures 6(b) and 6(c) show near-field TPL excitation images taken with the excitation pulses before and after the modulations, respectively. Note that Fig. 6(b) is identical to Fig. 5(c).
Fig. 6 (a) Spectral characteristics of the phase-modulated pulse used for the two-photon excitation. (b, c) Near-field two-photon luminescence images taken before and after the pulse modulation, respectively. Scale bars: 200 nm.
From a comparison of the images, we found that at most locations the signal intensity in Fig. 6(b) is nearly identical to that in Fig. 6(c). This observation is reasonable since the pulse width used for these measurements are the approximately the same. It should be noted here that the signal intensity at A in Fig. 6(c) is slightly intense (integrated intensity is 10% larger) than that in Fig. 6(b). Similarly, the enhancement was also observed at the other locations, and in some case the enhancement factor reached more than 3. These observations cannot be explained by the conventional excitation scheme based on the temporal width of the excitation pulse used. This counter intuitive observation for the non-linear process was also reported in two-photon fluorescence from atomic vapors [57]. In this system, optical response of the intermediate state was modulated by phase-tuning of the excitation pulses, and as a result two-photon absorption rate was enhanced several times. In the excitation process of the TPL, two-photon excitation process occurs in successive one-photon absorptions, and thus the second photo-absorption occurs through a real intermediate state. The excitation scheme is similar to the one for the atomic vapors, and thus the identical mechanism may be operative for the TPL.
From the figures, we also found that the intensity distribution of the images changes drastically at the marked area. It is to be noted that such a drastic change cannot be induced by excitation with temporally long pulses. We performed similar measurements on the other area of the sample and found that approximately 5-10% of the bright spots exhibit the similar propensity. We also found that the variation of the signal is dependent on the phase angle of the modulation, and the phase angle dependence of the signal varies on the sample locations. In some locations, the variation becomes significant near the phase angle 30°-100°. Figure 6(c) is one of the typical examples. These results may indicate that the interference of local excitations is involved in the signal variation. In a negatively chirped pulse, the high frequency component travels faster than the low frequency component, and thus the higher-order plasmon modes are excited first and the lower modes are excited later, as was discussed for plasmon excitations in the silver wires [58]. The phase velocity of the lower mode is faster than that of the higher modes, and it is feasible that the interference between the higher and lower modes occurs at some distance from the excitation location. Hence, it might be reasonable that the negatively chirped pulse induces the interference of the excited plasmons. For understanding the detailed mechanism underlying the behaviors, further investigation is indispensable and is currently underway.
4. Conclusion
In conclusion, we developed an advanced microscope by combining aperture-type near-field optical microscopy with a pulse shaping system. The microscope simultaneously achieves a 40 nm spatial resolution and a 15 fs time resolution. By using this apparatus, we visualized plasmonic optical fields and measured the lifetimes of plasmons excited on the rough gold film. We found that the enhanced optical field was localized at protrusions on the sample, and the depahsing time of plasmon on the film ranged from several fs to 15 fs. We studied TPL images of the sample with various excitation pulses and found that the spatial distribution of plasmonic optical fields varies depending on the characteristics of the excitation pulse used. The results indicate that the interference of the plasmonic waves plays an important role in determining the spatial distributions of the optical fields, and thus, optical control of plasmonic waves is feasible by using phase-modulated femtosecond pulses.
Acknowledgments
This work was supported by PRESTO program from Japan Science and Technology Agency, Grants-in-Aid for Scientific Research (Grant Nos. 24655020, 24350014, and 25109713) from the Japan Society for the Promotion of Science and from the Ministry of Education, Culture, Sports, Science and Technology, and Grant for Special Research Projects (2012A-506) from Waseda University.
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