Direct Near-Field Observation of Orientation-Dependent Optical

Article pubs.acs.org/JPCC

Direct Near-Field Observation of Orientation-Dependent Optical Response of Gold Nanorods Terefe G. Habteyes* Department of Chemistry and Chemical Biology and Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico 87131, United States S Supporting Information *

ABSTRACT: The orientation-dependent optical response of short gold nanorods (length less than 100 nm) has been directly observed in the near-field, mapping the in-plane and out-of-plane vector components selectively using interferometric apertureless near-field scanning optical microscope. For the gold nanorods dispersed randomly on oxide-coated silicon wafer, the optical amplitude and phase contrast that are characteristic of the longitudinal and transverse mode dipolar plasmon resonances have been clearly resolved when the long axes of the nanorods are aligned parallel and perpendicular to the electric field of the laser, respectively. The near-field amplitude ratio of the longitudinal to the transverse plasmon mode is much smaller than the corresponding ratio of the scattering cross section, indicating the more efficient coupling of the longitudinal mode to the farfield than the transverse mode. This near-field amplitude ratio increases with the length-to-width aspect ratio of the nanorods, and electromagnetic simulation suggests a similar trend in the scattering cross section. In addition, by choosing the polarization of the laser light such that either the probe or the sample is preferentially excited, the near-field profiles of the dipolar surface plasmon modes induced by the incident light and by the field localized at the probing tip are identified. In accordance with the reciprocity relations of the tip−sample optical coupling, identical near-field optical amplitude and phase contrast have been obtained when the plasmon modes are excited by the incident field and by the field localized at the tip.

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INTRODUCTION Colloidal gold nanorods have desirable optical properties for many applications including chemical sensing,1−5 surfaceenhanced spectroscopy,6−12 biomedical imaging/therapy,13−16 and plasmonic hybrid materials/devices.17−27 The polarizability of the nanorods along the long and short axes results in plasmon resonances with different frequencies, known respectively as longitudinal and transverse modes. The fact that the longitudinal plasmon resonance frequency can be tuned from the visible to the infrared spectral region by increasing the length to width aspect ratio of the nanorods28 through simple chemical synthesis allows controlling and tailoring the optical properties for targeted applications, drawing intense research interest in the synthesis,29−35 biological effects,36,37 surface functionalization,38−46 and optical characterization.47−58 Owing to their structural anisotropy, the scattering and absorption cross sections of gold nanorods depend on their orientation with respect to the excitation electric field. This orientation dependence has been well studied using dark-field imaging/scattering spectroscopy and shows a strong dependence of scattering intensity on the polarization of the excitation light.47,52,58,59 A similar trend has been observed in two-photon photoluminescence60 and photothermal imaging61 experiments. However, none of these experiments allow direct correlation between the nanostructures and the optical response. The © 2014 American Chemical Society

optical response that depends on particle size, shape, and orientation can directly be visualized by mapping the near-field distribution on the surfaces of the nanomaterials. Using aperture-based near-field scanning optical microscope (NSOM), Okamoto and co-workers have studied the mode profiles of the dipole and higher order plasmon resonances on relatively long gold nanorods (length on the order of 400 nm) with a spatial resolution beyond the diffraction limit of light.48,56 However, the spatial resolution of NSOM is still limited by the practical aperture size of the probe and may not be applicable to map the mode profiles on sub-100 nm plasmonic nanostructures. The fact that most commonly synthesized gold nanorods (and other colloidal nanostructures) for many applications have dimensions less than 100 nm makes an optical imaging technique that can resolve the field distribution with a few nanometers spatial resolution imperative. High spatial resolution (on the order of 20 nm) images of plasmon modes can be obtained using optical excitation in electron microscopy.62−64 However, in addition to the expensive vacuum requirement, this technique lacks polarization selectivity, and no phase information can be obtained. Received: January 28, 2014 Revised: April 9, 2014 Published: April 9, 2014 9119

dx.doi.org/10.1021/jp501007a | J. Phys. Chem. C 2014, 118, 9119−9127

The Journal of Physical Chemistry C

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The use of readily available sharp probes in scattering-type or apertureless near-field scanning optical microscope (ANSOM)65,66 achieves spatial resolution on the order of 10 nm and provides both near-field amplitude and phase contrast simultaneous with the topographic image as it has been demonstrated on various nanoscale materials.67−73 However, most of the near-field imaging studies using ANSOM have been focused on lithographically fabricated flat samples (with only few reports on chemically synthesized nanostructures69,74−76), whereas most of the promising applications of nanomaterial in chemistry and biology are based on colloidal nanoparticles, which have different crystallographic, morphological, and surface properties from that of lithographically fabricated nanostructures. Near-field optical characterization of plasmonic colloidal nanomaterials can lead to chemical mapping of surface properties measuring the Raman scattering by the stabilizing surface ligands. Here, we report the first near-field optical characterization of chemically synthesized short gold nanorods (∼40 nm width and ∼80 nm length) using ANSOM. The orientation-dependent optical response of gold nanorods randomly dispersed on oxide-coated silicon wafer is directly visualized by recording the topography, optical amplitude, and optical phase images simultaneously. Depending on the orientation of the nanorods, the dipolar characters of the longitudinal and transverse plasmon modes have been clearly visualized in both amplitude and phase images. Using the relative near-field amplitude of the longitudinal and the transverse plasmon modes, it is shown that the longitudinal plasmon mode couples to the far-field more efficiently than the transverse plasmon mode. Proportional to the corresponding scattering cross-section ratio, the longitudinal-to-transverse mode near-field amplitude ratio increases with increasing length-to-width aspect ratio of the nanorods. In addition, by taking advantage of the structural anisotropy of the nanorods and the preferential excitation of the sample or the probe depending on the laser polarization, the near-field distribution of the plasmon modes excited by the incident light and by the field localized at the probing tip has been identified. In accordance with the reciprocity relations of the tip−sample optical coupling, identical near-field optical amplitude and phase contrast have been obtained when the plasmon modes are excited by the incident field and by the field localized at the tip.

Figure 1. Schematic of the ANSOM setup. Linearly polarized laser beam is split 50:50 using a beam splitter (BS). The beam that passes through the BS is focused at the tip−sample interaction area using a parabolic mirror (PM). The scattered light is collected by the same PM and mixed with the reference beam and directed to a photodiode detector. The output of the detector is demodulated at nΩ (Ω ≡ cantilever oscillation frequency), and the data recorded at 4Ω are presented. Pseudoheterodyne interferometric detection is implemented by modulating mirror (M) at frequency F ∼300 Hz. ki, ks, and kr represent the propagation vectors of incident light, scattered light, and reference beam, respectively. The laser polarization is controlled by the half-waveplate (HWP) and that of the reference beam by a quarter-waveplate (QWP). The polarization of detected light is determined by the polarization of the reference beam. EP ≡ vertical polarization (parallel to tip-axis), ES ≡ in-plane polarization (perpendicular to tip-axis), and these polarizations are referred in the text simply as S and P.

when measured before the beamsplitter. The images are obtained using silicon probes immediately after etching the native oxide in hydrofluoric acid. The incidence angle with respect to surface normal is 60°, and for vertically polarized light, the electric field vector has components parallel (Ez ≡ EP) and perpendicular (Ey) to the tip axis. Similarly, the propagation vector (k) has vertical (kz) and horizontal (ky) projections. For the horizontal polarization, the electric field of the laser is entirely in the sample plane (Ex ≡ ES). Following past notations,72,75,77 in the subsequent discussion, P and S refer to the polarization of the incident and scattered light that corresponds to the Ez and Ex components, respectively. For example, SP polarization refers to excitation of the sample with an in-plane polarized light (electric field perpendicular to the tip axis, S-polarized) and detection of the vertical (electric field parallel to the tip, P-polarized) component of the scattered light. Colloidal gold nanorods (AuNRs) obtained from Nanopartz Inc. are dispersed on oxide-coated silicon wafer either by immersing the substrate in the solution or by drop-casting the solution after removing the excess surfactant (cetyltrimethylammonium bromide, CTAB) via centrifugation and resuspension procedures. A single particle dark-field scattering measurement is used to determine the overlap of the plasmon resonance with the excitation wavelength when the AuNRs are deposited on a glass substrate. The experimental results have been compared with the results of electromagnetic simulation carried out using the finite-difference time domain (FDTD) method and implemented with the Lumerical software. Unless stated otherwise, a total-field/scattered-field source scheme is used to introduce light energy into the simulation region where the grid size is 0.8 nm in x, y, and z directions. The bulk dielectric constants of

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EXPERIMENTAL AND THEORETICAL METHODS The ANSOM experimental setup is shown in the schematic in Figure 1. It is based on the atomic force microscope (AFM) that is customized for pseudoheterodyne interferometric detection of scattered light (Neaspec GmbH). A linearly polarized laser beam is split (50:50) into excitation and reference beams. The excitation beam is focused at the probe tip using a parabolic mirror. The scattered light is collected through the same parabolic mirror and mixed with the reference beam. The intensity of the reference beam is much higher than that of the scattered light, and its polarization determines the polarization of the detected scattered light. The AFM is operated in a tapping mode near the resonance oscillation frequency (Ω ≈ 250 kHz) of the cantilever at tapping amplitude of ∼50 nm. The scattered light plus reference beam is detected using a silicon photodiode. The output of the detector is demodulated at nΩ, and the optical images recorded at n = 4 are presented. The near-field images have been acquired at 632.8 nm (HeNe output) at ∼3 mW 9120



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gold78 are used to model the nanostructures. The nanorods are modeled as a cylinder capped with a hemisphere at each end and also as a cylinder with rounded ends.

reproduced modeling the nanorod as a prolate spheroid and applying electrostatic approximations.79 For a prolate spheriod with radii a and b (a > b), when the applied field is parallel to the long axis, the polarizability α is given as ε − εm α = 4πab2 3εm + 3L(ε − εm) (1)

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RESULTS AND DISCUSSION A representative scanning electron microscope image of the gold nanorods dispersed on oxide-coated silicon wafer is shown in Figure 2a. The average width and length of the gold

where ε is the dielectric function of the particle, εm is the dielectric constant of the medium where the particle is embedded, and L is a geometrical parameter that depends on the eccentricity e, e = (1 − a2/b2)1/2, of the prolate shape according to the relation given as L=

1 − e 2 ⎛⎜ 1 1 + e ⎞⎟ −1 + ln 2 ⎝ 2e 1 − e⎠ e

(2)

The position of the resonance peaks that depends on the medium dielectric constant can then be determined by calculating the scattering and absorption cross sections using the following relations.79 scattering cross section: absorption cross section:

σsca =

k4 2 |α | 6π

σabs = k Im(α)

(3) (4)

where k = 2π/λ. The scattering and absorption cross sections obtained from eqs 3 and 4 at λ = 632.8 nm are plotted in Figure S1. For a fixed aspect ratio (2.0), corresponding to the average width (43 nm) and length (86 nm) of the gold nanorods, the calculated resonance peak position matches the observed average peak for εm = 2.1. The resonance of the cylindrical shape gold nanostructure is expected to appear at longer wavelength than that of the prolate spheroid,80 and using FDTD simulation, the shift is estimated as ∼32 nm that can be compensated by lowering the εm value to 1.95 compared to the 1.6 average of the air and glass dielectric constants.81 The larger than the air and glass average value may partly be attributed to the presence of the stabilizing ligands (CTAB) on the AuNR surface. It is also known that when a dipole is placed at two half-spaces, the energy preferentially radiates into the higher dielectric constant medium, resulting in the higher than average effective dielectric constant.82−84 A match to the solution phase absorption peak is obtained at εm = 2.87, which is adjusted to 2.34 to compensate for the geometrical difference of the prolate spheroid from the cylindrical shape. In general, the theoretical analysis provides intuitive understanding of the local environment and the measured spectra shows that the longitudinal dipolar plasmon resonances of the gold nanorods have significant overlap with the 632.8 nm excitation wavelength. Simultaneously recorded topography and near-field optical images of the AuNRs are shown in Figures 3a and 3b, respectively. The optical image is obtained using excitation polarization as shown in the schematics in Figures 4a and 4b for nanorods oriented parallel and perpendicular to the excitation electric field (solid blue line), respectively. The vertical nearfield component that is orthogonal to the excitation electric field is selectively mapped. The nanorods are randomly oriented in the sample plane and larger near-field amplitude is observed when the long axis of the nanorods is oriented parallel to the laser field as can be seen by comparing the images in Figures 3a and 3b. The orientation-dependent optical response can be seen more clearly by comparing the enlarged topographic image (Figure 3c) with the optical amplitude and

Figure 2. (a) Scanning electron microscope image of gold nanorods dispersed on oxide-coated silicon wafer. (b) Population distribution of gold nanorods plotted as a function of length-to-width aspect ratio. (c) Scattering spectrum of single gold nanorod supported on a coverglass. The pink line is the HeNe laser line indicating the overlap with the plasmon resonance band.

nanorods are 43 ± 3 nm and 86 ± 6 nm, respectively, where the uncertainty represents one standard deviation. The distribution of the length-to-width aspect ratio is shown in Figure 2b, centering at ∼2.0 as specified by the supplier (results for higher aspect ratio nanorods are also presented for comparison). In solution phase (aqueous), the plasmon resonance absorption has a maximum at λ ≈ 660 nm. The spectrum shown in Figure 2c is for a single gold nanorod supported on a glass coverslip, and repeating the measurement on several individual nanorods, the average plasmon resonance is determined as 614 ± 10 nm with full width at half-maximum of ∼48 nm. The significant blue-shift (greater than 40 nm) of the plasmon resonance energy compared to that of solution phase is consistent with previous report61 and can be 9121



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regardless of the size variation, the near-field amplitude is more intense for the parallel orientation than for the perpendicular, indicating larger oscillator strength of the longitudinal plasmon mode than that of the transverse mode. For the two gold nanorods oriented orthogonal to each other (labeled as I and II in Figure 3d), the optical amplitude and phase images obtained with four polarization combinations of excitation and detection are displayed in Figure 5. The excitation polarization can be visualized referring to the corresponding schematic in Figures 4a−d. The results in Figure 5a are the same as the results in Figures 3d,e and are given here to facilitate the comparison with the optical images obtained with the other polarizations. The images displayed in Figure 5b are obtained with the same excitation polarization as in Figures 5a: while the vertical Ez component is mapped in Figure 5a, the horizontal Ex component is selectively mapped in Figure 5b. Excitation with the in-plane polarized light leaves the probe essentially unexcited as indicated by the absence of field enhancement in Figure S2a while exciting the sample strongly due to the favorable orientation of the electric field that is parallel either to the short or long axis of the nanorods. As a result, the observed amplitude and phase contrast are in good agreement with the results of the FDTD simulations (Figure 4i) performed in the absence of the probe. While the amplitude for the SP and SS polarization may appear similar, there is a stark difference in the phase images. In both theory and experiment when crossing the nanorod end to end (for nanorod I) and side to side (for nanorod II), there is an ∼180° phase jump for the SP polarization while for the SS polarization the phase contrast is approximately the same at the two ends or sides. The optical images in Figures 5c and 5d are obtained with the laser polarization that has significant projection of the electric field along the tip axis as can be seen in Figures 4c and 4d. That is, due to the oblique incidence, the excitation electric field has projections in the sample plane (y-axis) as well as parallel to the probe axis (z-axes), resulting in the excitation of both the probe and the sample. The excitation of the probe is indicated by the strong field enhancement that is localized at the tip of a conical silicon probe as seen in Figure S2b. For nanorod I (Figures 4c and 5c), the excitation field is parallel to the short axis of the nanorod, resulting in the excitation of the transverse plasmon mode that peaks at ∼516 nm as seen in Figure 4g. For nanorod II (Figures 4d and 5c), the excitation field has components along the short and long axes, exciting both the transverse and longitudinal plasmon modes as seen in the spectra in Figure 4h. In Figure 5c, the vertical component of the near-field is mapped. For nanorod I, the excitation field induces a dipole (along the short axis) that is oriented perpendicular to the sample plane; that is, the near-field localizes at the nanorod−substrate interface (bottom) and at the opposite nanorod−air interface (top). In the experiment, the field distribution on the top half of the curved surface of the nanorod can be accessed. In effect, the near-field image in Figure 5c shows the field distribution at one of the poles of the transverse dipolar plasmon mode. As expected, there is no phase jump across the length of the nanorod in Figure 5c, which is in agreement with the calculated φ(Ez) phase as seen in Figure 4k. For nanorod II, the laser field has projection along the long axis, and mapping the Ez component reflects the dipolar character of the longitudinal mode that localizes the field at the two ends. The larger field amplitude on the far-end (with respect to the laser propagation direction) is consistent

Figure 3. Simultaneously recorded topography (a) and near-field optical amplitude (b) images of gold nanorods randomly dispersed on oxide-coated silicon wafer. Excitation laser is in-plane polarized and the vertical near-field component is selectively mapped. (c, d, e) Zoom-in topography (c), optical amplitude (d), and optical phase (e) images for the area in the dashed rectangles in (a) and (b). The numbers on the images of the nanorods in (c) are the diameter of the nanorods in nanometers determined from the topographic height. (f) Normalized near-field amplitude plotted as a function of the angle between the nanorod and the electric field of the laser. The data points represent the near-field amplitude of different nanorods with different orientations, and the solid line is a cosine squared function.

phase images in Figures 3d and 3e. In Figure 3f, the normalized near-field amplitude is plotted as a function of the angle between the excitation electric field and the long axis of the nanorod (the angle for each nanorod is determined using the ImageJ software). Although the general trend follows a cosine squared function (solid line), the data points fluctuate strongly due to the variation in the aspect ratio of the nanorods (Figure 2b) that results in different extent of resonance overlap with the excitation laser. This near-field amplitude fluctuation is consistent with the variation of the scattering peaks due to different nanorods, ranging from 596 to 630 nm. Nevertheless, the dependence of the near-field amplitude on the orientation of the gold nanorods with respect to the excitation field is apparent. The nanorods labeled I and II in Figure 3d are approximately (with in 10°) oriented parallel and perpendicular to the electric field of the laser, respectively. As labeled in Figure 3c, the diameter of the nanorods ranges from 38 to 45 nm. There are three nanorods oriented parallel to the laser field and three nanorods perpendicular to the laser field, and 9122



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Figure 4. (a, b, c, d) Schematics showing the orientation of nanorods I and II with respect to the propagation vector (solid red arrow) and electric field (solid blue arrow) of the laser. The absorption, scattering, and extinction cross sections calculated using the FDTD method of electromagnetic simulation are plotted (e, f, g, h) below the corresponding excitation configuration. (i, j, k, l) Calculated near-field amplitude and phase of Ez (top) and Ex (bottom) components (λ = 632.8 nm). Because of the different spatial localization of the Ex and Ez components, the field distributions are calculated on the xy-plane at z = 0 for Ex and z = 21.5 nm (radius of the cylinder) for Ez. In all cases the long axis of the nanorod is along the horizontal axis, and to indicate the rotation with respect to the configuration in (a)−(d), the x- and y-axes labels are shown at the top of the simulated images.

excitation by this localized field, and the excitation of the longitudinal mode of the nanorod oriented perpendicular to the incident field indicates that the field localized at the probing tip has all polarization components. In other words, the structural anisotropy allows distinguishing the excitation due to the incident light and due to the local field. The similarity of the images obtained with the SP (Figure 5a) and PS (Figure 5d) polarization is in accordance with the reciprocity relations.85 In the SP polarization, the sample excites the tip and in the PS polarization the tip excites the sample, and the scattering is the result of a coupled probe−sample optical interaction.72 We note that similar reciprocity relationship has been observed in aperture-type near-field optical microscopy where the same images have been obtained in collection and transmission mode experiments.86 In general, the observed optical amplitude and phase contrasts in Figures 3 and 5 are characteristic of the longitudinal and transverse plasmon modes depending on the orientation of the nanorods with respect to the excitation field. Comparing the relative amplitude ratio of the longitudinal and transverse plasmon modes, important insight can be extracted as discussed next. The near-field amplitude ratio of the longitudinal to the transverse mode is 2.7 ± 0.4 as determined

with the result of the simulation given in Figure 4l for the Ez component, and it is due to the oblique incidence of the laser. The slight distortion of the phase contrast in Figure 5c compared to the calculated φ(Ez) image in Figure 4l reflects the strong tip−sample optical interaction due to the excitation of the probe by the vertically polarized incident laser field as seen in Figure S2b. The amplitude and phase contrast in Figure 5d are obtained with the same excitation polarization as in Figure 5c but detecting the horizontal Ex component that is orthogonal to the excitation field. The images appear exactly the same as those obtained with the SP polarization in Figure 5a, where the sample is excited with in-plane polarized light and the vertical component is detected. The calculated optical contrasts (Figures 4k and 4l for the Ex component) are significantly different from the observation, indicating that the observed field distribution is not due to direct excitation by the incident laser field. Referring to the schematics in Figures 4c and 4d, it can be seen that the incident laser field has no electric field component along the x-axis. As mentioned above, the P-polarization is favorable to create field localization at the probing tip that can then excite the surface plasmon mode of the sample. The observed near-field profiles in Figure 5d are the result of 9123



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Figure 5. Near-field optical amplitude and phase images obtained with different combinations of excitation and detection polarizations on two gold nanorods oriented perpendicular to each other. The in-plane projection (ky) of the propagation vector and the electric field vector (E) or its projections (Ey, Ez) are indicated on the left side of the images. (a) SP-polarization: excitation with in-plane (S) polarized light and detection of the vertical (P) component. (b) SS-polarization: excitation same as in (a) and detection of the in-plane component of the near-field. (c) PP-polarization: excitation with vertically polarized light and detection of the vertical component. (d) PS-polarization: excitation same as in (c) and detection of the in-plane component.

Figure 6. (a) Comparison of the near-field amplitude when the electric field is oriented parallel (circles) and perpendicular (diamonds) to the long axis of 43 nm × 86 nm gold nanorod based on the configuration shown in Figures 4a and 4b. The plots are obtained by using the maximum values on a plane 1 nm away from the end (for the longitudinal mode) and away from the side (for the transverse mode) of the nanorod. The black and red curves are obtained modeling the nanorod as a cylinder capped with hemispheres at the two ends and a cylinder with rounded ends, respectively. (b) Ratio of longitudinal (σL) to transverse (σT) mode cross sections.

from the fourth harmonic signal (the signal at the third harmonic is much stronger but the ratio remains the same) compared to a calculated value of 3.3 peak to peak near-field amplitude ratio, modeling the nanorod as a cylinder capped with a hemisphere (see Figure 6a). The measured scattering spectra (Figure 2c) indicate that the plasmon resonance of the longitudinal mode overlaps with the 632.8 nm excitation wavelength more significantly than that of the transverse mode, and the observed ratio is expected to be larger than the calculated peak-to-peak ratio. However, the calculated value depends sensitively on the aspect ratio and slight deviation of the geometry from the real structure. For example, when a rounded cylinder is used instead of the cylinder capped with a hemisphere, the ratio becomes 2.2 (compare the peak values of the red curves in Figure 6a), placing the observed value within the simulation uncertainty. As can be seen comparing the absorption and scattering cross sections of the longitudinal (Figure 4e) and transverse (Figure 4f) plasmon modes, the observed near-field amplitude ratio is much smaller than the scattering cross-section ratio (∼30) but comparable to that of the absorption cross section (∼5). This trend indicates that the longitudinal plasmon mode couples to the far-field more efficiently than the transverse mode. As the scattering cross section is the far-field limit of the electric field amplitude on the surface of the nanostructures,87 a

similar trend is expected in the ratios of the scattering cross sections and the near-field amplitudes of the two modes as a function of the aspect ratio of the nanorods. In order to evaluate this trend, the near-field imaging experiment has been carried out on longer nanorods (same diameter as the nanorods considered above) with aspect ratio 2.9 and absorption peak wavelength of 700 nm (on the silica surface, the resonance peak is expected to shift to about 660 nm as illustrated in Figure S1; this blue-shift combined with the broad spectral line width can result in significant overlap with the 632.8 nm excitation wavelength). As can be seen comparing the topographic image (Figure 7a) and the optical amplitude image (Figure 7b), large near-field amplitude is observed on the gold nanorods that are oriented parallel to the excitation electric field (labeled 1 in Figure 7a). In contrast, for the nanorods oriented perpendicular to the excitation field (labeled 2), the signal-to-background ratio is very small, and the longitudinal to transverse mode amplitude ratio is ∼5. This corresponds to an ∼1.8× increase in the near-field amplitude ratio with respect to the corresponding value for the nanorods with the aspect ratio 2.0, which is in very 9124



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longitudinal to the transverse mode increases with the lengthto-width aspect ratio of the nanorods, and the results of electromagnetic simulation indicate the same trend in the scattering cross section. In addition, by choosing the polarization of the excitation light such that either the probe or the sample is preferentially excited, the excitation of the plasmon modes by the incident light and by the field localized at the probing tip are identified, which is facilitated by the structural anisotropy of the nanorods. In accordance with the reciprocity relations of the tip−sample optical coupling, identical near-field optical amplitude and phase contrast have been obtained when the plasmon modes are excited by the incident field and by the field localized at the tip.

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ASSOCIATED CONTENT

S Supporting Information *

Results of calculations based on electrostatic approximations and FDTD simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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Figure 7. Results on gold nanorods with length-to-width aspect ratio of 2.9 (a, b) and 3.7 (c, d).

ACKNOWLEDGMENTS The author thanks Prof. Steven Brueck for the valuable discussion.

good agreement with 1.7× increase in the scattering crosssection ratio as shown in Figure 6b. Finally, in order to confirm that the observed near-field amplitude is indeed due to plasmon excitation, the experiment has been repeated on AuNRs with aspect ratio 3.7 and resonance peak at 820 nm. In this case, the excitation wavelength (632.8 nm) is clearly outside the resonance band, and as can be seen in Figure 7d, the near-field amplitude is not recognizable from the background for any of the nanorods even with PP polarization. As demonstrated in Figure 5c, for PP polarization, appreciable near-field amplitude should be observed on all the nanorods regardless of their orientation as the localized field at the probing tip has all polarization components. The fact that the signal-to-background ratio is near unity despite this favorable excitation condition confirms that the observed field distribution for the nanorods with the aspect ratios 2.0 and 2.9 are indeed due to excitation of the localized surface plasmon resonances.

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REFERENCES

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CONCLUSION The optical response of short colloidal gold nanorods has been studied through detailed near-field analysis, mapping both the near-field amplitude and phase optical characteristics of the inplane and out-of-plane vector components selectively. For the gold nanorods dispersed randomly on oxide-coated silicon wafer, the near-field distributions that are characteristic of the longitudinal and transverse mode dipolar plasmon resonances have been clearly resolved when the long axes of the nanorods are aligned parallel and perpendicular to the in-plane polarized excitation electric field, respectively. The observed relative nearfield amplitude indicates that the longitudinal plasmon mode couples more efficiently to the far-field than the transverse mode. It is also shown that the amplitude ratio of the 9125



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Direct Near-Field Observation of Orientation-Dependent Optical

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