The probe-sample optical interaction in apertureless near-field optical microscopy is studied at 633 nm and 808 nm excitation wavelengths using gold nanodisks as model systems. The near-field distributions of the dipolar and quadrupolar surface plasmon modes have been mapped successfully using metal coated probes with different polarization combinations of excitation and detection except when the incident and the scattered light polarizations are chosen to be parallel to the probe axis. For the parallel polarization of the incident and the scattered light, the pattern of the near-field distribution differs from the inherent plasmon mode structures of the sample, depending sensitively on the sample size and excitation energy. For a given excitation energy, the near-field amplitude shifts from one pole to the other as the sample size increases, having nearly equal amplitude at the two poles when the plasmon resonance peak spectrally overlaps with the excitation energy.
© 2013 OSA
Optical imaging with spatial resolution beyond the diffraction limit has long been the subject of active scientific research [1,2]. Apertureless (scattering type) near-field scanning optical microscope (ANSOM) [3,4] has been demonstrated to image the near-field characteristics of materials including dielectric contrasts [5–8], infrared properties [9–11] and plasmon modes [12–18] with spatial resolution independent of the wavelength of light, resolving optical and spectroscopic properties on the order of 10 nm. The basic principle of ANSOM imaging involves focusing external laser light at the tip of atomic force microscope (AFM) probe and measuring the scattered light as the sample is raster scanned in close proximity to the tip. For a given sample and AFM operating parameters, the nature and strength of the probe-sample interaction depends on the radius of curvature of the probe tip , the material properties of the probe , the polarization of the excitation light with respect to the probe axis [20–22] and the wavelength of the excitation light . The probe is most sensitive to the electric field of light that is parallel to the probe axis [20–22], and earlier ANSOM experiments were based on this vertical (P) polarization of the excitation light. While the P-polarization affords detection of scattered light with high sensitivity, the excitation of the probe leads to strong probe-sample optical interactions, resulting in significant distortion of the intrinsic near-field distribution on the sample, particularly when a metallic probe is used .
When the electric field of the excitation light is oriented perpendicular to the probe axis [in-plane (S) polarization], the tip is expected to remain essentially unexcited regardless of the materials properties of the probe [20–22]. The use of silicon probes with in-plane polarized excitation light while detecting the in-plane [24–26] or out-of-plane [13,14,17,24–26] vector components of the near-field has been demonstrated to result in near-field images of the plasmon modes with field distribution patterns consistent with theoretical predictions for the samples in the absence of the tip. However, there are only a few reports that suggest metal coated probes can be used to map plasmonic modes [15,27] and it still remains unclear to what extent the influence of metallic tips can be decoupled by polarization selection. More importantly, while excitation with in-plane polarized light is suitable for plasmon near-field imaging, systematic investigation of the probe-sample optical interactions, which is significant when the electric field of the excitation light is parallel to the probe axis, is lacking. The parallel (vertical) polarization produces a large enhancement of the field that is localized at the tip of a conical metallic probe, which is a desired characteristic for spectroscopic applications [28–34]. Understanding the probe-sample optical interaction is crucial not only for mapping near-field distribution but also for the advancement of tip-enhanced spectroscopy and nanoscopy.
In this work, using lithographically fabricated gold nanodisks as model systems, it is first demonstrated that metal coated probes can be used to successfully map the dipolar and quadrupolar plasmon near-field distributions when in-plane polarized light is used for excitation and the scattered light that corresponds to both the in-plane and the out-of-plane near-field vector components is detected. Second, considering an incident light with significant projection of its electric field parallel to the probe axis and selectively detecting the near-field vector that is oriented parallel to the probe axis, the probe-sample interaction is investigated at λ = 633 nm and 808 nm as the disk diameter is systematically increased from 64 nm to 210 nm. For a given excitation wavelength, the near-field distribution is observed to evolve as a function of disk diameter showing characteristic trend as the plasmon resonance overlaps with the excitation energy.
2. Method and layout of the experiment
Polarization selective ANSOM imaging has been performed by configuring the optical components around the Neaspec atomic force microscope customized for pseudoheterodyne interferometric detection of scattered light as shown in Fig. 1. The details of the experimental procedures are available elsewhere [35,36], and briefly described as follows. Linearly polarized laser beam is focused at the probe tip using a parabolic mirror (NA = 0.46). The scattered light is collected through the same parabolic mirror and mixed with a reference beam. Polarization selection of the scattered light is achieved by controlling the polarization of the reference beam using a quarter waveplate. The polarization selection is further refined using a polarizer mounted close to the detector. The AFM is operated in a tapping mode (tapping amplitude ≈50 nm) near the resonance oscillation frequency (Ω ≈240 kHz) of the cantilever. The scattered light + reference beam is detected using a silicon photodiode (New Focus, Model 2051). The output of the detector is demodulated at nΩ, and the optical images obtained at n > 2 are used in the subsequent discussions.
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 probe axis [see left panel of the inset in Fig. 1]. For the horizontal polarization, the electric field of the laser is entirely in the sample plane (Ex ≡ ES) as seen in the right panel of the inset in Fig. 1. In the subsequent discussion, P and S refer to the polarization of the incident and scattered light that corresponds to the Ez and Ex, respectively. For example, SP polarization refers to excitation of the sample with an in-plane (S) polarized light and detection of the vertical (P) component of the scattered light.
The near-field optical imaging has been implemented on two sets of gold nanodisks, shown in the schematics in Fig. 2(a). The gold disks are fabricated using electron beam lithography and liftoff procedures on oxide coated (~300 nm thick) silicon wafers without any metallic adhesion layer, which can result in strong chemical interface plasmon damping . The nominal height of the disks is 24 nm as determined from the AFM images. The scattering cross-sections of the gold nanodisks have been calculated using FDTD simulations (implemented using Lumerical software) and representative spectra are plotted in Fig. 2(b). The outputs of a HeNe laser (λ = 633 nm) and a laser diode (λ = 808 nm) are used as sources of excitations. The laser lines are indicated by the dashed vertical lines in Fig. 2(b), and it can be seen that the dipolar resonances of the disks of the smaller diameter set (64 – 104 nm) overlap with the 633 nm laser line, with the extent of the overlap depending on the diameter. Similarly, the dipolar resonances of the larger diameter set (140 – 210 nm) overlap with the 808 nm laser line with different extent depending on the diameter. In addition, the larger diameter disk sets have quadrupolar plasmon resonances that are excited due to the retardation effects , which are accessible at the 633 nm laser line.
3. Mapping the dipolar and quadrupolar plasmon modes
Representative near-field optical images of the smaller diameter (64 - 104 nm) nanodisks are presented in Fig. 3(b) along with the simultaneously recorded topographic image shown in Fig. 3(a). The optical images are acquired with the SP polarization scheme, and the near-field distribution of the individual disks has two lobes that are localized at diametrically opposite ends of the disks, where the diameter is parallel to the electric field vector (indicated by the arrow on the image) of the excitation light. The double lobe structures of the near-field images agree well with the results of FDTD simulation [Fig. 3(c)] calculated in the absence of the probe. These double lobe structures are due to the dipole plasmon resonance modes excited on the nanodisks. The slight variation in the optical images of the disks of the same nominal diameter can be attributed to structural defects and anisotropy in the lithographically fabricated nanostructures.
The near-field imaging with the SP polarization scheme is extended on the larger diameter (140 - 210 nm) disk sets, and the topographic and optical images are presented in Figs. 4(a) and 4(b), respectively. The near-field distribution on the nanodisks has four lobes with more pronounced amplitude at the far-edge of the disk with respect to the laser propagation direction. The observed field distribution pattern is in agreement with the results of FDTD simulation [Fig. 4(c)] and is characteristic of the quadrupolar plasmon mode structures of the nanodisks. Referring to the calculated scattering cross-sections plotted in Fig. 2(b), it can be seen that the quadrupolar mode structures become more apparent as the spectral overlap of the resonance peak with the laser line (633 nm) becomes significant.
The plasmon near-field images presented in Figs. 3(b) and 4(b) are consistent with the previous results obtained using silicon tips , and is in very good agreement with the calculated near-field distributions on the disks in the absence of the tip, indicating that the orthogonal SP polarization scheme can minimize the tip influence beyond recognition even when metallic probes are used. As seen in the inset of Fig. 1, the probe is not excited with an S-polarized incident light and the tip-sample interaction is not expected to strongly influence the near-field distribution on the sample. The above results have been reproduced using a silicon probe. While the field distribution pattern remains the same, the signal-to-noise ratio is much higher when metal coated probes are used. It is also verified that the sensitivity of silicon probes can be improved when the native oxide is removed using hydrofluoric acid.
As mentioned above, in the SP polarization scheme the scattered light that corresponds to the near-field vector oriented parallel to the probe axis is detected, i.e.; the excitation and detection polarizations are orthogonal as described previously in detail . In this case, since the probe is sensitive to the vertical Ez component, it may be thought that the tip is excited by the sample, resulting in the scattering of the near-field [13,15]. The results presented in Fig. 5, however, show that the conical probe scatters both the in-plane (Ex) and out-of-plane (Ez) electric field components. The amplitude and phase images obtained with the SS polarization are presented in Figs. 5(b) and 5(g), which can be compared to the corresponding images obtained with the SP polarization shown in Figs. 5(a) and 5(f). Closely observing the amplitude images obtained with the SP and SS polarization, it can be seen that the two poles are more clearly separated for the SS polarization than for the SP consistent with the results of the simulation presented in Fig. 5(e). However, a more stark difference is observed when the phase images are compared: there is an ~180° phase jump crossing from one pole to the other for the SP polarization [Fig. 5(f)], while no apparent phase change is observed in the case of SS polarization [Fig. 5(g)]. The observed phase contrast agrees with the theoretical prediction presented in Fig. 5(e), indicating that the detected (scattered) light in the SS polarization scheme corresponds to the near-field component oriented in the sample plane. The observation of the in-plane component of the plasmon near-field suggests that, similar to an isotropic spherical particle probe , a conical probe effectively scatters the near-field even when the field vector is perpendicular to the tip axis, which is consistent with recent reports [24,25]. In general, for S-polarized excitation light, the observed plasmon mode structures of the nanodisks are as expected for isolated nanodisks, which implies that the probe remains unexcited, serving as a passive scatterer. The amplitude and phase images obtained with the PS polarization [Figs. 5(c) and 5(h)] are similar to those obtained with the SP polarization [Figs. 5(a) and 5(f)] in agreement with previous report . In the case of the PS polarization, the incident field excites the probe resulting in field localization at the tip, which in turn excites the plasmon mode of the sample due to the near-field interaction. The strong similarity of the images obtained with the SP and PS polarizations is in accordance to the reciprocity theory , and the result confirms that the strong background can be removed by the orthogonal detection scheme [13,42] even when the probe is strongly excited. We note that excitation of the plasmon mode by the field localized at the tip is expected to result in nearly circular near-field distribution around the gold nanodisks. However, our interferometric detection scheme allows mapping only the x- and z-components.
4. Size and wavelength dependent probe-sample interaction
When the incident light is P-polarized and the P-polarization component of the scattered light is detected, the observed near-field distribution differs significantly from what is expected for the isolated nanodisks, depending sensitively on the sample size as seen in Figs. 5(d). For the smallest disk diameter (64 nm), the near-field amplitude obtained at λ = 633 nm is larger on the near-edge (NE) of the gold disk than on the far-edge (FE). For the context of NE and FE see label in Fig. 5(d) and the discussion in . As the diameter of the disks increases, the NE to FE amplitude ratio decreases, attaining larger amplitude at the far-edge for diameter greater than 88 nm as seen in the line profile plotted in Fig. 6(a). In contrast, as can be seen in the corresponding simulation results in Fig. 5(e), the calculated near-field amplitude for the isolated 64 nm diameter disk is greater at the FE than at the NE, and the NE to FE ratio increases as the diameter of the disks increases from 64 nm to 104 nm. Clearly, the probe-sample interaction has altered not only the pattern of the inherent near-field distribution of the sample when a single disk is considered but also the trend as the sample size changes. Repeating the experiment at λ = 808 nm reveals that the reversal of the relative amplitude as a function of the sample size depends on the wavelength of the excitation energy. While at 633 nm excitation the maximum amplitude is shifted from NE to FE at diameter 96 nm [see the inset and the corresponding line profile in Fig. 6(a)], at 808 nm, the FE amplitude becomes dominant for diameters greater than 154 nm.
In order to gain insight into the probe-sample optical interaction, the field distribution on the gold nanodisks has been calculated in the presence of a platinum probe with 1 μm cone length and 25 nm radius of curvature of the tip. Representative results for the 633 nm excitation are presented in Fig. 6(b) showing the near-field distribution when the probe is located at the near-edge (left panel) and far-edge (right panel) 3 nm above the top surface of a 64 nm diameter disk. The difference in the near-field distributions for the NE and FE probe location can be explained in terms of the probe-sample configuration with respect to the laser field, indicated on the right panel in Fig. 6(b). For the NE arrangement, the polarizations of the tip and the sample are nearly anti-parallel, which is not favorable for strong dipole-dipole interaction as described by Aizpurua and associates .
On the other hand, the far-edge probe-sample arrangement results in parallel orientation of the tip and sample polarizations, leading to a strong dipole-dipole interaction that localizes the field in the tip-sample gap as shown in Fig. 6(b) right panel. In this case the inherent optical response of the sample is influenced considerably due to the near-field interaction with the tip of the probe, as it is evident in the calculated near-field distribution. Approximating a platinum probe as a point dipole, a spectral red-shift of the dipole plasmon resonance energy of a gold nanodisk has been predicted for P-polarized excitation light . However, it should be noted that the spectral red-shift for the small nanodisks (diameter less than 88 nm) is expected to increase the plasmon resonance overlap with the 633 nm laser line [see Fig. 2(b)], and does not explain the smaller near-field amplitude at the FE than at the NE. Despite this apparent contradiction between the results of the measurement and the simulation when a fixed sample size is considered, interesting trend can be extracted by analyzing the relative near-field amplitude ratio as a function of the diameter of the gold nanodisks as described next.
We first introduce a parameter ρ that is defined as NE to FE electric field amplitude ratio, ρ = |E|NE/|E|FE (the deviation of ρ from unity indicates the extent of asymmetry in the dipolar near-field distribution). When ρ extracted from the 633 nm excitation experiment is plotted as a function of the diameter of the nanodisks, it decreases as shown in Fig. 6(c) [blue curve]. We note that for the 633 nm excitation ρ is plotted up to 104 nm diameter because for larger disks the quadrupolar field distribution affects the simple dipolar field localization at the NE and FE poles. Similar to the experimental result, the maximum calculated ρ value is obtained for 64 nm diameter disk and it decreases with increasing diameter. The calculated absolute ρ value is arbitrary as it depends on the tip-sample gap. Therefore, the theoretical plot is displaced along the y-axis such that the theoretical maximum is equal to the experimental maximum as plotted in Fig. 6(c). Each experimental data point is an average of up to 15 nominally the same but different disks and also of measurements with different tips. The error bar is mainly due to variation from particle to particle rather than from measurement to measurement, and therefore can be attributed to the sensitivity of the FE probe-sample coupling to slight structural variations/defects. It is interesting to note that when silicon tip is used, ρ < 1 for all the disks with slight increase with decreasing disk diameter [black curve in Fig. 6(c)], indicating insignificant influence of the tip on the optical characteristics of the sample.
To confirm that the trend observed in the 633 nm experiment is not simply due to tip-sample size mismatch, similar plots have been reproduced for the experimental and simulation results obtained at 808 nm excitation. As seen in Fig. 6(c), while at λ = 633 nm, ρ < 1 for diameter greater than 88 nm, at λ = 808 nm, ρ > 1 even for diameter 154 nm. Then similar to the trend observed at λ = 633 nm, as the plasmon resonance peak shifts towards the laser line with increasing diameter, ρ decreases, crosses unity and becomes nearly constant after apparently a complete spectral overlap of the plasmon resonance with the laser line [Fig. 6(c), red curve]. In fact, in agreement to the results of the calculation, ρ appears to increase after attaining a minimum at the plasmon resonance. The faster increase of the theoretical ρ values beyond the minimum than the experimental values can be attributed to the reduced signal-to-noise ratio as the spectral overlap with the laser line is decreasing with further increase of diameter. In general the observation at the two excitation wavelengths suggest that, close to resonance, the plasmon near-field amplitude becomes large dominating the tip influence and resulting in the near-field images characteristics of dipolar mode structure. The asymmetry of the field distribution (ρ < 1) can partly be attributed to the oblique incidence of the laser, which favors larger field amplitude at the far-edge as can be seen in Fig. 5(e) last three panels. Off-resonance, the field distribution is mainly characteristic of the tip-sample interaction, resulting in harmonic oscillator type of picture that is more evident in the simulation result, pink curve in Fig. 6(c).
Comparing the near-field images obtained at the two excitation wavelengths with PP polarization, it can be seen that the resolution of the dipolar mode structures is better at λ = 633 nm than at λ = 808 nm. At the 633 nm excitation, the dipolar plasmon mode structure becomes evident with increasing overlap of the resonance peak with the laser line as seen in Figs. 5(d) and 6(a). On the other hand, at 808 nm, the corresponding dipolar mode on the larger disks has not been clearly resolved for any of the disks; instead as the diameter increases, the near-field amplitude shifts from NE to FE before the two poles are discernible as seen in Fig. 5(k). The increased obscurity of the dipolar signature at longer wavelength can be attributed to weaker near-field amplitude on the larger disks and/or greater field enhancement at the tip. Results of FDTD simulation indicate that the near-field amplitude on the small disks at 633 nm is comparable to the near-field amplitude on the large disks at 808 nm. For the platinum probe, the calculated field enhancement at 808 nm is about twice as that calculated at 633 nm, which may result in stronger probe-sample interaction, resulting in greater influence on the inherent near-field distribution of the sample. However, as seen in Fig. 5(l), even for the SS polarization more pronounced asymmetry is observed in the dipolar modes on some of the disks than in the corresponding images obtained at 633 nm. This observation suggests that the poorer resolution at 808 nm can result from the poorer beam quality of the laser diode output and polarization mixing by the parabolic mirror  and other optical components on the beam path. In fact, without using polarizers 1 and 2 in Fig. 1, the dipolar modes have not been clearly resolved at 808 nm for any of the nanodisks. On the other hand, at 633 nm excitation, the near-field images obtained in the absence and presence of the polarizers are practically the same.
In summary, systematic analyses of the probe-sample optical interactions in apertureless near-field scanning optical microscopy are presented for 633 nm and 808 nm excitation wavelengths using lithographically fabricated gold nanodisks as model systems. The near-field images of the dipolar and quadrupolar plasmon modes of gold nanodisks (diameter 64 – 210 nm) have been successfully mapped using metal coated probes. When the excitation light is polarized perpendicular to the probe axis, the observed near-field distributions agree very well with the results of electromagnetic simulations (performed on the isolated gold nanodisks) for both vertical (out-of-plane) and in-plane detection polarizations. For the incident light polarized parallel to the probe axis, it is shown that the strong background associated to the strong excitation of the probe can be removed through orthogonal detection polarization scheme. For the parallel excitation and detection polarization combination (PP), the probe-sample optical interaction depends on the sample size and excitation wavelength, and the observed near-field distributions differ from the inherent near-field distribution of the sample. For a given excitation wavelength, the near-field distribution is observed to evolve as a function of disk diameter showing characteristic trend as the plasmon resonance overlaps with the excitation energy.
The nanofabrication of the gold nanodisks was performed as a user project at the Molecular Foundry, Lawrence Berkeley National Laboratory, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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