Near-Field Plasmonic Probe with Super Resolution and High

Dec 27, 2017 - Near-field scanning optical microscopy (NSOM) enables observation of light-matter interaction with a spatial resolution far below the d...
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Near-Field Plasmonic Probe with Super Resolution and High Throughput and Signal-to-Noise Ratio Ruei Han Jiang, Ta-Jen Yen, Chi Chen, Ding-Zheng Lin, He Chun Chou, and Jen-You Chu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04142 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Near-Field Plasmonic Probe with Super Resolution

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and High Throughput and Signal-to-Noise Ratio

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Ruei-Han Jiang,†,‡,§ Chi Chen,§ Ding-Zheng Lin,‡ He-Chun Chou,§ Jen-You Chu,‡* Ta-Jen

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Yen†,‡,*

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30013, Taiwan

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Research Institute, Hsinchu 31057, Taiwan

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Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu

Department of Materials and Chemical Research Laboratory, Industrial Technology and

Research Center for Applied Sciences, Academia Sinica, Taipei City, Taiwan

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ABSTRACT. Near-field scanning optical microscopy (NSOM) enables observation of

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light-matter interaction with a spatial resolution far below the diffraction limit without the need for

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a vacuum environment. However, modern NSOM techniques remain subject to a few fundamental

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restrictions. For example, concerning the aperture tip (a-tip), the throughput is extremely low, and

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the lateral resolution is poor; both are limited by the aperture size. Meanwhile, with regard to the

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scattering tip (s-tip), the signal-to-noise ratio (SNR) appears to be almost zero; consequently, one

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cannot directly use the measured data. In this work, we present a plasmonic tip (p-tip) developed

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by tailoring subwavelength annuli so as to couple internal radial illumination to surface plasmon

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polaritons (SPPs), resulting in an ultra-strong, superfocused spot. Our p-tip supports both a radial

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symmetric SPP excitation and a Fabry-Pérot resonance, and experimental results indicate an

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optical resolution of 10 nm, a topographic resolution of 10 nm, a throughput of 3.28 %, and an

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outstanding SNR of up to 18.2 (nearly free of background). The demonstrated p-tip outperforms

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state-of-the-art NSOM tips and can be readily employed in near-field optics, nanolithography,

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tip-enhanced Raman spectroscopy and other applications.

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KEYWORDS. Near-field scanning optical microscopy (NSOM), plasmonic tip (p-tip), surface

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plasmon polariton (SPP), Fabry-Pérot resonance, super resolution, throughput, signal-to-noise

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ratio (SNR)

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Spatially resolving objects at the nanoscale satisfies a wide range of demands in science and

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technology. To fulfill such demands, near-field scanning optical microscopy (NSOM) is a

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powerful technique that simultaneously enables optical imaging, spectroscopic analysis and

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chemical identification beyond the diffraction limit.1-9 To date, there are two major types of

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NSOM tips: an aperture-type tip (a-tip) and a scattering-type tip (s-tip; also known as an

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apertureless-type tip). The a-tip is constructed by introducing a tiny aperture to a fiber taper or to

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the tip apex of an atomic force microscope (AFM) probe. The size of the aperture fundamentally

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determines the optical resolution—the smaller the aperture, the finer the optical resolution.

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Nevertheless, the transmittance through the subwavelength aperture dramatically decreases with a

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factor of   , where d is the diameter of the aperture and λ, the excitation wavelength.2,10,11 

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Therefore, as a tradeoff between spatial resolution and optical throughput, the aperture size used in

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practice is larger than 60 nm, which fundamentally limits the optical resolution (60-100 nm) and

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topographic resolution (usually poorer than 100 nm).

 

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In contrast, the s-tip has no aperture but consists of an extremely sharp tip on a metallic or

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semiconducting probe, leading to better optical and topographic resolutions of approximately tens

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of nanometers.12,13 In such scattering-type NSOM, the near-field optical signal is collected from

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the scattered light in the far field, causing interference between the light scattered from the tip and

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the illuminated sample to result in common and severe disturbances during measurement.

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Consequently, complicated interferometry, a lock-in technique and a suitable theoretical

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model14-16 are typically required to extract the near-field signal without background noise via

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non-fundamental harmonic demodulations.17-22

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To enhance the spatial resolution, optical throughput and signal-to-noise ratio (SNR) beyond

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those of conventional a-tips and s-tips, several methods involving plasmonic nanofocusing with

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internal illumination23-33 or side illumination34,35 have recently been proposed. However, to date,

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no solution that meets the three aforementioned demands has been demonstrated. Herein, with an

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exquisite non-periodic multi-ring design, we present a high-efficiency plasmonic NSOM tip

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(p-tip) that supports a superfocusing mode under radially polarized (RP) excitation.36-44 This

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tailored p-tip can block the illumination background and then create a nanoemitter of a pure

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longitudinal dipole with a diameter as small as 8 nm. In our NSOM measurements, this novel p-tip

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experimentally yielded an outstanding DC SNR of up to 18.2, in contrast to the DC SNR of almost

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0 for the conventional s-tip. Finally, we demonstrated both 10 nm optical and 10 nm topographic

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resolutions. In short, our p-tip demonstrates super resolutions (both optical and topographic),

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extraordinary throughput, and outstanding SNR, outperforming the state-of-the-art a-tips and

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s-tips.

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The design of the p-tip and the excitation of the corresponding superfocusing mode are illustrated

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in Figure 1a. This p-tip was based on a commercially available SiO2 AFM probe (Nanosensor,

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uniqprobe®), then coated with a 120 nm-thick Au film, and finally inscribed with six annular slits.

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Here, the Au film supports the surface plasmon polariton (SPP) wave under the internal excitation

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of RP light from the backside. Then, the six annular slits couple and direct the excited SPP wave

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and ultimately boost the energy accumulation at the apex of the tip to achieve the superfocusing

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mode.45 Figure 1b shows a scanning electron microscopy (SEM) image of a fabricated tip, where a

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SiO2 conical tip, with a specifically aligned multi-ring structure, was utilized as the body material.

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The inset shows an enlarged image of the tip apex, which is coated with a very smooth Au layer

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and sharp tip apex. Information on the nanofabrication process of the plasmonic tip (S1.1) and the

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enhancement improvement with optimal slits (S1.2), are presented in the supplemental

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information (S1 Plasmonic tip design).

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Figure 1. Superfocusing mode of surface plasmons on the plasmonic tip. (a) Principle of the

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superfocusing mode excitation at the tip apex. The far-field RP beam excites radial SPPs on the

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plasmonic facet, which propagate along the shaft toward the tip apex, where they are reradiated

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into the far field. (b) SEM images of the SiO2 AFM probe with a Au coating prepared via

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sputtering and with a plasmonic facet prepared via FIB milling. (c) Spot size of the focused RP

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excitation. (d) Non-radiative SPP propagation leading to superfocusing and finally re-emission at

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the tip apex. (The images in (c) and (d) were collected using a high numerical aperture objective

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lens (100X, NA = 0.8)).

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Note that we optimized the performance of the p-tip by deliberately tailoring the locations of

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these annular slits to satisfy the phase matching condition between the phase delay of the SPP

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wave propagation, ∆φsp, and the phase delay of the excitation light propagation in media (i.e., SiO2

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in this case), ∆φm. Because |∆φsp-∆φm| equals 2nπ, which satisfies the condition of Fabry-Pérot

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resonance, the SPP wave propagates and constructively interferes at the tip apex, leading to the

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superfocusing mode with a spot size of 8 nm.45,46 For example, we internally illuminated RP light

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on the backside of the p-tip to excite the SPP wave on the Au film. The spot size was

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approximately 6 µm, as shown in Figure 1c. Subsequently, the SPP wave was directed and focused

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to the tip by the optimized annular slits and eventually coupled to far-field radiation due to the

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nanoantenna effect.47 As presented in Figure 1d, the spot size at the focal plane above the tip apex

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is substantially reduced to 400 nm in the far field, which is much smaller than the illuminated beam

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spot (i.e., 6 µm). The observed far-field radiating emission from the apex corresponds to

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3.28 % of the incident radiation. The p-tip possesses an optical throughput approximately four

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orders higher than that of a commercial a-tip with an 80 nm aperture (7.03 × 10 %) in

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measurement.11 The throughput measurement of the a-tip and p-tip are provided in the

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supplemental information (Figure S5).

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Next, to quantify the optical resolution and to identify the mode of the p-tip, we equipped a

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modified NSOM system with our p-tip and a-tip (as a control) (Figure S5) to scan a standard

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Fischer’s pattern to define both the topographic and optical resolutions.48 Fischer’s pattern is

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composed of the Au projection pattern (60 -100 nm) of a polymethylmethacrylate (PMMA) sphere

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(600 nm) on glass, the gap size of which ranges from 10 to 30 nm as a reference. First, using a

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commercial a-tip (Witec, aperture size ~ 80 nm), the topographic image achieved a resolution of

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60 ± 10 nm in the near-field scanning (Figure 2a). In addition, the NSOM image resolved the

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contour of the Au nanotriangles, with a darker optical signal with a 48 ± 10 nm resolution

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(Figure 2b). Note that the definition of resolution follows a standard edge criterion,49 in which the

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resolution corresponds to a width between 10 % and 90 % of the edge height, as shown in Figure

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2c. In addition, we also observed a common offset of 40 nm between the topographic and optical

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images using the a-tip but no offset in our p-tip mapping. In addition to the absence of an offset, the

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p-tip enables much greater topographic and optical resolutions, as presented in Figures 2d and 2e.

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The topographic and optical resolutions of the p-tip were 10 ± 1 nm and 10 ± 1 nm, respectively,

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based on our experimental data (see Figure 2f). Furthermore, another interesting phenomenon

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worth mentioning is the reverse tone of the NSOM images obtained using the a-tip (Figure 2b) and

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p-tip (Figure 2e). Regarding the a-tip, its aperture supports the transverse dipole,3,5,50 leading to a

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darker contrast with respect to the metal51. However, the p-tip activated the longitudinal dipole39,43

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at the tip apex; thus, the contrast in the metal became brighter.8,52,53 The detailed are presented in

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the supplemental information (S3 Characterizations of near-field signal contrast at the a-tip and

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p-tip).

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Figure 2. Mapping the standard Fischer’s pattern using an a-tip and a p-tip. (a) Topography and

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(b) corresponding NSOM image of Fischer’s pattern measured using an a-tip. (c) Height profile

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(blue circle, solid line) and corresponding NSOM intensity profile (red triangle, dashed line) of

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the sharpest edge obtained using an a-tip. (d) Topography and (e) corresponding NSOM image

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of Fischer’s pattern measured using a p-tip. (f) Height profile (blue circle, solid line) and

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corresponding NSOM intensity profile (red triangle, dashed line) of the sharpest edge obtained

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using a p-tip. (Note: the scale bars in the figures denote 100 nm, and the sharpest edges are

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indicated by blue dashed line in (a), (b), (d), and (e).)

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To further scrutinize the resolution and SNR of the fabricated p-tip, we employed the p-tip to

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image a plasmonic lens.54-55 This plasmonic lens, comprising six 100 nm-wide rings inscribed

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into a 100 nm-thick Au film sputtered onto a glass substrate, has a symmetric periodicity along

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the horizontal direction but asymmetric periodicity along the vertical direction, which makes it

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suitable for observing the interference of SPP waves. Figure 3a shows the SEM image, indicating

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that the periodicities of the plasmonic lens are P1 = 601 nm, P2 = 650 nm, and P3 = 550 nm. As

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a result, the gradual periodic change in the radial periodicity was distributed from 650 nm to 550

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nm. First, we applied a confocal microscope with RP illumination, as shown in Figure 3b, to

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observe this plasmonic lens in the far field. Due to the Abbé diffraction limitation, it is

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unsurprising that we found a blurred spot only in the center without resolving any detailed

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features, as shown in Figure 3c. Next, we inserted our p-tip into the same optical setup to image

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this plasmonic lens again. As displayed in Figure 3d, the topographic image clearly revealed the

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details of the plasmonic lens, which are almost identical to the SEM observation. In addition, the

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near-field optical image is also demonstrated in Figure 3e. We can observe the eccentric SPP

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focusing spot in the center plate, the tiny 30 nm-wide dark line in the slits and the clear SPP

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fringe in the surroundings. In our work, the experimental results by the p-tip is more sensitive to

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Ez field, showing a maximum spot in the center of plasmonic lens. On the contrary, the a-tip is

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more sensitive to Ex and Ey, showing a maximum annular ring in the center of plasmonic lens.56

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Next, we retracted the p-tip by approximately 20 µm to determine whether the scattered image

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originates from planar SPPs. As seen in the inset of Figure 3f, we found no near-field pattern but

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a small scattering background when the tip was retracted, suggesting an excellent SNR of our

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p-tip. The measured SNR is calculated in Figure 3f, manifesting an excellent value as high as

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18.2 for the zeroth order (DC) signal. Our p-tip thus substantially outperforms the conventional

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s-tip.57-59

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Figure 3. Near-background-free plasmonic NSOM mapping of the asymmetric plasmonic lens.

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(a) SEM image of the asymmetric plasmonic lens. (b) Schematic of the NSOM measurement

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setup with the p-tip. (c) Optical image measured in the same NSOM setup without the p-tip. (d)

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Topography and (e) corresponding NSOM image measured using the p-tip. (f) SNR analysis,

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where the red crossed line and blue crossed line in the optical images were measured with the tip

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in and with the tip retracted, respectively. (Note: the scale bars in the figures denote 2 µm.)

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With such a high resolution of the p-tip, we can observe the morphology of the asymmetric

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plasmonic lens and even study the detailed physical phenomena of the surface wave (Figure 4).

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In Figure 4a, the symmetric interference pattern is observed in the middle plate in the symmetric

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(P1) direction (see Figure S9a). However, the focusing spot is shifted approximately 94 nm to

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the shorter periodic slits in the asymmetric periodicity (P2 and P3) directions (see Figure S9b).

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The asymmetric periodicity caused the offset of the focus spot. The offset derivation is further

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scattered by the slits with different periodicities has different scattered angles. In Figure 4b, the

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20 nm to 40 nm dark line (red arrow) is observed in the slits. The astonishingly high transmission

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maximum was verified on the side of the metal, corresponding to the excitation of surface

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plasmons. The edge of the slits functioned as a wide range optical coupler; as a result, brighter

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lines were observed in the two sides of the slits.60-62 In the outer flat region (Figure 4c), the 608

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nm fringe spacing corresponded to the superposition of the longitudinal field at the tip and the

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evanescent, phase-retarded SP field. 19,63

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Figure 4. High-resolution AFM and corresponding NSOM mapping in (a) the plate in the center;

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(b) the periodic grating region, with the red arrows indicating local minima; and (c) the outer flat

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region of the plasmonic lens. (Note: the scan step size is 20 nm.) 11 ACS Paragon Plus Environment

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In conclusion, we demonstrated a p-tip possessing six subwavelength annular gratings on the

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plasmonic facet of the tip for efficiently directing and constructively interfering with the SPPs at

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the tip apex. Subsequently, these SPPs formed the superfocusing mode and reradiated into the far

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field as a nanoemitter without background noise. Therefore, the technique can be expected to not

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only expand the s-NSOM measurement but also remove the need for high-order harmonic

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analysis and complex convolution of the s-tip and illuminated nanostructures. With this p-tip, we

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performed near-field optical experiments using a standard Fischer’s pattern, demonstrating super

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resolutions (optical resolution, 10 nm; topographic resolution, 10 nm). In addition, we

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characterized the plasmonic lens to scrutinize the propagation and interference of the surface

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waves, successfully demonstrating a very high SNR (~ 18.2). The presented results proved that

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such a nanofabricated tip promises a near-background-free p-NSOM measurement. The p-tip

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offers several significant opportunities in nanoscale near-field mapping, such as in the

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investigation of light-matter interactions, plasmonics, metamaterials and two-dimensional

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optoelectronic devices. We believe that the demonstrated p-tip has considerable potential for

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applications in background-free NSOM, tip-enhanced spectroscopy and nanolithography.64-67

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ASSOCIATED CONTENT

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Supporting Information

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S1 Plasmonic tip design

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1.1 Nanofabrication process of the plasmonic tip

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1.2 The enhancement improvement with optimal slits

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1.3 Tip apex observation (1) SEM image (2) Tip frontend reconstruction from AFM image

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1.4 Theoretical focus size at different tip apex diameter

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S2 Near-field scanning optical measurement setup and throughput measurement of the a-tip and

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p-tip

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S3 Characterizations of near-field signal contrast at the a-tip and p-tip

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3.1 Approaching curve on Au region of Fischer pattern

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3.2 Theoretical near field contrast

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S4 Analysis of offset between the center of the inner plate and the intensity peak in the

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asymmetric and symmetric directions

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AUTHOR INFORMATION

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Corresponding Author

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*Correspondence and requests for materials should be addressed to Ta-Jen Yen

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([email protected]) and Jen-You Chu ([email protected]).

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Funding Sources

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This work was partially supported by the Ministry of Education’s “Aim for the Top University”

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program and by the Ministry of Science and Technology (MOST) under Contract Nos.

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104-2221-E-007-040-MY3,

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104-2112-M-001-004-MY3. The financial support of the ITRI/MRL of Taiwan (Project No.

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G301AA5610) is also acknowledged.

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Notes

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The authors have no conflicts of interest to declare.

106-2221-EE007-038-MY3,

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106-2923-E-007-003,

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ACKNOWLEDGMENT

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The authors acknowledge the help of Dr. Pi-Ju Cheng, Prof. Shu-Wei Chang, members of Prof.

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Chen’s Lab at the Research Center for Applied Sciences, Academia Sinica, members of the

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M100 group at the Department of Materials and Chemical Research Laboratory, Industrial

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Technology and Research Institute, and members of Prof. Yen’s Lab at the Department of

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Materials Science and Engineering, National Tsing Hua University, for their insightful

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discussions and support.

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