Near-Field Plasmonic Probe with Super Resolution ... - ACS Publications

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Cite This: Nano Lett. 2018, 18, 881−885

Near-Field Plasmonic Probe with Super Resolution and High Throughput and Signal-to-Noise Ratio Ruei-Han Jiang,†,‡,§ Chi Chen,§ Ding-Zheng Lin,‡ He-Chun Chou,§ Jen-You Chu,*,‡ and Ta-Jen Yen*,†,‡ †

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Materials and Chemical Research Laboratory, Industrial Technology and Research Institute, Hsinchu 31057, Taiwan § Research Center for Applied Sciences, Academia Sinica, Taipei City, Taiwan Downloaded via DURHAM UNIV on July 19, 2018 at 11:48:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Near-field scanning optical microscopy (NSOM) enables observation of light-matter interaction with a spatial resolution far below the diffraction limit without the need for a vacuum environment. However, modern NSOM techniques remain subject to a few fundamental restrictions. For example, concerning the aperture tip (atip), the throughput is extremely low, and the lateral resolution is poor; both are limited by the aperture size. Meanwhile, with regard to the scattering tip (s-tip), the signal-tonoise ratio (SNR) appears to be almost zero; consequently, one cannot directly use the measured data. In this work, we present a plasmonic tip (p-tip) developed by tailoring subwavelength annuli so as to couple internal radial illumination to surface plasmon polaritons (SPPs), resulting in an ultrastrong, superfocused spot. Our p-tip supports both a radial symmetric SPP excitation and a Fabry−Pérot resonance, and experimental results indicate an optical resolution of 10 nm, a topographic resolution of 10 nm, a throughput of 3.28%, and an outstanding SNR of up to 18.2 (nearly free of background). The demonstrated p-tip outperforms state-of-the-art NSOM tips and can be readily employed in near-field optics, nanolithography, tip-enhanced Raman spectroscopy, and other applications. KEYWORDS: Near-field scanning optical microscopy (NSOM), plasmonic tip (p-tip), surface plasmon polariton (SPP), Fabry−Pérot resonance, super resolution, throughput, signal-to-noise ratio (SNR)

S

NSOM, the near-field optical signal is collected from the scattered light in the far field, causing interference between the light scattered from the tip and the illuminated sample to result in common and severe disturbances during measurement. Consequently, complicated interferometry, a lock-in technique and a suitable theoretical model14−16 are typically required to extract the near-field signal without background noise via nonfundamental harmonic demodulations.17−22 To enhance the spatial resolution, optical throughput, and signal-to-noise ratio (SNR) beyond those of conventional a-tips and s-tips, several methods involving plasmonic nanofocusing with internal illumination23−33 or side illumination34,35 have recently been proposed. However, to date, no solution that meets the three aforementioned demands has been demonstrated. Herein, with an exquisite nonperiodic multiring design, we present a high-efficiency plasmonic NSOM tip (p-tip) that supports a superfocusing mode under radially polarized (RP) excitation.36−44 This tailored p-tip can block the illumination background and then create a nanoemitter of a pure longitudinal

patially resolving objects at the nanoscale satisfies a wide range of demands in science and technology. To fulfill such demands, near-field scanning optical microscopy (NSOM) is a powerful technique that simultaneously enables optical imaging, spectroscopic analysis and chemical identification beyond the diffraction limit.1−9 To date, there are two major types of NSOM tips: an aperture-type tip (a-tip) and a scattering-type tip (s-tip; also known as an apertureless-type tip). The a-tip is constructed by introducing a tiny aperture to a fiber taper or to the tip apex of an atomic force microscope (AFM) probe. The size of the aperture fundamentally determines the optical resolution: the smaller the aperture, the finer the optical resolution. Nevertheless, the transmittance through the subwavelength aperture dramatically decreases with a factor of

, where d is the

diameter of the aperture and λ is the excitation wavelength.2,10,11 Therefore, as a trade-off between spatial resolution and optical throughput, the aperture size used in practice is larger than 60 nm, which fundamentally limits the optical resolution (60−100 nm) and topographic resolution (usually poorer than 100 nm). In contrast, the s-tip has no aperture but consists of an extremely sharp tip on a metallic or semiconducting probe, leading to better optical and topographic resolutions of approximately tens of nanometers.12,13 In such scattering-type © 2017 American Chemical Society

Received: September 27, 2017 Revised: December 18, 2017 Published: December 27, 2017 881

DOI: 10.1021/acs.nanolett.7b04142 Nano Lett. 2018, 18, 881−885

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Nano Letters

enhancement improvement with optimal slits (S1.2), are presented in the Supporting Information (S1 Plasmonic tip design). Note that we optimized the performance of the p-tip by deliberately tailoring the locations of these annular slits to satisfy the phase matching condition between the phase delay of the SPP wave propagation, Δφsp, and the phase delay of the excitation light propagation in media (i.e., SiO2 in this case), Δφm. Because |Δφsp − Δφm| equals 2nπ, which satisfies the condition of Fabry−Pérot resonance, the SPP wave propagates and constructively interferes at the tip apex, leading to the superfocusing mode with a spot size of 8 nm.45,46 For example, we internally illuminated RP light on the backside of the p-tip to excite the SPP wave on the Au film. The spot size was approximately 6 μm, as shown in Figure 1c. Subsequently, the SPP wave was directed and focused to the tip by the optimized annular slits and eventually coupled to far-field radiation due to the nanoantenna effect.47 As presented in Figure 1d, the spot size at the focal plane above the tip apex is substantially reduced to 400 nm in the far field, which is much smaller than the illuminated beam spot (i.e., 6 μm). The observed far-field radiating emission from the apex corresponds to 3.28% of the incident radiation. The p-tip possesses an optical throughput approximately four orders higher than that of a commercial a-tip with an 80 nm aperture (7.03 × 10−4 %) in measurement.11 The throughput measurement of the a-tip and p-tip are provided in the Supporting Information (Figure S5). Next, to quantify the optical resolution and to identify the mode of the p-tip, we equipped a modified NSOM system with our p-tip and a-tip (as a control) (Figure S5) to scan a standard Fischer’s pattern to define both the topographic and optical resolutions.48 Fischer’s pattern is composed of the Au projection pattern (60−100 nm) of a poly(methyl methacrylate) (PMMA) sphere (600 nm) on glass, the gap size of which ranges from 10 to 30 nm as a reference. First, using a commercial a-tip (Witec, aperture size ∼80 nm), the topographic image achieved a resolution of 60 ± 10 nm in the near-field scanning (Figure 2a). In addition, the NSOM image resolved the contour of the Au nanotriangles with a darker optical signal with a 48 ± 10 nm resolution (Figure 2b). Note that the definition of resolution follows a standard edge criterion49 in which the resolution corresponds to a width between 10% and 90% of the edge height, as shown in Figure 2c. In addition, we also observed a common offset of 40 nm between the topographic and optical images using the a-tip but no offset in our p-tip mapping. In addition to the absence of an offset, the p-tip enables much greater topographic and optical resolutions, as presented in Figure 2d,e. The topographic and optical resolutions of the p-tip were 10 ± 1 and 10 ± 1 nm, respectively, based on our experimental data (see Figure 2f). Furthermore, another interesting phenomenon worth mentioning is the reverse tone of the NSOM images obtained using the a-tip (Figure 2b) and ptip (Figure 2e). Regarding the a-tip, its aperture supports the transverse dipole,3,5,50 leading to a darker contrast with respect to the metal.51 However, the p-tip activated the longitudinal dipole39,43 at the tip apex; thus, the contrast in the metal became brighter.8,52,53 The detailed are presented in the Supporting Information (S3 Characterizations of near-field signal contrast at the a-tip and p-tip). To further scrutinize the resolution and SNR of the fabricated p-tip, we employed the p-tip to image a plasmonic lens.54,55 This plasmonic lens, comprising six 100 nm-wide rings inscribed into a 100 nm-thick Au film sputtered onto a glass substrate, has a

dipole with a diameter as small as 8 nm. In our NSOM measurements, this novel p-tip experimentally yielded an outstanding DC SNR of up to 18.2, in contrast to the DC SNR of almost 0 for the conventional s-tip. Finally, we demonstrated both 10 nm optical and 10 nm topographic resolutions. In short, our p-tip demonstrates super resolutions (both optical and topographic), extraordinary throughput, and outstanding SNR, outperforming the state-of-the-art a-tips and s-tips. The design of the p-tip and the excitation of the corresponding superfocusing mode are illustrated in Figure 1a.

Figure 1. Superfocusing mode of surface plasmons on the plasmonic tip. (a) Principle of the superfocusing mode excitation at the tip apex. The far-field RP beam excites radial SPPs on the plasmonic facet, which propagate along the shaft toward the tip apex, where they are reradiated into the far field. (b) SEM images of the SiO2 AFM probe with a Au coating prepared via sputtering and with a plasmonic facet prepared via FIB milling. (c) Spot size of the focused RP excitation. (d) Nonradiative SPP propagation leading to superfocusing and finally re-emission at the tip apex. (The images in (c,d) were collected using a high numerical aperture objective lens (100×, NA = 0.8)).

This p-tip was based on a commercially available SiO2 AFM probe (Nanosensor, uniqprobe), then coated with a 120 nm thick Au film, and finally inscribed with six annular slits. Here, the Au film supports the surface plasmon polariton (SPP) wave under the internal excitation of RP light from the backside. Then, the six annular slits couple and direct the excited SPP wave and ultimately boost the energy accumulation at the apex of the tip to achieve the superfocusing mode.45 Figure 1b shows a scanning electron microscopy (SEM) image of a fabricated tip where a SiO2 conical tip with a specifically aligned multiring structure was utilized as the body material. The inset shows an enlarged image of the tip apex, which is coated with a very smooth Au layer and sharp tip apex. Information on the nanofabrication process of the plasmonic tip (S1.1) and the 882

DOI: 10.1021/acs.nanolett.7b04142 Nano Lett. 2018, 18, 881−885

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to Ez field, showing a maximum spot in the center of plasmonic lens. On the contrary, the a-tip is more sensitive to Ex and Ey, showing a maximum annular ring in the center of plasmonic lens.56 Next, we retracted the p-tip by approximately 20 μm to determine whether the scattered image originates from planar SPPs. As seen in the inset of Figure 3f, we found no near-field pattern but a small scattering background when the tip was retracted, suggesting an excellent SNR of our p-tip. The measured SNR is calculated in Figure 3f, manifesting an excellent value as high as 18.2 for the zeroth order (dc) signal. Our p-tip thus substantially outperforms the conventional stip.57−59 With such a high resolution of the p-tip, we can observe the morphology of the asymmetric plasmonic lens and even study the detailed physical phenomena of the surface wave (Figure 4). In Figure 4a, the symmetric interference pattern is observed in the middle plate in the symmetric (P1) direction (see Figure S9a). However, the focusing spot is shifted approximately 94 nm to the shorter periodic slits in the asymmetric periodicity (P2 and P3) directions (see Figure S9b). The asymmetric periodicity caused the offset of the focus spot. The offset derivation is further discussed in depth in the Supporting Information (Figures S9 and S10). The SPP wave scattered by the slits with different periodicities has different scattered angles. In Figure 4b, the 20−40 nm dark line (red arrow) is observed in the slits. The astonishingly high transmission maximum was verified on the side of the metal, corresponding to the excitation of surface plasmons. The edge of the slits functioned as a wide range optical coupler; as a result, brighter lines were observed in the two sides of the slits.60−62 In the outer flat region (Figure 4c), the 608 nm fringe spacing corresponded to the superposition of the longitudinal field at the tip and the evanescent, phaseretarded SP field.19,63 In conclusion, we demonstrated a p-tip possessing six subwavelength annular gratings on the plasmonic facet of the tip for efficiently directing and constructively interfering with the SPPs at the tip apex. Subsequently, these SPPs formed the superfocusing mode and reradiated into the far field as a nanoemitter without background noise. Therefore, the technique can be expected to not only expand the s-NSOM measurement but also remove the need for high-order harmonic analysis and complex convolution of the s-tip and illuminated nanostructures. With this p-tip, we performed near-field optical experiments using a standard Fischer’s pattern, demonstrating super resolutions (optical resolution, 10 nm; topographic resolution, 10 nm). In addition, we characterized the plasmonic lens to scrutinize the propagation and interference of the surface waves, successfully demonstrating a very high SNR (∼18.2). The presented results proved that such a nanofabricated tip promises a near-background-free p-NSOM measurement. The p-tip offers several significant opportunities in nanoscale nearfield mapping, such as in the investigation of light-matter interactions, plasmonics, metamaterials and two-dimensional optoelectronic devices. We believe that the demonstrated p-tip has considerable potential for applications in background-free NSOM, tip-enhanced spectroscopy and nanolithography.64−67

Figure 2. Mapping the standard Fischer’s pattern using an a-tip and a ptip. (a) Topography and (b) corresponding NSOM image of Fischer’s pattern measured using an a-tip. (c) Height profile (blue circle, solid line) and corresponding NSOM intensity profile (red triangle, dashed line) of the sharpest edge obtained using an a-tip. (d) Topography and (e) corresponding NSOM image of Fischer’s pattern measured using a p-tip. (f) Height profile (blue circle, solid line) and corresponding NSOM intensity profile (red triangle, dashed line) of the sharpest edge obtained using a p-tip. (Note: the scale bars in the figures denote 100 nm, and the sharpest edges are indicated by blue dashed line in (a,b,d,e).)

symmetric periodicity along the horizontal direction but asymmetric periodicity along the vertical direction, which makes it suitable for observing the interference of SPP waves. Figure 3a shows the SEM image, indicating that the periodicities of the plasmonic lens are P1 = 601 nm, P2 = 650 nm, and P3 = 550 nm. As a result, the gradual periodic change in the radial periodicity was distributed from 650 to 550 nm. First, we applied a confocal microscope with RP illumination, as shown in Figure 3b, to observe this plasmonic lens in the far field. Because of the Abbé diffraction limitation, it is unsurprising that we found a blurred spot only in the center without resolving any detailed features, as shown in Figure 3c. Next, we inserted our p-tip into the same optical setup to image this plasmonic lens again. As displayed in Figure 3d, the topographic image clearly revealed the details of the plasmonic lens, which are almost identical to the SEM observation. In addition, the near-field optical image is also demonstrated in Figure 3e. We can observe the eccentric SPP focusing spot in the center plate, the tiny 30 nm-wide dark line in the slits and the clear SPP fringe in the surroundings. In our work, the experimental results by the p-tip is more sensitive

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b04142. 883

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Figure 3. Near-background-free plasmonic NSOM mapping of the asymmetric plasmonic lens. (a) SEM image of the asymmetric plasmonic lens. (b) Schematic of the NSOM measurement setup with the p-tip. (c) Optical image measured in the same NSOM setup without the p-tip. (d) Topography and (e) corresponding NSOM image measured using the p-tip. (f) SNR analysis, where the red crossed line and blue crossed line in the optical images were measured with the tip in and with the tip retracted, respectively. (Note: the scale bars in the figures denote 2 μm.)

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focus size at different tip apex diameter. Near-field scanning optical measurement setup and throughput measurement of the a-tip and p-tip. Characterizations of near-field signal contrast at the a-tip and p-tip: approaching curve on Au region of Fischer pattern; theoretical near field contrast. Analysis of offset between the center of the inner plate and the intensity peak in the asymmetric and symmetric directions (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: (T.-J.Y.) [email protected]. *E-mail: (J.-Y.C.) [email protected]. ORCID

Ta-Jen Yen: 0000-0002-2710-2002 Funding

This work was partially supported by the Ministry of Education’s “Aim for the Top University” program and by the Ministry of Science and Technology (MOST) under Contract Nos. 104− 2221-E-007-040-MY3, 106-2221-EE007-038-MY3, 106-2923E-007-003, and 104-2112-M-001-004-MY3. The financial support of the ITRI/MCL of Taiwan (Project No. H301AA5610) is also acknowledged. Notes

The authors declare no competing financial interest.

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Figure 4. High-resolution AFM and corresponding NSOM mapping in (a) the plate in the center, (b) the periodic grating region with the red arrows indicating local minima, and (c) the outer flat region of the plasmonic lens. (Note: the scan step size is 20 nm.)

ACKNOWLEDGMENTS The authors acknowledge the help of Dr. Pi-Ju Cheng, Professor Shu-Wei Chang, members of Professor Chen’s Lab at the Research Center for Applied Sciences, Academia Sinica, members of the M100 group at the Department of Materials and Chemical Research Laboratory, Industrial Technology and Research Institute, and members of Professor Yen’s Lab at the Department of Materials Science and Engineering, National

Plasmonic tip design: nanofabrication process of the plasmonic tip; the enhancement improvement with optimal slits; tip apex observation (1) SEM image (2), tip frontend reconstruction from AFM image; theoretical 884

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Tsing Hua University, for their insightful discussions and support.

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DOI: 10.1021/acs.nanolett.7b04142 Nano Lett. 2018, 18, 881−885