3D printing of plasmonic nanofocusing tip enabling high resolution, high throughput and high contrast optical near-field imaging.pdf
Long et al. Light: Science & Applications   (2023) 12:219  Ofﬁcial journal of the CIOMP 2047-7538 https://doi.org/10.1038/s41377-023-01272-6 www.nature.com/lsa A R T I C L E O p e n A c c e s s 3D printing of plasmonic nanofocusing tip enabling high resolution, high throughput and high contrast optical near-ﬁeld imaging Li Long1, Qiurong Deng1, Rongtao Huang1, Jiafang Li 2 and Zhi-Yuan Li1,3✉ Abstract Scanning near-ﬁeld optical microscopy (SNOM) offers a means to reach a ﬁne spatial resolution down to ~ 10 nm, but unfortunately suffers from low transmission efﬁciency of optical signal. Here we present design and 3D printing of a ﬁber-bound polymer-core/gold-shell spiral-grating conical tip that allows for coupling the inner incident optical signal to the outer surface plasmon polariton with high efﬁciency, which then adiabatically transport, squeeze, and interfere constructively at the tip apex to form a plasmonic superfocusing spot with tiny size and high brightness. Numerical simulations and optical measurements show that this specially designed and fabricated tip has 10% transmission efﬁciency, ~ 5 nm spatial resolution, 20 dB signal-to-noise ratio, and 7000 pixels per second fast scanning speed. This high-resolution, high throughput, and high contrast SNOM would open up a new frontier of high spatial-temporal resolution detecting, imaging, and monitoring of single-molecule physical, chemical, and biological systems, and deepen our understanding of their basic science in the single-molecule level. Introduction Optical microscopy offers a bridge connecting macro- scopic and microscopic world through imaging, spectro- scopy, and other means in space-time domain. But it also determines the spatial resolution of optical imaging to be micrometer scale at best, thanks to the diffraction effect of light wave, and this becomes the biggest disadvantage of conventional optical microscopy. In recent decades there has been increasing demand to lift this limitation and push the resolution down to nanometer scale, thus effectively upgrading optical microscopy into optical nanoscopy. Meanwhile, it is also highly desired that the advancement in the imaging resolution of nanoscopy is not in the price of severe degradation in many other advantages of conventional optical microscopy. History tells us that this is never an easy task. Among different means of super-resolution optical imaging schemes and technologies, scanning near-ﬁeld optical microscopy (SNOM) is a purely optical technique that can effectively lift the diffraction limit of resolution1–4. In early 1980s, the great success of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) stimulated the invention of SNOM5–9, and since then a wide variety of SNOM instruments have been developed, advanced, and placed into applications for basic sciences10,11. The classical SNOM techniques can be classiﬁed into the “aperture SNOM” (a-SNOM)3–8 and “aptertureless or scattering SNOM” (s-SNOM)9,12–19 categories. As sche- matically illustrated in Fig. 1a, a-SNOM can have high resolution and high contrast by reducing the aperture size, but suffers low throughput, while the s-SNOM, as sche- matically illustrated in Fig. 1b, can have high resolution by reducing the tip apex size, but suffers both low through- put and low signal contrast. Many schemes have been developed and adopted to relieve the difﬁculty of very low transmission efﬁciency intrinsic with classical a-SNOM and s-SNOM instruments. 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Correspondence: Zhi-Yuan Li (phzyli@scut.edu.cn) 1School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510641, China 2School of Physics, Beijing Institute of Technology, Beijing 100081, China Full list of author information is available at the end of the article 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; tapered ﬁber tip in a-SNOM, Xu et al. adopted complex- shape aperture such as H-shaped and bowtie-shaped aperture other than regular circular aperture and the transmission efﬁciency could increase by one order of magnitude20. Zhang et al. integrated a 100 nm nanoa- perture surrounded by a circular through grating that resembles a plasmonic lens on the metal coating of a conic SNOM tip21. With a balance of luminous ﬂux and aperture size, these deliberately designed probes still have very low optical throughput, resulting in weak signal light in use. Besides, the resolution is limited due to aperture size, at about 50-100 nm. Two major problems exist for apertureless s-SNOM, one is the weak transmission efﬁciency of signal light for use, the other is the strong background signal and thus bad signal-to-noise ratio (SNR) level. Ropers et al. d a b X Y Z 5 µm µm  c e f g h i k j   Aperture (25–100 nm) Sample XYZ Scanning probe Apertureless tip Top view Single mode Fiber SPPs Superfocusing spot 20 m 2 m 25 m 5 m 5 m 500 nnm 500 nnm   Fig. 1 Principle for design and fabrication of plasmonic nanofucsing tip for SNOM near-ﬁeld imaging. a Principle of operation for a classical a-SNOM. b Principle of operation for a classic s-SNOM. c Principle of operation for the current gold spiral-grating conical tip. d Schematic of plasmon nanofocusing conical tip directly built on the facet of optical ﬁber to serve as a high-performance SNOM tip under internal illumination. e Top-view optical microscope image of the fabricated SNOM tip. The outer dashed circle means the optical ﬁber edge, and the inner dashed square denotes the location of the center conical tip. f, g Top view and side view respectively of the optical image against a nanofocusing light spot at 785 nm emitting from the tip apex, as observed by a dark-ﬁeld microscope objective. h–k Scanning electron microscopy (SEM) image of the fabricated spiral- grating conical tip in different scale ranges of view Long et al. Light: Science & Applications  (2023) 12:219  Page 2 of 9 SNOM imaging experiments. Ideally this standard sample has an inﬁnitely sharp edge of air and chrome in geometry, and should also show a sharp edge of bright and dark region with zero cross-over distance w in the SNOM imaging with ideally inﬁnitely ﬁne resolution. But when the SNOM has ﬁnite resolution, the sharp edge would has a smooth cross- over distance from dark to bright region in the image. In more mathematical terms, the line groove proﬁle can be thought of as the so-called edge response function (ERF), and the characteristic width w of an ERF is known as the line spread function (LSF). The LSF represents an image of a linear object, usually a Gaussian function centered on the material boundary. The width of the LSF determines the resolution according to some speciﬁc standard such as Rayleigh criterion popularly used to evaluate the resolution of conventional optical microscope, which is just about half wavelength. Careful theoretical calculations presented in Methods Sec. S7 and Fig. S5 indicate that the resolution of SNOM is nearly equal to the size of the SNOM tip nano- focusing spot used to illuminate the standard chrome grating sample. Based on these theoretical knowledges, we have the capability to determine the nanofocusing spot size as well as the SNOM imaging resolution. The standard chrome grating sample is observed by our home-modiﬁed SNOM instrument armed with the gold spiral-grating conical tip operating in the illumination mode. The near-ﬁeld optical imaging is made at a scan- ning speed of 15 μm s-1 and a step size of 2 nm, corre- sponding to a scanning pixel speed of 7000 pixel s-1 and 0.13 ms per pixel. This is a very fast near-ﬁeld image acquiring speed and it can be only attributed to the ~10% high throughput of the SNOM tip. The 2D pictures of near-ﬁeld scanning image obtained for this standard chrome grating sample is shown in Fig. 4d. The bright region for each perforated slit domain and dark region for other intact chrome thin ﬁlm domains is distinct. According to the edge curve distribution of Fig. S5b in the supporting material, we use the Boltzmann distribution to ﬁt the experimental data. The 1D scanning curve of near ﬁeld picture across a slit as displayed in Fig. 4e-f shows a near-ﬁeld imaging resolution of about 5.7 nm for this SNOM tip, which clearly shows that this tip can push the SNOM resolution well below 10 nm. To further demonstrate the excellent performance of our home-made SNOM tip, including its independence upon the incident light polarization, we change the standard sample to a logo pattern composed of four let- ters as “SCUT” perforated into a 80 nm thick chrome thin ﬁlm, again by using the FIB lithography. Each letter consists of line grooves (slits) orienting in different directions, including the orthogonal X and Y direction. The SEM image is shown in Fig. 5a, where the slit width is set to be 198 nm. A typical 2D SNOM imaging picture recorded at the same scanning speed of 15 μm s-1 and scanning step of 2 nm for this complicated sample is displayed in Fig. 5b. Since in principle SEM can only reﬂect the surface morphology of the sample, it cannot disclose the detailed structure inside the sample slit. To determine this subtle local 3D geometric conﬁguration, we image the slit with a standard AFM instrument. The 2D AFM picture for the letter “S” is shown in Fig. 5c, and the 1D curve representing two white dotted lines R1 and R2 across the letter are displayed in Fig. 5d. There appear apparent unevenness at the edges of the FIB sculptured lines, indicating that the slits do not have ideally sharp edges in geometry, but rather involve smooth cross-over regions. This geometry apparently will cause some pattern unevenness in the SNOM imaging picture that has been illustrated in Fig. 5b. The existence of the cross-over region within a slit makes the SNOM image of this slit, a bright line, narrower than the ideal slit with sharp edge and no cross-over region, in good agreement with the observed SNOM picture. This conﬁrms once again that this home-made SNOM tip indeed has a very ﬁne reso- lution so that it can reveal clearly such kind of 3D geo- metric unevenness within the inner space of a slit. To accurately examine the resolution of the home-made SNOM tip against the complex pattern, we select two cross sections within the letters as marked in Fig. 5b, one is L1 along the vertical Y direction, and the other is L2 along the horizontal X direction. The corresponding 1D curves plotting the near-ﬁeld imaging data are displayed in Fig. 5e, f. The results show that the spatial resolution in the 2D scan for the complex 2D logo pattern can reach a very high value up to 4.5 nm in Y direction and 4.7 nm in X direction. No wonder this tip can reveal the geometric unevenness within practical slits sculptured within the chrome thin ﬁlm in a level matching up the powerful classical AFM instrument. When looking back at the near-ﬁeld imaging picture displayed in Fig. 4d for 1D grating slits, the unevenness within the picture, i.e., some parts of slit wider and some parts of slit narrower, now can be attributed to the geometric unevenness in various regions of slits across the practical grating sample perforated in the chrome thin ﬁlm. These data suggest the FIB nanofabrication technol- ogy should be improved to provide better standard samples for more reliable SNOM experiments. Discussion In summary, we have presented the design and 3D printing nanofabrication of a ﬁber-bound polymer-core/ gold-shell spiral-grating conical tip to serve as SNOM near- ﬁeld imaging probe, in a wish to achieve simultaneously high resolution, high throughput, and high contrast that are highly desirable but very difﬁcult to accomplish in previous schemes of a-SNOM and s-SNOM. Our design of SNOM tip is to introduce a gold spiral grating at the surface of dielectric conical tip and help to offer ﬂexible momentum Long et al. Light: Science & Applications  (2023) 12:219  Page 7 of 9 matching condition for coupling the inner optical signal coming from optical ﬁber via SPP excitation to the outer air-metal interface of gold conical tip with high efﬁciency and independent on the incident light polarization. Such a 3H SNOM tip and SNOM instrument would open up a new frontier of high spatial-temporal resolution detecting, imaging, and monitoring of single-molecule physical, che- mical, and biological systems, and deepen our under- standing of their basic science in the single-molecule level. Materials and methods Sample fabrications The direct-laser writing (DLW) 3D printing nanofabrica- tion technology used in this study is based on two-photon polymerization (2PP) technique that is implemented by using a commercial instrument system (Photonic Profes- sional, Nanoscribe GmbH), and by a homemade magnetron- sputtering technique. In fabrication, a 780 nm femtosecond laser beam (with pulse width 120 fs and repetition rate 80 MHz) is focused into a negative photoresist (IP-L-780, Nanoscribe GmbH) by a high numerical aperture (NA) oil- immersion objective (63×, NA = 1.4, Zeiss). Detailed opera- tion steps can refer to supplementary information Sec. S1. Numerical simulation FDTD simulations of electromagnetic ﬁeld for 1D grating To calculate the electromagnetic ﬁeld distribution for 1D model gold grating, we adopt 2D FDTD technique. Periodic boundary are added along all-directions, the mesh size is 2 nm, and the plane wave of p-polarization is incident at a b c e Normalized intensity(a.u.) Height(nm) Normalized height(nm) Normalized intensity(a.u.) Normalized intensity(a.u.) d f 1 m 1 m     L1 L2       400 nm   0 80 0 0.2 0.4 0.6 0.8 1.0 1.2 0 100 200 300 -100 -200 -300 0 0.2 0.4 0.6 0.8 1.0 0 200 Y(nm) X(nm) X(nm) 400 600 4.5 nm 205 nm 4.7 nm 198 nm R2 R1 R2 R1 L1 L2   200 400 600 0 0.2 0.4 0.6 0.8 1.0 0 1 Fig. 5 Experimental characterization of home-made SNOM tip resolution by performing near-ﬁeld imaging upon 2D letters of lamellar slits perforated in chrome thin ﬁlm. a SEM image of the “SCUT” logo pattern sample sculptured and perforated into 80 nm thick chromium ﬁlm. b The SNOM scan image for the “SCUT” logo sample. c AFM height image engraved with the letter “S”. d AFM image for the 1D proﬁle taken along the white dotted lines R1 and R2 as marked in (c). e, f 1D near-ﬁeld image curves scanned along white dotted lines L1 and L2 as marked in (b) Long et al. Light: Science & Applications  (2023) 12:219  Page 8 of 9 different angles with the wavelength 785 nm. The grating period is 750 nm and the semi-ellipse corrugation width is 580 nm. The gold ﬁlm has a thickness 80 nm, and the refractive index of the polymer (photoresist) is 1.52. FDTD simulations of electromagnetic ﬁeld for 3D spiral- grating conical tip The 3D FDTD method is used for the calculation of electromagnetic ﬁelds for the 3D spiral-grating conical tip. Perfectly matched layers (PML) are added along all- directions. The grid size is set as 5 nm to reduce the computation cost. The thickness of gold thin ﬁlm is 80 nm and the refractive index of the polymer (photoresist) is 1.52. The conical tip is excited by a Gaussian laser beam of a ﬁeld intensity of 1 Vm−1, 10 μm in diameter size and 5 μm in full width at half maximum (FWHM) diameter, in agreement with experiments. Both femtosecond pulse and continuous wave laser are used for full examination of the optical signal transport, SPP excitation and couplings, SPP transport and focus at the tip apex to form a nanofocusing spot. And See more details on methods and materials from supplementary information Sec. S5. Funding The authors are grateful for the ﬁnancial support from Guangdong Innovative and Entrepreneurial Research Team Program (2016ZT06C594), National Key R&D Program of China (2018YFA 0306200), and Science and Technology Project of Guangdong (2020B010190001). Author details 1School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510641, China. 2School of Physics, Beijing Institute of Technology, Beijing 100081, China. 3State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China Author contributions Z. Y. L. supervised the project. Z. Y. L. and L. L. conceived the 1D scheme for tip design, L. L. performed the theoretical design of 3D tip, made the 3D printing nanofabrication of designed tip, and performed near-ﬁeld imaging experiments. Q. D. offered assistance in numerical simulations and SNOM experiments, R. H. offered assistance in experiments, and J. Li offered assistance in FIB fabrication of chrome samples. Z. Y. L. and L. L. provided theoretical and experimental analyses, and wrote the manuscript. All authors participated in the discussion of results and reviewed the manuscript. Data availability All data needed to evaluate the conclusions in the paper are present in the main text. Conﬂict of interest The authors declare no competing interests. Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41377-023-01272-6. Received: 4 August 2023 Revised: 8 August 2023 Accepted: 22 August 2023 References 1. Heinzelmann, H. & Pohl, D. W. Scanning near-ﬁeld optical microscopy. Appl. Phys. A 59, 89–101 (1994). 2. Betzig, E. & Trautman, J. K. Near-ﬁeld optics: microscopy, spectroscopy, and surface modiﬁcation beyond the diffraction limit. Science 257, 189–195 (1992). 3. Synge, E. H. XXXVIII. A suggested method for extending microscopic resolu- tion into the ultra-microscopic region. Lond., Edinb., Dublin Philos. Mag. J. Sci. 6, 356–362 (1928). 4. Ash, E. A. & Nicholls, G. Super-resolution aperture scanning microscope. Nature 237, 510–512 (1972). 5. Pohl, D. W., Denk, W. & Lanz, M. Optical stethoscopy: Image recording with resolution λ/20. Appl. Phys. Lett. 44, 651–653 (1984). 6. Lewis, A. et al. Development of a 500 Å spatial resolution light microscope: I. light is efﬁciently transmitted through λ/16 diameter apertures. Ultramicro- scopy 13, 227–231 (1984). 7. Betzig, E., Isaacson, M. & Lewis, A. Collection mode near‐ﬁeld scanning optical microscopy. Appl. Phys. Lett. 51, 2088–2090 (1987). 8. Betzig, E. et al. Breaking the diffraction barrier: optical microscopy on a nanometric scale. Science 251, 1468–1470 (1991). 9. Inouye, Y. & Kawata, S. Near-ﬁeld scanning optical microscope with a metallic probe tip. Opt. Lett. 19, 159–161 (1994). 10. Abate, Y. et al. Nanoscopy reveals surface-metallic black phosphorus. Light- SciAppl. 5, e16162 (2016). 11. Hachtel, J. A. et al. Spatially and spectrally resolved orbital angular momentum interactions in plasmonic vortex generators. LightSciAppl 8, 33 (2019). 12. Zenhausern, F., Martin, Y. & Wickramasinghe, H. K. Scanning interferometric apertureless microscopy: optical imaging at 10 angstrom resolution. Science 269, 1083–1085 (1995). 13. Knoll, B. & Keilmann, F. Near-ﬁeld probing of vibrational absorption for che- mical microscopy. Nature 399, 134–137 (1999). 14. Hillenbrand, R., Taubner, T. & Keilmann, F. Phonon-enhanced light–matter interaction at the nanometre scale. Nature 418, 159–162 (2002). 15. Rang, M. et al. Optical near-ﬁeld mapping of plasmonic nanoprisms. Nano Lett. 8, 3357–3363 (2008). 16. Chen, J. N. et al. Optical nano-imaging of gate-tunable graphene plasmons. Nature 487, 77–81 (2012). 17. Zhang, R. et al. Chemical mapping of a single molecule by plasmon-enhanced Raman scattering. Nature 498, 82–86 (2013). 18. Zhang, Y. et al. Visualizing coherent intermolecular dipole–dipole coupling in real space. Nature 531, 623–627 (2016). 19. Chen, X. Z. et al. Modern scattering-type scanning near-ﬁeld optical micro- scopy for advanced material research. Adv. Mater. 31, 1804774 (2019). 20. Wang, L. & Xu, X. F. High transmission nanoscale bowtie-shaped aperture probe for near-ﬁeld optical imaging. Appl. Phys. Lett. 90, 261105 (2007). 21. Wang, Y. et al. Plasmonic nearﬁeld scanning probe with high transmission. Nano Lett. 8, 3041–3045 (2008). 22. Ropers, C. et al. Grating-coupling of surface plasmons onto metallic tips: a nanoconﬁned light source. Nano Lett. 7, 2784–2788 (2007). 23. Berweger, S. et al. Adiabatic tip-plasmon focusing for nano-raman spectro- scopy. J. Phys. Chem. Lett. 1, 3427–3432 (2010). 24. Lindquist, N. C. et al. Three-dimensional plasmonic nanofocusing. Nano Lett. 10, 1369–1373 (2010). 25. Wang, X. K. et al. Terahertz near-ﬁeld microscopy based on an air-plasma dynamic aperture. LightSciAppl 11, 129 (2022). 26. Xu, Q. Y. et al. Effects of edge on graphene plasmons as revealed by infrared nanoimaging. LightSciAppl 6, e16204 (2017). 27. Li, J. F. et al. Direct laser writing of symmetry-broken spiral tapers for polarization-insensitive three-dimensional plasmonic focusing. Laser Photonics Rev. 8, 602–609 (2014). 28. Jiang, R. H. et al. Near-ﬁeld plasmonic probe with super resolution and high throughput and signal-to-noise ratio. Nano Lett. 18, 881–885 (2018). 29. Auslender, M. & Hava, S. Scattering-matrix propagation algorithm in full- vectorial optics of multilayer grating structures. Opt. Lett. 21, 1765–1767 (1996). 30. Kawata, S. et al. Finer features for functional microdevices. Nature 412, 697–698 (2001). 31. Long, L. et al. Plasmonic enhanced ﬂuorescence via 3D printing spiral conical tapered gold tip bound to optical ﬁber. APL Photonics 7, 046107 (2022). Long et al. Light: Science & Applications  (2023) 12:219  Page 9 of 9
Title: 3D printing of plasmonic nanofocusing tip enabling high resolution, high throughput and high contrast optical near-field imaging ​
Authors: Li Long, Qiurong Deng, Rongtao Huang, Jiafang Li, and Zhi-Yuan Li ​
Publisher: Science & Applications, Official journal of the CIOMP ​
Date: 2023-09-06 00:00:00
Abstract: Scanning near-field optical microscopy (SNOM) offers a means to reach a fine spatial resolution down to ~10 nm, but unfortunately suffers from low transmission efficiency of optical signal. ​ Here we present design and 3D printing of a fiber-bound polymer-core/gold-shell spiral-grating conical tip that allows for coupling the inner incident optical signal to the outer surface plasmon polariton with high efficiency, which then adiabatically transport, squeeze, and interfere constructively at the tip apex to form a plasmonic superfocusing spot with tiny size and high brightness. ​ Numerical simulations and optical measurements show that this specially designed and fabricated tip has 10% transmission efficiency, ~5 nm spatial resolution, 20 dB signal-to-noise ratio, and 7000 pixels per second fast scanning speed. ​ This high-resolution, high throughput, and high contrast SNOM would open up a new frontier of high spatial-temporal resolution detecting, imaging, and monitoring of single-molecule physical, chemical, and biological systems, and deepen our understanding of their basic science at the single-molecule level. ​