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Abstract
We demonstrate coupling and directivity enhancement of electromagnetic fields emerging from a single metallic nanoaperture at the tip of a single-mode optical fiber. We achieve this by using circular grooves flanking the nanoaperture perforated in a 100 nm thick gold film. The film with nanostructure is transferred to the fiber tip by aligned stripping with optical epoxy. When incident from both sides of the nanoaperture, enhancement factors of 2.2 and 2.4 in power coupling into the fiber and in beaming into free-space were obtained. Numerical simulations show that the optimum grating period is nearly identical to the surface plasmon polariton wavelength that can be supported at the gold-epoxy interface. This integrated platform couples light between the single mode fiber and the nanoapeture without the need for cumbersome optics, with applications for optical trapping and single-photon detection.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Substantial effort has been devoted to improving light transmission through a subwavelength aperture in a metal film since the reemergence of plasmonics field motivated by the advancement in nanotechnology and nanoscale microscopy in the past two decades. Although a single nanoaperture has the ability to increase the field focus intensity in subwavelength dimensions, which makes it an essential element in optical tweezers [1–4] and plasmon-enhanced photon emission [5–8], it also scatters light in a broad angular space resulting in poor directivity [9,10]. This limits use in applications requiring narrow-beam radiation, e.g. in luminescence detection like emission from quantum dots [11–14], nitrogen-vacancy centers [15, 16], molecules and upconverting nanoparticles [17,18].
Several approaches have been proposed to improve photon emission from the subwavelength apertures. One strategy is to deliberately texture a planar metal surface containing the nanoaperture. For instance, periodic sculpting of straight grooves on both sides of a nanoslit [11,19–22] and circular grooves around a central photons source [16, 17, 23–27] are two examples that show improvement in the transmitted field intensity and directivity. Another approach to improve emission from single emitters, which has been theoretically proposed and demonstrated, is the use of a tapered and corrugated metallic probe with surface concentric grooves at the tip to enhance radiation directivity and photon collection [28]. In these configurations, photon detection can be efficiently achieved by a proper alignment of an objective lens with the planar metal structure that contains the subwavelength aperture. Another efficient method to collect photons is to directly couple radiation from a nanoaperture in a metal film, which is integrated with an optical fiber waveguide, to form a flexible, alignment-free, and compact excite-collect system [29–35]. In these integrated schemes, the plasmonic nanoantenna will allow efficient radiation coupling, from any driving source, into the core of the fiber optic waveguide. The design of such nanostructures often incorporates surface plasmon polaritons (SPPs) that confine light to the metal-dielectric interface and thereby enhance interactions with the grating. However, integrating such subwavelength aperture antennas, with corrugations on the inner surface, with a reliable, low loss, light guiding channel, such as the single-mode optical fiber (SMF), is still challenging and has not yet been demonstrated.
Plasmonic gratings have been extensively studied due to their geometric structure effect on SPPs. The momentum of these electromagnetic surface waves, waves which are supported by and confined to a dielectric-metal interface, can be harnessed to enhance the emerging field intensity and directivity of a plasmonic nanoaperture by modulating the interface periodically (a grating). Analytical and numerical studies have demonstrated the optimum geometrical design parameters of a grating structure, e.g. period, groove depth, and first groove center radius for circular gratings [36,37].
In previous studies, separate dielectric and plasmonic gratings were utilized to couple light into an optical fiber channel, or a detector aperture (for example, a relatively high numerical aperture lens) [12,38–42]. An alternative approach, also using a grating structure, is to directly integrate a grating-flanked nanoaperture in a metal film with the SMF tip. This approach has the advantage over an objective lens scheme, where the radiation source can be placed very close to the fiber facet reducing collector-source alignment time and eliminating the cumbersome microscope lens system. Although low guiding losses characterize a regular communications SMF, it has a small NA that makes coupling of the highly divergent optical power from a nanoaperture into the fiber core difficult.
Here, using design supported with finite difference time domain (FDTD) analysis, we experimentally demonstrate the enhancement of a 1536 nm free-space wavelength electromagnetic field radiation emerging from a nanoaperture that is directly placed at the SMF tip. The aperture is a bow-tie-shaped and flanked with circular grooves in a 100 nm thick gold (Au) film. The grating can be tuned to a chosen wavelength through its geometric parameters. The interest in integrating the corrugated metal film with the fiber tip arises because it can effectively participate in developing single-photon sources that emit nonclassical or weak radiation, e.g., from quantum dots [43], single molecules [4], or lanthanide ions [44,45].
2. Plasmonic structure and integration with optical fiber
To realize the integration of a grating-supported plasmonic nanoaperture in Au film with the optical fiber tip, we followed three steps. First, we sputtered a 100 nm thick Au-film on a 1.0 cm × 1.0 cm smooth-surface glass substrate. Second, we used the focused ion beam (FIB) to perforate the structure that is shown in Fig. 1(a) in the Au film. The shown circular film has an inner and outer diameter of 130 μm and 144.8 μm. The bow-tie nanoaperture, Fig. 1(c), which has dimensions of 350 nm × 170 nm in the xy plane, a 100 nm thick, and an average gap of 40 nm along the z-axis, was milled at the center of the circular grooves that were dug centered in the circular Au film as shown in Fig. 1(b). A schematic view for the grating is shown in Fig. 2(a), where Λ (980 nm) is the grating period, t is the grating depth (∼ 25 nm), and a (980 nm) is the first groove center radius. We performed several simulations at different groove depths, however, limited our design to 25 nm groove depth to minimize direct transmission through the gold and ensure that the milling process could be prevented from going too deep given the tolerances of around 10 nm.
Fig. 1 Scanning electron microscope (SEM) images. Image (a) shows one of multiple 130 μm diameter circular Au films created by milling rings of outer diameter 144.8 μm using the FIB. The showing ring contains a grating (bullseye) structure at the center, which is shown enlarged in the image (b) flanking a center bow-tie hole. The hole is zoomed in and is shown in image (c). The grating has a period of 980 nm; an outer diameter equals 9.8 μm and depth of 25 nm. The aperture has the dimensions 350 nm × 170 nm in the xy plane and a 100 nm depth, and 40 nm average gap at the center. The dark spot at the center of the image (b) is due to charging in SEM imaging. The small wing appears at the top of the Au film ring is for discharging when imaging the fiber tip using the SEM. The section A-A in (b) is schematically illustrated in Fig. 2(b).
Fig. 2 (a) xz-plane view of the FDTD simulation region. (b) A schematic diagram showing a cross-section view (section A− A of Fig. 1(b)) of the grating and part of the SMF in an integrated form. Dimensions of Λ, a, t, and h are, 980 nm, 980 nm, 25 nm, and 100 nm. The grey region, labelled as Po, represents the total emerging power from the nanoaperture without grating, while the blue cone represents the maximum possible acceptable power by the SMF, Pc. The angle α represents the SMF acceptance angle. (c) FDTD simulation results for the ratio Pc/Po at different grating periods, Λ. Inset (i) shows the electric field intensity at 8 μm away from the bow-tie hole in a non-corrugated Au film. Inset (ii) shows the field intensity at 8 μm away from the bow-tie hole in an Au film with 5 annular grooves grating surrounding the hole, Λ = 980 nm. Inset (iii) shows the field intensity when no hole at the grating center. The color bar scale represents the field intensity in (V/m)2.
For designing a structure with appropriate dimensions prior to nanofabrication, we used the FDTD to determine the optimum period for the grating at 1536 nm wavelength. In the last step, we transferred nanofabricated structure to the SMF tip. It is worth noting that the SMF has a core radius of 4.1 μm, a cladding outer radius of 62.5 μm, and an NA of 0.14. However, we used the stripping approach with epoxy adhesive (refractive index, ) to transfer the Au film to the fiber tip. More details about the stripping process and alignment are discussed in a previous work [35].
Au has a complex dielectric permittivity ∊m = ∊′m + j∊″m = −112.80 + j10.97 at 1536 nm wavelength, where ∊′m and ∊″m are the real and imaginary parts of the permittivity [46]. At the interface between the Au film and the epoxy layer, an SPP is guided at the metal surface with a wave number determined from the materials’ dielectric constants as follows [47]:
where λ is the excitation free-space wavelength. At 1536 nm wavelength, Eq. (1) shows that a surface plasmon polariton mode with wavelength λsp = 2π/ℜ{Ksp} = 986 nm can be supported at the gold-epoxy interface, where ℜ means the real part. The grating is designed to match the momentum of the surface plasmon to the free-space beamed light. The corresponding skin depth at this wavelength for gold is 6 nm [48,49].3. FDTD numerical simulations
To find the period for the grating that gives the best improvement in the field intensity and directivity, we used FDTD (Lumerical, FDTD Solutions Version 8.18.1365). We maximize the far-field transmitted power that is contained within a cone defined by the SMF acceptance angle (the angle α in Fig. 2(b)) as a function in the grating period, Λ.
Considering the computational capacity, we enclosed only a 15 μm × 15 μm of the Au film containing both the grating and nanoaperture in the xy plane and a 12 μm along the z-axis, using perfectly matched layer boundaries. Along the z-axis, the simulation region, extends from −2.0 μm to 10.0 μm as illustrated in Fig. 2(a). Also, along the same axis, the Au film extends from 0.0 μm to 0.1 μm, and the epoxy layer extends from 0.1 μm to 10.0 μm. In addition to the far-field power calculations, this 3D simulation region allows for a direct numerical calculation of the electric field intensity at the expected position of the SMF tip (∼ 8 μm from the Au film surface). The maximum extension of the grating in the xy plane considered in the FDTD simulation is 13 μm, which is 2 μm smaller than the simulation region extension in the xy plane.
Figure 2(b) shows a cross-section schematic diagram for the grating (section A-A in Fig. 1(b)) together with part of the SMF in an integrated picture. Our estimation for the adhesion layer thickness, 8 μm, is based on SEM imaging (not shown here). The excitation source used in the FDTD simulation is a plane wave, of 1536 nm wavelength, polarized along the y-axis and positioned at z = −1.6 μm in the simulation region with a propagation vector in the +z-direction.
The simulation results depicted in Fig. 2(c) show the percentage of the maximum possible acceptable power by the fiber, Pc, relative to the total power, Po, launched by the nanoaperture without a grating at different grating periods, Λ. The graph shows a peak at 980 nm grating period that is in agreement with the SPP wavelength. It is noted that the wavelength of light in the epoxy is 17 nm longer than this period (i.e., the SPP wavelength is closer). Pc/Po at this period is 55 % which is three times its counterpart (Pc/Po = 18 %) when no grating is used with the nanoaperture. We also show, in the same figure, the electric field intensity in the dielectric medium (at 8.0 μm away from the Au film in the +z-direction) for three cases for 10 mW incident power: without grooves (inset i), with five grooves of Λ = 980 nm around the nanoaperture (inset ii), and with grooves but no hole at the center (inset iii). Inset (iii) of Fig. 2(c) shows that the 100 nm Au film is not completely opaque to electromagnetic power flow although its thickness is 13 times the skin depth. It also shows the confinement of the transmitted field caused by the grating. However, in all of the FDTD simulations, the optical fiber was not included and we calculated the far-field power assuming a similar extension of the propagation medium (NOA61 epoxy, 1.541 refractive index at 1536 nm). We note that the difference in the intensity distribution is not pronounced at this location where the optical fiber is located since it is away from the near-field but still quite close to the gold film. The main difference is in the intensity gain.
The SMF full acceptance angle 2α, which is defined by its NA, is ∼ 10.5°. This angle is small and presents a significant obstacle in coupling electromagnetic signals from a nanoscale regions into the SMF. The data points shown in Fig. 2(c) draw a line of a peak value at the 980 nm grating-period, which corresponds to about 33 % improvement in power confinement.
A light ray emerging from the aperture with a wave number Kd (see Fig. 2(b)) can be transformed into a plasmonic wave by increasing its momentum through the grating defined by the grating constant, ΔKx = 2π/Λ as follows [47]:
where Kx is the wave number component in the x-direction (see Fig. 2(b)). Quantitatively, from Eq. (1), at 1536 nm excitation-wavelength, the surface plasmon polariton wave length λsp is 986 nm, which is very close to the optimum grating period, Λ = 980 nm, that we obtained from the FDTD simulation.4. Experimental results and discussion
Based on the results obtained from the FDTD simulations, which are shown in Fig. 2(c), we fabricated two groups of circular Au films with a bow-tie-shaped hole at the center of each film, using the FIB. In one group, we sculpted the Au film with circular grooves surrounding the hole, and in the other group, we left the Au film surface smooth. As mentioned earlier, the stripping approach was used to transfer the structured Au film onto the tip of an SMF [35].
Figure 3(a) and (b) show schematic diagrams of an SMF tip with non-corrugated Au film (left fiber in Fig. 3(a), and a corrugated Au film (left fiber in Fig. 3(b)). The cleaved end SMF on the right side, which has the same core and cladding size as that of the left SMF, represents the fiber-coupled source beam guiding channel. The source power is 10 mW from a laser diode at a wavelength of 1536 nm. We used both configurations to measure the photon counts per second coupled into the SMF through the two different metal structures. However, during the experiments, we maintained the two fibers separated by a distance, d, of 25 μm, and aligned along the z-axis as shown in Fig. 3. It is worth mentioning here that each of the optical fibers was tightened to a 3-axis translation stage, with differential adjusters, to allow precise alignment through a long focal length microscope.
Fig. 3 Fiber-fiber experimental setup partial schematic diagram: (a) Au film, h = 100 nm, not corrugated, and (b) the film has 5 circular grooves of Λ = a = 980 nm and t ∼ 25 nm.
Similarly, Fig. 4 shows the photon collection through an objective lens (× 20, NA = 0.4) and the excitation is through the nanoaperture integrated optical fiber as indicated by the blue arrow in the figure. The separation, s, in this case, is dictated by the lens focal length, ∼ 2.5 mm. It is worth noting in Fig. 3 and in Fig. 4 that the outgoing wavelength is labeled as λg to emphasize the fact that emission from the nanoaperture can be different from the excitation source wavelength, for example for luminescence studies. Also, we refer to these two schematic diagrams as “fiber-fiber” and “fiber-lens” partial setups for the sake of differentiation.
Fig. 4 Fiber-lens experimental setup partial schematic diagram: (a) Gold film, h = 100 nm, not corrugated, and (b) the film has 5 circular grooves of Λ = a = 980 nm and t ∼ 25 nm.
Figure 5 shows the average photon counts obtained from multiple experimental measurements. The data show clearly the effect caused by the plasmonic grating on the emerging field from the nanoaperture. Even though the 100 nm thick gold film is not 100 % opaque, the grating has effectively influenced the transmitted field and enhanced its intensity in a narrower beam. The differences in photon counts in both experiments indicate that an enhancement factor of > 2 (2.2 for the fiber-fiber experimental setup of Fig. 3, and 2.4 for the fiber-lens configuration of Fig. 4) is achieved due to the presence of the circular grating in the Au film around the nanoaperture. The photon counts measured through the objective lens was higher due to its sizeable numerical aperture even though there is a large discrepancy in the refraction index of glass and air. Although, the effect of the grating on one side of the metal film is obvious, a dual grating system, circular grooves on both sides of the gold film, would have a better coupling efficiency [50]. Compared to past works that show greater than ten-fold enhancement of directivity gain, the factor of 2–3 seen here may appear moderate. This is mainly limited by the numerical aperture of the fiber that accepts larger angles and therefore does not benefit further from a narrower beam.
Fig. 5 Average photon-counts, with error bars, obtained from multiple experimental measurements. (a) Fiber-Collection configuration, and (b) Lens-Collection configuration. The bars in the figure were labelled as NG and WG to denote no grooves and with grooves nanoantenna.
Fresnel reflection is small at the fiber-epoxy interface (∼ 7.2 × 10−4, for 1.46 fiber core index and assuming s-polarized electric field), due to the slight difference in the index of refraction between the two media. The experimentally measured data are in reasonable agreement with the FDTD data that show an enhancement factor of 3.
The obtained results are promising particularly when considering the possibility of the radiation field source located within the nanoaperture [32,35,51]; for instance, through optical trapping of a quantum emitter where a low-loss SMF channel can efficiently collect the emerging radiation through the grating.
5. Conclusion
We demonstrated ∼ 2 − 3 times enhancement in coupling through a bow-tie nanoaperture at the tip of optical fiber, beaming to the SMF waveguide. Due to the low optical transmissivity of a single nanoaperture in a metallic screen, we measured the photon counts per second to observe the grating influence on power coupling into the detection channel. We demonstrated two experiments for observing radiation emission enhancement from the nanoaperture. As a result, the field strength beamed within the acceptance angle of the optical fiber, or of an objective lens, was enhanced by a factor of greater than 2 in both cases. Furthermore, we used the FDTD numerical technique to show a reasonable agreement with the experimental results obtained at the optimum grating period. This achievement can have a positive effect in supporting light emission and detection of nanoscale luminescent objects, e.g. light emitting quantum dots, single molecules, and upconverting nanoparticles.
Funding
NSERC Discovery Grant, JE is also supported by the Higher Institute of Science and Technology, Misurata, Libya.
Acknowledgments
JE acknowledges instrumentation support from E. Humphrey, M. Wang, and J. Rudge at the Advanced Microscopy Facility (user facility) at the University of Victoria, BC, Canada.
References
1. M. L. Juan, M. Righini, and R. Quidant, “Plasmon nano-optical tweezers,” Nat. Photonics 5, 349 (2011). [CrossRef]
2. J. R. Moffitt, Y. R. Chemla, S. B. Smith, and C. Bustamante, “Recent advances in optical tweezers,” Annu. Rev. Biochem. 77, 205–228 (2008). [CrossRef] [PubMed]
3. R. Gordon, “Biosensing with nanoaperture optical tweezers,” Opt. & Laser Technol. 109, 328–335 (2019). [CrossRef]
4. R. Regmi, A. A. Al Balushi, H. Rigneault, R. Gordon, and J. Wenger, “Nanoscale volume confinement and fluorescence enhancement with double nanohole aperture,” Sci. Reports 5, 15852 (2015). [CrossRef]
5. J. Wenger, D. Gérard, J. Dintinger, O. Mahboub, N. Bonod, E. Popov, T. W. Ebbesen, and H. Rigneault, “Emission and excitation contributions to enhanced single molecule fluorescence by gold nanometric apertures,” Opt. Express 16, 3008–3020 (2008). [CrossRef] [PubMed]
6. D. Gérard, J. Wenger, N. Bonod, E. Popov, H. Rigneault, F. Mahdavi, S. Blair, J. Dintinger, and T. W. Ebbesen, “Nanoaperture-enhanced fluorescence: Towards higher detection rates with plasmonic metals,” Phys. Rev. B 77, 045413 (2008). [CrossRef]
7. G. Lu, W. Li, T. Zhang, S. Yue, J. Liu, L. Hou, Z. Li, and Q. Gong, “Plasmonic-enhanced molecular fluorescence within isolated bowtie nano-apertures,” ACS Nano 6, 1438–1448 (2012). [CrossRef] [PubMed]
8. R. A. Jensen, I.-C. Huang, O. Chen, J. T. Choy, T. S. Bischof, M. Lončar, and M. G. Bawendi, “Optical trapping and two-photon excitation of colloidal quantum dots using bowtie apertures,” ACS Photonics 3, 423–427 (2016). [CrossRef]
9. A. Degiron, H. Lezec, N. Yamamoto, and T. Ebbesen, “Optical transmission properties of a single subwavelength aperture in a real metal,” Opt. Commun. 239, 61–66 (2004). [CrossRef]
10. Y. Pang and R. Gordon, “Nanophotonics using a subwavelength aperture in a metal film,” Nanotechnol. Rev. 1, 339–362 (2012). [CrossRef]
11. Y. C. Jun, K. C. Huang, and M. L. Brongersma, “Plasmonic beaming and active control over fluorescent emission,” Nat. Commun. 2, 283 (2011). [CrossRef] [PubMed]
12. M. Davanço, M. T. Rakher, D. Schuh, A. Badolato, and K. Srinivasan, “A circular dielectric grating for vertical extraction of single quantum dot emission,” Appl. Phys. Lett. 99, 041102 (2011). [CrossRef]
13. S. Ates, L. Sapienza, M. Davanco, A. Badolato, and K. Srinivasan, “Bright single-photon emission from a quantum dot in a circular bragg grating microcavity,” IEEE J. Sel. Top. Quantum Electron. 18, 1711–1721 (2012). [CrossRef]
14. E. I. Moiseev, N. Kryzhanovskaya, Y. S. Polubavkina, M. V. Maximov, M. M. Kulagina, Y. M. Zadiranov, A. A. Lipovskii, I. S. Mukhin, A. M. Mozharov, F. E. Komissarenko, et al., “Light outcoupling from quantum dot-based microdisk laser via plasmonic nanoantenna,” ACS Photonics 4, 275–281 (2017). [CrossRef]
15. S. K. Andersen, S. Kumar, and S. I. Bozhevolnyi, “Ultrabright linearly polarized photon generation from a nitrogen vacancy center in a nanocube dimer antenna,” Nano Lett. 17, 3889–3895 (2017). [CrossRef] [PubMed]
16. L. Li, E. H. Chen, J. Zheng, S. L. Mouradian, F. Dolde, T. Schröder, S. Karaveli, M. L. Markham, D. J. Twitchen, and D. Englund, “Efficient photon collection from a nitrogen vacancy center in a circular bullseye grating,” Nano Lett. 15, 1493–1497 (2015). [CrossRef] [PubMed]
17. H. Aouani, O. Mahboub, N. Bonod, E. Devaux, E. Popov, H. Rigneault, T. W. Ebbesen, and J. Wenger, “Bright unidirectional fluorescence emission of molecules in a nanoaperture with plasmonic corrugations,” Nano Lett. 11, 637–644 (2011). [CrossRef] [PubMed]
18. J. Dong, Z. Zhang, H. Zheng, and M. Sun, “Recent progress on plasmon-enhanced fluorescence,” Nanophotonics 4, 472–490 (2015). [CrossRef]
19. H. J. Lezec, A. Degiron, E. Devaux, R. Linke, L. Martin-Moreno, F. Garcia-Vidal, and T. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297, 820–822 (2002). [CrossRef] [PubMed]
20. L. Martin-Moreno, F. Garcia-Vidal, H. Lezec, A. Degiron, and T. Ebbesen, “Theory of highly directional emission from a single subwavelength aperture surrounded by surface corrugations,” Phys. Rev. Lett. 90, 167401 (2003). [CrossRef] [PubMed]
21. D. Lin, C. Chang, Y. Chen, D. Yang, M. Lin, J. Yeh, J. Liu, C. Kuan, C. Yeh, and C. Lee, “Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings,” Opt. Express 14, 3503–3511 (2006). [CrossRef] [PubMed]
22. Y. Lim, J. Hahn, S. Kim, J. Park, H. Kim, and B. Lee, “Plasmonic light beaming manipulation and its detection using holographic microscopy,” IEEE J. Quantum Electron. 46, 300–305 (2010). [CrossRef]
23. C. Genet and T. Ebbesen, “Light in tiny holes,” in Nanoscience And Technology: A Collection of Reviews from Nature Journals, (World Scientific, 2010), pp. 205–212.
24. A. Angelini, E. Barakat, P. Munzert, L. Boarino, N. De Leo, E. Enrico, F. Giorgis, H. P. Herzig, C. F. Pirri, and E. Descrovi, “Focusing and extraction of light mediated by bloch surface waves,” Sci. Reports 4, 5428 (2014). [CrossRef]
25. A. A. Saleh, S. Sheikhoelislami, S. Gastelum, and J. A. Dionne, “Grating-flanked plasmonic coaxial apertures for efficient fiber optical tweezers,” Opt. Express 24, 20593–20603 (2016). [CrossRef] [PubMed]
26. S. K. Andersen, S. Bogdanov, O. Makarova, Y. Xuan, M. Y. Shalaginov, A. Boltasseva, S. I. Bozhevolnyi, and V. M. Shalaev, “Hybrid plasmonic bullseye antennas for efficient photon collection,” Acs Photonics 5, 692–698 (2018). [CrossRef]
27. F. Werschler, B. Lindner, C. Hinz, F. Conradt, P. Gumbsheimer, Y. Behovits, C. Negele, T. de Roo, O. Tzang, S. Mecking, et al., “Efficient emission enhancement of single cdse/cds/pmma quantum dots through controlled near-field coupling to plasmonic bullseye resonators,” Nano Lett. 18, 5396–5400 (2018). [CrossRef]
28. H. Shen, G. Lu, Y. He, Y. Cheng, H. Liu, and Q. Gong, “Directional and enhanced spontaneous emission with a corrugated metal probe,” Nanoscale 6, 7512–7518 (2014). [CrossRef] [PubMed]
29. L. Neumann, Y. Pang, A. Houyou, M. L. Juan, R. Gordon, and N. F. van Hulst, “Extraordinary optical transmission brightens near-field fiber probe,” Nano Lett. 11, 355–360 (2010). [CrossRef] [PubMed]
30. S. Kang, H.-E. Joe, J. Kim, Y. Jeong, B.-K. Min, and K. Oh, “Subwavelength plasmonic lens patterned on a composite optical fiber facet for quasi-one-dimensional bessel beam generation,” Appl. Phys. Lett. 98, 241103 (2011). [CrossRef]
31. G. Kostovski, P. R. Stoddart, and A. Mitchell, “The optical fiber tip: An inherently light-coupled microscopic platform for micro-and nanotechnologies,” Adv. Mater. 26, 3798–3820 (2014). [CrossRef] [PubMed]
32. R. M. Gelfand, S. Wheaton, and R. Gordon, “Cleaved fiber optic double nanohole optical tweezers for trapping nanoparticles,” Opt. Lett. 39, 6415–6417 (2014). [CrossRef] [PubMed]
33. M. Principe, M. Consales, A. Micco, A. Crescitelli, G. Castaldi, E. Esposito, V. La Ferrara, A. Cutolo, V. Galdi, and A. Cusano, “Optical fiber meta-tips,” Light. Sci. & Appl. 6, e16226 (2017). [CrossRef]
34. H. Lee, J. Park, and K. Oh, “Recent progress in all-fiber non-gaussian optical beam shaping technologies,” J. Light. Technol. (2018). [CrossRef]
35. J. M. Ehtaiba and R. Gordon, “Template-stripped nanoaperture tweezer integrated with optical fiber,” Opt. Express 26, 9607–9613 (2018). [CrossRef] [PubMed]
36. O. Mahboub, S. C. Palacios, C. Genet, F. Garcia-Vidal, S. G. Rodrigo, L. Martin-Moreno, and T. Ebbesen, “Optimization of bull’s eye structures for transmission enhancement,” Opt. Express 18, 11292–11299 (2010). [CrossRef] [PubMed]
37. U. Bog, K. Huska, F. Maerkle, A. Nesterov-Mueller, U. Lemmer, and T. Mappes, “Design of plasmonic grating structures towards optimum signal discrimination for biosensing applications,” Opt. Express 20, 11357–11369 (2012). [CrossRef] [PubMed]
38. T. Sharma, H. Kwon, J. Park, S. Han, G. Son, Y. Jung, and K. Yu, “Coupling performance enhancement using soi grating coupler design,” Opt. Commun. 427, 452–456 (2018). [CrossRef]
39. R. Chriki, A. Yanai, J. Shappir, and U. Levy, “Enhanced efficiency of thin film solar cells using a shifted dual grating plasmonic structure,” Opt. Express 21, A382–A391 (2013). [CrossRef] [PubMed]
40. A. Kumar, J.-C. Weeber, A. Bouhelier, F. Eloi, S. Buil, X. Quélin, M. Nasilowski, B. Dubertret, J.-P. Hermier, and G. C. Des Francs, “Spatially uniform enhancement of single quantum dot emission using plasmonic grating decoupler,” Sci. Reports 5, 16796 (2015). [CrossRef]
41. B. Chen, A. Wood, A. Pathak, J. Mathai, S. Bok, H. Zheng, S. Hamm, S. Basuray, S. Grant, K. Gangopadhyay, et al., “Plasmonic gratings with nano-protrusions made by glancing angle deposition for single-molecule super-resolution imaging,” Nanoscale 8, 12189–12201 (2016). [CrossRef] [PubMed]
42. D. T. Vu, H.-W. Chiu, R. Nababan, Q. M. Le, S.-W. Kuo, L.-K. Chau, C.-C. Ting, H.-C. Kan, and C. C. Hsu, “Enhancing upconversion luminescence emission of rare earth nanophosphors in aqueous solution with thousands fold enhancement factor by low refractive index resonant waveguide grating,” ACS Photonics 8, 3263–3271 (2018). [CrossRef]
43. R. S. Daveau, K. C. Balram, T. Pregnolato, J. Liu, E. H. Lee, J. D. Song, V. Verma, R. Mirin, S. W. Nam, L. Midolo, et al., “Efficient fiber-coupled single-photon source based on quantum dots in a photonic-crystal waveguide,” Optica 4, 178–184 (2017). [CrossRef] [PubMed]
44. J.-C. Boyer and F. C. Van Veggel, “Absolute quantum yield measurements of colloidal nayf 4: Er3+, yb3+ upconverting nanoparticles,” Nanoscale 2, 1417–1419 (2010). [CrossRef] [PubMed]
45. E. P. Schartner, D. Jin, H. Ebendorff-Heidepriem, J. A. Piper, Z. Lu, and T. M. Monro, “Lanthanide upconversion within microstructured optical fibers: improved detection limits for sensing and the demonstration of a new tool for nanocrystal characterization,” Nanoscale 4, 7448–7451 (2012). [CrossRef] [PubMed]
46. P. B. Johnson and R.-W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6, 4370 (1972). [CrossRef]
47. H. Raether, “Surface plasmons on smooth surfaces,” in Surface plasmons on smooth and rough surfaces and on gratings, (Springer, 1988), pp. 1–37.
48. X. Shou, A. Agrawal, and A. Nahata, “Role of metal film thickness on the enhanced transmission properties of a periodic array of subwavelength apertures,” Opt. Express 13, 9834–9840 (2005). [CrossRef] [PubMed]
49. D. Grupp, H. Lezec, T. Ebbesen, K. Pellerin, and T. Thio, “Crucial role of metal surface in enhanced transmission through subwavelength apertures,” Appl. Phys. Lett. 77, 1569–1571 (2000). [CrossRef]
50. H. Shen, G. Lu, T. Zhang, J. Liu, Y. He, Y. Wang, and Q. Gong, “Molecule fluorescence modified by a slit-based nanoantenna with dual gratings,” JOSA B 30, 2420–2426 (2013). [CrossRef]
51. J. Berthelot, S. Aćimović, M. Juan, M. Kreuzer, J. Renger, and R. Quidant, “Three-dimensional manipulation with scanning near-field optical nanotweezers,” Nat. Nanotechnol. 9, 295–299 (2014). [CrossRef] [PubMed]
