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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 27 — Dec. 19, 2011
  • pp: 25990–25999
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Efficient apertureless scanning probes using patterned plasmonic surfaces

Youngkyu Lee, Andrea Alu, and John X.J. Zhang  »View Author Affiliations


Optics Express, Vol. 19, Issue 27, pp. 25990-25999 (2011)
http://dx.doi.org/10.1364/OE.19.025990


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Abstract

We present a novel concept to design apertureless plasmonic probes for near-field scanning optical microscopy (NSOM) with enhanced optical power throughput and near-field enhancement. Specifically, we combine unidirectional surface plasmon polariton (SPP) generation along the tip lateral walls with nanofocusing of SPPs through adiabatic propagation towards an apertureless tip. Three key design parameters are considered: the nanoslit width, the pitch period of nanogrooves for unidirectional plasmonic excitation and the pyramidal geometry of the NSOM probe for SPP focusing. Optimal design parameters are obtained with 2D analysis and two realistic probe geometries with patterned plasmonic surfaces are proposed using the optimized designs. The electromagnetic properties of the designed probes are characterized in the near-field and compared to those of a conventional single-aperture probe with same pyramidal shape. The optimized probes feature FWHM around 150nm, comparable with conventional NSOM designs, but over 3 orders of magnitude larger field enhancement, without degrading its spatial resolution. Our ideas effectively combine the resolution of apertureless probes with throughput levels much larger than those available even in aperture-based devices.

© 2011 OSA

1. Introduction

The resolution of conventional optical microscopy is governed by Rayleigh diffraction and limited by the wavelength of operation [1

1. E. Hecht and A. R. Ganesan, Optics (Pearson Education, 2001).

,2

2. E. Abbe, “Beiträge zur theorie des mikroskops und der mikroskopischen wahrnehmung,” Archiv. Mikros. Anat. 9, 413 (1873). [CrossRef]

]. Various pursuits and efforts have been made to overcome this fundamental optical limitation [3

3. E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier: optical microscopy on a nanometric scale,” Science 251(5000), 1468–1470 (1991). [CrossRef] [PubMed]

5

5. A. Lewis and K. Lieberman, “Near-field optical imaging with a non-evanescently excited high-brightness light source of sub-wavelength dimensions,” Nature 354(6350), 214–216 (1991). [CrossRef]

], especially operating in the near-field of the object to be imaged. In particular, near-field scanning optical microscopy (NSOM) shows great promise to overcome the diffraction limit, by capturing the evanescent portion of the spatial spectrum of an image, before it rapidly decays away from the object. A conventional single aperture NSOM probe features spatial scanning resolution around 100 nm, which is largely determined by the aperture size, rather than by the wavelength of operation. Classic NSOM measurements can therefore break the diffraction limit; however, aperture NSOM probes still suffer from low optical throughput, which ultimately limits the resolution due to a low signal-to-noise ratio [6

6. R. Bachelot, F. H'dhili, D. Barchiesi, G. Lerondel, R. Fikri, P. Royer, N. Landraud, J. Peretti, F. Chaput, G. Lampel, J.-P. Boilot, and K. Lahlil, “Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films,” J. Appl. Phys. 94(3), 2060–2072 (2003). [CrossRef]

].

In this paper, we put forward new concepts to employ SPP generation and focusing to improve apertureless NSOM measurements: our goal is to extract the energy from the probe before it gets to the bottom of the tip, where the guided energy is deeply below cut-off due to the small cross-section of the probe. By introducing an optimally designed slit at a much larger cross-section, and directing the generated SPPs towards the apertureless tip, we aim at obtaining throughputs much larger than what available in conventional aperture-based probes, and then routing the energy towards an apertureless tip to ensure large resolution. We use two schemes to achieve this ambitious goal: nanofocusing of SPP propagation using proper shaping of the probe tip and enhanced plasmonic generation through nanopatterning of the probe tip, in order to achieve enhanced optical throughput in the near-field region. Specifically, sharp pyramidal probes, which can be fabricated by anisotropic wet-chemical etching process, are combined with gratings supporting unidirectional SPP propagation, in order to excite enhanced electromagnetic modes confined around the probe tip, which may drastically increase light focusing compared to conventional NSOM probes. Two different types of plasmonic probes are designed; each one integrates a slit grating with different unidirectional SPP excitation techniques. To verify our designs, we first explore the two-dimensional (2D) excitation of plasmonic surfaces by a metal slit in a planar geometry; then we apply the optimized design parameters to 3D realistic probe designs to explore the effects of geometrical, unidirectional SPP nanofocusing for NSOM measurements. Both proposed designs achieve electric field enhancement over 1000 times compared to a conventional single aperture probe, without compromising spatial resolution. In addition, the designed probes offer potential operations over a wide spectral range.

2. Unidirectional surface plasmon polariton excitation

Unidirectional excitation of SPPs along a plasmonic surface may be obtained by properly designing a grating, or the nanoslit width given a specific angle of illumination: the nanograting design may be tailored to reflect SPPs excited in the unwanted direction with respect to the slit [15

15. F. López-Tejeira, S. G. Rodrigo, L. Martín-Moreno, F. J. García-Vidal, E. Devaux, T. W. Ebbesen, J. R. Krenn, I. P. Radko, S. I. Bozhevolnyi, M. U. González, J. C. Weeber, and A. Dereux, “Efficient unidirectional nanoslit couplers for surface plasmons,” Nat. Phys. 3(5), 324–328 (2007). [CrossRef]

17

17. S. B. Choi, D. J. Park, Y. K. Jeong, Y. C. Yun, M. S. Jeong, C. C. Byeon, J. H. Kang, Q.-H. Park, and D. S. Kim, “Directional control of surface plasmon polariton waves propagating through an asymmetric Bragg resonator,” Appl. Phys. Lett. 94(6), 063115 (2009). [CrossRef]

], using stop-band design concepts; the nanoslit can also efficiently couple incident waves into SPPs preferably in one direction, exploiting the asymmetry in its geometry. Before applying these techniques to specific 3D probe designs, in order to enhance their throughput efficiency, we numerically analyze 2D plasmonic surfaces using finite-integration (FIT) numerical simulations.

2.1 Unidirectional SPP excitation for oblique illumination of a single slit

2.2 Unidirectional SPP excitation using a groove array

2.3 Two-dimensional numerical simulations

D=|Hyright||Hyleft|
(3)

Hyright is the magnetic field in component in the y direction calculated at the right side of the slit and Hyleft is the magnetic field at the left side of the slit.

3. Nanofocusing probe design using unidirectional SPP generation

4. Near-field light enhancement at the probe tip through coupled SPPs

Three different types of NSOM probes, i.e., probe A, probe B and a classic single aperture probe are numerically characterized in the near-field of the tip, consistent with Fig. 5. The calculated electric field intensity and the electric energy density of each probe are shown in Fig. 6
Fig. 6 |E|field distributions of 3 different probes in x-z plane (left column) and |E|2distribution along x axis with tip-sample distance of 20 nm (right column): (a) a classic single aperture probe with 100 nm-sized rectangular aperture, (b) type A probe, and (c) type B probe.
. For the electromagnetic characterization, full width and half maximum (FWHM) is defined by the distribution of the electric energy density near the probe tip. The electric energy density is compared to a canonical single aperture probe at 20 nm distance from the tip. Compared to the conventional single aperture probe, type A and type B probes feature electric field enhancements by a factor of 2119 and 1023, respectively, consistent with the 2D results reported in Section 2. As predicted above, type A shows the best performance in terms of optical throughput in the near-field region. Electrical energy density of type A is boosted more than 4×106times, without compromising FWHM. This value is about 4 times higher than that of type B probe, and this value is in well agreement with the predicted values from 2D simulations. The calculated FWHMs of the three probes are: 148.84 nm (canonical aperture probe), 138.02 nm (type A) and 171.3 nm (type B). In Fig. 7
Fig. 7 Near-field characteristics of three probes: type A, type B and classical single aperture probe. All data are measured at 20 nm above the tip apex.
, the calculated FWHM of each probe corresponding to various tip-sample distances is shown. Within 100 nm from the probe tip, the calculated FWHM of probe type A and B is under 200 nm, and the two designed probes still hold huge energy density enhancement (over six orders of magnitude compared to the simple aperture probe).

From these numerical results, it is found that the FWHM of the designed probe is largely determined by the tip size, as already reported in [7

7. M. I. Stockman, “Nanofocusing of optical energy in tapered plasmonic waveguides,” Phys. Rev. Lett. 93(13), 137404 (2004). [CrossRef] [PubMed]

], rather than by the type of excitation. The SPPs mainly contribute to dramatically enhance the field throughput at the aperture, with substantial advantages in terms of signal-to-noise ratio, without compromising the measurement resolution. It is expected that with smaller tip sizes, the SPPs may be focused even more efficiently. In our different designs, we have achieved FWHM around 50 nm for both type A and type B probes using sharper probe tips of 30 nm. Even though higher field enhancement and confinement may be achieved through the utilization of sharper tip geometries, in this study, we have focused on a 100nm probe tip, which is reliably attainable and reproducible with current nanofabrication technology [25

25. J. S. Hyun, J. S. Moon, J. H. Parka, J. W. Kima, Y. D. Kimaand, and J. H. Boo, “Fabrication of near-field optical probes using advanced functional thin films for MEMS and NEMS applications,” Mater. Sci. Eng. 149, 292–298 (2008).

]. This will pave the way to the near-future experimental demonstration of these concepts, which we are planning in our group.

5. Discussion and conclusions

We have presented here optimized plasmonic probe designs for enhanced near-field optical throughput. Without using a conventional aperture geometry [12

12. Y. Wang, Y. Y. Huang, and X. J. Zhang, “Plasmonic nanograting tip design for high power throughput near-field scanning aperture probe,” Opt. Express 18(13), 14004–14011 (2010). [CrossRef] [PubMed]

] or external illumination [10

10. C. C. Neacsu, S. Berweger, R. L. Olmon, L. V. Saraf, C. Ropers, and M. B. Raschke, “Near-field localization in plasmonic superfocusing: a nanoemitter on a tip,” Nano Lett. 10(2), 592–596 (2010). [CrossRef] [PubMed]

], we have numerically proven that simple nanopatterns, such as a slit and slit with an array of reflective grooves [15

15. F. López-Tejeira, S. G. Rodrigo, L. Martín-Moreno, F. J. García-Vidal, E. Devaux, T. W. Ebbesen, J. R. Krenn, I. P. Radko, S. I. Bozhevolnyi, M. U. González, J. C. Weeber, and A. Dereux, “Efficient unidirectional nanoslit couplers for surface plasmons,” Nat. Phys. 3(5), 324–328 (2007). [CrossRef]

,16

16. H. Kim and B. Lee, “Unidirectional surface plasmon polariton excitation on single slit with oblique backside illumination,” Plasmonics 4(2), 153–159 (2009). [CrossRef]

], may be used for near-field enhancement purposes, in combination with SPP nanofocusing associated with the NSOM probe tip. Under radially polarized illumination, the designed plasmonic probes show extremely high field enhancement at the apex of probe tip. The electric field enhancement is over three orders of magnitude higher than a classic single aperture probe for both proposed designs. The FWHM of all designed probes is comparable with the conventional single aperture probe within 100 nm from the probe tip, while providing huge electrical field enhancement. It is interesting to notice that the designed probes, especially type A, whose functionality does not depend on destructive interference due to gratings, provide the possibility of wideband operation if the slit width is properly selected. Even if the frequency of illumination is deviated from the optimal design frequency, the slit will still couple most part of the impinging wave into unidirectional SPPs and the generated SPPs will be focused at the apex of the probe by the sharp metallic probe tip. Figure 8(a) and (b)
Fig. 8 Spectral characteristics of three probes: type A, type B and classical single aperture probe. (a) Maximum electric energy density and (b) FWHM along x-axis are calculated at tip-sample distance of 20 nm.
shows the spectral characteristics of the three probe designs in the near-field region.

It is obvious that design B probes are more sensitive to frequency variation, due to frequency selective properties of the groove array, consistent with the periodic features of the directivity in Fig. 3(b). Both designs, however, can provide large throughput enhancement (of 4 and 6 orders of magnitude, respectively) over a wide frequency range without degrading FWHM. It is worth mentioning that, although type A probes offer consistently superior performance, in their practical realization type B designs can be advantageous. For instance, in configurations and experimental realizations in which it is difficult to control the slit angle or when perfectly collimated incident light is not obtainable, type B probe designs may offer more robust operation.

In conclusion, we have reported an extensive numerical study to optimize NSOM probe designs for nanoscale slits and grooves that may couple the impinging wave into unidirectional SPPs. We have shown that these concepts may provide large near-field enhancement compared to opening an aperture at the probe tip. This makes physical sense, since the probe geometry is effectively a waveguide below cut-off, whose throughput is largely affected by the small aperture size. By opening a slit away from the tip, where the waveguide cross section is larger, most of the impinging energy may be coupled to SPP modes, which do not have a cut-off and can propagate and focus at the probe tip. The optimized slit shape and groove design provide unidirectional SPP excitation, which further enhances the coupling efficiency towards the tip. The proposed designs can achieve extremely high near-field enhancement while minimizing the unwanted scattering of usual apertureless plasmonic probe that require external illumination. Unlike our proposed designs, a conventional apertureless scanning probe with direct tip illumination often shows limited performance when strong background noise is considered, associated with the probe body scattering and direct sample reflected scattered light. To overcome these drawbacks, recent progress has been made based on nonlocal probe tip geometry [10

10. C. C. Neacsu, S. Berweger, R. L. Olmon, L. V. Saraf, C. Ropers, and M. B. Raschke, “Near-field localization in plasmonic superfocusing: a nanoemitter on a tip,” Nano Lett. 10(2), 592–596 (2010). [CrossRef] [PubMed]

] and highly polarized SPPs launched from gratings, enabling a significant increase of signal to noise ratio; however, additional signal discrimination method based on the orientation of polarization of scattered field is still required in this configuration. In contrast, our approach allows minimal background noise and may not require any signal discrimination technique, which usually comes at the expense of NSOM sensitivity. In addition, our designs are particularly appealing for nanolithographic processes and may be realized within current nanofabrication technology. Pyramidal hollow tips suited for scanning probe can be formed by wet chemical etching of (100) and (111) single crystal silicon in TMAH etchant [25

25. J. S. Hyun, J. S. Moon, J. H. Parka, J. W. Kima, Y. D. Kimaand, and J. H. Boo, “Fabrication of near-field optical probes using advanced functional thin films for MEMS and NEMS applications,” Mater. Sci. Eng. 149, 292–298 (2008).

]. Tip formation with anisotropic etching technology and thermal oxidation yields a sharp tip diameter of less than 100 nm. Further apex sharpening with thermal oxidation and film deposition technology may be possible and result in tips with apex radius below 30 nm. Armed with in situ nanofabrication technology, we may realize a scanning probe featuring high optical resolution with high optical power throughput that is hardly achieved from conventional aperture-based scanning probes.

Acknowledgements

This research was performed at Biomedical Engineering, Microelectronics Research Center (MRC) at UT Austin. The authors thank Dr. Kazunori Hoshino and Dr. Ashwini Gopal for stimulating discussions on nanophotonic structure design and nanofabrication. We gratefully acknowledge the financial support from National Science Foundation (NSF CAREER Award Grant No. 0846313, PI: Zhang, NSF CAREER Award Grant No. 0953311, PI: Alù) and DARPA Young Faculty Award (N66001-10-1-4049, PI: Zhang).

References and links

1.

E. Hecht and A. R. Ganesan, Optics (Pearson Education, 2001).

2.

E. Abbe, “Beiträge zur theorie des mikroskops und der mikroskopischen wahrnehmung,” Archiv. Mikros. Anat. 9, 413 (1873). [CrossRef]

3.

E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier: optical microscopy on a nanometric scale,” Science 251(5000), 1468–1470 (1991). [CrossRef] [PubMed]

4.

K. Lieberman, S. Harush, A. Lewis, and R. Kopelman, “A light source smaller than the optical wavelength,” Science 247(4938), 59–61 (1990). [CrossRef] [PubMed]

5.

A. Lewis and K. Lieberman, “Near-field optical imaging with a non-evanescently excited high-brightness light source of sub-wavelength dimensions,” Nature 354(6350), 214–216 (1991). [CrossRef]

6.

R. Bachelot, F. H'dhili, D. Barchiesi, G. Lerondel, R. Fikri, P. Royer, N. Landraud, J. Peretti, F. Chaput, G. Lampel, J.-P. Boilot, and K. Lahlil, “Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films,” J. Appl. Phys. 94(3), 2060–2072 (2003). [CrossRef]

7.

M. I. Stockman, “Nanofocusing of optical energy in tapered plasmonic waveguides,” Phys. Rev. Lett. 93(13), 137404 (2004). [CrossRef] [PubMed]

8.

V. S. Volkov, S. I. Bozhevolnyi, S. G. Rodrigo, L. Martín-Moreno, F. J. García-Vidal, E. Devaux, and T. W. Ebbesen, “Nanofocusing with channel plasmon polaritons,” Nano Lett. 9(3), 1278–1282 (2009). [CrossRef] [PubMed]

9.

K. C. Vernon, D. K. Gramotnev, and D. F. P. Pile, “Adiabatic nanofocusing of plasmons by a sharp metal wedge on a dielectric substrate,” J. Appl. Phys. 101(10), 104312 (2007). [CrossRef]

10.

C. C. Neacsu, S. Berweger, R. L. Olmon, L. V. Saraf, C. Ropers, and M. B. Raschke, “Near-field localization in plasmonic superfocusing: a nanoemitter on a tip,” Nano Lett. 10(2), 592–596 (2010). [CrossRef] [PubMed]

11.

Y. Wang, W. Srituravanich, C. Sun, and X. Zhang, “Plasmonic nearfield scanning probe with high transmission,” Nano Lett. 8(9), 3041–3045 (2008). [CrossRef] [PubMed]

12.

Y. Wang, Y. Y. Huang, and X. J. Zhang, “Plasmonic nanograting tip design for high power throughput near-field scanning aperture probe,” Opt. Express 18(13), 14004–14011 (2010). [CrossRef] [PubMed]

13.

L. Yin, V. K. Vlasko-Vlasov, J. Pearson, J. M. Hiller, J. Hua, U. Welp, D. E. Brown, and C. W. Kimball, “Subwavelength focusing and guiding of surface plasmons,” Nano Lett. 5(7), 1399–1402 (2005). [CrossRef] [PubMed]

14.

W. Chen, D. C. Abeysinghe, R. L. Nelson, and Q. Zhan, “Plasmonic lens made of multiple concentric metallic rings under radially polarized illumination,” Nano Lett. 9(12), 4320–4325 (2009). [CrossRef] [PubMed]

15.

F. López-Tejeira, S. G. Rodrigo, L. Martín-Moreno, F. J. García-Vidal, E. Devaux, T. W. Ebbesen, J. R. Krenn, I. P. Radko, S. I. Bozhevolnyi, M. U. González, J. C. Weeber, and A. Dereux, “Efficient unidirectional nanoslit couplers for surface plasmons,” Nat. Phys. 3(5), 324–328 (2007). [CrossRef]

16.

H. Kim and B. Lee, “Unidirectional surface plasmon polariton excitation on single slit with oblique backside illumination,” Plasmonics 4(2), 153–159 (2009). [CrossRef]

17.

S. B. Choi, D. J. Park, Y. K. Jeong, Y. C. Yun, M. S. Jeong, C. C. Byeon, J. H. Kang, Q.-H. Park, and D. S. Kim, “Directional control of surface plasmon polariton waves propagating through an asymmetric Bragg resonator,” Appl. Phys. Lett. 94(6), 063115 (2009). [CrossRef]

18.

Y. Takakura, “Optical resonance in a narrow slit in a thick metallic screen,” Phys. Rev. Lett. 86(24), 5601–5603 (2001). [CrossRef] [PubMed]

19.

J. Soohoo and G. E. Mevers, “Cavity mode analysis using the fourier transform method,” Proc. IEEE 62(12), 1721–1723 (1974). [CrossRef]

20.

R. Petit, Electromagnetic Theory of Gratings (Springer-Verlag, New York, 1980).

21.

H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, Berlin, 1988).

22.

S. Astilean, Ph. Lalanneb, and M. Palamarua, “Light transmission through metallic channels much smaller than the wavelength,” Opt. Commun. 175(4-6), 265–273 (2000). [CrossRef]

23.

E. D. Palik and G. Ghosh, Handbook of Optical Constants of Solids, E.D. Palik, ed. (Academic, Orlando, Fla., 1985).

24.

H. A. Bethe, “Theory of diffraction by small holes,” Phys. Rev. 66(7-8), 163–182 (1944). [CrossRef]

25.

J. S. Hyun, J. S. Moon, J. H. Parka, J. W. Kima, Y. D. Kimaand, and J. H. Boo, “Fabrication of near-field optical probes using advanced functional thin films for MEMS and NEMS applications,” Mater. Sci. Eng. 149, 292–298 (2008).

OCIS Codes
(180.4243) Microscopy : Near-field microscopy

ToC Category:
Microscopy

History
Original Manuscript: September 22, 2011
Revised Manuscript: October 15, 2011
Manuscript Accepted: October 17, 2011
Published: December 6, 2011

Virtual Issues
Vol. 7, Iss. 2 Virtual Journal for Biomedical Optics

Citation
Youngkyu Lee, Andrea Alu, and John X.J. Zhang, "Efficient apertureless scanning probes using patterned plasmonic surfaces," Opt. Express 19, 25990-25999 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-25990


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References

  1. E. Hecht and A. R. Ganesan, Optics (Pearson Education, 2001).
  2. E. Abbe, “Beiträge zur theorie des mikroskops und der mikroskopischen wahrnehmung,” Archiv. Mikros. Anat.9, 413 (1873). [CrossRef]
  3. E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, “Breaking the diffraction barrier: optical microscopy on a nanometric scale,” Science251(5000), 1468–1470 (1991). [CrossRef] [PubMed]
  4. K. Lieberman, S. Harush, A. Lewis, and R. Kopelman, “A light source smaller than the optical wavelength,” Science247(4938), 59–61 (1990). [CrossRef] [PubMed]
  5. A. Lewis and K. Lieberman, “Near-field optical imaging with a non-evanescently excited high-brightness light source of sub-wavelength dimensions,” Nature354(6350), 214–216 (1991). [CrossRef]
  6. R. Bachelot, F. H'dhili, D. Barchiesi, G. Lerondel, R. Fikri, P. Royer, N. Landraud, J. Peretti, F. Chaput, G. Lampel, J.-P. Boilot, and K. Lahlil, “Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films,” J. Appl. Phys.94(3), 2060–2072 (2003). [CrossRef]
  7. M. I. Stockman, “Nanofocusing of optical energy in tapered plasmonic waveguides,” Phys. Rev. Lett.93(13), 137404 (2004). [CrossRef] [PubMed]
  8. V. S. Volkov, S. I. Bozhevolnyi, S. G. Rodrigo, L. Martín-Moreno, F. J. García-Vidal, E. Devaux, and T. W. Ebbesen, “Nanofocusing with channel plasmon polaritons,” Nano Lett.9(3), 1278–1282 (2009). [CrossRef] [PubMed]
  9. K. C. Vernon, D. K. Gramotnev, and D. F. P. Pile, “Adiabatic nanofocusing of plasmons by a sharp metal wedge on a dielectric substrate,” J. Appl. Phys.101(10), 104312 (2007). [CrossRef]
  10. C. C. Neacsu, S. Berweger, R. L. Olmon, L. V. Saraf, C. Ropers, and M. B. Raschke, “Near-field localization in plasmonic superfocusing: a nanoemitter on a tip,” Nano Lett.10(2), 592–596 (2010). [CrossRef] [PubMed]
  11. Y. Wang, W. Srituravanich, C. Sun, and X. Zhang, “Plasmonic nearfield scanning probe with high transmission,” Nano Lett.8(9), 3041–3045 (2008). [CrossRef] [PubMed]
  12. Y. Wang, Y. Y. Huang, and X. J. Zhang, “Plasmonic nanograting tip design for high power throughput near-field scanning aperture probe,” Opt. Express18(13), 14004–14011 (2010). [CrossRef] [PubMed]
  13. L. Yin, V. K. Vlasko-Vlasov, J. Pearson, J. M. Hiller, J. Hua, U. Welp, D. E. Brown, and C. W. Kimball, “Subwavelength focusing and guiding of surface plasmons,” Nano Lett.5(7), 1399–1402 (2005). [CrossRef] [PubMed]
  14. W. Chen, D. C. Abeysinghe, R. L. Nelson, and Q. Zhan, “Plasmonic lens made of multiple concentric metallic rings under radially polarized illumination,” Nano Lett.9(12), 4320–4325 (2009). [CrossRef] [PubMed]
  15. F. López-Tejeira, S. G. Rodrigo, L. Martín-Moreno, F. J. García-Vidal, E. Devaux, T. W. Ebbesen, J. R. Krenn, I. P. Radko, S. I. Bozhevolnyi, M. U. González, J. C. Weeber, and A. Dereux, “Efficient unidirectional nanoslit couplers for surface plasmons,” Nat. Phys.3(5), 324–328 (2007). [CrossRef]
  16. H. Kim and B. Lee, “Unidirectional surface plasmon polariton excitation on single slit with oblique backside illumination,” Plasmonics4(2), 153–159 (2009). [CrossRef]
  17. S. B. Choi, D. J. Park, Y. K. Jeong, Y. C. Yun, M. S. Jeong, C. C. Byeon, J. H. Kang, Q.-H. Park, and D. S. Kim, “Directional control of surface plasmon polariton waves propagating through an asymmetric Bragg resonator,” Appl. Phys. Lett.94(6), 063115 (2009). [CrossRef]
  18. Y. Takakura, “Optical resonance in a narrow slit in a thick metallic screen,” Phys. Rev. Lett.86(24), 5601–5603 (2001). [CrossRef] [PubMed]
  19. J. Soohoo and G. E. Mevers, “Cavity mode analysis using the fourier transform method,” Proc. IEEE62(12), 1721–1723 (1974). [CrossRef]
  20. R. Petit, Electromagnetic Theory of Gratings (Springer-Verlag, New York, 1980).
  21. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, Berlin, 1988).
  22. S. Astilean, Ph. Lalanneb, and M. Palamarua, “Light transmission through metallic channels much smaller than the wavelength,” Opt. Commun.175(4-6), 265–273 (2000). [CrossRef]
  23. E. D. Palik and G. Ghosh, Handbook of Optical Constants of Solids, E.D. Palik, ed. (Academic, Orlando, Fla., 1985).
  24. H. A. Bethe, “Theory of diffraction by small holes,” Phys. Rev.66(7-8), 163–182 (1944). [CrossRef]
  25. J. S. Hyun, J. S. Moon, J. H. Parka, J. W. Kima, Y. D. Kimaand, and J. H. Boo, “Fabrication of near-field optical probes using advanced functional thin films for MEMS and NEMS applications,” Mater. Sci. Eng.149, 292–298 (2008).

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