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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 15 — Jul. 19, 2010
  • pp: 15741–15746
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Design and characterization of a micron-focusing plasmonic device

Fenghuan Hao, Rui Wang, and Jia Wang  »View Author Affiliations


Optics Express, Vol. 18, Issue 15, pp. 15741-15746 (2010)
http://dx.doi.org/10.1364/OE.18.015741


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Abstract

We design and experimentally demonstrate a focusing plasmonic device consisting of a central slit surrounded by the grooves with the fixed width and depth in Au/Cr metal layers. By modulating the phases of the radiation lights decoupled by the grooves from surface plasmon polaritons waves, the focal length can be controlled in the range of several wavelengths. A micron-focusing device is fabricated by the focused ion beam and the focusing performance is characterized using the scanning near-field optical microscope. The experimental results show a significant focusing effect and have agreement with the theoretical analysis and simulation results. Such a focusing plasmonic device with the predicted focal length has potential practical applications in integrated photonics due to the simplicity of the design and fabrication.

© 2010 OSA

1. Introduction

Since the directional beaming effect of a subwavelength metal aperture surrounded with the periodic grooves was firstly observed by H. Z. Lezec et al [1

1. H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297(5582), 820–822 (2002). [CrossRef] [PubMed]

], which is generally interpreted as the interactions of surface plasmon polaritons with metal surface corrugations [2

2. F. J. Garcia-Vidal, L. Martin-Moreno, H. J. Lezec, and T. W. Ebbesen, “Focusing light with a single subwavelength aperture flanked by surface corrugations,” Appl. Phys. Lett. 83, 4500–4502 (2003). [CrossRef]

, 3

3. L. Martín-Moreno, F. J. García-Vidal, H. J. Lezec, A. Degiron, and T. W. Ebbesen, “Theory of highly directional emission from a single subwavelength aperture surrounded by surface corrugations,” Phys. Rev. Lett. 90(16), 167401 (2003). [CrossRef] [PubMed]

], there has been great interest in manipulating light based on surface plasmon polaritons (SPPs) [4

4. Z. J. Sun and H. K. Kim, “Refractive transmission of light and beam shaping with metallic nano-optic lenses,” Appl. Phys. Lett. 85(4), 642–644 (2004). [CrossRef]

13

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

]. In recent years, types of slit-grooves structures are demonstrated to generate a subwavelength-sized focusing spot with the focal length of several micrometers in simulations and experiments [14

14. H. F. Shi, C. L. Du, and X. G. Luo, “Focal length modulation based on a metallic slit surrounded with grooves in curved depths,” Appl. Phys. Lett. 91(9), 093111 (2007). [CrossRef]

17

17. F. Hao, R. Wang, and J. Wang, “A novel design method of focusing-control device by modulating SPPs scattering,” Plasmonics 5(1), 45–49 (2010). [CrossRef]

], which have great application potential in imaging, optical storage and integrated optical circuits. In this letter, we experimentally demonstrate a focusing plasmonic device with a focal length of several wavelengths, which is designed using the method reported in ref [17

17. F. Hao, R. Wang, and J. Wang, “A novel design method of focusing-control device by modulating SPPs scattering,” Plasmonics 5(1), 45–49 (2010). [CrossRef]

]. The plasmonic device consists of the fixed-width and depth grooves surrounding the central slit, which is much easier to fabricate compared with the focusing structures consisting of the stepped grooves [14

14. H. F. Shi, C. L. Du, and X. G. Luo, “Focal length modulation based on a metallic slit surrounded with grooves in curved depths,” Appl. Phys. Lett. 91(9), 093111 (2007). [CrossRef]

, 16

16. B. H. Jia, H. F. Shi, J. F. Li, Y. Q. Fu, C. L. Du, and M. Gu, “Near-field visualization of focal depth modulation by step corrugated plasmonic slits,” Appl. Phys. Lett. 94(15), 151912 (2009). [CrossRef]

]or the chirped dielectric surface gratings [15

15. S. Kim, Y. Lim, H. Kim, J. Park, and B. Lee, “Optical beam focusing by a single subwavelength metal slit surrounded by chirped dielectric surface gratings,” Appl. Phys. Lett. 92(1), 013103 (2008). [CrossRef]

]. The micron-focusing device is fabricated using the focused ion beam (FIB), and the transmitted fields are detected with the scanning near-field optical microscope (SNOM) and compared with the simulation results.

2. Principle and simulation

The geometric configuration of the plasmonic device is shown in Fig. 1
Fig. 1 The schematic diagram of the plasmonic device.
. On the SiO2 substrate are the Au/Cr dual metal layers which have the same thickness of 100 nm. The central slit has the width of 200 nm; the width and the depth of the grooves are 150 nm and 100 nm respectively. The p-polarized incident lights with a wavelength of 633 nm are illuminated on the slit. As described in ref [17

17. F. Hao, R. Wang, and J. Wang, “A novel design method of focusing-control device by modulating SPPs scattering,” Plasmonics 5(1), 45–49 (2010). [CrossRef]

], the SPPs waves are excited at the slit exit and then propagate along Au film surface. The grooves scatter the SPPs waves into radiation waves with the specific phases. To focus the transmitted lights with the focal length of f, the radiation waves from grooves and the slit should interfere constructively at the focusing spot, so the phase from the slit (denoted as φo) and the phase from the groove with the position of x (denoted as φx) should satisfy the following equation:
2πλ(f2+x2)1/2f2πλ+φxφo=2mπ,m=0,±1,±2,±3...,
(1)
in whichλ=633nm is the incident wavelength.

According to the method reported in ref [17

17. F. Hao, R. Wang, and J. Wang, “A novel design method of focusing-control device by modulating SPPs scattering,” Plasmonics 5(1), 45–49 (2010). [CrossRef]

], the dependence of the phase difference φxφo on the groove position x is explored by investigating the simplified structure which is composed of a pair of grooves symmetrically surrounding the central slit. The phases at the centers of the slit exit and the groove exit are used to represent the phases of radiation lights from the slit and the groove. With the grooves positions varying from 440 nm to 3240 nm with the step of 80 nm, the phases of Ex field (φx andφo) at the centers of the groove exit and the slit exit are calculated with finite-difference time-domain (FDTD) method. The dependence of the phase difference φxφoon the groove position x is shown in Fig. 2
Fig. 2 (a) The dependence of the phase difference on the groove position and (b) the linear fit
. Under linear approximation, the relation is expressed in the following formula:

φxφo=0.0106x3.34.
(2)

Therefore by solving Eqs. (1) and (2), the grooves positions of the focusing device with a specific focal length of f can be obtained easily. Here a focusing device with a focal length of 1.8 μm as an illustrative device is designed. The transmitted field through the device is simulated with FDTD method and shown in Fig. 3
Fig. 3 The electric field intensity distribution of the transmitted lights through the plasmonic device.
. The simulation results indicate that the transmitted field is focused efficiently by the designed device with the focal length of 1.65 μm and the focusing spot has the size of 400 nm.

3. Experiment

In this section, we experimentally investigate a micron-focusing plasmonic device which is designed based on above analysis and simulations.

Two metal layers of 100-nm-thick chromium (Cr) and 100-nm-thick gold (Au) are deposited successively by the electron beam evaporation on the SiO2 substrate. The plasmonic device with a focal length of 1.8 μm corresponded to the incident wavelength of 633 nm is fabricated using the FIB. The structure parameters of the slit and the grooves are adopted the same as those in the simulations. The SEM image of the focusing plasmonic device is shown in Fig. 4
Fig. 4 The SEM image of a micron-focusing plasmonic device with a focal length of 1.8 μm.
.

The He-Ne laser beam (λ=633nm) is incident normally on the plasmonic device with the polarization vertical to the long edges of the slit and the grooves. The SNOM (NTEGRA, NT-MDT) is employed to detect the transmitted field at the horizontal planes with different heights from the device surface. Figure 5
Fig. 5 The intensity distribution of the transmitted lights at the horizontal planes with the distances of (a) z=0.5 μm, (b) z=1.6 μm, (c) z=2.5 μm, and (d) z=3.5 μm away from the device surface. (e) the intensity distribution along the solid line in Fig. 5 (b).
show the field distributions at the planes with the heights of z=0.5 μm, 1.6 μm (corresponding to the simulation result of the focal length), 2.5μm and 3.5 μm. It can be seen that with increasing the distance from the plasmonic device, the transmitted lights focus and then diverge. A line-shaped apparent focus spot with a stronger intensity is formed at the plane z=1.6 μm. At the plane z=2.5 μm the focus spot has a decreased intensity and a low contrast; at the plane z=3.5 μm the optical field diverges and no focus spot is formed. Figure 5(e) demonstrates the field profile along the solid line in Fig. 5 (b) and the corresponding simulation results. The experimental results show a basic agreement with the calculated values. The full width at half maximum of the focus spot is 550 nm, a little larger than the calculated results.

To verify the focal length of the plasmonic device, we use the SNOM to measure the field intensity at a center point of the slit at the different heights from the device surface (along the optical axis of the device). The dependence of the field intensity on the distance from the device surface is displayed in Fig. 6
Fig. 6 Intensity distribution along the optical axis of the plasmonic device.
. For the distance z<0.5 μm, the field intensity decreases with the distance increasing which is due to the attenuation of the evanescent waves. For the distance z> 0.5 μm, the strongest intensity is observed at the point with the height of z=1.5 μm, which corresponds to the focus spot and has an accordance with simulation results.

4. Conclusions

In this work, a focusing plasmonic device with a focal length of 1.8 μm is designed and fabricated. The transmitted fields through the plasmonic device are measured by the SNOM, and the measured results show agreement with analysis and simulation results.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (No. 60678028). The authors are grateful to Kaiwu Peng at National Nano-Center in China for FIB fabrication.

References and links

1.

H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297(5582), 820–822 (2002). [CrossRef] [PubMed]

2.

F. J. Garcia-Vidal, L. Martin-Moreno, H. J. Lezec, and T. W. Ebbesen, “Focusing light with a single subwavelength aperture flanked by surface corrugations,” Appl. Phys. Lett. 83, 4500–4502 (2003). [CrossRef]

3.

L. Martín-Moreno, F. J. García-Vidal, H. J. Lezec, A. Degiron, and T. W. Ebbesen, “Theory of highly directional emission from a single subwavelength aperture surrounded by surface corrugations,” Phys. Rev. Lett. 90(16), 167401 (2003). [CrossRef] [PubMed]

4.

Z. J. Sun and H. K. Kim, “Refractive transmission of light and beam shaping with metallic nano-optic lenses,” Appl. Phys. Lett. 85(4), 642–644 (2004). [CrossRef]

5.

H. Shi, C. Wang, C. Du, X. Luo, X. Dong, and H. Gao, “Beam manipulating by metallic nano-slits with variant widths,” Opt. Express 13(18), 6815–6820 (2005). [CrossRef] [PubMed]

6.

L. B. Yu, D. Z. Lin, Y. C. Chen, Y. C. Chang, K. T. Huang, J. W. Liaw, J. T. Yeh, J. M. Liu, C. S. Yeh, and C. K. Lee, “Physical origin of directional beaming emitted from a subwavelength slit,” Phys. Rev. B 71(4), 041405 (2005). [CrossRef]

7.

D. Z. Lin, C. K. Chang, Y. C. Chen, D. L. Yang, M. W. Lin, J. T. Yeh, J. M. Liu, C. H. Kuan, C. S. Yeh, and C. K. Lee, “Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings,” Opt. Express 14(8), 3503–3511 (2006). [CrossRef] [PubMed]

8.

S. Kim, H. Kim, Y. Lim, and B. Lee, “Off-axis directional beaming of optical field diffracted by a single subwavelength metal slit with asymmetric dielectric surface gratings,” Appl. Phys. Lett. 90(5), 51113 (2007). [CrossRef]

9.

L. Feng, K. A. Tetz, B. Slutsky, V. Lomakin, and Y. Fainman, “Fourier plasmonics: Diffractive focusing of in-plane surface plasmon polariton waves,” Appl. Phys. Lett. 91(8), 081101 (2007). [CrossRef]

10.

J. Zhang and G. P. Wang, “Dual-wavelength light beaming from a metal nanoslit flanked by dielectric gratings,” J. Opt. Soc. Am. B 25(8), 1356–1361 (2008). [CrossRef]

11.

H. Kim, J. Park, and B. Lee, “Tunable directional beaming from subwavelength metal slits with metal-dielectric composite surface gratings,” Opt. Lett. 34(17), 2569–2571 (2009). [CrossRef] [PubMed]

12.

J. Zhang and G. P. Wang, “Determination of thickness and dielectric constant of thin films by dual-wavelength light beaming effect of a metal nanoslit,” J. Appl. Phys. 106(3), 034305 (2009). [CrossRef]

13.

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

14.

H. F. Shi, C. L. Du, and X. G. Luo, “Focal length modulation based on a metallic slit surrounded with grooves in curved depths,” Appl. Phys. Lett. 91(9), 093111 (2007). [CrossRef]

15.

S. Kim, Y. Lim, H. Kim, J. Park, and B. Lee, “Optical beam focusing by a single subwavelength metal slit surrounded by chirped dielectric surface gratings,” Appl. Phys. Lett. 92(1), 013103 (2008). [CrossRef]

16.

B. H. Jia, H. F. Shi, J. F. Li, Y. Q. Fu, C. L. Du, and M. Gu, “Near-field visualization of focal depth modulation by step corrugated plasmonic slits,” Appl. Phys. Lett. 94(15), 151912 (2009). [CrossRef]

17.

F. Hao, R. Wang, and J. Wang, “A novel design method of focusing-control device by modulating SPPs scattering,” Plasmonics 5(1), 45–49 (2010). [CrossRef]

OCIS Codes
(050.1970) Diffraction and gratings : Diffractive optics
(240.6680) Optics at surfaces : Surface plasmons
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Diffraction and Gratings

History
Original Manuscript: May 24, 2010
Revised Manuscript: June 20, 2010
Manuscript Accepted: June 29, 2010
Published: July 9, 2010

Citation
Fenghuan Hao, Rui Wang, and Jia Wang, "Design and characterization of a micron-focusing plasmonic device," Opt. Express 18, 15741-15746 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-15-15741


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References

  1. H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297(5582), 820–822 (2002). [CrossRef] [PubMed]
  2. F. J. Garcia-Vidal, L. Martin-Moreno, H. J. Lezec, and T. W. Ebbesen, “Focusing light with a single subwavelength aperture flanked by surface corrugations,” Appl. Phys. Lett. 83, 4500–4502 (2003). [CrossRef]
  3. L. Martín-Moreno, F. J. García-Vidal, H. J. Lezec, A. Degiron, and T. W. Ebbesen, “Theory of highly directional emission from a single subwavelength aperture surrounded by surface corrugations,” Phys. Rev. Lett. 90(16), 167401 (2003). [CrossRef] [PubMed]
  4. Z. J. Sun and H. K. Kim, “Refractive transmission of light and beam shaping with metallic nano-optic lenses,” Appl. Phys. Lett. 85(4), 642–644 (2004). [CrossRef]
  5. H. Shi, C. Wang, C. Du, X. Luo, X. Dong, and H. Gao, “Beam manipulating by metallic nano-slits with variant widths,” Opt. Express 13(18), 6815–6820 (2005). [CrossRef] [PubMed]
  6. L. B. Yu, D. Z. Lin, Y. C. Chen, Y. C. Chang, K. T. Huang, J. W. Liaw, J. T. Yeh, J. M. Liu, C. S. Yeh, and C. K. Lee, “Physical origin of directional beaming emitted from a subwavelength slit,” Phys. Rev. B 71(4), 041405 (2005). [CrossRef]
  7. D. Z. Lin, C. K. Chang, Y. C. Chen, D. L. Yang, M. W. Lin, J. T. Yeh, J. M. Liu, C. H. Kuan, C. S. Yeh, and C. K. Lee, “Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings,” Opt. Express 14(8), 3503–3511 (2006). [CrossRef] [PubMed]
  8. S. Kim, H. Kim, Y. Lim, and B. Lee, “Off-axis directional beaming of optical field diffracted by a single subwavelength metal slit with asymmetric dielectric surface gratings,” Appl. Phys. Lett. 90(5), 51113 (2007). [CrossRef]
  9. L. Feng, K. A. Tetz, B. Slutsky, V. Lomakin, and Y. Fainman, “Fourier plasmonics: Diffractive focusing of in-plane surface plasmon polariton waves,” Appl. Phys. Lett. 91(8), 081101 (2007). [CrossRef]
  10. J. Zhang and G. P. Wang, “Dual-wavelength light beaming from a metal nanoslit flanked by dielectric gratings,” J. Opt. Soc. Am. B 25(8), 1356–1361 (2008). [CrossRef]
  11. H. Kim, J. Park, and B. Lee, “Tunable directional beaming from subwavelength metal slits with metal-dielectric composite surface gratings,” Opt. Lett. 34(17), 2569–2571 (2009). [CrossRef] [PubMed]
  12. J. Zhang and G. P. Wang, “Determination of thickness and dielectric constant of thin films by dual-wavelength light beaming effect of a metal nanoslit,” J. Appl. Phys. 106(3), 034305 (2009). [CrossRef]
  13. W. B. Chen, D. C. Abeysinghe, R. L. Nelson, and Q. W. Zhan, “Plasmonic lens made of multiple concentric metallic rings under radially polarized illumination,” Nano Lett. 9(12), 4320–4325 (2009). [CrossRef] [PubMed]
  14. H. F. Shi, C. L. Du, and X. G. Luo, “Focal length modulation based on a metallic slit surrounded with grooves in curved depths,” Appl. Phys. Lett. 91(9), 093111 (2007). [CrossRef]
  15. S. Kim, Y. Lim, H. Kim, J. Park, and B. Lee, “Optical beam focusing by a single subwavelength metal slit surrounded by chirped dielectric surface gratings,” Appl. Phys. Lett. 92(1), 013103 (2008). [CrossRef]
  16. B. H. Jia, H. F. Shi, J. F. Li, Y. Q. Fu, C. L. Du, and M. Gu, “Near-field visualization of focal depth modulation by step corrugated plasmonic slits,” Appl. Phys. Lett. 94(15), 151912 (2009). [CrossRef]
  17. F. Hao, R. Wang, and J. Wang, “A novel design method of focusing-control device by modulating SPPs scattering,” Plasmonics 5(1), 45–49 (2010). [CrossRef]

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