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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 2 — Jan. 28, 2013
  • pp: 1639–1644
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Plasmonic analog of microstrip transmission line and effect of thermal annealing on its propagation loss

Yiting Chen, Jing Wang, Xi Chen, Min Yan, and Min Qiu  »View Author Affiliations


Optics Express, Vol. 21, Issue 2, pp. 1639-1644 (2013)
http://dx.doi.org/10.1364/OE.21.001639


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Abstract

We fabricated a plasmonic analog of the microwave microstrip transmission line and measured its propagation loss before and after thermal annealing. It is found that its propagation loss at 980 nm wavelength can be reduced by more than 50%, from 0.45 to 0.20 dB/μm, after thermal annealing at 300 °C. The reduction in loss can be attributed to the improved gold surface condition and probably also to the change in the metal’s inner structure. Less evident loss reduction is noticed at 1550 nm, which is owing to extremely small portion of the modal electric field located in the metal regions at this wavelength.

© 2013 OSA

1. Introduction

2. Waveguide and the modes

The fabricated waveguide is schematically shown in Fig. 1(a). Basically it consists of two gold layers separated by a dielectric layer of Al2O3. Refer to the figure: the thicknesses of the three layers from top to bottom are h1 = 80 nm, h2 = 120 nm and h3 = 100 nm respectively; the width of the top strip is w = 400 nm. The waveguide is sitting on a glass substrate. The gold and Al2O3 layers are deposited through electron-beam vaporization in a high vacuum at the same rate of 0.1 nm/s. The length of the waveguide (along z) is 30 μm. Patterning of the top-layer gold strip is achieved through electron-beam lithography. The top-view scanning-electron-microscope (SEM) image of the fabricated sample will be presented in Section 3.

Fig. 1 (a) Geometry of the waveguide under study. (b) Fundamental guided mode at 1550 nm. The color shading indicates the z-component of the Poynting vector, while the vectors show the transverse electric field. (c) The same but for a slot waveguide of similar size (see text).

3. Experimental results and discussions

The measurement of its propagation loss is done in a way similar to the cut-back method in the fiber-optics community. A tapered fiber tip is used to excite the waveguide from its side; the output light intensity scattered from the end of waveguide is captured by a CCD camera, while the excitation spot is carefully varied along the waveguide. The propagation loss can then be extracted by fitting the light output versus propagation distance to an exponentially decaying function [21

21. Y. Ma, X. Li, H. Yu, L. Tong, Y. Gu, and Q. Gong, “Direct measurement of propagation losses in silver nanowires,” Opt. Lett. 35, 1160–1162 (2010). [CrossRef] [PubMed]

, 22

22. Q. Li, S. Wang, Y. Chen, M. Yan, L. Tong, and M. Qiu, “Experimental demonstration of plasmon propagation, coupling, and splitting in silver nanowire at 1550-nm wavelength,” IEEE of Selected Topics J. in Quantum Electronics 17, 1107–1111 (2011). [CrossRef]

]. The error in propagation loss from repeated measurements, even with different fiber tips, is well within 10%. The measured output intensity (I0) of the waveguide at 980 nm is shown by the black squares in Fig. 2(a). A least-square curve fitting suggests that the waveguide has a propagation loss of 0.45 dB/μm. Similarly at 1550 nm the output is recorded by the black squares in Fig. 2(b), and the fitted loss is 0.31 dB/μm. The experimentally obtained propagation loss values are higher than those calculated based on the Drude model pamameter data [20

20. M. A. Ordal, R. J. Bell, R. W. Alexander Jr, L. L. Long, and M. R. Querry, “Optical properties of fourteen metals in the infrared and far infrared: Al, Co, Cu, Au, Fe, Pb, Mo, Ni, Pd, Pt, Ag, Ti, V, and W.,” Appl. Opt. 24, 4493–4499 (1985). [CrossRef] [PubMed]

], but the difference is acceptable. Besides, it can introduce some difference to the gold parameters due to different sample preparation methods. In Fig. 2(c) we show 9 snapshots for the coupling experiment at 980 nm at various excitation spots. The SPP waveguide and the fiber tip are clearly seen in the top bright-field image.

Fig. 2 Output intensity v.s. propagation distance measured at 980 nm (a) and 1550 nm (b) before (black squares) and after (red circles) annealing. The black and red lines are the fitted curves for the decaying light intensity in the waveguide before and after annealing, respectively. (c) Bright- and dark-field images for coupling at 980 nm recorded by a CCD camera.

It has already been known that thermal annealing can significantly change the quality of a gold film, by increasing the grain size and flatting the film surface, which was previously confirmed by examining SEM images [23

23. D. Porath, Y. Goldstein, A. Grayevsky, and O. Millo, “Scanning tunneling microscopy studies of annealing of gold films,” Surf. Sci. 321, 81–88 (1994). [CrossRef]

, 24

24. M. Bechelany, X. Maeder, J. Riesterer, J. Hankache, D. Lerose, S. Christiansen, J. Michler, and L. Philippe, “Synthesis Mechanisms of Organized Gold Nanoparticles: Influence of Annealing Temperature and Atmosphere,” Cryst. Growth Des. 10, 587–596 (2010). [CrossRef]

], TEM images and X-ray diffraction analysis [25

25. Y. Golan, L. Margulis, and I. Rubinstein, “Vacuum-deposited gold films,” Surf. Sci. 264, 312–326 (1992). [CrossRef]

, 26

26. C.E.D. Chidsey, D.N. Loiacono, T. Sleator, and S. Nakahara, “STM study of the surface morphology of gold on mica,” Surf. Sci. 200, 45–66 (1988). [CrossRef]

]. Here we demonstrate the effect of thermal annealing on light propagation in our fabricated plasmonic waveguide, as a consequence of the change in gold quality. The annealing treatments were done in a temperature-controlled oven at atmospheric pressure. First, the sample was heated to 300 °C from room temperature (25 °C) and the temperature was maintained for 18 hours before cooling. The heating and cooling rates are about 1.5 and 0.5 °C/min, respectively; they are slow enough so no thermal shocks are induced. The propagation losses at 980 and 1550 nm are then measured using the same technique described above. The output intensity measurements are shown by the red circles in Fig. 2(a) and 2(b). By curve fitting, we obtain the propagation losses at 980 nm and 1550 nm as 0.20 dB/μm and 0.28 dB/μm respectively.

Quite surprisingly, the propagation loss at 980 nm has been decreased by more than 50% (from 0.45 to 0.20 dB/μm). We attribute the reduction of the propagation loss to the structural improvement in the gold strip and possibly also in the bottom gold film. Indeed the top-view SEM images of the waveguide before and after annealing (Fig. 3) show clear difference: before annealing (Fig. 3(a)) the surface of gold strip appear grainy with grain sizes in 15–30 nm; after annealing (Fig. 3(b)) the strip surface as well as its sides appear much smoother, and the grain sizes also increase, which agree well with the result achieved in paper [23

23. D. Porath, Y. Goldstein, A. Grayevsky, and O. Millo, “Scanning tunneling microscopy studies of annealing of gold films,” Surf. Sci. 321, 81–88 (1994). [CrossRef]

26

26. C.E.D. Chidsey, D.N. Loiacono, T. Sleator, and S. Nakahara, “STM study of the surface morphology of gold on mica,” Surf. Sci. 200, 45–66 (1988). [CrossRef]

]. Apart from the surface, the possible change in the inner structure of the gold parts may also have contributed to the improvement of propagation.

Fig. 3 (a) SEM images of a section of the fabricated waveguide. (b) The waveguide after thermal annealing at 300 °C.

To further investigate the effect of annealing temperature, we put the same waveguide back to the oven and heated it at 400 °C again for 18 hours. Compared to what we have obtained for 300 °C, no obvious difference is found, both from the top-view SEM image of the waveguide and from the measured propagation losses at the two wavelengths. Then we pushed the annealing temperature to 500 °C (also 18 hours). The top gold strip then undergoes severe distortion: the width of the gold strip at different positions experiences shrinking by different degrees. The nonuniformity of the width leads to increased propagation losses at both 980 and 1550 nm.

4. Conclusion

Acknowledgment

This work is supported by the Swedish Foundation for Strategic Research (SSF), the Swedish Research Council (VR), and VR’s Linnaeus center in Advanced Optics and Photonics (ADOPT).

References and links

1.

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424, 824–830 (2003). [CrossRef] [PubMed]

2.

D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4, 83–91 (2010). [CrossRef]

3.

B. Steinberger, A. Hohenau, H. Ditlbacher, A. L. Stepanov, A. Drezet, F. R. Aussenegg, A. Leitner, and J. R. Krenn, “Dielectric stripes on gold as surface plasmon waveguides,” Appl. Phys. Lett. 88, 094104 (2006). [CrossRef]

4.

T. Holmgaard and S. I. Bozhevolnyi, “Theoretical analysis of dielectric-loaded surface plasmon-polariton waveguides,” Phys. Rev. B 75, 245405 (2007). [CrossRef]

5.

M. Quinten, A. Leitner, J. R. Krenn, and F. R. Aussenegg, “Electromagnetic energy transport via linear chains of silver nanoparticles,” Opt. Lett. 23, 1331–1333 (1998). [CrossRef]

6.

S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. Requicha, “Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides,” Nat. Mater. 2, 229–232 (2003). [CrossRef] [PubMed]

7.

D. F. P. Pile, T. Ogawa, D. K. Gramotnev, T. Okamoto, M. Haraguchi, M. Fukui, and S. Matsuo, “Theoretical and experimental investigation of strongly localized plasmons on triangular metal wedges for subwavelength waveguiding,” Appl. Phys. Lett. 87, 061106 (2005). [CrossRef]

8.

S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J. Y. Laluet, and T. W. Ebbesen, “Channel plasmon subwavelength waveguide components including interferometers and ring resonators,” Nature 440, 508–511 (2006). [CrossRef] [PubMed]

9.

M. Yan and M. Qiu, “Guided plasmon polariton at 2D metal corners,” J. Opt. Soc. Am. B 24, 2333–2342 (2007). [CrossRef]

10.

J. Takahara, S. Yamagishi, H. Taki, A. Morimoto, and T. Kobayashi, “Guiding of a one-dimensional optical beam with nanometer diameter,” Opt. Lett. 22, 475–477 (1997). [CrossRef] [PubMed]

11.

R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics 2, 496 – 500 (2008). [CrossRef]

12.

M. Yan and M. Qiu, “Compact optical waveguides based on hybrid index and surface- plasmon-polariton guidance mechanisms,” Active and Passive Electronic Components 2007, 52461 (2007). [CrossRef]

13.

L. Liu, Z. Han, and S. He, “Novel surface plasmon waveguide for high integration,” Opt. Express 13, 6645–6650 (2005). [CrossRef] [PubMed]

14.

D. F. P. Pile, T. Ogawa, D. K. Gramotnev, Y. Matsuzaki, K. C. Vernon, K. Yamaguchi, T. OKamoto, M. Haraguchi, and M. Fukui, “Two-dimensionally localized modes of a nanoscale gap plasmon waveguide,” Appl. Phys. Lett. 87, 261114 (2005). [CrossRef]

15.

M. C. Gather, K. Meerholz, N. Danz, and K. Leosson, “Net optical gain in a plasmonic waveguide embedded in a fluorescent polymer,” Nat. Photonics 4, 457–461 (2010). [CrossRef]

16.

M. Rocca, F. Moresco, and U. Valbusa, “Temperature dependence of surface plasmons on ag(001),” Phys. Rev. B 45, 1399–1402 (1992). [CrossRef]

17.

C. Rhodes, S. Franzen, J. P. Maria, M. Losego, D. N. Leonard, B. Laughlin, G. Duscher, and S. Weibel, “Surface plasmon resonance in conducting metal oxides,” J. Appl. Phys. 100, 054905 (2006). [CrossRef]

18.

S. Kumar, Y. Lu, A. Huck, and U. L. Andersen, “Propagation of plasmons in designed single crystalline silver nanostructures,” Opt. Express 20, 24614–24622 (2012). [CrossRef] [PubMed]

19.

P. Kusar, C. Gruber, A. Hohenau, and J. R. Krenn, “Measurement and Reduction of Damping in Plasmonic Nanowires,” Nano Lett. 12, 661–665 (2012). [CrossRef] [PubMed]

20.

M. A. Ordal, R. J. Bell, R. W. Alexander Jr, L. L. Long, and M. R. Querry, “Optical properties of fourteen metals in the infrared and far infrared: Al, Co, Cu, Au, Fe, Pb, Mo, Ni, Pd, Pt, Ag, Ti, V, and W.,” Appl. Opt. 24, 4493–4499 (1985). [CrossRef] [PubMed]

21.

Y. Ma, X. Li, H. Yu, L. Tong, Y. Gu, and Q. Gong, “Direct measurement of propagation losses in silver nanowires,” Opt. Lett. 35, 1160–1162 (2010). [CrossRef] [PubMed]

22.

Q. Li, S. Wang, Y. Chen, M. Yan, L. Tong, and M. Qiu, “Experimental demonstration of plasmon propagation, coupling, and splitting in silver nanowire at 1550-nm wavelength,” IEEE of Selected Topics J. in Quantum Electronics 17, 1107–1111 (2011). [CrossRef]

23.

D. Porath, Y. Goldstein, A. Grayevsky, and O. Millo, “Scanning tunneling microscopy studies of annealing of gold films,” Surf. Sci. 321, 81–88 (1994). [CrossRef]

24.

M. Bechelany, X. Maeder, J. Riesterer, J. Hankache, D. Lerose, S. Christiansen, J. Michler, and L. Philippe, “Synthesis Mechanisms of Organized Gold Nanoparticles: Influence of Annealing Temperature and Atmosphere,” Cryst. Growth Des. 10, 587–596 (2010). [CrossRef]

25.

Y. Golan, L. Margulis, and I. Rubinstein, “Vacuum-deposited gold films,” Surf. Sci. 264, 312–326 (1992). [CrossRef]

26.

C.E.D. Chidsey, D.N. Loiacono, T. Sleator, and S. Nakahara, “STM study of the surface morphology of gold on mica,” Surf. Sci. 200, 45–66 (1988). [CrossRef]

27.

T. Andersson and C. G. Granqvist, “Morphology and size distributions of islands in discontinuous films,” J. Appl. Phys. 48, 1673–1679 (1977). [CrossRef]

28.

M. Bowker, “Surface science: The going rate for catalysts,” Nat. Mater. 1, 205–206 (2002). [CrossRef]

29.

P. Meakin, “The growth of rough surfaces and interfaces,” Phys. Rep. 235, 189–289 (1993). [CrossRef]

OCIS Codes
(120.6810) Instrumentation, measurement, and metrology : Thermal effects
(220.3740) Optical design and fabrication : Lithography
(230.7370) Optical devices : Waveguides
(240.6680) Optics at surfaces : Surface plasmons
(260.3910) Physical optics : Metal optics
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Optics at Surfaces

History
Original Manuscript: November 26, 2012
Revised Manuscript: December 23, 2012
Manuscript Accepted: December 23, 2012
Published: January 15, 2013

Citation
Yiting Chen, Jing Wang, Xi Chen, Min Yan, and Min Qiu, "Plasmonic analog of microstrip transmission line and effect of thermal annealing on its propagation loss," Opt. Express 21, 1639-1644 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-2-1639


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References

  1. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature424, 824–830 (2003). [CrossRef] [PubMed]
  2. D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics4, 83–91 (2010). [CrossRef]
  3. B. Steinberger, A. Hohenau, H. Ditlbacher, A. L. Stepanov, A. Drezet, F. R. Aussenegg, A. Leitner, and J. R. Krenn, “Dielectric stripes on gold as surface plasmon waveguides,” Appl. Phys. Lett.88, 094104 (2006). [CrossRef]
  4. T. Holmgaard and S. I. Bozhevolnyi, “Theoretical analysis of dielectric-loaded surface plasmon-polariton waveguides,” Phys. Rev. B75, 245405 (2007). [CrossRef]
  5. M. Quinten, A. Leitner, J. R. Krenn, and F. R. Aussenegg, “Electromagnetic energy transport via linear chains of silver nanoparticles,” Opt. Lett.23, 1331–1333 (1998). [CrossRef]
  6. S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. Requicha, “Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides,” Nat. Mater.2, 229–232 (2003). [CrossRef] [PubMed]
  7. D. F. P. Pile, T. Ogawa, D. K. Gramotnev, T. Okamoto, M. Haraguchi, M. Fukui, and S. Matsuo, “Theoretical and experimental investigation of strongly localized plasmons on triangular metal wedges for subwavelength waveguiding,” Appl. Phys. Lett.87, 061106 (2005). [CrossRef]
  8. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J. Y. Laluet, and T. W. Ebbesen, “Channel plasmon subwavelength waveguide components including interferometers and ring resonators,” Nature440, 508–511 (2006). [CrossRef] [PubMed]
  9. M. Yan and M. Qiu, “Guided plasmon polariton at 2D metal corners,” J. Opt. Soc. Am. B24, 2333–2342 (2007). [CrossRef]
  10. J. Takahara, S. Yamagishi, H. Taki, A. Morimoto, and T. Kobayashi, “Guiding of a one-dimensional optical beam with nanometer diameter,” Opt. Lett.22, 475–477 (1997). [CrossRef] [PubMed]
  11. R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, “A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation,” Nat. Photonics2, 496 – 500 (2008). [CrossRef]
  12. M. Yan and M. Qiu, “Compact optical waveguides based on hybrid index and surface- plasmon-polariton guidance mechanisms,” Active and Passive Electronic Components2007, 52461 (2007). [CrossRef]
  13. L. Liu, Z. Han, and S. He, “Novel surface plasmon waveguide for high integration,” Opt. Express13, 6645–6650 (2005). [CrossRef] [PubMed]
  14. D. F. P. Pile, T. Ogawa, D. K. Gramotnev, Y. Matsuzaki, K. C. Vernon, K. Yamaguchi, T. OKamoto, M. Haraguchi, and M. Fukui, “Two-dimensionally localized modes of a nanoscale gap plasmon waveguide,” Appl. Phys. Lett.87, 261114 (2005). [CrossRef]
  15. M. C. Gather, K. Meerholz, N. Danz, and K. Leosson, “Net optical gain in a plasmonic waveguide embedded in a fluorescent polymer,” Nat. Photonics4, 457–461 (2010). [CrossRef]
  16. M. Rocca, F. Moresco, and U. Valbusa, “Temperature dependence of surface plasmons on ag(001),” Phys. Rev. B45, 1399–1402 (1992). [CrossRef]
  17. C. Rhodes, S. Franzen, J. P. Maria, M. Losego, D. N. Leonard, B. Laughlin, G. Duscher, and S. Weibel, “Surface plasmon resonance in conducting metal oxides,” J. Appl. Phys.100, 054905 (2006). [CrossRef]
  18. S. Kumar, Y. Lu, A. Huck, and U. L. Andersen, “Propagation of plasmons in designed single crystalline silver nanostructures,” Opt. Express20, 24614–24622 (2012). [CrossRef] [PubMed]
  19. P. Kusar, C. Gruber, A. Hohenau, and J. R. Krenn, “Measurement and Reduction of Damping in Plasmonic Nanowires,” Nano Lett.12, 661–665 (2012). [CrossRef] [PubMed]
  20. M. A. Ordal, R. J. Bell, R. W. Alexander, L. L. Long, and M. R. Querry, “Optical properties of fourteen metals in the infrared and far infrared: Al, Co, Cu, Au, Fe, Pb, Mo, Ni, Pd, Pt, Ag, Ti, V, and W.,” Appl. Opt.24, 4493–4499 (1985). [CrossRef] [PubMed]
  21. Y. Ma, X. Li, H. Yu, L. Tong, Y. Gu, and Q. Gong, “Direct measurement of propagation losses in silver nanowires,” Opt. Lett.35, 1160–1162 (2010). [CrossRef] [PubMed]
  22. Q. Li, S. Wang, Y. Chen, M. Yan, L. Tong, and M. Qiu, “Experimental demonstration of plasmon propagation, coupling, and splitting in silver nanowire at 1550-nm wavelength,” IEEE of Selected Topics J. in Quantum Electronics17, 1107–1111 (2011). [CrossRef]
  23. D. Porath, Y. Goldstein, A. Grayevsky, and O. Millo, “Scanning tunneling microscopy studies of annealing of gold films,” Surf. Sci.321, 81–88 (1994). [CrossRef]
  24. M. Bechelany, X. Maeder, J. Riesterer, J. Hankache, D. Lerose, S. Christiansen, J. Michler, and L. Philippe, “Synthesis Mechanisms of Organized Gold Nanoparticles: Influence of Annealing Temperature and Atmosphere,” Cryst. Growth Des.10, 587–596 (2010). [CrossRef]
  25. Y. Golan, L. Margulis, and I. Rubinstein, “Vacuum-deposited gold films,” Surf. Sci.264, 312–326 (1992). [CrossRef]
  26. C.E.D. Chidsey, D.N. Loiacono, T. Sleator, and S. Nakahara, “STM study of the surface morphology of gold on mica,” Surf. Sci.200, 45–66 (1988). [CrossRef]
  27. T. Andersson and C. G. Granqvist, “Morphology and size distributions of islands in discontinuous films,” J. Appl. Phys.48, 1673–1679 (1977). [CrossRef]
  28. M. Bowker, “Surface science: The going rate for catalysts,” Nat. Mater.1, 205–206 (2002). [CrossRef]
  29. P. Meakin, “The growth of rough surfaces and interfaces,” Phys. Rep.235, 189–289 (1993). [CrossRef]

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