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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 11 — May. 24, 2010
  • pp: 11202–11208
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Impact of titanium adhesion layers on the response of arrays of metallic split-ring resonators (SRRs)

Basudev Lahiri, Rafal Dylewicz, Richard M. De La Rue, and Nigel P. Johnson  »View Author Affiliations


Optics Express, Vol. 18, Issue 11, pp. 11202-11208 (2010)
http://dx.doi.org/10.1364/OE.18.011202


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Abstract

At higher frequencies (visible and infrared) both the dimensions and the individual metal properties play an important role in determining the resonant response of arrays of SRRs. As a result, a substantial difference between the responses of gold- and Al-based SRR arrays has been observed. Additionally, deposition of gold SRRs onto a substrate typically involves the use of an additional adhesion layer. Titanium (Ti) is the most common adhesive thin-film material used to attach gold onto dielectric/semiconductor substrates. In this paper we investigate the impact of the Ti adhesion layer on the overall response of Au-based nano-scale SRRs. The results quantify the extent to which the overall difference in the resonance frequencies between Au- and Al-based SRRs is due to the presence of the Ti. We show that even a 2-nm-thick Ti layer can red-shift the position of SRR resonance by 20 nm. Finally, we demonstrate that by intentional addition of titanium in the Au-based SRRs, their overall resonant response can be tuned widely in frequency, but at the expense of resonance magnitude.

© 2010 OSA

1. Introduction

2. Fabrication and measurements

In this paper, simulated and experimental responses of SRRs with characteristic planar dimensions as small as 200 nm were considered. The devices were composed of either Al or Au layers, together with a variable amount of Ti adhesion layer - while keeping the overall SRR metallization thickness constant at 50 nm. Medium doped n-type silicon substrates with resistivity ~100 Ωcm were used in all cases. The fabrication was performed using direct writing with electron beam lithography. The multi-component metallic layers were deposited onto the silicon using electron-beam evaporation. The patterns were written over an area of around 300 µm × 300 µm. The reflectance measurements were performed at normal incidence with a × 10 microscope objective (NA = 0.25), using a white light source and monochromator with an InGaAs detector operating in the range from 0.8 µm to 1.6 µm - together with a lock-in amplifier. The measurements were taken for two orthogonal linear polarizations of the incident light (TE and TM polarization) - and were then normalized with respect to the reflectivity of a bare silicon substrate. Examples of the measured responses of closely similar-sized Au- and Al-based SRRs are shown in Fig. 1
Fig. 1 SEM micrographs of fabricated SRRs and their corresponding experimental reflectance spectra for TE/TM polarization: (a) 50-nm-thick Al SRRs without Ti adhesion layer; (b) 48-nm-thick Au SRRs with 2 nm of Ti layer.
. The difference in resonance positions between these two cases, reaching as much as ~200 nm, requires an analysis based on the different material properties and the additional inclusion of the effect of the Ti-layer for Au-based devices.

The values of the plasma frequency (ωp) and the collision frequency (ωτ) for bulk gold are 2175 THz and 6.5 THz, whereas those of aluminium are 3750 THz and 19.4 THz [8

8. M. A. Ordal, L. L. Long, R. J. Bell, S. E. Bell, R. R. Bell, R. W. Alexander Jr, and C. A. Ward, “Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared,” Appl. Opt. 22(7), 1099–1119 (1983). [CrossRef] [PubMed]

,9

9. I. El-Kady, M. M. Sigalas, R. Biswas, K. M. Ho, and C. M. Soukoulis, “Metallic Photonic crystals at optical wavelengths,” Phys. Rev. B 62(23), 15299 (2000). [CrossRef]

]. As experimentally shown both in Fig. 1 and in reference [3

3. B. Lahiri, S. G. McMeekin, A. Z. Khokhar, R. M. De La Rue, and N. P. Johnson, “Magnetic response of split ring resonators (SRRs) at visible frequencies,” Opt. Express 18(3), 3210–3218 (2010). [CrossRef] [PubMed]

], the higher plasma frequency of Al-based SRRs results in a shorter wavelength of resonance, as compared to Au-based SRRs. On the other hand, the higher collision frequency of Al results in lower magnitude in, as well as the broadening of, the resonance peak of Al-based SRRs.

3. Numerical simulations

The resonant response of the SRRs was calculated using a fully three-dimensional (3-D) simulation of the fabricated device, after convergence tests were performed. The numerical simulations were performed with a commercial finite-difference time-domain method (FDTD) software package from Lumerical. The transmission and reflection characteristics were calculated from a single unit cell for two different optical polarizations, over a wide range of wavelengths. A Drude model was used to describe the frequency-dependent material properties. An integer number of cells with a small mesh-size of 2 nm was applied within the metallic layers, while non-uniform and coarser meshing was used in the surrounding regions. The bulk values of ωp and ωτ (2175 and 6.5 THz) for gold were adjusted to effective values (2400 and 25 THz), following the approach presented in [10

10. S. Linden, C. Enkrich, M. Wegener, J. Zhou, T. Koschny, and C. M. Soukoulis, “Magnetic Response of Metamaterials at 100 THz,” Science 306(5700), 1351–1353 (2004). [CrossRef] [PubMed]

], so that the simulation spectrum was in close agreement with the experimental spectrum for minimal Ti content Au structures. For very thin films, the fraction of the oscillating electric field outside the metallic structure is larger, which results in an additional dynamic inductance contribution [10

10. S. Linden, C. Enkrich, M. Wegener, J. Zhou, T. Koschny, and C. M. Soukoulis, “Magnetic Response of Metamaterials at 100 THz,” Science 306(5700), 1351–1353 (2004). [CrossRef] [PubMed]

]. At the same time, the effective collision frequency is higher than the bulk value due to the additional scattering experienced by electrons in such thin metal films. In the absence of direct experimental data for our deposited Ti layers, bulk values were used (383 and 136 THz). As a first approximation, an effective medium for the composite was assumed by taking linear combinations of the respective plasma frequencies and collision frequencies for Au and Ti, in proportion to the volume fractions of metal (corresponding to their relative thicknesses). Such a phenomenological approach was used due to the limitations of the software in accurate modeling of the behavior of very thin layers (i.e. a ~2-nm-thick Ti adhesion layer).

4. Effect of titanium adhesion layer

In order to examine how much the Ti adhesion layer contributes to the overall response of the Au SRRs, two different sets of experiments were performed. In the first set of experiments, the overall thickness of the Au/Ti SRR was kept constant at 50 nm, while the thickness of the Ti adhesion layer was increased at the expense of Au. The effect of increasing Ti content in an array of such mixed Au/Ti SRRs is shown in Fig. 2
Fig. 2 Reflection spectra of dual-layer SRRs with increased Ti quantity, where the overall Au/Ti thickness is kept constant: (a) experimental spectra at two different polarizations; (b) calculated spectra for the extended wavelength range.
. By increasing the fractional amount of Ti in the layer composition, both plasmonic and magnetic peaks move towards longer wavelengths - and diminish in amplitude. This flattening effect can be explained by the very high damping frequency of Ti.

From Ordal et al. [8

8. M. A. Ordal, L. L. Long, R. J. Bell, S. E. Bell, R. R. Bell, R. W. Alexander Jr, and C. A. Ward, “Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared,” Appl. Opt. 22(7), 1099–1119 (1983). [CrossRef] [PubMed]

] the collision frequency (ωτ) and plasma frequency (ωp) are associated with the complex dielectric function according to:
ωτ=ωε21ε1     
(1)
and
ωp2=(1ε1)(ω2+ωτ2)
(2)
For ω = 6451.61 cm−1, which is equivalent to a free-space wavelength of 1.55 µm, we obtain the corresponding ε1 and ε2 values for titanium of -ε1 = 6.56 and ε2 = 33.3, based on [8

8. M. A. Ordal, L. L. Long, R. J. Bell, S. E. Bell, R. R. Bell, R. W. Alexander Jr, and C. A. Ward, “Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared,” Appl. Opt. 22(7), 1099–1119 (1983). [CrossRef] [PubMed]

]. Putting the values of ω, ε1 and ε2 into Eqs. (1) and (2), we obtain the collision frequency for titanium, ωτ = 136 THz, and plasma frequency for titanium, ωp = 383 THz. Thus, the calculated plasma frequency for Ti is almost one order of magnitude lower than for the other metals. Additionally, Ti is characterized by a very high damping frequency of 136 THz, which is associated with absorption and losses. As more Ti is added to the system, the reflectance response is both red-shifted and reduced in magnitude (see Fig. 2)

In a second set of experiments, the thickness of the Ti adhesion layer was gradually increased, while the thickness of the Au layer was kept constant at 48 nm. In this case, the total thickness of the SRRs also increased progressively. In Fig. 3
Fig. 3 Reflection spectra of dual-layer SRRs with increased Ti fraction and total system thickness, while the Au thickness is kept constant: (a) experimental spectra at two different polarizations; (b) calculated spectra for the extended wavelength range.
, a similar trend is observable to that in Fig. 2, i.e. by increasing the amount of Ti in the system, the overall resonant response degrades. However, by maintaining the Au thickness at a constant value, the reflectance response of the SRRs becomes larger in amplitude - and the flattening of the curves for high Ti content layers is no longer apparent. The increasing thickness of the SRRs apparently increases the effective plasma frequency and gives a blue-shift to the peak positions. In this experiment, a trade-off occurs between the increased Ti thickness - which pushes the response towards longer wavelengths while progressively suppressing the peaks - and the increased total thickness, which causes a blue-shift in the resonance peaks. For both simulation and experiment it can be confirmed, from the results shown in Figs. 2 and 3, that an increase of Ti fraction has reduced the observable response nearly to zero, once it forms more than 40% of the overall SRR thickness. The primary conclusion from these results is that the greater absorption in the Ti dominates the overall behavior, once it is present in sufficiently large amounts - even when the amount of Au is undiminished.

The FDTD simulation results presented in Figs. 2b and 3b substantially reproduce the experimental trend for an increased Ti content in dual-layer arrays at wavelengths up to λ = 1600 nm, while additionally showing decreasing reflectance in the extended wavelength range up to λ = 2440 nm. Good agreement with the experimental behavior shown in Figs. 2a and 3a is observed, especially for the magnetic (LC) resonances (around 1600 nm), whereas some discrepancy is evident between the experimental results and the simulated plasmonic peak positions. This discrepancy may be due to the non-ideal shape of the fabricated SRRs (see Fig. 1), in contrast with ideal shape used for the numerical calculations (see left inset in Figs. 2 and 3). The effect of the Ti-layer induced red-shift of both the plasmonic and the magnetic resonances of the Au/Ti bi-layer SRRs is shown in Fig. 4
Fig. 4 Calculated Ti-induced change in resonance position and amplitude for dual-layer SRR as a function of Ti content. The overall thickness of Au/Ti SRR is kept constant: results for (a) TE and (b) TM polarization.
. It can be seen that the presence of even a 2-nm-thick Ti layer can shift the SRR response by as much as 20 nm.

For a strongly composition-dependent behavior, the movement of the resonance towards longer wavelengths is clearly observed, as well as the strongly diminishing amplitude of these effects, for both light polarizations. For Ti-content > 40%, the reflectance at both the plasmonic and magnetic resonance peaks is reduced to about half of its initial/maximum value. Although the amplitude is reduced, addition of Ti (up to 40%) provides additional tuning possibilities for both the electric and magnetic resonance peaks of SRRs, as shown in Fig. 4.

5. Skin depth

Finally, to understand the impact of the Ti adhesion layer on the observed difference between the responses of 50-nm-thick Al-based SRRs and that of mixed Au/Ti-based SRRs (see Fig. 1), an additional experiment was performed. A Ti adhesion layer of 2 nm was added into the fabrication process, while realizing 48-nm-thick Al SRRs and the response was compared to that of a purely Al-based SRR (50 nm) of Fig. 1. The experimental results are shown in Fig. 6
Fig. 6 Comparison between the experimental reflectance spectra of 50-nm-thick Al with that of 2 nm Ti + 48 nm Al SRRs: (a) results for TE polarization; (b) results for TM polarization.
, where a red-shift of about 20 nm is observed for both TE/TM peaks, when compared to results for pure Al SRRs. This result further corroborates the calculated results presented in Fig. 4, where a similar Ti-induced shift was observed in case of Au-based SRRs.

Hence, the difference in the responses of the Au- and Al-based SRRs (see Fig. 1) is mainly caused by their material properties, i.e. because of the lower value of the Au plasma resonance frequency compared with that of Al, and partially by a Ti-induced red-shift. Therefore, at optical frequencies, apart from the size of the SRRs, the plasma frequency of the particular metal used determines the position of the resonances - with the collision frequency determining the broadening as well as the amplitude of reflectance.

6. Conclusions

Acknowledgements

The authors wish to acknowledge support from the European Commission through the Metamorphose NoE, ECONAM and the COST action MP0702 and also the staff and facilities of the James Watt Nanofabrication Centre at Glasgow University.

References and links

1.

J. Zhou, T. Koschny, M. Kafesaki, E. N. Economou, J. B. Pendry, and C. M. Soukoulis, “Saturation of the magnetic response of split-ring resonators at optical frequencies,” Phys. Rev. Lett. 95(22), 223902 (2005). [CrossRef] [PubMed]

2.

S. Tretyakov, “On geometrical scaling of split-ring and double-bar resonators at optical frequencies,” Metamaterials (Amst.) 1(1), 40–43 (2007). [CrossRef]

3.

B. Lahiri, S. G. McMeekin, A. Z. Khokhar, R. M. De La Rue, and N. P. Johnson, “Magnetic response of split ring resonators (SRRs) at visible frequencies,” Opt. Express 18(3), 3210–3218 (2010). [CrossRef] [PubMed]

4.

B. Kante, A. de Lustrac, and J. M. Lourtioz, “In-plane coupling and field enhancement in infrared metamaterial surfaces,” Phys. Rev. B 80(3), 035108 (2009). [CrossRef]

5.

F. Gadot, B. Belier, A. Aassime, J. Mangeney, A. Lustrac, J.-M. Lourtioz, A de Lustrac, and J.-M Lourtioz, “Infrared response of a metamaterial made of gold wires and split ring resonators deposited on silicon,” Opt. Quantum Electron. 39(4-6), 273–284 (2007). [CrossRef]

6.

B. Lahiri, A. Z. Khokhar, R. M. De La Rue, S. G. McMeekin, and N. P. Johnson, “Asymmetric split ring resonators for optical sensing of organic materials,” Opt. Express 17(2), 1107–1115 (2009). [CrossRef] [PubMed]

7.

B. Kante, J.-M. Lourtioz, and A. de Lustrac, “Infrared metafilms on a dielectric substrate,” Phys. Rev. B 80(20), 205120 (2009). [CrossRef]

8.

M. A. Ordal, L. L. Long, R. J. Bell, S. E. Bell, R. R. Bell, R. W. Alexander Jr, and C. A. Ward, “Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared,” Appl. Opt. 22(7), 1099–1119 (1983). [CrossRef] [PubMed]

9.

I. El-Kady, M. M. Sigalas, R. Biswas, K. M. Ho, and C. M. Soukoulis, “Metallic Photonic crystals at optical wavelengths,” Phys. Rev. B 62(23), 15299 (2000). [CrossRef]

10.

S. Linden, C. Enkrich, M. Wegener, J. Zhou, T. Koschny, and C. M. Soukoulis, “Magnetic Response of Metamaterials at 100 THz,” Science 306(5700), 1351–1353 (2004). [CrossRef] [PubMed]

11.

E. V. Ponizovskaya and A. M. Bratkovsky, “Metallic negative index nanostructures at optical frequencies: losses and effect of gain medium,” Appl. Phys. A Mater. Sci. Process. 87(2), 161–165 (2007). [CrossRef]

12.

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]

13.

V. D. Kumar and K. Asakawa, “Investigation of a slot nanoantenna in optical frequency range,” Photonics Nanostruct. Fundam. Appl 7(3), 161–168 (2009). [CrossRef]

14.

A. David Olver, Microwave and Optical Transmission (John Wiley & Sons Ltd, 1992) Chap. 8.

OCIS Codes
(160.3820) Materials : Magneto-optical materials
(160.3900) Materials : Metals
(160.4670) Materials : Optical materials
(160.4760) Materials : Optical properties
(160.3918) Materials : Metamaterials
(160.4236) Materials : Nanomaterials
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Metamaterials

History
Original Manuscript: March 23, 2010
Revised Manuscript: April 14, 2010
Manuscript Accepted: April 30, 2010
Published: May 12, 2010

Citation
Basudev Lahiri, Rafal Dylewicz, Richard M. De La Rue, and Nigel P. Johnson, "Impact of titanium adhesion layers on the response of arrays of metallic split-ring resonators (SRRs)," Opt. Express 18, 11202-11208 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-11-11202


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References

  1. J. Zhou, T. Koschny, M. Kafesaki, E. N. Economou, J. B. Pendry, and C. M. Soukoulis, “Saturation of the magnetic response of split-ring resonators at optical frequencies,” Phys. Rev. Lett. 95(22), 223902 (2005). [CrossRef] [PubMed]
  2. S. Tretyakov, “On geometrical scaling of split-ring and double-bar resonators at optical frequencies,” Metamaterials (Amst.) 1(1), 40–43 (2007). [CrossRef]
  3. B. Lahiri, S. G. McMeekin, A. Z. Khokhar, R. M. De La Rue, and N. P. Johnson, “Magnetic response of split ring resonators (SRRs) at visible frequencies,” Opt. Express 18(3), 3210–3218 (2010). [CrossRef] [PubMed]
  4. B. Kante, A. de Lustrac, and J. M. Lourtioz, “In-plane coupling and field enhancement in infrared metamaterial surfaces,” Phys. Rev. B 80(3), 035108 (2009). [CrossRef]
  5. F. Gadot, B. Belier, A. Aassime, J. Mangeney, A. Lustrac, J.-M. Lourtioz, A de Lustrac, and J.-M Lourtioz, “Infrared response of a metamaterial made of gold wires and split ring resonators deposited on silicon,” Opt. Quantum Electron. 39(4-6), 273–284 (2007). [CrossRef]
  6. B. Lahiri, A. Z. Khokhar, R. M. De La Rue, S. G. McMeekin, and N. P. Johnson, “Asymmetric split ring resonators for optical sensing of organic materials,” Opt. Express 17(2), 1107–1115 (2009). [CrossRef] [PubMed]
  7. B. Kante, J.-M. Lourtioz, and A. de Lustrac, “Infrared metafilms on a dielectric substrate,” Phys. Rev. B 80(20), 205120 (2009). [CrossRef]
  8. M. A. Ordal, L. L. Long, R. J. Bell, S. E. Bell, R. R. Bell, R. W. Alexander, and C. A. Ward, “Optical properties of the metals Al, Co, Cu, Au, Fe, Pb, Ni, Pd, Pt, Ag, Ti, and W in the infrared and far infrared,” Appl. Opt. 22(7), 1099–1119 (1983). [CrossRef] [PubMed]
  9. I. El-Kady, M. M. Sigalas, R. Biswas, K. M. Ho, and C. M. Soukoulis, “Metallic Photonic crystals at optical wavelengths,” Phys. Rev. B 62(23), 15299 (2000). [CrossRef]
  10. S. Linden, C. Enkrich, M. Wegener, J. Zhou, T. Koschny, and C. M. Soukoulis, “Magnetic Response of Metamaterials at 100 THz,” Science 306(5700), 1351–1353 (2004). [CrossRef] [PubMed]
  11. E. V. Ponizovskaya and A. M. Bratkovsky, “Metallic negative index nanostructures at optical frequencies: losses and effect of gain medium,” Appl. Phys. A Mater. Sci. Process. 87(2), 161–165 (2007). [CrossRef]
  12. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]
  13. V. D. Kumar and K. Asakawa, “Investigation of a slot nanoantenna in optical frequency range,” Photonics Nanostruct. Fundam. Appl 7(3), 161–168 (2009). [CrossRef]
  14. A. David Olver, Microwave and Optical Transmission (John Wiley & Sons Ltd, 1992) Chap. 8.

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