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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 22 — Nov. 4, 2013
  • pp: 27326–27337
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Towards CMOS-compatible nanophotonics: Ultra-compact modulators using alternative plasmonic materials

Viktoriia E. Babicheva, Nathaniel Kinsey, Gururaj V. Naik, Marcello Ferrera, Andrei V. Lavrinenko, Vladimir M. Shalaev, and Alexandra Boltasseva  »View Author Affiliations


Optics Express, Vol. 21, Issue 22, pp. 27326-27337 (2013)
http://dx.doi.org/10.1364/OE.21.027326


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Abstract

We propose several planar layouts of ultra-compact plasmonic modulators that utilize alternative plasmonic materials such as transparent conducting oxides and titanium nitride. The modulation is achieved by tuning the carrier concentration in a transparent conducting oxide layer into and out of the plasmon resonance with an applied electric field. The resonance significantly increases the absorption coefficient of the modulator, which enables larger modulation depth. We show that an extinction ratio of 46 dB/µm can be achieved, allowing for a 3-dB modulation depth in much less than one micron at the telecommunication wavelength. Our multilayer structures can be integrated with existing plasmonic and photonic waveguides as well as novel semiconductor-based hybrid photonic/electronic circuits.

© 2013 Optical Society of America

1. Introduction

Plasmonics enables the merging of two major technologies: nanometer-scale electronics and ultra-fast photonics [1

1. J. A. Dionne and H. A. Atwater, “Plasmonics: metal-worthy methods and materials in nanophotonics,” MRS Bull. 37(08), 717–724 (2012). [CrossRef]

]. Metal-dielectric interfaces can support the waves known as surface plasmon polaritons (SPPs) that are tightly coupled to the interface which allow for the manipulation of light at the nanoscale, overcoming the diffraction limit. Plasmonic technologies can lead to a new generation of fast, on-chip, nanoscale devices with unique capabilities [2

2. M. L. Brongersma and V. M. Shalaev, “Applied physics. the case for plasmonics,” Science 328(5977), 440–441 (2010). [CrossRef] [PubMed]

, 3

3. V. J. Sorger, R. F. Oulton, R.-M. Ma, and X. Zhang, “Toward integrated plasmonic circuits,” MRS Bull. 37(08), 728–738 (2012). [CrossRef]

]. To provide basic nanophotonic circuit functionalities, elementary plasmonic devices such as waveguides, modulators, sources, amplifiers, and photodetectors are required. Various designs of plasmonic waveguides have been proposed to achieve the highest mode localization and the lowest propagation losses [3

3. V. J. Sorger, R. F. Oulton, R.-M. Ma, and X. Zhang, “Toward integrated plasmonic circuits,” MRS Bull. 37(08), 728–738 (2012). [CrossRef]

]. In addition to waveguides, modulators are the most fundamental component for digital signal encoding and are paramount to the development of nanophotonic circuits. In this regard, opto-electronic modulators can be designed to achieve operational speeds on the order of a few 10’s of GHz. Many plasmonic waveguide and modulator structures have been proposed and experimentally verified, but most of these structures use metals such as gold or silver, which are not CMOS-compatible, limiting their applicability in realistic consumer devices [4

4. M. L. Brongersma and P. G. Kik, Surface Plasmon Nanophotonics (Springer, 2007).

23

23. R. Thomas, Z. Ikonic, and R. W. Kelsall, “Electro-optic metal–insulator–semiconductor–insulator–metal Mach-Zehnder plasmonic modulator,” Photon. Nanostructures 10(1), 183–189 (2012). [CrossRef]

].

TCOs can provide extraordinary tuning and modulation of their complex refractive indices by changing the carrier concentration with the application of an electric field [17

17. A. Melikyan, N. Lindenmann, S. Walheim, P. M. Leufke, S. Ulrich, J. Ye, P. Vincze, H. Hahn, T. Schimmel, C. Koos, W. Freude, and J. Leuthold, “Surface plasmon polariton absorption modulator,” Opt. Express 19(9), 8855–8869 (2011). [CrossRef] [PubMed]

20

20. C. Huang, R. J. Lamond, S. K. Pickus, Z. R. Li, and V. J. Sorger, “A sub-λ-size modulator beyond the efficiency-loss limit,” IEEE Photon. J. 5(4), 2202411 (2013). [CrossRef]

, 35

35. E. Feigenbaum, K. Diest, and H. A. Atwater, “Unity-order index change in transparent conducting oxides at visible frequencies,” Nano Lett. 10(6), 2111–2116 (2010). [CrossRef] [PubMed]

37

37. V. Babicheva and A. Lavrinenko, “Plasmonic modulator optimized by patterning of active layer and tuning permittivity,” Opt. Commun. 285(24), 5500–5507 (2012). [CrossRef]

]. The resulting electric field causes a charge accumulation, or depletion, in the TCO layer (depending on the direction of electric field) which in turn changes the plasma frequency of the TCO, and consequently, its permittivity. In particular, an increase of approximately one order of magnitude can be achieved in a 5 nm thick accumulation layer for a metal-insulator-metal (MIM) structure using indium-tin-oxide (ITO) [35

35. E. Feigenbaum, K. Diest, and H. A. Atwater, “Unity-order index change in transparent conducting oxides at visible frequencies,” Nano Lett. 10(6), 2111–2116 (2010). [CrossRef] [PubMed]

]. A similar decrease in the carrier concentration within a 10-nm thick film for metal-oxide-semiconductor (MOS) stack was demonstrated for an ITO film [19

19. V. J. Sorger, N. D. Lanzillotti-Kimura, R.-M. Ma, and X. Zhang, “Ultra-compact silicon nanophotonic modulator with broadband response,” Nanophotonics 1(1), 1–6 (2012). [CrossRef]

, 20

20. C. Huang, R. J. Lamond, S. K. Pickus, Z. R. Li, and V. J. Sorger, “A sub-λ-size modulator beyond the efficiency-loss limit,” IEEE Photon. J. 5(4), 2202411 (2013). [CrossRef]

]. The modulating speed is only RC limited and is expected to exceed 10’s of GHz. TCO based modulators have been shown to achieve extinction ratios on the order of 18 dB/µm [36

36. Z. Lu, W. Zhao, and K. Shi, “Ultracompact electroabsorption modulators based on tunable epsilon-near-zero-slot waveguides,” IEEE Photon. J. 4(3), 735–740 (2012). [CrossRef]

]. In addition, TCO permittivity tuning can provide further improvements and increase of propagation length [37

37. V. Babicheva and A. Lavrinenko, “Plasmonic modulator optimized by patterning of active layer and tuning permittivity,” Opt. Commun. 285(24), 5500–5507 (2012). [CrossRef]

]. A small absolute value of TCO permittivity can be utilized to achieve plasmonic resonances and consequently high extinction ratio [17

17. A. Melikyan, N. Lindenmann, S. Walheim, P. M. Leufke, S. Ulrich, J. Ye, P. Vincze, H. Hahn, T. Schimmel, C. Koos, W. Freude, and J. Leuthold, “Surface plasmon polariton absorption modulator,” Opt. Express 19(9), 8855–8869 (2011). [CrossRef] [PubMed]

, 36

36. Z. Lu, W. Zhao, and K. Shi, “Ultracompact electroabsorption modulators based on tunable epsilon-near-zero-slot waveguides,” IEEE Photon. J. 4(3), 735–740 (2012). [CrossRef]

]. Thus, TCOs are promising candidates for adding electro-optical capabilities to plasmonic devices.

2. Towards fully CMOS-compatible plasmonics

For a device to be fully CMOS-compatible, both the material and the processing technique used to synthesize this material should be compatible with the standards in CMOS production lines. Currently, TiN is routinely used in CMOS processing, but the optical properties of this material are quite poor [38

38. A. Kerber and E. A. Cartier, “Reliability challenges for CMOS technology qualifications with Hafnium Oxide/Titanium Nitride gate stacks,” IEEE Trans. Device Mater. Reliab. 9(2), 147–162 (2009). [CrossRef]

41

41. A. Emboras, R. M. Briggs, A. Najar, S. Nambiar, C. Delacour, P. Grosse, E. Augendre, J. M. Fedeli, B. de Salvo, H. A. Atwater, and R. Espiau de Lamaestre, “Efficient coupler between silicon photonic and metal-insulator-silicon-metal plasmonic waveguides,” Appl. Phys. Lett. 101(25), 251117 (2012). [CrossRef]

]. This is because the primary consideration has been the electrical properties of the material, not the optical properties. In this study, we use experimentally obtained optical properties of TiN films which have been optimized for plasmonic applications [30

30. G. Naik, J. L. Schroeder, X. Ni, A. V. Kildishev, T. D. Sands, and A. Boltasseva, “Titanium nitride as a plasmonic material for visible and near-infrared wavelengths,” Opt. Mater. Express 2(4), 478–489 (2012). [CrossRef]

]. These films were deposited using a high temperature (800 °C) reactive DC magnetron sputtering technique. This high temperature sputtering process is not utilized in the current semiconductor manufacturing processes for TiN deposition. Thus, we claim devices based on CMOS-compatible materials, but acknowledge that the entire process is not currently CMOS-compatible. However, it has been shown that plasmonic TiN can also be grown at lower temperatures [30

30. G. Naik, J. L. Schroeder, X. Ni, A. V. Kildishev, T. D. Sands, and A. Boltasseva, “Titanium nitride as a plasmonic material for visible and near-infrared wavelengths,” Opt. Mater. Express 2(4), 478–489 (2012). [CrossRef]

, 33

33. G. V. Naik, V. M. Shalaev, and A. Boltasseva, “Alternative plasmonic materials: beyond gold and silver,” Adv. Mater. 25(24), 3264–3294 (2013). [CrossRef] [PubMed]

]. Thus, through an optimization process of the low temperature TiN (less than 400°) currently available in the CMOS industry, TiN which possesses the required optical properties can be made available in future CMOS production lines. This is in stark contrast to the noble metals which are not allowed in the CMOS process. A similar situation was encountered for low-loss doped silica glass which is normally obtained through high-temperature annealing. Nevertheless, in 2003 a new material platform, namely Hydex®, was synthesized to bring this glass into full CMOS-compatibility where it was subsequently used for integrated nonlinear optics experiments [42

42. B. Little, “A VLSI photonics platform,” in Optical Fiber Communication Conference, (Optical Society of America, 2003), paper ThD1.

, 43

43. M. Ferrera, L. Razzari, D. Duchesne, R. Morandotti, Z. Yang, M. Liscidini, J. E. Sipe, S. Chu, B. E. Little, and D. J. Moss, “Low-power continuous-wave nonlinear optics in doped silica glass integrated waveguide structures,” Nat. Photonics 2(12), 737–740 (2008). [CrossRef]

].

The TCOs discussed in this paper, Tin-doped Indium Oxide (ITO), Gallium-doped Zinc Oxide (GZO), Aluminum-doped Zinc-Oxide (AZO) and others, may be deposited at relatively low temperatures (less than 300°C), which makes it possible to integrate them as a final stage in the standard silicon process [31

31. G. V. Naik, J. Liu, A. V. Kildishev, V. M. Shalaev, and A. Boltasseva, “Demonstration of Al:ZnO as a plasmonic component for near-infrared metamaterials,” Proc. Natl. Acad. Sci. U.S.A. 109(23), 8834–8838 (2012). [CrossRef] [PubMed]

]. Due to their low temperature deposition they will not impact the CMOS produced structures below. Similar nondestructive methods of integration with CMOS circuitry have been utilized to include lithium niobate crystals and electro-optic polymers on CMOS produced photonic chips [50

50. K. Noguchi, O. Mitomi, and H. Miyazawa, “Millimeter-wave Ti:LiNbO3 optical modulators,” J. Lightwave Technol. 16(4), 615–619 (1998). [CrossRef]

, 51

51. T. Fujiwara, A. Watanabe, and H. Mori, “Measurement of uniformity of driving voltage in Ti:LiNbO3 waveguides using Mach-Zehnder interferometers,” IEEE Photon. Technol. Lett. 2(4), 260–261 (1990). [CrossRef]

]. Such methods also consider these techniques to be CMOS-compatible.

3. Multilayer structures

We consider the use of the above mentioned CMOS-compatible materials in several modulator configurations. Stripe waveguides have low propagation loss and are relatively simple to fabricate using the planar process [6

6. P. Berini, “Long-range surface plasmon polaritons,” Adv. Opt. Photon. 1(3), 484–588 (2009). [CrossRef]

, 7

7. A. Boltasseva, T. Nikolajsen, K. Leosson, K. Kjaer, M. S. Larsen, and S. I. Bozhevolnyi, “Integrated optical components utilizing long-range surface plasmon polaritons,” J. Lightwave Technol. 23(1), 413–422 (2005). [CrossRef]

]. Therefore, we propose several modulator geometries which are based on stripe waveguides. This also allows for the modulator to be easily integrated with long-range SPP (LR-SPP) stripe waveguides [6

6. P. Berini, “Long-range surface plasmon polaritons,” Adv. Opt. Photon. 1(3), 484–588 (2009). [CrossRef]

] to decrease both propagation and coupling losses, leading to a fully plasmonic integrated modulator design. A schematic showing the basic outline of the modulator integration with LR-SPP stripe waveguides is shown in Fig. 1
Fig. 1 General scheme of a compact modulator integrated with low-loss plasmonic waveguides. In this geometry, a stripe waveguide (grey) is used to bring a long ranging SPP mode to and from the modulator structure where an applied voltage modulates the SPP wave.
.

To further reduce the mode size of the three alternatives and increase the modulating capability, we consider including high-index claddings. However, due to the increased propagation losses in the modulator it is unclear whether structures with a high-index cladding will outperform the low-index equivalents. For this reason, we consider two sub-groups of devices: one with low-index claddings Figs. 2(a)
Fig. 2 Illustration of the low-index (a, b, c) and high-index (d, e, f) multilayer modulator designs considered in this work. They are vertically divided by their configuration. The first column, GZO only structures (a) and (d), uses the GZO as both the plasmonic layer and the dynamic layer. The second column, single interface structures (b) and (e), introduces a thick TiN layer, which supports single interface SPPs and use the GZO layer to perform modulation. Finally, the third column of thin TiN structures (c) and (f), uses a thin stripe of TiN to support the long ranging SPP mode and the GZO layer to modulate the signal.
-2(c), and another with high-index claddings Figs. 2(d)-2(f). In all structures, thin plasma-enhanced chemical vapor deposition (PE-CVD) nonstoichiometric silicon nitride (SiN) or thin low-pressure chemical vapor deposition (LP-CVD) Si3N4 layers, are used for electrical isolation between the contacts to allow for modulation.

4. Defining performance metrics

With these considerations, six basic geometries were chosen as templates for modulator designs, operating at the telecom wavelength of λ = 1.55 μm. We consider zinc oxide (ZnO), LP-CVD Si3N4 and PE-CVD silicon nitride (denoted in by SiN), as low-index materials. The refractive indices used in the calculations are the following: nZnO = 1.93 [53

53. M. Bass, C. DeCusatis, G. Li, V. N. Mahajan, and E. V. Stryland, Handbook of Optics, Volume II: Design, Fabrication and Testing, Sources and Detectors, Radiometry and Photometry (McGraw Hill, 1994).

], nSiN = 1.76 and nSi3N4 = 1.97 (value retrieved from in loco ellipsometry measurements of PE-CVD and LP-CVD films, respectively) at λ = 1.55 μm. It should be mentioned that LP-CVD Si3N4 requires high temperature deposition which may degrade the properties of TCO layer. Hence, only PE-CVD SiN can be deposited after the TCO layer. We consider silicon as a high-index cladding nSi = 3.48 at λ = 1.55 µm for both crystalline and amorphous [54

54. Sopra data sheet, http://www.sspectra.com/sopra.html.

]. In all cases we neglect optical losses in the silicon as they are much lower than losses associated with plasmonic structures.

The dispersion equation was solved for the multilayer structures with varying carrier concentrations in the TCO. The permittivity of the GZO layer was taken from experimentally grown films [52

52. J. Kim, G. Naik, N. Emani, U. Guler, and A. Boltasseva, “Plasmonic resonances in nanostructured transparent conducting oxide films,” IEEE J. Sel. Top. Quantum Electron. 19, 4601907 (2012).

] and a carrier concentration in the GZO was determined using a Drude-Lorentz model fitting: N0 = 9.426 × 1020 cm−3. For this work we consider a range of carrier concentrations between N = 0.5N0…2N0. The upper limit of 2N0 (1.88 × 1021 cm−3) is shown to be achievable by the recently reported film which obtained a carrier concentration of 1.46 × 1021 cm−3 for GZO [55

55. D. C. Look, T. C. Droubay, and S. A. Chambers, “Stable highly conductive ZnO via reduction of Zn vacancies,” Appl. Phys. Lett. 101(10), 102101 (2012). [CrossRef]

]. In addition, numerous studies have focused on increasing the carrier concentration of TCO films which is further proof that the parameters used in our analysis are realistic [33

33. G. V. Naik, V. M. Shalaev, and A. Boltasseva, “Alternative plasmonic materials: beyond gold and silver,” Adv. Mater. 25(24), 3264–3294 (2013). [CrossRef] [PubMed]

, 52

52. J. Kim, G. Naik, N. Emani, U. Guler, and A. Boltasseva, “Plasmonic resonances in nanostructured transparent conducting oxide films,” IEEE J. Sel. Top. Quantum Electron. 19, 4601907 (2012).

, 55

55. D. C. Look, T. C. Droubay, and S. A. Chambers, “Stable highly conductive ZnO via reduction of Zn vacancies,” Appl. Phys. Lett. 101(10), 102101 (2012). [CrossRef]

57

57. H. Kim, M. Osofsky, S. M. Prokes, O. J. Glembocki, and A. Piqué, “Optimization of Al-doped ZnO films for low loss plasmonic materials at telecommunication wavelengths,” Appl. Phys. Lett. 102(17), 171103 (2013). [CrossRef]

]. The calculated permittivity of GZO for these carrier concentrations is shown in Fig. 3(a)
Fig. 3 (a) GZO permittivity versus its carrier concentration, λ = 1.55 μm. The permittivity of the GZO layer was taken from [52] and a carrier concentration in the GZO was determined using a Drude-Lorentz model fitting: N0 = 9.426 × 1020 cm−3 (black dotted line). (b) TiN permittivity extracted from spectroscopic ellipsometry measurements.
.

The permittivity of TiN is taken as experimentally measured εTiN = – 83.3 + 21.3i at λ = 1.55 μm, Fig. 3(b). The TiN film was deposited at 800°C and the optical properties of the 20 nm thick film was measured using spectroscopic ellipsometer (J.A. Woollam Co). The high deposition temperature poses some fabrication and integration restrictions, similar to LP-CVD Si3N4. The materials beneath the TiN layer must withstand the TiN-deposition and etch conditions without degradation. Since the properties of the TCO degrade at high temperatures, the TCO layer must be deposited only after the deposition and patterning of the TiN layer.

In all cases, we consider the one-dimensional structure as an approximation to the two-dimensional stripe waveguide. This assumption does not substantially affect the theoretical performance of the devices [5

5. S. I. Bozhevolnyi, Plasmonic Nanoguides and Circuits (Pan Stanford Publishing, 2009).

, 58

58. B. Lamprecht, J. R. Krenn, G. Schider, H. Ditlbacher, M. Salerno, N. Felidj, A. Leitner, F. R. Aussenegg, and J. C. Weeber, “Surface plasmon propagation in microscale metal stripes,” Appl. Phys. Lett. 79(1), 51–53 (2001). [CrossRef]

]. The thickness of the internal GZO, TiN, SiN, Si3N4 layers is 10 nm. The top and bottom cladding layers are assumed to be infinitely thick.

5. Defining the figure of merit

Before defining the FoM used in our analysis, a few fundamental parameters should be discussed. First of these is the mode size. Due to the complex field profiles of the multilayer structures, we define the mode size such that 86% of electrical energy is localized within the region as shown in Fig. 4
Fig. 4 Depiction of the mode profile illustrating the definition of the mode size. Due to the complexity of the structure and high concentration of electrical energy in the GZO layer, the traditional definition of the mode size cannot be utilized. Here we define the mode size as the distance/range, which encompasses 86% of the electric field energy, a condition similar to that of the 1/e definition for a single interface waveguide.
. This is similar to the case of a single interface where the 1/e point of the electric field corresponds to an 86% localization of electrical energy.

The second parameter is the attenuation of the signal in decibels, which is calculated from the absorption coefficient as α=8.68Im(βeff) [17

17. A. Melikyan, N. Lindenmann, S. Walheim, P. M. Leufke, S. Ulrich, J. Ye, P. Vincze, H. Hahn, T. Schimmel, C. Koos, W. Freude, and J. Leuthold, “Surface plasmon polariton absorption modulator,” Opt. Express 19(9), 8855–8869 (2011). [CrossRef] [PubMed]

], where βeff is the complex propagation constant of plasmonic wave in the multilayer structures. Therefore, the extinction ratio (ER) of the modulator is defined as
ER=αmaxαmin,
(1)
where αmin is propagation loss in the off-state and αmax is the maximum of the propagation loss in the on-state. Here, we define the on-state as the carrier concentration which results in the maximum absorption in each modulator design.

We define the off-state as the minimum in the absorption. However, two solutions are possible. For this discussion, we consult Fig. 5
Fig. 5 Multilayer structures along with graphs of the absorption coefficient (a, c) and mode size (b, d) versus GZO carrier concentration. Structures with high-index cladding (lower) show much higher absorption than structures with a low-index cladding (upper). The absorption maximum is accompanied by the highest mode localization, which occurs at the plasmon resonance for the structure. At lower carrier concentrations in the GZO, modes are increased due to smaller magnitude of its real permittivity.
which illustrates the absorption coefficient and mode size as a function of carrier concentration for each of the six structures. Note that the final structure, Fig. 2(f) supports both a symmetric and asymmetric mode. As shown in Figs. 5(a) and 5(c), a reduction in the absorption can be achieved either by increasing or decreasing the carrier concentration from the on-state (αmax). However, as shown in Figs. 5(b) and 5(d), for N<N(αmax), the plasmonic mode becomes delocalized from the waveguide, resulting in a drastic increase in the mode size. This scenario is highly undesirable because 1) the delocalized light can result in significant cross-talk between devices, and 2) the light may recouple to the plasmonic waveguide resulting in reduced modulation capability. Thus, we refer to the off-state as Noff = 2N0 = 1.88 × 1021 cm−3.This definition is valid for all the proposed structures apart from Si3N4/GZO/SiN/ZnO in Fig. 2(a) due to its small range containing a bound mode (see Fig. 5(b)). Thus, for this layout, αmin = α (N = 8 × 1020 cm−3) is defined.

Finally, we define a FoM for such multilayer modulator structures as
FoM=ERαminλeff,offwoff
(2)
where ER is the extinction ratio, αmin is the off-state absorption coefficient, λeff,off=2π/Re(βeff) is the effective wavelength in the modulator in the off-state, and woff is the off-state mode size. This FoM reflects the trade-off between the modulation depth and the loss of the signal in the off-state (αmin), while giving additional weight to devices which can fulfill the promise of plasmonics, compactness.

6. Modulator performance

The high-index structure shown in Fig. 2(f), achieves off-state losses of 0.29 dB/μm while maintaining a large ER of 46 dB/µm. This structure obtained the largest FoM = 51, approximately twice the FoM of all other structures shown here. This large ER requires only a 65 nm of modulator length to achieve a 3 dB signal modulation. This is the only structure considered in the subsequent integration analysis.

7. Waveguide-modulator integration and losses

To achieve the highest performance of the integrated structure, the ability to efficiently couple into the device is critical. Ease of integration and coupling losses were considered from the beginning of the design, evident by our use of the low-loss stripe waveguide geometry as a template. Because of this, very efficient coupling can be achieved both into and out of the modulator. A schematic of the high-index “thin TiN” modulator structure integrated with high-index cladded TiN stripe interconnects is shown in Fig. 6
Fig. 6 Schematic of plasmonic modulators integrated with TiN stripe waveguides providing long range SPP propagation to and from the modulator (side view). To create the electrical isolation and prevent shorting of the modulator structure, the silicon layers are doped as shown. However, even with large doping required in the n + region, the losses associated with silicon are several orders of magnitude below the plasmonic losses and are neglected in this analysis.
. To achieve a similar mode size between the high-index modulator and interconnects, silicon was also used as the cladding for the waveguides. However, to prevent electrical shorting of the modulator structure, p-n junctions must be formed through doping of the silicon [59

59. S. Kang and Y. Leblebici, CMOS Digital Integrated Circuits Analysis & Designs (McGraw Hill, 2003).

]. Despite this doping, losses in the silicon are still several orders of magnitude below plasmonic losses, and are neglected in this analysis.

We calculated the coupling losses for the design shown in Fig. 6, and the results are shown on Fig. 7
Fig. 7 Single interface coupling loss between the high-index waveguide and high-index “thin TiN” modulator sections versus carrier concentration in the GZO layer.
. For the off-state (N = 1.88 × 1021 cm−3) coupling losses are shown to be approximately 0.7 dB for each interface. This, along with the low propagation loss in the modulator structure ensures high signal throughput in the off-state. As the carrier concentration is reduced, the coupling loss monotonically increases towards the maximum in the modulator absorption. The increase in coupling loss is a result of the highly localized field at the plasmon resonance (maximum absorption). In this situation, the field is almost entirely located within the GZO layer leading to a small mode overlap integral and high coupling losses (Fig. 8
Fig. 8 Example mode profiles in the integrated modulator geometry high-index “thin TiN”. Note that the field decay outside the stripe waveguide is slow and therefore appears constant in this graph. The carrier concentration in the GZO layer used for the calculations corresponds to the maximum absorption in the modulator, i.e. plasmonic resonance condition N = Non. Under these conditions the majority of the field is localized within the GZO layer.
). This effect can be beneficial for modulator performance in specific applications as it provides additional losses in the on-state and fewer losses in the off-state.

8. Conclusion

In this paper, we have analyzed several multilayer structures with alternative plasmonic materials to be utilized in ultra-compact, CMOS-compatible plasmonic modulators. Various materials were studied as constituent building blocks of the investigated geometries including different dielectrics (silicon nitride, silicon, zinc oxide) and plasmonic materials such as transparent conducting oxides and titanium nitride. Applying an electric field across the TCO layer allows for the permittivity to be tuned, resulting in a change of the absorption coefficient of the waveguide. Therefore, active modulation is achieved. Numerous modulator layouts are investigated and the typical trade-off between compactness and propagation loss is analyzed. Amongst all the reported structures, one stands out with a remarkable FoM (even in comparison with the best state-of-the-art devices). This FoM takes into account the modulation depth (ER = 46 dB/μm), propagation losses in the off-state (α = 0.29 dB/μm), and off-state mode size (woff = 1.3 µm). The corresponding geometry may allow for ultra-compact modulation with effective length much less than 1 µm. The proposed approach based on the cost-effective planar fabrication processes and the ability to easily integrate with existing semiconductor systems could enable new devices for applications in on-chip optics, sensing, optoelectronics, data storage, and information processing.

Acknowledgments

We thank Jieran Fang, Jongbum Kim, and Naresh K. Emani for helpful discussions. V.E.B. acknowledges financial support from 2012 SPIE Optics and Photonics Education Scholarship, Otto Mønsteds and Thomas B. Thriges foundations. M.F. wishes to acknowledge the Marie Curie Outgoing International Fellowship (contract no. 329346). A.V.L. acknowledges partial financial support from the Danish Research Council for Technology and Production Sciences via the THz COW project. We acknowledge support from the following grants: ARO grant 57981-PH (W911NF-11-1-0359), NSF MRSEC grant DMR-1120923 and NSF PREM DRM-0611430.

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R. Charbonneau, C. Scales, I. Breukelaar, S. Fafard, N. Lahoud, G. Mattiussi, and P. Berini, “Passive integrated optics elements based on long-range surface plasmon polaritons,” J. Lightwave Technol. 24(1), 477–494 (2006). [CrossRef]

10.

V. J. Sorger, Z. Ye, R. F. Oulton, Y. Wang, G. Bartal, X. Yin, and X. Zhang, “Experimental demonstration of low-loss optical waveguiding at deep sub-wavelength scales,” Nat. Commun. 2, 331 (2011). [CrossRef]

11.

V. S. Volkov, Z. Han, M. G. Nielsen, K. Leosson, H. Keshmiri, J. Gosciniak, O. Albrektsen, and S. I. Bozhevolnyi, “Long-range dielectric-loaded surface plasmon polariton waveguides operating at telecommunication wavelengths,” Opt. Lett. 36(21), 4278–4280 (2011). [CrossRef] [PubMed]

12.

T. Nikolajsen, K. Leosson, and S. I. Bozhevolnyi, “Surface plasmon polariton based modulators and switches operating at telecom wavelengths,” Appl. Phys. Lett. 85(24), 5833–5835 (2004). [CrossRef]

13.

T. Nikolajsen, K. Leosson, and S. I. Bozhevolnyi, “In-line extinction modulator based on long-range surface plasmon polaritons,” Opt. Commun. 244(1-6), 455–459 (2005). [CrossRef]

14.

J. A. Dionne, K. Diest, L. A. Sweatlock, and H. A. Atwater, “PlasMOStor: a metal-oxide-Si field effect plasmonic modulator,” Nano Lett. 9(2), 897–902 (2009). [CrossRef] [PubMed]

15.

W. Cai, J. S. White, and M. L. Brongersma, “Compact, high-speed and power-efficient electrooptic plasmonic modulators,” Nano Lett. 9(12), 4403–4411 (2009). [CrossRef] [PubMed]

16.

K. F. MacDonald and N. I. Zheludev, “Active plasmonics: current status,” Laser Photon. Rev. 4(4), 562–567 (2010). [CrossRef]

17.

A. Melikyan, N. Lindenmann, S. Walheim, P. M. Leufke, S. Ulrich, J. Ye, P. Vincze, H. Hahn, T. Schimmel, C. Koos, W. Freude, and J. Leuthold, “Surface plasmon polariton absorption modulator,” Opt. Express 19(9), 8855–8869 (2011). [CrossRef] [PubMed]

18.

A. V. Krasavin and A. V. Zayats, “Photonic signal processing on electronic scales: electro-optical field-effect nanoplasmonic modulator,” Phys. Rev. Lett. 109(5), 053901 (2012). [CrossRef] [PubMed]

19.

V. J. Sorger, N. D. Lanzillotti-Kimura, R.-M. Ma, and X. Zhang, “Ultra-compact silicon nanophotonic modulator with broadband response,” Nanophotonics 1(1), 1–6 (2012). [CrossRef]

20.

C. Huang, R. J. Lamond, S. K. Pickus, Z. R. Li, and V. J. Sorger, “A sub-λ-size modulator beyond the efficiency-loss limit,” IEEE Photon. J. 5(4), 2202411 (2013). [CrossRef]

21.

V. E. Babicheva, I. V. Kulkova, R. Malureanu, K. Yvind, and A. V. Lavrinenko, “Plasmonic modulator based on gain-assisted metal–semiconductor–metal waveguide,” Photon. Nanostructures 10(4), 389–399 (2012). [CrossRef]

22.

A. Melikyan, L. Alloatti, A. Muslija, D. Hillerkuss, P. Schindler, J. Li, R. Palmer, D. Korn, S. Muehlbrandt, D. Van Thourhout, B. Chen, R. Dinu, M. Sommer, C. Koos, M. Kohl, W. Freude, and J. Leuthold, “Surface plasmon polariton high-speed modulator,” CLEO: 2013, OSA Technical Digest, paper CTh5D.2 (2013).

23.

R. Thomas, Z. Ikonic, and R. W. Kelsall, “Electro-optic metal–insulator–semiconductor–insulator–metal Mach-Zehnder plasmonic modulator,” Photon. Nanostructures 10(1), 183–189 (2012). [CrossRef]

24.

A. Boltasseva and H. A. Atwater, “Low-loss plasmonic metamaterials,” Science 331(6015), 290–291 (2011). [CrossRef] [PubMed]

25.

P. R. West, S. Ishii, G. V. Naik, N. K. Emani, V. M. Shalaev, and A. Boltasseva, “Searching for better plasmonic materials,” Laser Photon. Rev. 4(6), 795–808 (2010). [CrossRef]

26.

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(5), 054905 (2006). [CrossRef]

27.

G. V. Naik and A. Boltasseva, “Semiconductors for plasmonics and metamaterials,” Phys. Status Solidi RRL 4(10), 295–297 (2010). [CrossRef]

28.

G. V. Naik and A. Boltasseva, “A comparative study of semiconductor-based plasmonic metamaterials,” Metamaterials (Amst.) 5(1), 1–7 (2011). [CrossRef]

29.

G. Naik, J. Kim, and A. Boltasseva, “Oxides and nitrides as alternative plasmonic materials in the optical range,” Opt. Mater. Express 4(6), 1090–1099 (2011). [CrossRef]

30.

G. Naik, J. L. Schroeder, X. Ni, A. V. Kildishev, T. D. Sands, and A. Boltasseva, “Titanium nitride as a plasmonic material for visible and near-infrared wavelengths,” Opt. Mater. Express 2(4), 478–489 (2012). [CrossRef]

31.

G. V. Naik, J. Liu, A. V. Kildishev, V. M. Shalaev, and A. Boltasseva, “Demonstration of Al:ZnO as a plasmonic component for near-infrared metamaterials,” Proc. Natl. Acad. Sci. U.S.A. 109(23), 8834–8838 (2012). [CrossRef] [PubMed]

32.

J. B. Khurgin and A. Boltasseva, “Reflecting upon the losses in plasmonics and metamaterials,” MRS Bull. 37(08), 768–779 (2012). [CrossRef]

33.

G. V. Naik, V. M. Shalaev, and A. Boltasseva, “Alternative plasmonic materials: beyond gold and silver,” Adv. Mater. 25(24), 3264–3294 (2013). [CrossRef] [PubMed]

34.

J. Narayan, P. Tiwari, X. Chen, J. Singh, R. Chowdhury, and T. Zheleva, “Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition,” Appl. Phys. Lett. 61(11), 1290–1292 (1992). [CrossRef]

35.

E. Feigenbaum, K. Diest, and H. A. Atwater, “Unity-order index change in transparent conducting oxides at visible frequencies,” Nano Lett. 10(6), 2111–2116 (2010). [CrossRef] [PubMed]

36.

Z. Lu, W. Zhao, and K. Shi, “Ultracompact electroabsorption modulators based on tunable epsilon-near-zero-slot waveguides,” IEEE Photon. J. 4(3), 735–740 (2012). [CrossRef]

37.

V. Babicheva and A. Lavrinenko, “Plasmonic modulator optimized by patterning of active layer and tuning permittivity,” Opt. Commun. 285(24), 5500–5507 (2012). [CrossRef]

38.

A. Kerber and E. A. Cartier, “Reliability challenges for CMOS technology qualifications with Hafnium Oxide/Titanium Nitride gate stacks,” IEEE Trans. Device Mater. Reliab. 9(2), 147–162 (2009). [CrossRef]

39.

R. Chau, M. Doczy, B. Doyle, and J. Kavalieros, “Metal-gate electrode for CMOS transistor applications,” US Patent 6 696 345, Feb. 24 (2004).

40.

J. K. Brask, T. E. Glassman, M. L. Doczy, and M. V. Metz, “Method for making a semiconductor device having a high-k gate dielectric,” US Patent 6 716 707, Sept. 30 (2004).

41.

A. Emboras, R. M. Briggs, A. Najar, S. Nambiar, C. Delacour, P. Grosse, E. Augendre, J. M. Fedeli, B. de Salvo, H. A. Atwater, and R. Espiau de Lamaestre, “Efficient coupler between silicon photonic and metal-insulator-silicon-metal plasmonic waveguides,” Appl. Phys. Lett. 101(25), 251117 (2012). [CrossRef]

42.

B. Little, “A VLSI photonics platform,” in Optical Fiber Communication Conference, (Optical Society of America, 2003), paper ThD1.

43.

M. Ferrera, L. Razzari, D. Duchesne, R. Morandotti, Z. Yang, M. Liscidini, J. E. Sipe, S. Chu, B. E. Little, and D. J. Moss, “Low-power continuous-wave nonlinear optics in doped silica glass integrated waveguide structures,” Nat. Photonics 2(12), 737–740 (2008). [CrossRef]

44.

M.-S. Kwon, J.-S. Shin, S.-Y. Shin, and W.-G. Lee, “Characterizations of realized metal-insulator-silicon-insulator-metal waveguides and nanochannel fabrication via insulator removal,” Opt. Express 20(20), 21875–21887 (2012). [CrossRef] [PubMed]

45.

C. Delacour, S. Blaize, P. Grosse, J. M. Fedeli, A. Bruyant, R. Salas-Montiel, G. Lerondel, and A. Chelnokov, “Efficient directional coupling between silicon and copper plasmonic nanoslot waveguides: toward metal-oxide-silicon nanophotonics,” Nano Lett. 10(8), 2922–2926 (2010). [CrossRef] [PubMed]

46.

A. Emboras, A. Najar, S. Nambiar, P. Grosse, E. Augendre, C. Leroux, B. de Salvo, and R. E. de Lamaestre, “MNOS stack for reliable, low optical loss, Cu based CMOS plasmonic devices,” Opt. Express 20(13), 13612–13621 (2012). [CrossRef] [PubMed]

47.

S. Zhu, G. Q. Lo, and D. L. Kwong, “Electro-absorption modulation in horizontal metal-insulator-silicon-insulator-metal nanoplasmonic slot waveguides,” Appl. Phys. Lett. 99(15), 151114 (2011). [CrossRef]

48.

S. Zhu, G. Q. Lo, and D. L. Kwong, “Components for silicon plasmonic nanocircuits on horizontal Cu-SiO2-Si-SiO2-Cu nanoplasmonic waveguides,” Opt. Express 20, 1896–1898 (2012). [PubMed]

49.

R. Geffken and S. Luce, “Method of forming a self-aligned copper diffusion barrier in vias,” US Patent 5 985 762, Nov. 16 (1999).

50.

K. Noguchi, O. Mitomi, and H. Miyazawa, “Millimeter-wave Ti:LiNbO3 optical modulators,” J. Lightwave Technol. 16(4), 615–619 (1998). [CrossRef]

51.

T. Fujiwara, A. Watanabe, and H. Mori, “Measurement of uniformity of driving voltage in Ti:LiNbO3 waveguides using Mach-Zehnder interferometers,” IEEE Photon. Technol. Lett. 2(4), 260–261 (1990). [CrossRef]

52.

J. Kim, G. Naik, N. Emani, U. Guler, and A. Boltasseva, “Plasmonic resonances in nanostructured transparent conducting oxide films,” IEEE J. Sel. Top. Quantum Electron. 19, 4601907 (2012).

53.

M. Bass, C. DeCusatis, G. Li, V. N. Mahajan, and E. V. Stryland, Handbook of Optics, Volume II: Design, Fabrication and Testing, Sources and Detectors, Radiometry and Photometry (McGraw Hill, 1994).

54.

Sopra data sheet, http://www.sspectra.com/sopra.html.

55.

D. C. Look, T. C. Droubay, and S. A. Chambers, “Stable highly conductive ZnO via reduction of Zn vacancies,” Appl. Phys. Lett. 101(10), 102101 (2012). [CrossRef]

56.

M. A. Noginov, L. Gu, J. Livenere, G. Zhu, A. K. Pradhan, R. Mundle, M. Bahoura, Y. A. Barnakov, and V. A. Podolskiy, “Transparent conductive oxides: plasmonic materials for telecom wavelengths,” Appl. Phys. Lett. 99(2), 021101 (2011). [CrossRef]

57.

H. Kim, M. Osofsky, S. M. Prokes, O. J. Glembocki, and A. Piqué, “Optimization of Al-doped ZnO films for low loss plasmonic materials at telecommunication wavelengths,” Appl. Phys. Lett. 102(17), 171103 (2013). [CrossRef]

58.

B. Lamprecht, J. R. Krenn, G. Schider, H. Ditlbacher, M. Salerno, N. Felidj, A. Leitner, F. R. Aussenegg, and J. C. Weeber, “Surface plasmon propagation in microscale metal stripes,” Appl. Phys. Lett. 79(1), 51–53 (2001). [CrossRef]

59.

S. Kang and Y. Leblebici, CMOS Digital Integrated Circuits Analysis & Designs (McGraw Hill, 2003).

OCIS Codes
(230.7370) Optical devices : Waveguides
(240.6680) Optics at surfaces : Surface plasmons
(250.7360) Optoelectronics : Waveguide modulators
(250.5403) Optoelectronics : Plasmonics
(250.4110) Optoelectronics : Modulators

ToC Category:
Plasmonics

History
Original Manuscript: July 26, 2013
Revised Manuscript: October 17, 2013
Manuscript Accepted: October 18, 2013
Published: November 4, 2013

Virtual Issues
Surface Plasmon Photonics (2013) Optics Express

Citation
Viktoriia E. Babicheva, Nathaniel Kinsey, Gururaj V. Naik, Marcello Ferrera, Andrei V. Lavrinenko, Vladimir M. Shalaev, and Alexandra Boltasseva, "Towards CMOS-compatible nanophotonics: Ultra-compact modulators using alternative plasmonic materials," Opt. Express 21, 27326-27337 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-22-27326


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References

  1. J. A. Dionne and H. A. Atwater, “Plasmonics: metal-worthy methods and materials in nanophotonics,” MRS Bull.37(08), 717–724 (2012). [CrossRef]
  2. M. L. Brongersma and V. M. Shalaev, “Applied physics. the case for plasmonics,” Science328(5977), 440–441 (2010). [CrossRef] [PubMed]
  3. V. J. Sorger, R. F. Oulton, R.-M. Ma, and X. Zhang, “Toward integrated plasmonic circuits,” MRS Bull.37(08), 728–738 (2012). [CrossRef]
  4. M. L. Brongersma and P. G. Kik, Surface Plasmon Nanophotonics (Springer, 2007).
  5. S. I. Bozhevolnyi, Plasmonic Nanoguides and Circuits (Pan Stanford Publishing, 2009).
  6. P. Berini, “Long-range surface plasmon polaritons,” Adv. Opt. Photon.1(3), 484–588 (2009). [CrossRef]
  7. A. Boltasseva, T. Nikolajsen, K. Leosson, K. Kjaer, M. S. Larsen, and S. I. Bozhevolnyi, “Integrated optical components utilizing long-range surface plasmon polaritons,” J. Lightwave Technol.23(1), 413–422 (2005). [CrossRef]
  8. R. Charbonneau, N. Lahoud, G. Mattiussi, and P. Berini, “Demonstration of integrated optics elements based on long-ranging surface plasmon polaritons,” Opt. Express13(3), 977–984 (2005). [CrossRef] [PubMed]
  9. R. Charbonneau, C. Scales, I. Breukelaar, S. Fafard, N. Lahoud, G. Mattiussi, and P. Berini, “Passive integrated optics elements based on long-range surface plasmon polaritons,” J. Lightwave Technol.24(1), 477–494 (2006). [CrossRef]
  10. V. J. Sorger, Z. Ye, R. F. Oulton, Y. Wang, G. Bartal, X. Yin, and X. Zhang, “Experimental demonstration of low-loss optical waveguiding at deep sub-wavelength scales,” Nat. Commun.2, 331 (2011). [CrossRef]
  11. V. S. Volkov, Z. Han, M. G. Nielsen, K. Leosson, H. Keshmiri, J. Gosciniak, O. Albrektsen, and S. I. Bozhevolnyi, “Long-range dielectric-loaded surface plasmon polariton waveguides operating at telecommunication wavelengths,” Opt. Lett.36(21), 4278–4280 (2011). [CrossRef] [PubMed]
  12. T. Nikolajsen, K. Leosson, and S. I. Bozhevolnyi, “Surface plasmon polariton based modulators and switches operating at telecom wavelengths,” Appl. Phys. Lett.85(24), 5833–5835 (2004). [CrossRef]
  13. T. Nikolajsen, K. Leosson, and S. I. Bozhevolnyi, “In-line extinction modulator based on long-range surface plasmon polaritons,” Opt. Commun.244(1-6), 455–459 (2005). [CrossRef]
  14. J. A. Dionne, K. Diest, L. A. Sweatlock, and H. A. Atwater, “PlasMOStor: a metal-oxide-Si field effect plasmonic modulator,” Nano Lett.9(2), 897–902 (2009). [CrossRef] [PubMed]
  15. W. Cai, J. S. White, and M. L. Brongersma, “Compact, high-speed and power-efficient electrooptic plasmonic modulators,” Nano Lett.9(12), 4403–4411 (2009). [CrossRef] [PubMed]
  16. K. F. MacDonald and N. I. Zheludev, “Active plasmonics: current status,” Laser Photon. Rev.4(4), 562–567 (2010). [CrossRef]
  17. A. Melikyan, N. Lindenmann, S. Walheim, P. M. Leufke, S. Ulrich, J. Ye, P. Vincze, H. Hahn, T. Schimmel, C. Koos, W. Freude, and J. Leuthold, “Surface plasmon polariton absorption modulator,” Opt. Express19(9), 8855–8869 (2011). [CrossRef] [PubMed]
  18. A. V. Krasavin and A. V. Zayats, “Photonic signal processing on electronic scales: electro-optical field-effect nanoplasmonic modulator,” Phys. Rev. Lett.109(5), 053901 (2012). [CrossRef] [PubMed]
  19. V. J. Sorger, N. D. Lanzillotti-Kimura, R.-M. Ma, and X. Zhang, “Ultra-compact silicon nanophotonic modulator with broadband response,” Nanophotonics1(1), 1–6 (2012). [CrossRef]
  20. C. Huang, R. J. Lamond, S. K. Pickus, Z. R. Li, and V. J. Sorger, “A sub-λ-size modulator beyond the efficiency-loss limit,” IEEE Photon. J.5(4), 2202411 (2013). [CrossRef]
  21. V. E. Babicheva, I. V. Kulkova, R. Malureanu, K. Yvind, and A. V. Lavrinenko, “Plasmonic modulator based on gain-assisted metal–semiconductor–metal waveguide,” Photon. Nanostructures10(4), 389–399 (2012). [CrossRef]
  22. A. Melikyan, L. Alloatti, A. Muslija, D. Hillerkuss, P. Schindler, J. Li, R. Palmer, D. Korn, S. Muehlbrandt, D. Van Thourhout, B. Chen, R. Dinu, M. Sommer, C. Koos, M. Kohl, W. Freude, and J. Leuthold, “Surface plasmon polariton high-speed modulator,” CLEO: 2013, OSA Technical Digest, paper CTh5D.2 (2013).
  23. R. Thomas, Z. Ikonic, and R. W. Kelsall, “Electro-optic metal–insulator–semiconductor–insulator–metal Mach-Zehnder plasmonic modulator,” Photon. Nanostructures10(1), 183–189 (2012). [CrossRef]
  24. A. Boltasseva and H. A. Atwater, “Low-loss plasmonic metamaterials,” Science331(6015), 290–291 (2011). [CrossRef] [PubMed]
  25. P. R. West, S. Ishii, G. V. Naik, N. K. Emani, V. M. Shalaev, and A. Boltasseva, “Searching for better plasmonic materials,” Laser Photon. Rev.4(6), 795–808 (2010). [CrossRef]
  26. 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(5), 054905 (2006). [CrossRef]
  27. G. V. Naik and A. Boltasseva, “Semiconductors for plasmonics and metamaterials,” Phys. Status Solidi RRL4(10), 295–297 (2010). [CrossRef]
  28. G. V. Naik and A. Boltasseva, “A comparative study of semiconductor-based plasmonic metamaterials,” Metamaterials (Amst.)5(1), 1–7 (2011). [CrossRef]
  29. G. Naik, J. Kim, and A. Boltasseva, “Oxides and nitrides as alternative plasmonic materials in the optical range,” Opt. Mater. Express4(6), 1090–1099 (2011). [CrossRef]
  30. G. Naik, J. L. Schroeder, X. Ni, A. V. Kildishev, T. D. Sands, and A. Boltasseva, “Titanium nitride as a plasmonic material for visible and near-infrared wavelengths,” Opt. Mater. Express2(4), 478–489 (2012). [CrossRef]
  31. G. V. Naik, J. Liu, A. V. Kildishev, V. M. Shalaev, and A. Boltasseva, “Demonstration of Al:ZnO as a plasmonic component for near-infrared metamaterials,” Proc. Natl. Acad. Sci. U.S.A.109(23), 8834–8838 (2012). [CrossRef] [PubMed]
  32. J. B. Khurgin and A. Boltasseva, “Reflecting upon the losses in plasmonics and metamaterials,” MRS Bull.37(08), 768–779 (2012). [CrossRef]
  33. G. V. Naik, V. M. Shalaev, and A. Boltasseva, “Alternative plasmonic materials: beyond gold and silver,” Adv. Mater.25(24), 3264–3294 (2013). [CrossRef] [PubMed]
  34. J. Narayan, P. Tiwari, X. Chen, J. Singh, R. Chowdhury, and T. Zheleva, “Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition,” Appl. Phys. Lett.61(11), 1290–1292 (1992). [CrossRef]
  35. E. Feigenbaum, K. Diest, and H. A. Atwater, “Unity-order index change in transparent conducting oxides at visible frequencies,” Nano Lett.10(6), 2111–2116 (2010). [CrossRef] [PubMed]
  36. Z. Lu, W. Zhao, and K. Shi, “Ultracompact electroabsorption modulators based on tunable epsilon-near-zero-slot waveguides,” IEEE Photon. J.4(3), 735–740 (2012). [CrossRef]
  37. V. Babicheva and A. Lavrinenko, “Plasmonic modulator optimized by patterning of active layer and tuning permittivity,” Opt. Commun.285(24), 5500–5507 (2012). [CrossRef]
  38. A. Kerber and E. A. Cartier, “Reliability challenges for CMOS technology qualifications with Hafnium Oxide/Titanium Nitride gate stacks,” IEEE Trans. Device Mater. Reliab.9(2), 147–162 (2009). [CrossRef]
  39. R. Chau, M. Doczy, B. Doyle, and J. Kavalieros, “Metal-gate electrode for CMOS transistor applications,” US Patent 6 696 345, Feb. 24 (2004).
  40. J. K. Brask, T. E. Glassman, M. L. Doczy, and M. V. Metz, “Method for making a semiconductor device having a high-k gate dielectric,” US Patent 6 716 707, Sept. 30 (2004).
  41. A. Emboras, R. M. Briggs, A. Najar, S. Nambiar, C. Delacour, P. Grosse, E. Augendre, J. M. Fedeli, B. de Salvo, H. A. Atwater, and R. Espiau de Lamaestre, “Efficient coupler between silicon photonic and metal-insulator-silicon-metal plasmonic waveguides,” Appl. Phys. Lett.101(25), 251117 (2012). [CrossRef]
  42. B. Little, “A VLSI photonics platform,” in Optical Fiber Communication Conference, (Optical Society of America, 2003), paper ThD1.
  43. M. Ferrera, L. Razzari, D. Duchesne, R. Morandotti, Z. Yang, M. Liscidini, J. E. Sipe, S. Chu, B. E. Little, and D. J. Moss, “Low-power continuous-wave nonlinear optics in doped silica glass integrated waveguide structures,” Nat. Photonics2(12), 737–740 (2008). [CrossRef]
  44. M.-S. Kwon, J.-S. Shin, S.-Y. Shin, and W.-G. Lee, “Characterizations of realized metal-insulator-silicon-insulator-metal waveguides and nanochannel fabrication via insulator removal,” Opt. Express20(20), 21875–21887 (2012). [CrossRef] [PubMed]
  45. C. Delacour, S. Blaize, P. Grosse, J. M. Fedeli, A. Bruyant, R. Salas-Montiel, G. Lerondel, and A. Chelnokov, “Efficient directional coupling between silicon and copper plasmonic nanoslot waveguides: toward metal-oxide-silicon nanophotonics,” Nano Lett.10(8), 2922–2926 (2010). [CrossRef] [PubMed]
  46. A. Emboras, A. Najar, S. Nambiar, P. Grosse, E. Augendre, C. Leroux, B. de Salvo, and R. E. de Lamaestre, “MNOS stack for reliable, low optical loss, Cu based CMOS plasmonic devices,” Opt. Express20(13), 13612–13621 (2012). [CrossRef] [PubMed]
  47. S. Zhu, G. Q. Lo, and D. L. Kwong, “Electro-absorption modulation in horizontal metal-insulator-silicon-insulator-metal nanoplasmonic slot waveguides,” Appl. Phys. Lett.99(15), 151114 (2011). [CrossRef]
  48. S. Zhu, G. Q. Lo, and D. L. Kwong, “Components for silicon plasmonic nanocircuits on horizontal Cu-SiO2-Si-SiO2-Cu nanoplasmonic waveguides,” Opt. Express20, 1896–1898 (2012). [PubMed]
  49. R. Geffken and S. Luce, “Method of forming a self-aligned copper diffusion barrier in vias,” US Patent 5 985 762, Nov. 16 (1999).
  50. K. Noguchi, O. Mitomi, and H. Miyazawa, “Millimeter-wave Ti:LiNbO3 optical modulators,” J. Lightwave Technol.16(4), 615–619 (1998). [CrossRef]
  51. T. Fujiwara, A. Watanabe, and H. Mori, “Measurement of uniformity of driving voltage in Ti:LiNbO3 waveguides using Mach-Zehnder interferometers,” IEEE Photon. Technol. Lett.2(4), 260–261 (1990). [CrossRef]
  52. J. Kim, G. Naik, N. Emani, U. Guler, and A. Boltasseva, “Plasmonic resonances in nanostructured transparent conducting oxide films,” IEEE J. Sel. Top. Quantum Electron.19, 4601907 (2012).
  53. M. Bass, C. DeCusatis, G. Li, V. N. Mahajan, and E. V. Stryland, Handbook of Optics, Volume II: Design, Fabrication and Testing, Sources and Detectors, Radiometry and Photometry (McGraw Hill, 1994).
  54. Sopra data sheet, http://www.sspectra.com/sopra.html .
  55. D. C. Look, T. C. Droubay, and S. A. Chambers, “Stable highly conductive ZnO via reduction of Zn vacancies,” Appl. Phys. Lett.101(10), 102101 (2012). [CrossRef]
  56. M. A. Noginov, L. Gu, J. Livenere, G. Zhu, A. K. Pradhan, R. Mundle, M. Bahoura, Y. A. Barnakov, and V. A. Podolskiy, “Transparent conductive oxides: plasmonic materials for telecom wavelengths,” Appl. Phys. Lett.99(2), 021101 (2011). [CrossRef]
  57. H. Kim, M. Osofsky, S. M. Prokes, O. J. Glembocki, and A. Piqué, “Optimization of Al-doped ZnO films for low loss plasmonic materials at telecommunication wavelengths,” Appl. Phys. Lett.102(17), 171103 (2013). [CrossRef]
  58. B. Lamprecht, J. R. Krenn, G. Schider, H. Ditlbacher, M. Salerno, N. Felidj, A. Leitner, F. R. Aussenegg, and J. C. Weeber, “Surface plasmon propagation in microscale metal stripes,” Appl. Phys. Lett.79(1), 51–53 (2001). [CrossRef]
  59. S. Kang and Y. Leblebici, CMOS Digital Integrated Circuits Analysis & Designs (McGraw Hill, 2003).

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