OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 6 — Mar. 12, 2012
  • pp: 5867–5881
« Show journal navigation

Components for silicon plasmonic nanocircuits based on horizontal Cu-SiO2-Si-SiO2-Cu nanoplasmonic waveguides

Shiyang Zhu, G. Q. Lo, and D. L. Kwong  »View Author Affiliations


Optics Express, Vol. 20, Issue 6, pp. 5867-5881 (2012)
http://dx.doi.org/10.1364/OE.20.005867


View Full Text Article

Acrobat PDF (3526 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We report systematic results on the development of horizontal Cu-SiO2-Si-SiO2-Cu nanoplasmonic waveguide components operating at 1550-nm telecom wavelengths, including straight waveguides, sharp 90° bends, power splitters, and Mach-Zehnder interferometers (MZIs). Owing to the relatively low loss for propagating (~0.3 dB/µm) and for 90° sharply bending (~0.73 dB/turn), various ultracompact power splitters and MZIs are experimentally realized on a silicon-on-insulator (SOI) platform using standard CMOS technology. The demonstrated splitters exhibit a relatively low excess loss and the MZIs exhibit good performance such as high extinction ratio of ~18 dB and low normalized insertion loss of ~1.7 dB. The experimental results of these devices agree well with those predicted from numerical simulations with suitable Cu permittivity data.

© 2012 OSA

1. Introduction

The Cu-SiO2-Si-SiO2-Cu plasmonic waveguide as well as several waveguide components have been experimentally demonstrated on a silicon-on-insulator (SOI) platform using standard CMOS technology, including sharp 90° bends [10

10. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration,” Opt. Express 19(9), 8888–8902 (2011). [CrossRef] [PubMed]

], power splitters [13

13. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Nanoplasmonic power splitters based on the horizontal nanoplasmonic slot waveguide,” Appl. Phys. Lett. 99(3), 031112 (2011). [CrossRef]

], waveguide-ring resonators [14

14. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Experimental demonstration of horizontal nanoplasmonic slot waveguide-ring resonators with submicron radius,” IEEE Photon. Technol. Lett. 23(24), 1896–1898 (2011). [CrossRef]

], and Si electro-absorption modulators [15

15. S. Y. 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]

]. In this paper, more basic building blocks, e.g. Mach-Zehnder interferometers (MZIs), are systematically investigated. The experiment results are compared with those predicted from numerical simulation. The results reported in this paper can be used to guide the design of Cu-SiO2-Si-SiO2-Cu waveguide-based complex integrated plasmonic nanocircuits.

2. Experimental

The devices are fabricated on SOI wafers with 340-nm-thick top-Si and 2-µm-thick buried SiO2 using standard CMOS technology. The Cu-covered plasmonic components are inserted in the conventional Si-waveguide network, as shown in Fig. 1(a)
Fig. 1 (a) Microscope picture of one fabricated device (a 1 × 4 power splitter), the Cu-covered plasmonic component is inserted in the Si-waveguide network; (b) Schematic Si pattern of the 1 × 4 power splitter, each branch of the plasmonic splitter is connected with a Si waveguide through a 1-µm-long tapered coupler, the yellow rectangle is the SiO2 window which will be covered by Cu; (c) Schematic cross section of the Cu-SiO2-Si-SiO2-Cu waveguide; and (d) Cross-sectional transmission electron microcopy (XTEM) image of the fabricated plasmonic waveguide, showing a quasi-rectangular Si core (~94-nm × ~327-nm) surrounded by a ~28-nm-thick thermal SiO2 layer and covered by a thick Cu layer.
for an example of a 1 × 4 power splitter. In terms of fabrication, the only difference between the different plasmonic devices is the pattern of Si core, which is defined along with the Si channel waveguide. For example, Fig. 1(b) shows a schematic Si pattern of the 1 × 4 power splitter. After Si pattering, a thin Si3N4 film and a thick SiO2 film were sequentially deposited. A SiO2 window, the yellow rectangle in Fig. 1(b), is then opened to expose the plasmonic area, which includes the plasmonic component and 1-µm-long tapered couplers to connect with the Si channel waveguides. After thermal oxidization of the exposed Si core, a thick Cu film was deposited, followed by copper chemical mechanical polishing (Cu-CMP) to remove the Cu outside the window. The detailed fabrication conditions have been described elsewhere [10

10. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration,” Opt. Express 19(9), 8888–8902 (2011). [CrossRef] [PubMed]

]. Although the width of Si core (WP) in the layout keeps 0.2 µm, its final width can be tuned by the fabrication conditions, in particular, the expose condition during the ultraviolet (UV) lithography and the photo-resist trimming process. In this work, the final Si cores have widths of 64, 80, 94, and 102 nm, respectively, surrounded by a ~28-nm-thick thermal SiO2 layer, as shown in Fig. 1(d) for one example of WP = 94 nm. It should be noted that the Si core as narrow as < 10 nm and the surrounding insulator as thin as ~1 nm are technically available using standard CMOS technology as that for Si nanowire field-effect transistors [16

16. J. W. Peng, S. J. Lee, G. C. A. Liang, N. Singh, S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Improved carrier injection in gate-all-around Schottky barrier silicon nanowaire field-effect transistors,” Appl. Phys. Lett. 93(7), 073503 (2008). [CrossRef]

].

A commercial software FullWAVE/RSOFT [17] is used for three-dimensional (3D) finite-difference time-domain (FDTD) simulation. The Si core is approximated to an ideal 340-nm-high rectangle on the SiO2 substrate, surrounded by a uniform thermal SiO2 layer. 1550-nm TE light is launched at the input 500-nm-wide Si channel waveguide and transports into the plasmonic waveguide through a 1-µm-long tapered coupler. The detailed settings for the 3D FDTD simulation have been described elsewhere [10

10. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration,” Opt. Express 19(9), 8888–8902 (2011). [CrossRef] [PubMed]

]. The refractive indices of Si and SiO2 at 1550 nm are set to 1.445 and 3.455, respectively. Cu complex permittivity data are cited from different references [18

18. A. D. Rakic, A. B. Djurisic, J. M. Elazar, and M. L. Majewski, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt. 37(22), 5271–5283 (1998). [CrossRef] [PubMed]

20

20. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, Boston, 1985).

], and the one that match our experimental results best is adopted.

3. Straight waveguides

The build-in Cu permittivity in RSoft [17] is ~-109 + 9.8i at 1550 nm, which is cited from Ref [18

18. A. D. Rakic, A. B. Djurisic, J. M. Elazar, and M. L. Majewski, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt. 37(22), 5271–5283 (1998). [CrossRef] [PubMed]

]. Numerical simulation using this Cu permittivity predicts a propagation loss much larger than the measured value [10

10. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration,” Opt. Express 19(9), 8888–8902 (2011). [CrossRef] [PubMed]

]. We notice that there is a large discrepancy in Cu permittivity data reported in the literature, for example, the Cu permittivity is ~-122 + 6.2i at 1550 nm in Ref [19

19. S. Roberts, “Optical properties of copper,” Phys. Rev. 118(6), 1509–1518 (1960). [CrossRef]

], and is ~-79.2 + 10.8i at 1550 nm in Ref [20

20. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, Boston, 1985).

]. This discrepancy may arise from the different Cu fabrication conditions in different references. On the other hand, it reflects that the Cu permittivity may be sensitive to the fabrication condition. In this work, we simply adopt a standard Cu process well developed for the Cu/low-k dielectrics interconnections in the CMOS technology [21

21. G. S. Mathad, Copper Interconnects, New Contact Metallurgies, Structures, and Low-k Interlevel Dielectrics (The Electrochemical Society, Inc., New Jersey, USA, 2003).

], i.e., 100-nm-thick Cu sputtering as a seed layer first, followed by 1-μm-thick Cu electroplating, and then annealing at 200°C for 30 min. We do not measure the optical property of this Cu film directly. Instead, we test various reported Cu permittivity data in numerical simulation to calculate the propagation loss. We find that the permittivity of ~-122 + 6.2i at 1550 nm reported in Ref [19

19. S. Roberts, “Optical properties of copper,” Phys. Rev. 118(6), 1509–1518 (1960). [CrossRef]

]. matches our experimental results best. Therefore, this Cu permittivity is adopted hereafter in this paper.

Figure 3(a)
Fig. 3 (a) Propagation loss, and (b) Real effective index (neff) at 1550 nm wavelength for the Cu-SiO2-Si-SiO2-Cu plasmonic waveguides with various SiO2 thicknesses. The curves are obtained from 3D FDTD simulation with Cu permittivity of −122 + 6.2i [19]. The experimental propagation losses are read from Fig. 2 and from Ref [10]. The experimental neff data are extracted from the plasmonic MZIs.
plots the propagation loss as a function of WP. Like the conventional gap-SPP waveguides [6

6. S. I. Bozhevolnyi and J. Jung, “Scaling for gap plasmon based waveguides,” Opt. Express 16(4), 2676–2684 (2008). [CrossRef] [PubMed]

], the propagation loss increases with the slot between two metal interfaces decreasing (the light confinement level increases accordingly), reflecting the abovementioned fundamental tradeoff for all kinds of plasmonic waveguides. For plasmonic waveguides with a certain slot width (WP and the surrounding SiO2 thickness), the propagation loss is mainly determined by the Cu permittivity. In this work, the Cu fabrication condition is not intentionally modified to reduce the propagation loss. However, we regard that it may be possible to further reduce the propagation loss by optimizing the Cu fabrication condition such as the sputtering and annealing parameters.

Figure 3(b) plots the real effective indices (neff) of the Cu-SiO2-Si-SiO2-Cu waveguides as a function of WP, obtained from the 3D FDTD simulation as well as the experimental data extracted from the plasmonic MZIs. Unlike the conventional gag-SPP waveguide whose neff decreases monotonously with the gap width increasing [6

6. S. I. Bozhevolnyi and J. Jung, “Scaling for gap plasmon based waveguides,” Opt. Express 16(4), 2676–2684 (2008). [CrossRef] [PubMed]

], the neff-WP relation of our plasmonic waveguides depends on the surrounding SiO2 thickness. For waveguides with a thick SiO2 layer (e.g., ≥ 12 nm), neff increases with WP increasing from 10 to 160 nm. For waveguides with a mediate SiO2 layer (e.g. ~6 nm), neff is almost independent on WP. Whereas for waveguides with a very thin SiO2 layer (e.g., ≤ 2 nm), neff decreases with WP increasing from 10 to 160 nm. This observation can be attributed to two contradictory effects of WP on neff. On one hand, the total slot width increases with WP increasing, thus leading to neff decreasing as the conventional gap-SPP waveguides. On the other hand, the ratio of SPP power in the Si core increases with WP increasing, which causes neff increasing. For waveguides with a very thin SiO2 layer, the first effect dominates, making neff decreasing with WP increasing. Whereas for waveguides with a thick SiO2 layer, the second effect dominates, making neff increasing with WP increasing.

4. Multiple sharp 90° bends

Our plasmonic waveguide supports sharply bending with a relatively low loss [9

9. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Fully complementary metal-oxide-semiconductor compatible nanoplasmonic slot waveguides for silicon electronic photonic integrated circuits,” Appl. Phys. Lett. 98(2), 021107 (2011). [CrossRef]

, 10

10. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration,” Opt. Express 19(9), 8888–8902 (2011). [CrossRef] [PubMed]

], thus enabling to route SPP signals flexibly in plasmonic nanocircuits. Figure 4(g)
Fig. 4 (a)-(f) SEM images (the Si core patterns) of a set of bent plasmonic waveguides which contains 2, 4, 6, 8, 10, and 12 sharp 90° bends, respectively, the total plasmonci waveguide length is 13 µm; (g) Normalized magnetic field (Hy) distribution in a sharp 90° bend with 64-nm WP and 28-nm SiO2, obtained from the 3D FDTD simulation. The power indicated after the junction is the pure bending loss, calculated by comparing the powers monitored before and after the junction and subtracting the propagation loss through the same distance in the straight plasmonic waveguide; (h) Transmission spectra measured on a set of bent plasmonic waveguides with 64-nm WP and a 13-µm-long plasmonic straight waveguide (represented by “0”); and (i) Transmitted powers measured on bent plasmonic waveguides with different WPs as a function of the number of bends, normalized by the corresponding 13-µm-long straight plsmonic waveguide. The bending loss is estimated from a linearly fitting line through zero.
illuminates the normalized magnetic field (Hy) distribution in a sharp 90° bend with 64-nm WP and 28-nm SiO2. The power monitored after the junction is normalized by the power monitored before the junction (where “0 dB” is indicated). The pure bending loss is calculated by subtracting the propagation loss through the same distance in the straight plasmonic waveguide (2 µm here) from the above normalized power. It is −0.77 dB, as indicated in the figure.

The bending property of our plasmonic waveguide is more like the gap-SPP waveguide [6

6. S. I. Bozhevolnyi and J. Jung, “Scaling for gap plasmon based waveguides,” Opt. Express 16(4), 2676–2684 (2008). [CrossRef] [PubMed]

], rather than the hybrid [7

7. 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(8), 496–500 (2008). [CrossRef]

] and DLSPP [8

8. T. Holmgaard and S. I. Bozhevolnyi, “Theoretical analysis of dielectric –loaded surface plasmon-polariton waveguides,” Phys. Rev. B 75(24), 245405 (2007). [CrossRef]

] waveguides. The latter two types of plasmonic waveguides do not support the sharp 90° bending because of their relatively weaker light confinement compared with our and gap-SPP waveguides.

5. Symmetric and asymmetric power splitters

The Cu-SiO2-Si-SiO2-Cu plasmonic waveguides support large-angle power splitting with relatively low excess loss [13

13. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Nanoplasmonic power splitters based on the horizontal nanoplasmonic slot waveguide,” Appl. Phys. Lett. 99(3), 031112 (2011). [CrossRef]

]. Figures 5(a)
Fig. 5 (a) SEM image (the Si core pattern) of a symmetric 1 × 2 splitter with 90° opening angle; (b) Normalized Hy distribution in the 1 × 2 splitter with 64-nm WP and 28-nm SiO2, obtained from the 3D FDTD simulation, the indicated values after the junction are normalized by the input power and subtracted the propagation loss through the same distance in the straight waveguide; (c) The spectra measured from output ports of a 1 × 2 splitter with 64-nm WP, normalized by that measured on a corresponding 3-µm-long straight plasmonic waveguide. The indicated values are averaged from three identical splitters; (d)-(f) are the corresponding figures for a symmetric 1 × 4 splitter.
and 5(d) show SEM images of symmetric 1 × 2 and 1 × 4 splitters with a 90° open angle, respectively. The total length of each plasmonic route is 3 µm. Figures 5(b) and 5(e) illuminate the normalized Hy distributions in the splitters with 64-nm WP and 28-nm SiO2, obtained from 3D FDTD simulation. The power indicated in each output branch is the power normalized by the input power and subtracted the propagation loss of the same distance in the straight plasmonic waveguide. They are −3.63 dB for the 1 × 2 splitter and −6.95 dB for the 1 × 4 splitter. Because the 1 × 2 splitter contains an additional 45° sharp bend, the theoretical excess loss for one Y-splitter is estimated to be ~0.46 dB. Figures 4(c) and 4(f) depict the spectra measured from each output port of the 1 × 2 and 1 × 4 splitters with 64-nm WP, respectively. We can see that the spectra are almost wavelength independent and the difference among different output ports in the splitters is within the measurement error, confirming the symmetric nature of our splitters. To reduce the experimental uncertainty, three identical splitters are measured. The averaged output power after subtracting that measured on the 3-µm-long straight plasmonic waveguide is −3.8 ± 0.4 dB for the 1 × 2 splitter and −7.5 ± 0.2 dB for the 1 × 4 splitter. Both are in good agreement with the simulation results. The experimental excess loss for one Y-splitter is estimated to be ~0.74 dB. The splitters with larger WPs are also examined. They exhibit similar splitting behaviors (not shown here). The apparent insensitivity of the excess loss on WP may also be attributed to the relatively thick SiO2 layer, as the above-observed apparent insensitivity of the pure bending loss on WP.

Asymmetric splitters which deliver unequal power to each output port can be flexibly designed based on our plasmonic waveguide. Figure 6
Fig. 6 (a) SEM image (the Si core pattern) of an asymmetric 1 × 2 splitter with 30° opening angle; (b) Normalized Hy distribution in the splitter with 64-nm WP and 28-nm SiO2, obtained from the 3D FDTD simulation. The indicated values after the junction are normalized by the input power and subtracted the propagation loss through the same distance in the straight waveguide; (c) The spectra measured from each output port of a splitter with 64-nm WP, normalized by that measured from the 3-µm-long straight plasmonic waveguide. The indicated values are averaged from three identical splitters; (d)-(f) are the corresponding figures for an asymmetric 1 × 2 splitter with 60° opening angle; (g)-(i) are the corresponding figures for an asymmetric 1 × 2 splitter with 90° opening angle (the ⊥-splitter); (j)-(l) are the corresponding figures for a symmetric 1 × 2 T-splitter for comparison.
shows 1 × 2 asymmetric plasmonic splitters fabricated in this work, where the one output branch (out-1) is straight up from the input arm while the other output branch (out-2) is obliquely connected to the input arm with a certain angle. The angle is 30° in Fig. 6(a), 60° in Fig. 6(d), and 90° (i.e., ⊥-splitter) in Fig. 6(g), respectively. Figure 9(j)
Fig. 9 (a) Schematic top view of plasmonic MZIs designed in this work, ΔL varies from 0 to 0.8 µm with a step of 0.1 µm; (b) SEM image (Si core pattern) of MZI with ΔL = 0; (c) Normalized Hy distribution in the MZI with 64-nm WP and 28-nm SiO2, obtained from the 3D FDTD simulation. The indicated value at the output plasmonic waveguide is normalized by the input power and subtracted the propagation loss of 5-µm-long straight plasmonic waveguide; (d)-(e) are the corresponding figures for the MZI with ΔL = 0.4 µm; and (f)-(g) are the corresponding figures for the MZI with ΔL = 0.8 µm.
is a symmetric T-splitter for comparison. The total length of each plasmonic route is 3 µm. The simulation results of these splitters with 64-nm WP and 28-nm SiO2 are illuminated in Figs. 6(b), 6(e), 6(h), and 6(k), respectively, and the measurement results are plotted in Figs. 6(c), 6(f), 6(i), and 6(l), respectively.

From simulation, we can see that the power delivered to the out-2 branch decreases significantly from 43.1% to 36.1%, while the power delivered to the out-1 branch increases slightly from 48.5% to 51.6%, with the angle increasing from 30° to 90°. It indicates that only a small fraction of the decreased power in the out-2 branch due to the angle increasing is rerouted to the out-1 branch, making the overall excess loss increasing from 0.38 dB to 0.57 dB with the angle increasing from 30° to 90°. In particular, the ⊥-splitter delivers 51.6% power to the out-1 branch and 36.1% power to the out-2 branch, for comparison, the T-splitter delivers 36.8% power to each branch and the remaining 26.4% power is lost mainly due to reflection at the junction. Moreover, we find that the ratio of powers delivered to two branches of the above 1 × 2 asymmetric splitters increases, as well as the excess loss of T-splitter increases, with the slot width (both WP and the surrounding SiO2 thickness) of the plasmonic waveguide increasing (not shown here), in agreement with the report that the reflection from the junction increases with the slot width increasing [23

23. W. Cai, W. Shin, S. Fan, and M. L. Brongersma, “Elements for plasmonic nanocircuits with three-dimensional slot waveguides,” Adv. Mater. (Deerfield Beach Fla.) 22(45), 5120–5124 (2010). [CrossRef] [PubMed]

].

From experiment, we can see that the transmission spectra measured from each output port of 1 × 2 asymmetric splitters are also almost wavelength independent in the spectral range of 1520−1620 nm. The power delivered to the out-1 branch increases and the power delivered to the out-2 branch decreases, with the angle increasing, in agreement with the simulation results. However, except the 30° splitter, the power delivered to the out-1 branch is larger, and that delivered to the out-2 branch is smaller, than that predicted from simulation. This discrepancy may partially be attributed to the enlarged junction in the fabricated splitters due to the fabrication limitation, as observed in Figs. 6(d) and 6(g). Consistently, the larger excess loss of the T-splitter than that predicted from simulation may also partially arise from the enlarged junction [13

13. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Nanoplasmonic power splitters based on the horizontal nanoplasmonic slot waveguide,” Appl. Phys. Lett. 99(3), 031112 (2011). [CrossRef]

], as observed in Fig. 6(j). The enlarged junction makes the splitter behaving as a splitter with a larger slot width, thus leading to a large reflection of SPP wave at the junction. For the 30° splitter shown in Fig. 6(a), on the other hand, the two output branches near the junction are not physically separated due to the limitation of our UV lithography. The two output branches are apparently almost symmetric near the junction, thus resulting in almost equally power splitting with low excess loss of ~0.5 dB.

To verify the above hypothesis, the ⊥- and T-splitters with an enlarged junction are simulated. For simplification, the Si core of the enlarge junction is approximated to an equilateral triangle with the side length of 0.3 μm. The simulation results are given in Fig. 7
Fig. 7 3D FDTD simulation results of (a) ⊥-splitter and (b) T-splitter, which have the same parameters as those shown in Figs. 6 (h) and (k), respectively, but with an enlarged junction which is approximated to an equilateral triangle with the side length of 0.3 μm for simplification.
. Compared with the corresponding values indicated in Fig. 6(h) and 6(k), the power delivered to the out-1 branch becomes larger and that delivered to the out-2 branch becomes smaller for the ⊥-splitter, and the power delivered to the each branch becomes smaller for the T-splitter. The trend agrees qualitatively with the above hypothesis although the predicted values still deviate relatively largely from the experimental data. We expect that this deviation may be reduced if a more accurate morphology of the Si core as well as the surrounding SiO2 is taken into account in the numerical simulation.

According to the above result that a substantial fraction of power can be delivered to the orthogonal branch, more complex splitters can be designed, as shown in Fig. 8
Fig. 8 (a) SEM image (the Si core pattern) of a 1 × 3 cross splitter with one junction; (b) Normalized Hy distribution in the splitter with 64-nm WP and 28-nm SiO2, obtained from the 3D FDTD simulation. The indicated values after the junction are normalized by the input power and subtracted the propagation loss through the same distance in the straight plasmonic waveguide; (c) The spectra measured from each output port of the splitter, subtracted by the straight waveguide with the same plasmonic route length. The indicated values are averaged from three identical splitters; (d)-(f) are the corresponding figures for a 1 × 3 splitter with two junctions; (g)-(i) are the corresponding figures for a 1 × 5 splitter with four junctions.
just for some examples. Figure 8(a) is a 1×3 cross splitter with one junction. Figure 8(d) is a 1×3 splitter with two junctions, which actually consists of two sequent ⊥-splitters. Figure 8(g) is a 1×5 splitter with 4 junctions, which actually consists of four sequent ⊥-splitters. Their simulation results are illuminated in Figs. 8(b), 8(e), and 8(h), respectively. The value indicated at each output branch is the power normalized by the input power and subtracted the propagation loss through the same distance in the straight waveguide (be noted that each plasmonic route in these splitters has a different length). We can see that the SPP power can be delivered to the orthogonal branches accordingly, in consistence with the result of ⊥-splitter shown in Fig. 6(g). Figures 8(c), 8(f), and 8(i) show the measurement results of these three splitters, respectively. The transmission spectra measured from each output ports are almost wavelength independent. The indicated values are averaged from three identical splitters, normalized by the straight plasmonic waveguide with the same length. In agreement with that predicted from simulation, the SPP power is delivered to each branch accordingly. Like the asymmetric splitters shown in Fig. 6, the power delivered to the straight branch is larger, whereas the power delivered to the orthogonal braches is smaller, than that predicted from simulation partially due to the enlarged junctions. The splitters with large WPs are also measured (not shown here). They deliver even larger fraction of power to the straight branch and even smaller fraction of power to the orthogonal branches than those indicated in Fig. 8, in agreement with the abovementioned effect that the large slot width of the plasmonic waveguide results in a large reflection of SPP power at the junction.

6. Plasmonic Mach-Zehnder interferometers

MZI is a basic building block for many optical devices. Figure 9(a) shows schematically a typical design of an ultracompact plasmonic MZI, where the opening angle of the splitter and the combiner is 120°, the upper oblique arm is 1-µm long, and the bottom oblique arm is (1 + ΔL)-µm long. ΔL varies from 0 to 0.8 µm with a step of 0.1 µm. The input plasmonic waveguide before splitting and the output plasmonic waveguide after combing are both 1-µm long. Therefore, the total plasmonic route through the upper arm is 6 µm, and that through the bottom arm is (6 + ΔL) µm. Figures 9(b), 9(d), and 9(f) show SEM images of the MZIs with ΔL = 0, 0.4, and 0.8 µm, respectively. Figures 9(c), 9(e), and 9(g) illuminate the simulation results for these three MZIs, respectively. We can see that two plasmonic waves propagating along the upper and bottom arms are constructively recombined at the combiner in the cases of ΔL = 0 and 0.8 µm, whereas they are destructively recombined at the combiner in the case of ΔL = 0.4 µm. The transmitted powers normalized by the input power and subtracted the propagation loss of the 6-µm-long straight plasmonic waveguide are −2.58, −22.7, and −2.93 dB for MZIs with ΔL = 0, 0.4, and 0.8 µm, respectively. It indicates that the plasmonic MZI has the same working mechanism as the conventional dielectric waveguide based MZIs. Therefore, the normalized transmission, T(λ), can be expressed as follows [24

24. G. T. Reed, Silicon Photonics: The State of the Art (John Wiley &Sons, Ltd, 2008), Chap. 7.

]:
T(λ)=14α1[1+α22+2α2cos(2πλneffΔL)]
(1)
where α1 is the normalized transmission loss due to splitting, combining, and bending. α2 ( = 10-α/10 × ΔL, where α is the propagation loss in dB/µm unit) is the power difference after propagating through two arms.

Figure 10(a)
Fig. 10 (a) The normalized transmission spectra measured on MZIs with ΔL of 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8 µm, respectively; (b) The transmitted powers obtained from measurement and simulation as a function of ΔL, as well as the fitting curve based on Eq. (1) with the best fitting parameters of α1 = 0.69 and neff = 1.85.
depicts the measured transmission spectra of a set of MZIs with 64-nm WP whose ΔL is 0, 0.1, 0.2, …, and 0.8 µm, respectively, normalized by the 6-µm-long straight plasmonic waveguide. As expected from Eq. (1), the transmission spectrum of the balanced MZI (i.e., ΔL = 0) is wavelength independent, whereas the spectra of the unbalanced MZIs (i.e., ΔL ≠ 0) become wavelength dependent, especially in the case of ΔL = 0.4 µm. But, no clear peak can be observed in the spectra due to the large free spectral range (FSR, = λr2/(neffΔL)) and the narrow spectral range (~1.52-1.62 µm) of our measurement setup. FSR decreases with ΔL increasing. For our MZI with the largest ΔL of 0.8 µm, FSR is estimated to be ~1.5 µm around 1550-nm wavelengths, which is much larger than the spectral range of 0.1 µm. Figure 10(b) plots the measured transmitted powers as a function of ΔL, normalized by the 6-µm-long straight plasmonic waveguide. Each data point is averaged from three identical MZIs and the standard deviation is indicated as the error bar. The transmitted powers obtained from the 3D FDTD simulation are also shown for comparison. They agree well with the measurement values. The measurement data are fitted using Eq. (1) by setting α2 = 10-0.0304 × ΔL (because the corresponding straight plasmonic waveguide has propagation loss of ~0.304 dB/µm, as indicated in Fig. 2) and setting neff and α1 as the fitting parameters. The best fit parameters are found to be α1 = 0.69 and neff = 1.85. The MZIs with large WPs are also examined. They exhibit similar property as that shown in Fig. 10 (not shown here). From best fitting using Eq. (1), the real effective indices are extracted to be ~1.85, ~1.95, ~2.00, and ~2.05 for plasmonic waveguides with WP of 64, 81, 94, and 102 nm, respectively. These neff values are plotted in Fig. 3(b) as a function of WP. We can see that they agree well with those extracted from the 3D FDTD simulation of the straight plasmonic waveguides.

From Fig. 10(b), we can see that the fabricated MZIs have normalized insertion loss (IL) of ~1.7 dB and extinction ratio (ER) of ~18 dB. The ER is mainly limited by the power difference between the upper and bottom arms. Theoretically, it can be as large as ~35 dB (in the case of ΔL = 0.43 around 1550-nm wavelengths). The normalized IL is due to splitter, combining, and bending. The value of 1.7 dB insertion loss quantitatively agrees well with the experimental results of the symmetric 1 × 2 splitter shown in Fig. 5 and the sharp bend shown in Fig. 4.

Our plasmonic waveguide allow us to design more compact MZIs. One example is shown in Fig. 11(a)
Fig. 11 (a) Schematic top view of ultracompact plasmonic MZIs with R of 0.9 µm and ΔL varying from 0 to 0.3 µm; (b) SEM image (Si core patterns) of MZI with ΔL = 0; (c) Normalized Hy distribution in the MZI with 64-nm WP and 28-nm SiO2, obtained from the 3D FDTD simulation, The indicated value at the output plasmonic waveguide is normalized by the input power and subtracted the propagation loss of 3.8-µm-long straight plasmonic waveguide; and (d)-(e) are the corresponding figures for MZI with ΔL = 0.2 µm.
schematically, where two arms are the upper and bottom arcs of a ring with radius R. The output plasmonic waveguide deviates from the parallel line of the input plasmonic waveguide by ΔL. In this work, R is set to 0.9 µm and ΔL is set to 0, 0.1, 0.15, 0.2, 0.25, and 0.3 µm, respectively. The input plasmonic waveguide before splitting and the output plasmonic waveguide after combing are both 1-µm long. Therefore, the total length of plasmonic route through the upper arm is ~(4.8 - ΔL) µm, and that through the bottom arm is ~(4.8 + ΔL) µm. Figures 11(a) and 11(d) shows SEM images of the fabricated MZIs with ΔL = 0 and 0.2 µm, respectively. Their corresponding simulation results are illuminated in Figs. 11(c) and 11(e), respectively. Similar to the MZIs shown in Fig. 9, the two SPP waves propagating along two arms are constructively recombined at the combiner in the case of ΔL = 0, and are destructively recombined in the case of ΔL = 0.2 µm. The normalized output power is −3.21 and −19.0 dB, respectively.

Figure 12(a)
Fig. 12 (a) The transmission spectra measured on MZIs with ΔL = 0, 0.1, 0.15, 0.2, 0.25, and 0.3, respectively, normalized by the 4.8-µm-long straight plasmonic waveguide; (b) The transmitted powers obtained from measurement and simulation as a function of ΔL, as well as the fitting curve based on Eq. (1) with the best fitting parameters of α1 = 0.45 and neff = 1.75.
plots the spectra measured on a set of MZIs with ΔL = 0, 0.1, …, and 0.3 µm, respectively, normalized by the 4.8-µm-long straight plasmonic waveguide. Similar to Fig. 10(a), the spectrum of the balanced MZI is almost wavelength independent, whereas the spectra of the unbalanced MZIs are wavelength dependent. Again, due to the very large FSR and the narrow spectral range, no clear peak can be observed in the spectra. Figure 12(b) plots the normalized output powers as a function of ΔL, as well as those obtained from 3D FDTD simulation. The fitting curve using Eq. (1) (be noted that ΔL in Eq. (1) should be replaced by 2Rarcsin(ΔL/R) here) is also shown. The best fitting parameters are α1 = 0.45 and neff = 1.75. The large normalized IL of this type of MZIs is due to the large excess loss of the T-splitter and combiner, as shown in Fig. 6(j). The footprint of this type of MZIs can be reduced simply by reducing the radius of the ring.

7. Conclusions

Various horizontal Cu-SiO2-Si-SiO2-Cu plasmonic waveguide components are systematically investigated. The propagation losses measured from the fabricated straight plasmonic waveguides agree with those predicted from 3D FDTD simulation using the Cu permittivity reported in Ref [19

19. S. Roberts, “Optical properties of copper,” Phys. Rev. 118(6), 1509–1518 (1960). [CrossRef]

]. The plasmonic waveguide supports sharp 90° bend with a relatively low bending loss of ~0.73 dB/turn. However, the bending loss extracted from the bent plasmonic waveguides with multiple sharp 90° bends suffers from large uncertainty due to the weak F-P resonances induced by weak reflection of SPP power at the junction. Therefore, a new testing structure should be designed to extract the bending loss accurately.

Owing to the low bending loss, various ultracompact power splitters and MZIs can be flexibly designed for plasmonic nanocircuits. The fabricated 1 × 2 Y-splitter has excess loss of ~0.74 dB and MZIs has normalized insertion loss of ~1.7 dB and extinction ratio of ~18 dB, in good agreement with those predicted from the 3D FDTD simulation. For asymmetric, T-, ⊥-, and cross- splitters, the ratio of powers delivered to output ports depends on the slot width of the plasmonic waveguide, and the power delivered to each output port differs quantitatively from that predicted from simulation partially due to the enlarged junction caused by the fabrication limitation. The plasmonic waveguide with narrower slot width has better performance for rerouting, but has a larger propagation loss. It indicates that a main challenge for plasmonic nanocircuits is to reduce the propagation loss without sacrificing the confinement level.

Acknowledgments

This work was supported by Singapore SERC/A*STAR Grant 092-154-0098, and Singapore A*STAR Influse Exploratory Grant I02-0331-12.

References and links

1.

M. Dragoman and D. Dragoman, “Plasmonics: applications to nanoscale terahertz and optical devices,” Prog. Quantum Electron. 32(1), 1–41 (2008). [CrossRef]

2.

D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics 4(2), 83–91 (2010). [CrossRef]

3.

R. F. Oulton, G. Bartal, D. F. P. Pile, and X. Zhang, “Confinement and propagation characteristics of subwavelength plasmonic modes,” New J. Phys. 10(10), 105018 (2008). [CrossRef]

4.

E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science 311(5758), 189–193 (2006). [CrossRef] [PubMed]

5.

R. Zia, J. A. Schuller, A. Chandran, and M. L. Brongersma, “Plasmonics: the next chip-scale technology,” Mater. Today 9(7–8), 20–27 (2006). [CrossRef]

6.

S. I. Bozhevolnyi and J. Jung, “Scaling for gap plasmon based waveguides,” Opt. Express 16(4), 2676–2684 (2008). [CrossRef] [PubMed]

7.

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(8), 496–500 (2008). [CrossRef]

8.

T. Holmgaard and S. I. Bozhevolnyi, “Theoretical analysis of dielectric –loaded surface plasmon-polariton waveguides,” Phys. Rev. B 75(24), 245405 (2007). [CrossRef]

9.

S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Fully complementary metal-oxide-semiconductor compatible nanoplasmonic slot waveguides for silicon electronic photonic integrated circuits,” Appl. Phys. Lett. 98(2), 021107 (2011). [CrossRef]

10.

S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration,” Opt. Express 19(9), 8888–8902 (2011). [CrossRef] [PubMed]

11.

S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Theoretical investigation of silicon MOS-type plasmonic slot waveguide based MZI modulators,” Opt. Express 18(26), 27802–27819 (2010). [CrossRef] [PubMed]

12.

S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Theoretical investigation of silicide Schottky barrier detector integrated in horizontal metal-insulator-silicon-insulator-metal nanoplasmonic slot waveguide,” Opt. Express 19(17), 15843–15854 (2011). [CrossRef] [PubMed]

13.

S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Nanoplasmonic power splitters based on the horizontal nanoplasmonic slot waveguide,” Appl. Phys. Lett. 99(3), 031112 (2011). [CrossRef]

14.

S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Experimental demonstration of horizontal nanoplasmonic slot waveguide-ring resonators with submicron radius,” IEEE Photon. Technol. Lett. 23(24), 1896–1898 (2011). [CrossRef]

15.

S. Y. 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]

16.

J. W. Peng, S. J. Lee, G. C. A. Liang, N. Singh, S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Improved carrier injection in gate-all-around Schottky barrier silicon nanowaire field-effect transistors,” Appl. Phys. Lett. 93(7), 073503 (2008). [CrossRef]

17.

http://www.rsoftinc.com

18.

A. D. Rakic, A. B. Djurisic, J. M. Elazar, and M. L. Majewski, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt. 37(22), 5271–5283 (1998). [CrossRef] [PubMed]

19.

S. Roberts, “Optical properties of copper,” Phys. Rev. 118(6), 1509–1518 (1960). [CrossRef]

20.

E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, Boston, 1985).

21.

G. S. Mathad, Copper Interconnects, New Contact Metallurgies, Structures, and Low-k Interlevel Dielectrics (The Electrochemical Society, Inc., New Jersey, USA, 2003).

22.

S. Y. Zhu, Q. Fang, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Propagation losses in undoped and n-doped polycrystalline silicon wire waveguides,” Opt. Express 17(23), 20891–20899 (2009). [CrossRef] [PubMed]

23.

W. Cai, W. Shin, S. Fan, and M. L. Brongersma, “Elements for plasmonic nanocircuits with three-dimensional slot waveguides,” Adv. Mater. (Deerfield Beach Fla.) 22(45), 5120–5124 (2010). [CrossRef] [PubMed]

24.

G. T. Reed, Silicon Photonics: The State of the Art (John Wiley &Sons, Ltd, 2008), Chap. 7.

OCIS Codes
(230.7370) Optical devices : Waveguides
(240.6680) Optics at surfaces : Surface plasmons
(250.5300) Optoelectronics : Photonic integrated circuits
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Optoelectronics

History
Original Manuscript: November 28, 2011
Revised Manuscript: January 27, 2012
Manuscript Accepted: January 27, 2012
Published: February 27, 2012

Citation
Shiyang Zhu, G. Q. Lo, and D. L. Kwong, "Components for silicon plasmonic nanocircuits based on horizontal Cu-SiO2-Si-SiO2-Cu nanoplasmonic waveguides," Opt. Express 20, 5867-5881 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-6-5867


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. M. Dragoman and D. Dragoman, “Plasmonics: applications to nanoscale terahertz and optical devices,” Prog. Quantum Electron.32(1), 1–41 (2008). [CrossRef]
  2. D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics4(2), 83–91 (2010). [CrossRef]
  3. R. F. Oulton, G. Bartal, D. F. P. Pile, and X. Zhang, “Confinement and propagation characteristics of subwavelength plasmonic modes,” New J. Phys.10(10), 105018 (2008). [CrossRef]
  4. E. Ozbay, “Plasmonics: merging photonics and electronics at nanoscale dimensions,” Science311(5758), 189–193 (2006). [CrossRef] [PubMed]
  5. R. Zia, J. A. Schuller, A. Chandran, and M. L. Brongersma, “Plasmonics: the next chip-scale technology,” Mater. Today9(7–8), 20–27 (2006). [CrossRef]
  6. S. I. Bozhevolnyi and J. Jung, “Scaling for gap plasmon based waveguides,” Opt. Express16(4), 2676–2684 (2008). [CrossRef] [PubMed]
  7. 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(8), 496–500 (2008). [CrossRef]
  8. T. Holmgaard and S. I. Bozhevolnyi, “Theoretical analysis of dielectric –loaded surface plasmon-polariton waveguides,” Phys. Rev. B75(24), 245405 (2007). [CrossRef]
  9. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Fully complementary metal-oxide-semiconductor compatible nanoplasmonic slot waveguides for silicon electronic photonic integrated circuits,” Appl. Phys. Lett.98(2), 021107 (2011). [CrossRef]
  10. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, “Silicon-based horizontal nanoplasmonic slot waveguides for on-chip integration,” Opt. Express19(9), 8888–8902 (2011). [CrossRef] [PubMed]
  11. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Theoretical investigation of silicon MOS-type plasmonic slot waveguide based MZI modulators,” Opt. Express18(26), 27802–27819 (2010). [CrossRef] [PubMed]
  12. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Theoretical investigation of silicide Schottky barrier detector integrated in horizontal metal-insulator-silicon-insulator-metal nanoplasmonic slot waveguide,” Opt. Express19(17), 15843–15854 (2011). [CrossRef] [PubMed]
  13. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Nanoplasmonic power splitters based on the horizontal nanoplasmonic slot waveguide,” Appl. Phys. Lett.99(3), 031112 (2011). [CrossRef]
  14. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Experimental demonstration of horizontal nanoplasmonic slot waveguide-ring resonators with submicron radius,” IEEE Photon. Technol. Lett.23(24), 1896–1898 (2011). [CrossRef]
  15. S. Y. 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]
  16. J. W. Peng, S. J. Lee, G. C. A. Liang, N. Singh, S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Improved carrier injection in gate-all-around Schottky barrier silicon nanowaire field-effect transistors,” Appl. Phys. Lett.93(7), 073503 (2008). [CrossRef]
  17. http://www.rsoftinc.com
  18. A. D. Rakic, A. B. Djurisic, J. M. Elazar, and M. L. Majewski, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt.37(22), 5271–5283 (1998). [CrossRef] [PubMed]
  19. S. Roberts, “Optical properties of copper,” Phys. Rev.118(6), 1509–1518 (1960). [CrossRef]
  20. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, Boston, 1985).
  21. G. S. Mathad, Copper Interconnects, New Contact Metallurgies, Structures, and Low-k Interlevel Dielectrics (The Electrochemical Society, Inc., New Jersey, USA, 2003).
  22. S. Y. Zhu, Q. Fang, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Propagation losses in undoped and n-doped polycrystalline silicon wire waveguides,” Opt. Express17(23), 20891–20899 (2009). [CrossRef] [PubMed]
  23. W. Cai, W. Shin, S. Fan, and M. L. Brongersma, “Elements for plasmonic nanocircuits with three-dimensional slot waveguides,” Adv. Mater. (Deerfield Beach Fla.)22(45), 5120–5124 (2010). [CrossRef] [PubMed]
  24. G. T. Reed, Silicon Photonics: The State of the Art (John Wiley &Sons, Ltd, 2008), Chap. 7.

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited