OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 14 — Jul. 2, 2012
  • pp: 15232–15246
« Show journal navigation

Performance of ultracompact copper-capped silicon hybrid plasmonic waveguide-ring resonators at telecom wavelengths

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


Optics Express, Vol. 20, Issue 14, pp. 15232-15246 (2012)
http://dx.doi.org/10.1364/OE.20.015232


View Full Text Article

Acrobat PDF (2007 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Ultracompact Cu-capped Si hybrid plasmonic waveguide-ring resonators (WRRs) with ring radii of 1.09–2.59 μm are fabricated on silicon on insulator substrates using standard complementary metal-oxide-semiconductor technology and characterized over the telecom wavelength range of 1.52–1.62 μm. The dependence of the spectral characteristics on the key structural parameters such as the Si core width, the ring radius, the separation gap between the ring and bus waveguides, and the ring configuration is systematically studied. A WRR with 2.59-μm radius and 0.250-μm nominal gap exhibits good performances such as normalized insertion loss of ~0.1 dB, extinction ratio of ~12.8 dB, free spectral range of ~47 nm, and quality factor of ~275. The resonance wavelength is redshifted by ~4.6 nm and an extinction ratio of ~7.5 dB is achieved with temperature increasing from 27 to 82°C. The corresponding effective thermo-optical coefficient (dng/dT) is estimated to be ~1.6 × 10−4 K−1, which is contributed by the thermo-optical effect of both the Si core and the Cu cap, as revealed by numerical simulations. Combined with the compact size and the high thermal conductivity of Cu, various effective thermo-optical devices based on these Cu-capped plasmonic WRRs could be realized for seamless integration in existing Si electronic-photonic integrated circuits.

© 2012 OSA

1. Introduction

A potential approach for future minimization of optical devices beyond the diffraction limit is to utilize surface plasmon polaritons (SPPs), which are electromagnetic surface waves coherently coupled to collective electron oscillations in metal-dielectric interfaces [1

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

]. Plasmonic waveguides, which comprise one or more metal-dielectric interfaces, are a basic element to accommodate various passive and active plasmonic devices. In the past decades, many waveguding structures have been explored such as channel plasmon polariton (CPP) [2

2. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J. Y. Laluet, and T. W. Ebbesen, “Channel plasmon subwavelength waveguide components including interferometers and ring resonators,” Nature 440(7083), 508–511 (2006). [CrossRef] [PubMed]

], metal-insulator-metal (MIM) [3

3. 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]

], dielectric-loaded SPP (DLSPP) [4

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

], and hybrid plasmonic waveguides [5

5. 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]

], etc. A typical challenge in all these plasmonic waveguides is the tradeoff between strong optical field confinement and long propagation distance. Moreover, the plasmonic waveguide should be flexible enough to allow active functions such as modulation and detection to be implemented. For monolithic integration in existing Si electronic-photonic integrated circuits (EPICs), the fabrication of these plasmonic waveguides as well as waveguide-based devices should be compatible with the mature complementary metal-oxide-semiconductor (CMOS) technology. The hybrid plasmonic waveguide, which consist of a high-index dielectric core separated from a metal surface by a nanoscale low-index dielectric gap, is regarded as an attractive candidate to meet the above requirements. Theoretical analyses show that the nanoscale gap can support a relatively low-loss compact plasmonic-like mode whereas the high-index dielectric core, which is typically patterned to a rectangle or a cylinder, supports a photonic-like mode [5

5. 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]

7

7. D. Dai and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express 17(19), 16646–16653 (2009). [CrossRef] [PubMed]

]. For seamless integration in Si EPICs, the high-index dielectric core is naturally Si and the low-index dielectric gap is naturally SiO2 or Si3N4. Such a vertical metal-SiO2-Si hybrid waveguide, as well as various waveguide-based devices such as couplers, power splitters, and filters, etc., has been extensively studied theoretically [7

7. D. Dai and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express 17(19), 16646–16653 (2009). [CrossRef] [PubMed]

9

9. H. S. Chu, Y. A. Akimov, P. Bai, and E. P. Li, “Hybrid dielectric-loaded plasmonic waveguide and wavelength selective components for efficiently controlling light at subwavelength scale,” J. Opt. Soc. Am. B 28(12), 2895–2901 (2011). [CrossRef]

]. Experimentally, conductor-gap-silicon (CGS) plasmonic waveguides have been demonstrated using patterned Au as the metal [10

10. M. Wu, Z. Han, and V. Van, “Conductor-gap-silicon plasmonic waveguides and passive components as subwavelength scale,” Opt. Express 18(11), 11728–11736 (2010). [CrossRef] [PubMed]

]. The nanoscale Au patterning requires expensive electron beam lithography (EBL), which is not an industry-standard technology. A self-aligned approach is proposed to avoid EBL [11

11. I. Goykhman, B. Desiatov, and B. Levy, “Experimental demonstration of locally oxidized hybrid silicon-plasmonic waveguide,” Appl. Phys. Lett. 97(14), 141106 (2010). [CrossRef]

], but it is only viable for Si waveguides formed by the local oxidation process which suffer from blunt sidewalls. However, theoretical studies have revealed that the metal layer in hybrid plasmonic waveguides can be infinitely wide because the lateral field confinement of the plasmonic-like mode can be solely achieved by the Si core, similarly to the photonic mode in standard Si waveguides [8

8. H. S. Chu, E. P. Li, P. Bai, and R. Hegde, “Optical performance of single-mode hybrid dielectric-loaded plasmonic waveguide-based components,” Appl. Phys. Lett. 96(22), 221103 (2010). [CrossRef]

, 9

9. H. S. Chu, Y. A. Akimov, P. Bai, and E. P. Li, “Hybrid dielectric-loaded plasmonic waveguide and wavelength selective components for efficiently controlling light at subwavelength scale,” J. Opt. Soc. Am. B 28(12), 2895–2901 (2011). [CrossRef]

]. Moreover, although Au is a good metal for plasmonics, it is not a CMOS compatible material. In terms of CMOS compatibility, Al or Cu should be used as the metal. At the telecom wavelengths of 1550 nm, Cu is found to be much better than Al as it provides much lower propagation loss in the same waveguide configuration [12

12. 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]

]. Considering the above two points, a vertical Cu-SiO2-Si hybrid plasmonic waveguide is proposed and has been experimentally demonstrated on silicon-on-insulator (SOI) substrates using standard CMOS technology [13

13. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Monolithic integration of hybrid plasmonic waveguide components into a fully CMOS-compatible SOI platform,” IEEE Photon. Technol. Lett. (Accepted).

]. The Cu layer above the Si core, which is defined by the industry-standard ultraviolet (UV) lithography, is much wider than the underlying Si core. The fabricated Cu-SiO2-Si plasmonic waveguide exhibits a relatively low propagation loss of ~0.12 dB/μm at telecom wavelengths and a high coupling efficiency of ~86% with the conventional Si strip waveguide [13

13. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Monolithic integration of hybrid plasmonic waveguide components into a fully CMOS-compatible SOI platform,” IEEE Photon. Technol. Lett. (Accepted).

]. It indicates that this hybrid plasmonic waveguide is a more feasible candidate for seamless integration in existing Si EPICs than the CGS waveguide.

2. Design, fabrication, and measurement

The schematic layout and cross section of the Cu-SiO2-Si plasmonic WRRs studied in this paper are depicted in Fig. 1(a)
Fig. 1 Vertical Cu-SiO2-Si hybrid plasmonic waveguide-based WRRs fabricated in this paper. (a) Schematic layout, (b) Cross section, and (c) SEM image of the patterned Si core of a typical WRR with R of 2.59 μm. The parameters in the layout are WP = 0.18 μm, R = 1.09, 1.59, 2.09, or 2.59 μm, and gap = 0.2 or 0.25 μm. The parameters which are determined by fabrication are hSi = ~340 nm and tSiO2 = ~17 nm.
and 1(b), respectively. The Cu-capped rectangular window is defined as the plasmonic area. The straight bus plasmonic waveguide has length of 7 μm and width (WP) of 0.18 μm, which is linked with input/output 0.5-μm-wide Si strip waveguides through 1-μm-long tapered couplers. A microring adjacent to the bus waveguide has the same width of 0.18 μm and the central radius (R) of 1.09, 1.59, 2.09, or 2.59 μm, respectively. The separation gap between the ring and bus waveguide is 0.2 or 0.25 μm, respectively.

The devices are fabricated on SOI wafers with 340-nm top-Si and 2-µm buried SiO2. During patenting the Si core of the plasmonic WRRs and tapered couplers (as well as the input/output Si strip waveguides simultaneously) using UV lithography, the expose condition is intentionally varied to result in different critical dimensions of the Si core. A photoresist trimming process is carried out to further reduce the critical dimension. Using this method, WRRs with different Si core widths are fabricated using the same mask. Figure 1(c) shows a scanning electron microscope (SEM) image of a patterned Si core of a typical plasmonic WRR with 2.59-μm R, for example. Then, SiO2/Si3N4 deposition, SiO2 chemical mechanical polishing (CMP) (using Si3N4 as the CMP stopping layer), SiO2/Si3N4 deposition again, SiO2 window opening (using Si3N4 as the SiO2 dry etching stopping layer), wet etching of remaining Si3N4 in the windows, thermal oxidization to grow a thin SiO2 layer, Cu deposition, and Cu-CMP (to remove Cu outside the windows) are carried out sequentially. The details of the fabrication have been described elsewhere [13

13. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Monolithic integration of hybrid plasmonic waveguide components into a fully CMOS-compatible SOI platform,” IEEE Photon. Technol. Lett. (Accepted).

]. The thickness (tSiO2) of the thin SiO2 layer between the Si core and the metal is a critical parameter to determine the property of the hybrid plasmonic waveguide [5

5. 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]

], thus it needs to be precisely controlled. In this work, for simplicity, this layer is a ~17-nm-thick thermal oxide, as shown in Fig. 2
Fig. 2 XTEM images of vertical Cu-SiO2-Si hybrid plasmonic waveguides with different Si core widths, which are caused by the different expose conditions during UV lithography. A thin Si3N4 layer between Si and SiO2 in the bottom Si core region is for the fabrication process control, not for the function. The Si core height is ~300 nm and the thermal SiO2 layer between the Si core and the Cu cap is ~17 nm. These four kinds of plasmonic WRRs are referred as S1, S2, S3, and S4, respectively.
. It is worthy to be noted that if this thin SiO2 layer is replaced by a functional dielectric, e.g., a dielectric with a large electro-optic (EO) or TO coefficient, effective active plasmonic devices may be realized because of the strong mode confinement in this layer. Figure 2 shows cross sectional transmission electron microscope (XTEM) images of four fabricated plasmonic WRRs. Their Si core widths at the middle of the height (WP) are ~163 nm, ~156 nm, ~133 nm, and ~126 nm, respectively. The Si cores are not a perfect rectangle and contain round shoulders at upper two corners due to the imperfection of the Si dry etching process. Because the plasmonic mode is strongly confined near the upper surface of the Si core, the round shoulder makes the effective Si core width smaller than the above WP value measured on the middle height of the Si core, especially for the devices with narrower WP. For the S4 device shown in Fig. 2(d), the flat region of the Si core top is only ~27 nm. WRRs with WP narrower than S4 device are also fabricated, but they exhibit poor resonant characteristics, which can be explained by the fact that their flat region of the Si core top has approached to zero so that no plasmonic mode can be excited by the transverse-magnetic (TM)-polarized light (the electric field is perpendicular to the chip surface plane). Hereafter, WRRs with Si cores shown in Figs. 2(a), 2(b), 2(c), and 2(d) are referred as S1, S2, S3, and S4 respectively for analysis in detail. Because the Si core shrinks from the nominal width of 180 nm in the mask to a certain width as shown in Fig. 2, the gap between the bus and ring waveguides is widened accordingly. Assuming that the Si core shrinks equally at both sides during fabrication, the gaps of these four WRRs measured at the middle height of the Si core are widened by ~17 nm, ~24 nm, ~47 nm, and ~54 nm, respectively. Be noted that due to the round shoulder of the Si core, the effective gap is larger and the effective Si core width is smaller than the abovementioned values.

3. Results and discussions

Because the plasmonic WRRs have 7-μm-long bus waveguide, the transmission spectra measured on the plasmonic WRRs are normalized by that measured on the corresponding 7-μm-long straight plasmonic waveguide. The normalized transmission spectrum can in general be expressed by [22

22. A. Yariv, “Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett. 36(4), 321–322 (2000). [CrossRef]

]:
T(λ)=α2+t22αtcosθ1+α2t22αtcosθ
(1)
where θ=(2π/λ)neff2πR is the phase change around the ring, α2 is a power loss factor per roundtrip around the ring, t=|t|exp(iθ) is the field transmission through the coupling region in the bus waveguide, λ is the free-space wavelength, and neff is the real effective index of the plasmonic waveguide. As other plasmonic WRRs [17

17. H. Hassan, J.-C. Weeber, L. Markey, and A. Dereux, “Thermo-optical control of dielectric loaded plasmonic racetrack resonators,” J. Appl. Phys. 110(2), 023106 (2011). [CrossRef]

], it is convenient to express the coefficient α as α2=αb×αprop, where αb refers to pure bending loss (αb = 1 corresponds to no bending loss) and αprop describes the propagation loss along the perimeter of the ring. αprop can be calculated using the propagation loss extracted from the straight waveguide by assuming that the curve waveguide has the same propagation loss as the straight counterpart. Other parameters including neff, αb, |t|, and θ are used as fitting parameters. The value of α and |t| is between 0 – 1. A larger α value means a less lossy resonator and a large |t| value means a weaker coupling between the bus and ring waveguides.

3.1. Plasmonic WRRs with different radii and silicon core widths

Figure 4
Fig. 4 Power transmission spectra for plasmonic WRRs with the same nominal gap of 200 nm but with different Si core widths (S1, S2, S3, and S4) and different ring radii (R = 1.09, 1.59, 2.09, and 2.59 μm). The experimental curves (in black) are normalized by the transmission spectrum of the corresponding 7-μm-long straight plasmonic waveguide. The fitting curves (in red) are based on Eq. (1). The fitting parameters of neff, αb, |t|, and θ are indicated for each plasmonic WRR.
plots the normalized transmission spectra of plasmonic WRRs with the same nominal gap of 200 nm. Due to the abovementioned Si core shrinkage, the real gaps (measured at the middle height of the Si core) become ~217, ~224, ~247, and ~254 nm for the S1, S2, S3, and S4 WRRs, respectively. From Fig. 4, we can see that the small-ring WRRs exhibit only one resonance while the large-R WRRs exhibit two resonances in the spectral range of 1520–1620 nm. The free spectral range (FSR) between these two resonances is solely determined by neff:
FSR=λ22πRneff
(2)
The insertion loss (IL) and the extinction ratio (ER) of the spectrum are determined by α and |t| together, whereas the peak position is determined by θ when neff is determined. However, α and |t| cannot be distinguished from a single spectrum because they are interchangeable in Eq. (1). To circumvent this issue and to make the fitting less ambiguous, all WRRs are considered together with the following assumptions:

  • (1) The plasmonic waveguides with the same Si core width have the same neff, regardless the radius;
  • (2) A smaller R ring has a larger bending loss, i.e., a smaller αb value;
  • (3) A smaller R ring has a larger |t| value because it has a shorter effective interaction length, thus leading to more weakly coupled as compared with the larger-R counterpart.
  • (4) A ring with a narrower Si core width has a larger |t| value because it has a larger gap induced by the abovementioned Si core shrinkage, thus leading to more weakly coupled as compared with the large-WP counterpart.
  • (5) For WRRs with the same radius, the bending loss is larger (thus the αb value is smaller) for the narrower WP.

The fitting curves as well as the fitting parameters of neff, αb, |t|, and θ for each WRR are shown in Fig. 4 accordingly. Because the value of αb increases and the value of αprop decreases with R increasing, α2=αb×αprop depends on R more weakly than |t|, thus the different spectral characteristics for different-R WRRs is mainly attributed to the variation of the |t| value with R and WP. The different spectral characteristics for various WRRs observed in Fig. 4 are explained as below:

  • (1) The S1 WRRs shown in Figs. 4(a)4(d) are overcoupling as |t| < α. Since the value of |t| increases with R decreasing, the smaller-R WRR exhibits a larger ER. In particular, the WRR with 1.09-μm R shown in Fig. 4(a) exhibits ER = ~13.2 dB, IL = ~2.3 dB, FSR = ~106 nm, full width at half maximum (FWHM) = ~30 nm, and Q (=λr/FWHM, where λr is the resonant wavelength) = ~53, respectively.
  • (2) On the other hand, the S4 WRRs shown in Figs. 4(m)4(p) are undercouping as |t| > α. Since the value of |t| increases with R decreasing, the larger-R WRR exhibits a larger ER. In particular, the WRR with 2.59-μm R shown in Fig. 4(p) exhibits ER = ~8.1 dB, IL = ~2.4 dB, FSR = ~52 nm, FWHM = ~22.9 nm, and Q = ~69, respectively.
  • (3) For S2 WRRs shown in Figs. 4(e)4(h) and S3 WRRs shown in Figs. 4(i)4(l), the largest ER is observed in the cases of R = 1.59 μm and R = 2.59 μm, respectively, indicating that the critical coupling condition of |t| ≈α is roughly obtained in these two cases. The WRRs with a larger R are overcoupling whereas the WRRs with a smaller R are undercoupling. In particular, the S2 WRR with 1.59-μm R shown in Fig. 4(f) exhibits ER = ~16.3 dB, IL = ~2.4 dB, FSR = ~76 nm, FWHM = ~24 nm, and Q = ~67, respectively, and the S3 WRR with 2.59-μm R exhibits ER = ~26.2 dB, IL = ~3.3 dB, FSR = ~52 nm, FWHM = ~16 nm, and Q = ~95, respectively.
  • (4) For WRRs with the Si core width narrower than S4, no clear resonant peak is observed in the transmission spectra, even in the case of R = 2.59 μm (not shown here).
  • (5) The plasmonic waveguide with a smaller Si core width has a small neff, in agreement with the result obtained from numerical simulations, as discussed in Section 3.4.

3.2. Plasmonic WRRs with a large gap

Because the |t| value increases quickly with the gap between the bus and ring waveguides increasing, according to the above analysis, it is predicted that the S1 WRRs will have a larger ER, while S4 WRRs will have a smaller ER if the gap of WRRs increases. The plasmonic WRRs with the nominal gap of 250 nm (whereas the other structural parameters keep the same as before) are fabricated and characterized. Again, due to the abovementioned Si core shrinkage, the real gaps (measured at the middle height of Si core) become ~267, ~274, ~297, and ~304 nm for the S1, S2, S3, and S4 WRRs, respectively. The transmission spectra measured on these WRRs are plotted in Fig. 5
Fig. 5 Power transmission spectra for plasmonic WRRs with the same nominal gap of 250 nm but with different Si core widths (S1, S2, S3, and S4) and different ring radii (R = 1.09, 1.59, 2.09, and 2.59 μm). The experimental curves (in black) are normalized by the transmission spectrum of the corresponding 7-μm-long straight plasmonic waveguide. The fitting curves (in red) are based on Eq. (1). The fitting parameters of neff, αb, |t|, and θ are indicated for each plasmonic WRR.
, verifying the above prediction. The same fitting method is used to fit the measured spectra. The fitting curves as well as the fitting parameters for each WRR are shown in Fig. 5 accordingly. We can see that the ER value increases with R increasing for all four kinds of WRRs, indicating that all of these four kinds of WRRs are undercoupling. In particular, the S1 WRR with 2.59-μm R shown in Fig. 5(d) exhibits ER = ~11.5 dB, IL = ~0.1 dB, FSR = ~44 nm, FWHM = ~6.0 nm, and Q = ~260, respectively, and the S2 WRR with 2.59-μm R shown in Fig. 5(h) exhibits ER = ~12.8 dB, IL = ~0.1 dB, FSR = ~47 nm, FWHM = ~5.7 nm, and Q = ~275, respectively. Because the |t| value increases with WP decreasing, the S3 and S4 WRRs exhibit a small ER value, especially for the small-R WRRs whose |t| value is much larger than the α value. Compared with the WRRs shown in Fig. 4, we can see that the insertion loss is significantly reduced and the Q-value is significantly improved with the gap increasing. The property can be mainly attributed to the large |t| value for these WRRs. The large |t| value also results in the moderate ER value for these WRRs. It is expected that the ER value could be significantly improved by adjusting the gap to match the critical coupling condition of t ≈α.

3.3. Plasmonic WRRs with a dual-ring configuration

3.4. Wavelength dependence of the real effective index

The real effective index of the straight plasmonic waveguide calculated using the full-vectorial finite-difference method is ~2.27 at 1550 nm, as shown in Fig. 8(a)
Fig. 8 (a) Electrical field |Ey| distribution of the fundamental quasi-TM mode in the Cu-SiO2-Si hybrid plasmonic waveguide; (b) Real effective index of straight waveguides with different Si core widths versus wavelength, obtained by the 3-D FDTD numerical simulation.
. This value is substantially smaller than those extracted from the above WRRs. For comparison, the horizontal Cu-SiO2-Si-SiO2-Cu plasmonic waveguide has the similar neff value for that obtained from numerical simulation of the straight waveguide theoretically and that extracted from the fabricated WRRs experimentally [19

19. 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]

]. It is well known that the effective index neff in Eqs. (1) and (2) should be replaced by the group index ng if the waveguide is dispersive [14

14. W. Bogaerts, P. De Heyn, T. Van Vaerenbergh, K. De Vos, S. Kumar Selvaraja, T. Claes, P. Dumon, P. Bienstman, D. Van Thourhout, and R. Baets, “Silicon microring resonators,” Laser Photonics Rev. 6(1), 47–73 (2012). [CrossRef]

], which is defined by

ng=neffλdneffdλ
(3)

3.5. Thermo-optical response of plasmonic WRRs

The plasmonic WRRs are measured again when the chip holder is heated to a certain temperature (keeping for a sufficiently long time before measurement to make sure that the holder and the chip have the same temperature). Figure 9
Fig. 9 Transmission spectra of the S1 single-ring WRR with 2.59-μm R and 250-nm nominal gap measured at temperature of 27, 50, 82, and 101°C, respectively.
plots the transmission spectra of the S1 single-ring WRR with 2.59-μm R and 250-nm nominal gap measured at 27°C, 50°C, 82°C, and 101°C, respectively. At 27°C, the central resonance (λr) is peaked at 1536.2 nm, whereas at 82°C, λr is redshifted by ~4.6 nm. This redshift corresponds to an increase of the transmitted power level by ~7.5 dB. λr shifts with temperature almost linearly, as shown in Fig. 10(a)
Fig. 10 (a) Two resonant wavelengths,λr, read from Fig. 9 versus temperature, the thermo-optical coefficient dλr/dT is extracted from linearly fitting; (b) Group index ng of the plasmonic WRR versus temperature, ng is obtained by fitting the spectra in Fig. 9 using Eq. (1), the thermo-optical coefficient dng/dT is then extracted from linearly fitting.
. From linearly fitting the experimental data read from Fig. 9, the TO coefficient (dλr/dT) is estimated to ~80 pm/°C. It is worthy to be noted that this TO coefficient is smaller than that for a conventional strip-Si WRR (~107 pm/°C) but is larger than that for the DLSPP WRR reported in Ref [17

17. H. Hassan, J.-C. Weeber, L. Markey, and A. Dereux, “Thermo-optical control of dielectric loaded plasmonic racetrack resonators,” J. Appl. Phys. 110(2), 023106 (2011). [CrossRef]

]. (~60 pm/°C). In Eq. (1), we can see that λr is determined by ng and θ. Assuming that θ is temperature-independent, the shift of λr is solely contributed by the temperature-induced ng variation. To extract the dng/dT value, the spectra in Fig. 9 are fitted using Eq. (1) by varying the ng value only whereas keeping other fitting parameters of αb, |t|, and θ the same. Figure 10(b) shows that ng depends on temperature almost linearly with the TO coefficient (dng/dT) of ~1.69 × 10−4/°C.

Other plasmonic WRRs are also measured at different temperatures and their transmission spectra are analyzed using the same method. The extracted TO coefficient dng/dT values for these plasmonic WRRs are plotted in Fig. 11
Fig. 11 Thermo-optical coefficients (dng/dT) extracted from different plasmonic WRRs studied in this work, which are extracted from linearly fitting the data points of ng versus temperature, as shown in Fig. 10(b). The standard deviation of the linear fitting is indicated as the error bar.
. The relatively large discrepancy of the dng/dT data may be attributed to the temperature variation (~ ± 1°C) during measurement and the uncertainty of λr read from the spectra, especially for those with large FWHM. Nevertheless, the average dng/dT value of ~1.6 × 10−4 /°C can be estimated from Fig. 11 for all plasmonic WRRs studied in this work. Moreover, the dng/dT values extracted from the dual-ring WRRs is slightly larger than those extracted from the single-ring counterparts. This observation is consistent with the above observation that the ng values extracted from the dual-ring WRRs are larger than those extracted from the single-ring counterparts.

Because both Si and Cu have large TO effect, it is expected that dneff/dT is contributed by both the Si core and the Cu cap. To distinguish the Cu-cap contribution to dneff/dT, the above plasmonic waveguide is simulated again by setting the Si and SiO2 indices at 500°K while the Cu permittivity at 300°K. The calculated neff is ~2.297, which corresponds to the dneff/dT value of ~1.10 × 10−4 /°C. It indicates that the Cu cap provides ~10% contribution in the overall dneff/dT.

As shown in Fig. 8(a), the plasmonic mode is mainly confined in the thin SiO2 layer between the Si core and the Cu cap. However, SiO2 has a very small TO coefficient. It is expected the overall dneff/dT can be significantly improved if this layer is replaced by a dielectric with higher TO coefficient, such as silicon-rich SiO2 [25

25. S. Y. Seo, J. Lee, J. H. Shin, E. S. Kang, and B. S. Bae, “The thermo-optic effect of Si nanocrystals in silicon-rich silicon oxide thin films,” Appl. Phys. Lett. 85(13), 2526–2528 (2004). [CrossRef]

].

To design real thermo-optical devices, a heater should be implemented to control temperature through an external current or voltage. For the Cu-SiO2-Si plasmonic waveguide studied in this work, the Cu layer may be directly used as the heater, but due to the very low resistivity of Cu, the Cu layer should be thin enough to offer a sufficiently large resistance. The other approach is to add an additional layer such as TiN for heating, as the conventional Si-waveguide-based TO devices. An advantage offered by the hybrid plasmonic waveguide is that the TiN layer can be placed just above the Cu cap because the Cu cap can isolate the optical field completely. Combined with the high thermal conductivity of Cu and the ultracompact size of the plasmonic WRRs, TO devices based on the vertical Cu-SiO2-Si plasmonic WRRs should be more effective and fast than those based on the conventional Si-waveguide WRRs.

4. Conclusions

We have experimentally investigated the optical properties and thermo-optical effect of ultracompact plasmonic WRRs based on the Cu-SiO2-Si hybrid plasmonic waveguide. The effect of various structural parameters such as the ring radius, the separation gap, the Si core width, and the ring configuration (i.e., single-ring or dual-ring) on the spectral characteristics of WRRs is evaluated. Some of the demonstrated plasmonic WRRs exhibit very good performance. For example, a single-ring WRR with 2.59-μm R, 156-nm WP, and 250-nm nominal gap exhibits ER of ~12.8 dB, IL of ~0.1 dB, FSR of ~47 nm, FWHM of ~5.7 nm, and Q of ~275, respectively. ER larger than 20 dB has been achieved using the dual-ring configuration. The plasmonic WRRs exhibit a redshift of the resonance of ~4.6 nm for a temperature increase of ~55°C, which leads to a ~7.5-dB variation of the transmitted power. The overall TO coefficient dng/dT is estimated to ~1.6 × 10−4 /°C, which is contributed by the thermo-optical effect of both the Si core and the Cu cap. It is expected that the dng/dT value could be significantly improved if a dielectric with large TO coefficient replaces the thin SiO2 layer between the Si core and the Cu cap. Combined with the Si-CMOS compatibility and the ultracompact size, the Cu-capped hybrid plasmonic WRRs are a very promising element for various effective TO devices for seamless integration in existing Si EPICs.

Acknowledgments

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

References and links

1.

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

2.

S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J. Y. Laluet, and T. W. Ebbesen, “Channel plasmon subwavelength waveguide components including interferometers and ring resonators,” Nature 440(7083), 508–511 (2006). [CrossRef] [PubMed]

3.

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]

4.

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

5.

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]

6.

R. Salvador, A. Martinez, C. Garcia-Meca, R. Ortuno, and J. Marti, “Analysis of hybrid dielectric plasmonic waveguides,” IEEE J. Sel. Top.Quant. Electron. 14, 1496–1501 (2008).

7.

D. Dai and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express 17(19), 16646–16653 (2009). [CrossRef] [PubMed]

8.

H. S. Chu, E. P. Li, P. Bai, and R. Hegde, “Optical performance of single-mode hybrid dielectric-loaded plasmonic waveguide-based components,” Appl. Phys. Lett. 96(22), 221103 (2010). [CrossRef]

9.

H. S. Chu, Y. A. Akimov, P. Bai, and E. P. Li, “Hybrid dielectric-loaded plasmonic waveguide and wavelength selective components for efficiently controlling light at subwavelength scale,” J. Opt. Soc. Am. B 28(12), 2895–2901 (2011). [CrossRef]

10.

M. Wu, Z. Han, and V. Van, “Conductor-gap-silicon plasmonic waveguides and passive components as subwavelength scale,” Opt. Express 18(11), 11728–11736 (2010). [CrossRef] [PubMed]

11.

I. Goykhman, B. Desiatov, and B. Levy, “Experimental demonstration of locally oxidized hybrid silicon-plasmonic waveguide,” Appl. Phys. Lett. 97(14), 141106 (2010). [CrossRef]

12.

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]

13.

S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Monolithic integration of hybrid plasmonic waveguide components into a fully CMOS-compatible SOI platform,” IEEE Photon. Technol. Lett. (Accepted).

14.

W. Bogaerts, P. De Heyn, T. Van Vaerenbergh, K. De Vos, S. Kumar Selvaraja, T. Claes, P. Dumon, P. Bienstman, D. Van Thourhout, and R. Baets, “Silicon microring resonators,” Laser Photonics Rev. 6(1), 47–73 (2012). [CrossRef]

15.

S. Randhawa, S. Lachèze, J. Renger, A. Bouhelier, R. E. de Lamaestre, A. Dereux, and R. Quidant, “Performance of electro-optical plasmonic ring resonators at telecom wavelengths,” Opt. Express 20(3), 2354–2362 (2012). [CrossRef] [PubMed]

16.

R. M. Briggs, J. Grandidier, S. P. Burgos, E. Feigenbaum, and H. A. Atwater, “Efficient coupling between dielectric-loaded plasmonic and silicon photonic waveguides,” Nano Lett. 10(12), 4851–4857 (2010). [CrossRef] [PubMed]

17.

H. Hassan, J.-C. Weeber, L. Markey, and A. Dereux, “Thermo-optical control of dielectric loaded plasmonic racetrack resonators,” J. Appl. Phys. 110(2), 023106 (2011). [CrossRef]

18.

B. Min, E. Ostby, V. Sorger, E. Ulin-Avila, L. Yang, X. Zhang, and K. Vahala, “High-Q surface-plasmon-polariton whispering-gallery microcavity,” Nature 457(7228), 455–458 (2009). [CrossRef] [PubMed]

19.

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]

20.

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]

21.

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

22.

A. Yariv, “Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett. 36(4), 321–322 (2000). [CrossRef]

23.

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]

24.

S. Y. 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(6), 5867–5881 (2012). [CrossRef] [PubMed]

25.

S. Y. Seo, J. Lee, J. H. Shin, E. S. Kang, and B. S. Bae, “The thermo-optic effect of Si nanocrystals in silicon-rich silicon oxide thin films,” Appl. Phys. Lett. 85(13), 2526–2528 (2004). [CrossRef]

OCIS Codes
(160.6840) Materials : Thermo-optical materials
(240.6680) Optics at surfaces : Surface plasmons
(250.5300) Optoelectronics : Photonic integrated circuits
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Integrated Optics

History
Original Manuscript: March 26, 2012
Revised Manuscript: May 5, 2012
Manuscript Accepted: May 8, 2012
Published: June 22, 2012

Citation
Shiyang Zhu, G. Q. Lo, and D. L. Kwong, "Performance of ultracompact copper-capped silicon hybrid plasmonic waveguide-ring resonators at telecom wavelengths," Opt. Express 20, 15232-15246 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-14-15232


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. D. K. Gramotnev and S. I. Bozhevolnyi, “Plasmonics beyond the diffraction limit,” Nat. Photonics4(2), 83–91 (2010). [CrossRef]
  2. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J. Y. Laluet, and T. W. Ebbesen, “Channel plasmon subwavelength waveguide components including interferometers and ring resonators,” Nature440(7083), 508–511 (2006). [CrossRef] [PubMed]
  3. 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]
  4. T. Holmgaard and S. I. Bozhevolnyi, “Theoretical analysis of dielectric-loaded surface plasmon-polariton waveguide,” Phys. Rev. B75(24), 245405 (2007). [CrossRef]
  5. 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]
  6. R. Salvador, A. Martinez, C. Garcia-Meca, R. Ortuno, and J. Marti, “Analysis of hybrid dielectric plasmonic waveguides,” IEEE J. Sel. Top.Quant. Electron.14, 1496–1501 (2008).
  7. D. Dai and S. He, “A silicon-based hybrid plasmonic waveguide with a metal cap for a nano-scale light confinement,” Opt. Express17(19), 16646–16653 (2009). [CrossRef] [PubMed]
  8. H. S. Chu, E. P. Li, P. Bai, and R. Hegde, “Optical performance of single-mode hybrid dielectric-loaded plasmonic waveguide-based components,” Appl. Phys. Lett.96(22), 221103 (2010). [CrossRef]
  9. H. S. Chu, Y. A. Akimov, P. Bai, and E. P. Li, “Hybrid dielectric-loaded plasmonic waveguide and wavelength selective components for efficiently controlling light at subwavelength scale,” J. Opt. Soc. Am. B28(12), 2895–2901 (2011). [CrossRef]
  10. M. Wu, Z. Han, and V. Van, “Conductor-gap-silicon plasmonic waveguides and passive components as subwavelength scale,” Opt. Express18(11), 11728–11736 (2010). [CrossRef] [PubMed]
  11. I. Goykhman, B. Desiatov, and B. Levy, “Experimental demonstration of locally oxidized hybrid silicon-plasmonic waveguide,” Appl. Phys. Lett.97(14), 141106 (2010). [CrossRef]
  12. 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]
  13. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Monolithic integration of hybrid plasmonic waveguide components into a fully CMOS-compatible SOI platform,” IEEE Photon. Technol. Lett. (Accepted).
  14. W. Bogaerts, P. De Heyn, T. Van Vaerenbergh, K. De Vos, S. Kumar Selvaraja, T. Claes, P. Dumon, P. Bienstman, D. Van Thourhout, and R. Baets, “Silicon microring resonators,” Laser Photonics Rev.6(1), 47–73 (2012). [CrossRef]
  15. S. Randhawa, S. Lachèze, J. Renger, A. Bouhelier, R. E. de Lamaestre, A. Dereux, and R. Quidant, “Performance of electro-optical plasmonic ring resonators at telecom wavelengths,” Opt. Express20(3), 2354–2362 (2012). [CrossRef] [PubMed]
  16. R. M. Briggs, J. Grandidier, S. P. Burgos, E. Feigenbaum, and H. A. Atwater, “Efficient coupling between dielectric-loaded plasmonic and silicon photonic waveguides,” Nano Lett.10(12), 4851–4857 (2010). [CrossRef] [PubMed]
  17. H. Hassan, J.-C. Weeber, L. Markey, and A. Dereux, “Thermo-optical control of dielectric loaded plasmonic racetrack resonators,” J. Appl. Phys.110(2), 023106 (2011). [CrossRef]
  18. B. Min, E. Ostby, V. Sorger, E. Ulin-Avila, L. Yang, X. Zhang, and K. Vahala, “High-Q surface-plasmon-polariton whispering-gallery microcavity,” Nature457(7228), 455–458 (2009). [CrossRef] [PubMed]
  19. 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]
  20. 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]
  21. S. Roberts, “Optical properties of copper,” Phys. Rev.118(6), 1509–1518 (1960). [CrossRef]
  22. A. Yariv, “Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett.36(4), 321–322 (2000). [CrossRef]
  23. 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]
  24. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Components for silicon plasmonic nanocircuits based on horizontal Cu-SiO2-Si-SiO2-Cu nanoplasmonic waveguides,” Opt. Express20(6), 5867–5881 (2012). [CrossRef] [PubMed]
  25. S. Y. Seo, J. Lee, J. H. Shin, E. S. Kang, and B. S. Bae, “The thermo-optic effect of Si nanocrystals in silicon-rich silicon oxide thin films,” Appl. Phys. Lett.85(13), 2526–2528 (2004). [CrossRef]

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