REPORT◥

OPTOELECTRONICS

High-speed plasmonic modulator in asingle metal layerMasafumi Ayata,1* Yuriy Fedoryshyn,1 Wolfgang Heni,1 Benedikt Baeuerle,1

Arne Josten,1 Marco Zahner,1 Ueli Koch,1 Yannick Salamin,1 Claudia Hoessbacher,1

Christian Haffner,1 Delwin L. Elder,2 Larry R. Dalton,2 Juerg Leuthold1*

Plasmonics provides a possible route to overcome both the speed limitations of electronics andthe critical dimensions of photonics.We present an all-plasmonic 116–gigabits per second electro-optical modulator in which all the elements—the vertical grating couplers, splitters, polarizationrotators, and active section with phase shifters—are included in a single metal layer.The devicecan be realized on any smooth substrate surface and operates with low energy consumption.Our results show that plasmonics is indeed a viable path to an ultracompact, highest-speed, andlow-cost technology that might find many applications in a wide range of fields of sensing andcommunications because it is compatible with and can be placed on a wide variety of materials.

Applied plasmonics has been proposed as analternative to conventional photonics andelectronics to overcome their bottlenecks interms of speed and energy efficiency, aswell as manufacturing cost and footprint

(1–3). A large variety of plasmonic structures anddevices—including passive waveguides and activecomponents such as light emitters, detectors, andmodulators—have been investigated and demon-strated (4–22). Plasmonic devices are continu-ously gaining interest for applications rangingfrom sensing to telecommunications (23–28).Among the many plasmonic devices, the opti-

cal modulator plays a central role by mapping areceived electrical signal onto an optical carrier(29). Various types of plasmonic modulators havebeen introduced that exploit the free-carrier dis-persion effect (11–15), thermo-optic effect (16, 17 ),mechanical effect (18, 19), or Pockels effect (20–22)with the strong confinement of light in the activeregion so that they can be operated on a smalldevice footprint. In telecommunications, thePockels-effect Mach-Zehnder modulators havebeen preferred because they allow one to encodeelectrical information either onto the phase orthe amplitude of an optical carrier, thus making ita key building block of any state-of-the-art mod-ulator for advanced modulation formats (30).To date, Pockels-effect plasmonic devices havedemonstrated operation beyond 100 Gbit/s withpower consumption of 25 fJ/bit (21, 31). However,these devices rely partially on photonic wave-guides to bring light onto the chip, which resultsin a large footprint and requires dedicated sub-strates. An all-metallic Mach-Zehnder modulatorthat could be placed on almost any surface and

that could couple the optical signal directly to afiber would mark quite an advance.We present an integrated plasmonic Mach-

Zehnder modulator that only relies on metallicstructures fabricated on a glass layer and per-forms high-speed modulation. The entire deviceis produced in a single metal layer covered witha nonlinear optical material, and it occupies anarea of 36 mm by 6 mm. High-speed modulationenables data transmission beyond 116 Gbit/s ata bit-error ratio (BER) of less than 1.7 × 10−3.The input and output optical signals are fedin and fed out through a single multicore fiber(MCF) via a new coupler that performs verticalgrating coupling, polarization rotation, and split-ting in one compact metallic structure so thatdevices can be arranged with a pitch only limit-ed by the distance of the next fiber core.The all-metallic high-speed plasmonic mod-

ulator is arranged in a Mach-Zehnder interfer-ometer (MZI) configuration with vertical metallicgrating couplers and polarization rotators (Fig. 1).Modulation occurs in the slots of the two MZIarms, which are filled with a nonlinear optical(NLO) material. When electrical signals are fedto the central part of the device (the grating cou-pler serves as a signal pad that becomes the innerplasmonic rail, whereas the outer plasmonic railsalso serve as ground pads), electric fields are ap-plied across the two slots in opposite directions.Due to the Pockels effect, the light propagatesfaster in one of the MZI arms and slower in theother, depending on the directions of the appliedelectric fields. At the output of the MZI, both por-tions of the optical carrier from the two arms arerotated back to p-polarization while having con-structive or destructive interference based on thesign of the applied electrical signals. The opticalsignal, now encoded with information, is then cou-pled out to another core of the MCF via the outputvertical metallic grating coupler. The entire device,comprising the vertical grating couplers, the po-

larization rotators, and the plasmonic Mach-Zehnder phase shifters, can be realized in a singlemetal layer (Fig. 2A), and it occupies an area de-fined by the distance between two cores of a MCF.In an arrangement where space matters and

the size of devices is far below a fiber diameter,a multicore fiber offers a solution for couplinglight in and out. However, efficient coupling witha multicore fiber requires a vertical couplingscheme rather than the conventional tilted cou-pling schemes for taking advantage of the shortestpossible pitch (14, 19, 32). Yet, coupling from avertically aligned fiber mode with a 6-mm diam-eter to a plasmonic waveguide mode of a 100-nmwidth with a high efficiency is challenging. Con-ventional one-step-etch gratings would equallymap the vertical-incident light into the forwardand backward propagating modes (fig. S1). Toenhance the coupling efficiency, we favor stair-case gratings and break the horizontal symmetry(Fig. 2B). The staircase grating is composed oftwo diffraction points, creating waves that inter-fere constructively only in one direction but de-structively in the other direction (33, 34).We should note that only the p-polarized light

can be coupled to the surface plasmon modewith a metallic grating coupler. For an efficientoperation of the plasmonicmodulator, however,it is necessary to squeeze the plasmonicmode asa gap plasmon into ametal-insulator-metal (MIM)waveguide (14, 18–21). Therefore, the polarizationhas to be rotated from p to s polarization. Afterlight is coupled to the device via the grating cou-pler, the surface charges distribute mainly on thetop surface of the central part of the waveguide,leading to p-polarization (Fig. 2C). The ratio be-tween the two polarizations of the propagatinglight can be controlled by changing the widthsof the signal padws and the gap wg of the wave-guide (34). Therefore, whenws andwg decrease,the surface charges move along the sides andthe s-polarization becomes dominant (Fig. 2D).Finally, the propagating mode is converted intotwo MIM waveguide modes with s-polarizationby increasing ws (Fig. 2E). In combination witha focusing structure for the grating coupler, boththe grating coupler and the polarization rotatorcan be arranged within a length of 15 mm.The devices were fabricated in a single layer of

gold with a thickness of 200 nm and patterned byelectron-beam lithography followed by dry etch-ing of the metal. By replacing gold with copper,the device can be fabricated in a complementarymetal-oxide semiconductor–compatible process.The devices were spin-coated with a nonlinearoptical material (35), and poling processes wererealized by applying a voltage between the groundelectrodes. A detailed discussion on fabrication isprovided in the supplementary materials (34).Figure 3A shows the optical power transmission

spectrum for different bias voltages, and Fig. 3Bgives theoptical transfer functionversus the appliedvoltage. An extinction ratio of more than 15 dBhas been found when switching from –6 to +6 V.The optical power transmission spectrumvarieswith the wavelength, and the spectrum showsa peak power around 1.55 mm (Fig. 3A). This

RESEARCH

Ayata et al., Science 358, 630–632 (2017) 3 November 2017 1 of 3

1ETH Zurich, Institute of Electromagnetic Fields (IEF), 8092Zurich, Switzerland. 2University of Washington, Departmentof Chemistry, Seattle, WA 98195-1700, USA.*Corresponding author. Email: mayata@ethz.ch (M.A.);juergleuthold@ethz.ch (J.L.)

on Septem

ber 30, 2020

http://science.sciencemag.org/

Dow

nloaded from

frequency dependence is due to the metallicgratings, which are optimized for that peakwavelength. Moreover, the spectrum featuresa wavelength dependence because of a built-inasymmetry in the two arms of the waveguides.Comparing the experimental transfer functionin Fig. 3B with simulations, we extract the r33electro-optic coefficient of the material and findthat it is on the order of 100 pm/V ± 15 pm/V.Such a high extinction ratio with this voltagewithin only a few mm can be achievable becausethe device is arranged under an open-circuitMZIconfiguration in combination with a very stronglight-matter interaction and a slow-down effectof plasmonic mode in the slots (21).The frequency response of the device (Fig. 3C)

exhibits no speed limitation up to 70 GHz, whichis the highest radio frequency (RF) signal currentlyavailable in our laboratory. The modulation band-width of a modulator can be estimated by the RCconstant (the product of the circuit resistance andcapacitance). Due to a very small capacitance (14 fF)(see the supplementary materials) and small re-sistances of the device, the device should not beconstrained for operation beyond 200 GHz. Thisindicates that the modulator is capable of deal-ing with fast electrical signals and thus should beable to provide the highest data transmission.High-speed data experiments of the device have

been performed using a continuous-wave (CW)optical signal at a wavelength of 1558 nm fedinto the device (Fig. 4A). An arbitrary waveformgenerator was used to generate the 72 Gbit/stwo-level pulse-amplitude modulation (PAM2)and 116 Gbit/s PAM4 modulation formats, re-spectively. This high-speed electrical signal wasguided through electrical prober needles to theexternal electrical contact pads of the plasmonicmodulator. The electrical signal was then en-coded onto the surface plasmonic polariton,which was fed back to the multicore fiber. Thetransmitted signal was amplified and sampledby a coherent receiver before the digitized datawere processed offline. The wide and open eyediagram with clearly distinct constellations ofa PAM2 signal at 72 Gbit/s can be seen in Fig. 4B.A low BER of less than 5.56 × 10−6 was measuredat this signal rate. The device was also tested forits performance with a higher-order modulationformat. A PAM4 signal with a symbol rate of58 GBd and a data rate of 116 Gbit/s was trans-mitted and detected with a BER of 1.68 × 10−3.Both BERs are below the hard decision forwarderror correction (HD-FEC) limit of 3.8 × 10−3,with a 7% overhead. Another important figureis the energy consumption of the modulator. As-suming that the energy consumption for a tran-sition from one signal to the next costs CV2/2 (36),the estimated energy consumption is 110 fJ/bitwith a driving voltage of ±2.8 V at 72 Gbit/s forthe PAM2 signal. If the advanced PAM4 signalis applied, the energy consumption drops to30 fJ/bit at 116 Gbit/s (37). The fiber-to-fiber losseswere found to be ~30 dB rather than 17 dB aspredicted by simulations (34). These excess lossesare related to the applied fabrication technologyand can be mitigated. Likewise, there is room for

Ayata et al., Science 358, 630–632 (2017) 3 November 2017 2 of 3

Fig. 1. All-metallic high-speed modulator. Schematic of the operation principle of an all-metallicmodulator in a MZI configuration with vertical metallic grating couplers and polarization rotators.A continuous-wave optical carrier is coupled into the device through the vertical metallic gratingcoupler from one core of the multicore fiber. The polarization of the optical carrier is rotated fromp-polarization (p-pol) to s-polarization (s-pol) and split into the arms of the MZI in the polarizationrotator section. The electrical information is then encoded by means of a phase shift through thePockels effect onto the two optical signals in the MZI arms. At the output, the two optical signalsfrom the arms interfere either constructively or destructively, and they are coupled back to anothercore of the multicore fiber.

Fig. 2. Vertical grating coupler and polarization rotator of the all-metallic device. (A) False-color scanning electron micrograph of the device fabricated in a single 200-nm-thick layer of gold(colored in yellow) deposited on thermally grown glass layer (dark blue). (B) Magnified image ofa vertical metallic grating coupler with staircase gratings. The polarization rotation in the all-metallicdevice is illustrated by means of the plots of the electric field distributions. (C) A p-polarized surfaceplasmon polariton (SPP) on the top surface of the device after coupling through the verticalmetallic grating coupler. (D) Conversion from p-polarized into s-polarized SPP mode in thepolarization rotator section. (E) s-polarized SPP mode coupled into the metallic slot waveguidesbehind the polarization rotator section. The positions of the cross sections are shown in (A).

RESEARCH | REPORTon S

eptember 30, 2020

http://science.sciencem

ag.org/D

ownloaded from

further reducing the losses, as discussed in thesupplementary materials.Theworkpresentedhere shows that all-metallic

devices can replace advanced optical configura-tions. Our all-metallic approach allows for a sim-

ple fabrication process with just one single metallayer and relaxed manufacturing requirementsthat addresses both operation speed and devicefootprint. The vertical multicore coupling solu-tion enables a practical and dense way to direct-

ly interface plasmonics with electronics and mightbe an interesting packaging solution far beyondcommunications and sensing.

REFERENCES AND NOTES

1. E. Ozbay, Science 311, 189–193 (2006).2. H. A. Atwater, Sci. Am. 296, 56–63 (2007).3. M. L. Brongersma, V. M. Shalaev, Science 328, 440–441

(2010).4. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J. Y. Laluet,

T. W. Ebbesen, Nature 440, 508–511 (2006).5. R. F. Oulton et al., Nature 461, 629–632 (2009).6. E. Bermúdez-Ureña et al., Nano Lett. 17, 747–754 (2017).7. A. L. Falk et al., Nat. Phys. 5, 475–479 (2009).8. M. W. Knight, H. Sobhani, P. Nordlander, N. J. Halas, Science

332, 702–704 (2011).9. I. Goykhman, B. Desiatov, J. Khurgin, J. Shappir, U. Levy,

Nano Lett. 11, 2219–2224 (2011).10. S. Muehlbrandt et al., Optica 3, 741–747 (2016).11. J. A. Dionne, K. Diest, L. A. Sweatlock, H. A. Atwater, Nano Lett.

9, 897–902 (2009).12. V. J. Sorger, N. D. Lanzillotti-Kimura, R. M. Ma, X. Zhang,

Nanophotonics 1, 17–22 (2012).13. S. Zhu, G. Q. Lo, D. L. Kwong, Opt. Express 21, 8320–8330

(2013).14. H. W. Lee et al., Nano Lett. 14, 6463–6468 (2014).15. A. Olivieri et al., Nano Lett. 15, 2304–2311 (2015).16. F. H. Ren, X. Y. Wang, A. X. Wang, Appl. Phys. Lett. 102,

181101 (2013).17. J. T. Kim, Opt. Lett. 39, 3997–4000 (2014).18. S. Randhawa et al., Opt. Express 20, 2354–2362 (2012).19. B. S. Dennis et al., Nat. Photonics 9, 267–273 (2015).20. A. Melikyan et al., Nat. Photonics 8, 229–233 (2014).21. C. Haffner et al., Nat. Photonics 9, 525–528 (2015).22. F. H. Ren et al., Opt. Commun. 352, 116–120 (2015).23. J. Mitchell, Sensors 10, 7323–7346 (2010).24. K. F. MacDonald, Z. L. Samson, M. I. Stockman,

N. I. Zheludev, Nat. Photonics 3, 55–58 (2009).25. V. J. Sorger, R. F. Oulton, R. M. Ma, X. Zhang, MRS Bull. 37,

728–738 (2012).26. K. C. Y. Huang et al., Nat. Photonics 8, 244–249 (2014).27. D. Rodrigo et al., Science 349, 165–168 (2015).28. A. V. Krasavin, A. V. Zayats, Proc. IEEE 104, 2338–2348

(2016).29. G. T. Reed, G. Mashanovich, F. Y. Gardes, D. J. Thomson,

Nat. Photonics 4, 518–526 (2010).30. P. J. Winzer, R. J. Essiambre, J. Lightwave Technol. 24,

4711–4728 (2006).31. C. Hoessbacher et al., Opt. Express 25, 1762–1768

(2017).32. A. Kriesch et al., Nano Lett. 13, 4539–4545 (2013).33. W. Yao et al., Nano Lett. 15, 3115–3121 (2015).34. See supporting information in the supplementary materials.35. D. L. Elder et al., Chem. Mater. 29, 6457–6471 (2017).36. D. A. B. Miller, Proc. IEEE 97, 1166–1185 (2009).37. C. Haffner et al., Proc. IEEE 104, 2362–2379 (2016).

ACKNOWLEDGMENTS

This work was partially carried out at the Binnig and RohrerNanotechnology Center. We acknowledge support from EuropeanUnion grant 688166 PLASMOfab and European Research Councilgrant 670478 PLASILOR. We thank A. Messner for support ontemperature stability tests. Additional data can be found in thesupplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/358/6363/630/suppl/DC1Supplementary TextFigs. S1 to S5Tables S1 and S2References (38–51)

5 May 2017; accepted 26 September 201710.1126/science.aan5953

Ayata et al., Science 358, 630–632 (2017) 3 November 2017 3 of 3

V = +6V

V = -6V

Exp.Fit.

Fig. 4. High-speed data transmission experiment. (A) Schematic of the data transmissionexperiment. (B) The eye diagrams and constellation diagrams of the 72 GBd PAM2 and 58 GBd PAM4signals corresponding to a 72 Gbit/s and 116 Gbit/s data stream.

Fig. 3. Characterization of theall-metallic modulator.(A) Optical power transmissionspectrum for different biasvoltages. (B) Optical powertransfer function of the deviceover the applied voltage.(C) Normalized frequencyresponse of the device.

RESEARCH | REPORTon S

eptember 30, 2020

http://science.sciencem

ag.org/D

ownloaded from

High-speed plasmonic modulator in a single metal layer

Salamin, Claudia Hoessbacher, Christian Haffner, Delwin L. Elder, Larry R. Dalton and Juerg LeutholdMasafumi Ayata, Yuriy Fedoryshyn, Wolfgang Heni, Benedikt Baeuerle, Arne Josten, Marco Zahner, Ueli Koch, Yannick

DOI: 10.1126/science.aan5953 (6363), 630-632.358Science 

, this issue p. 630ScienceGHz, which could provide a flexible platform for future ultrafast plasmonic technology.a device with a footprint less than the cross-sectional area of a human hair and with modulation rates exceeding 100

fabricated a plasmonic modulator from a single layer of gold using a substrate-independent process. They createdet al.components to the nanometer scale, far below the hundreds of wavelengths typically set by conventional optics. Ayata

Plasmonics converts light into propagating electrical signals. This approach could allow us to shrink opticalUltrafast plasmonic modulation

ARTICLE TOOLS http://science.sciencemag.org/content/358/6363/630

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2017/11/02/358.6363.630.DC1

REFERENCES

http://science.sciencemag.org/content/358/6363/630#BIBLThis article cites 46 articles, 7 of which you can access for free

PERMISSIONS http://www.sciencemag.org/help/reprints-and-permissions

Terms of ServiceUse of this article is subject to the

is a registered trademark of AAAS.ScienceScience, 1200 New York Avenue NW, Washington, DC 20005. The title (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement ofScience

Science. No claim to original U.S. Government WorksCopyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of

on Septem

ber 30, 2020

http://science.sciencemag.org/

Dow

nloaded from