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Low-loss plasmon-assisted electro-optic modulator

Abstract

For nearly two decades, researchers in the field of plasmonics1—which studies the coupling of electromagnetic waves to the motion of free electrons near the surface of a metal2—have sought to realize subwavelength optical devices for information technology3,4,5,6, sensing7,8, nonlinear optics9,10, optical nanotweezers11 and biomedical applications12. However, the electron motion generates heat through ohmic losses. Although this heat is desirable for some applications such as photo-thermal therapy, it is a disadvantage in plasmonic devices for sensing and information technology13 and has led to a widespread view that plasmonics is too lossy to be practical. Here we demonstrate that the ohmic losses can be bypassed by using ‘resonant switching’. In the proposed approach, light is coupled to the lossy surface plasmon polaritons only in the device’s off state (in resonance) in which attenuation is desired, to ensure large extinction ratios between the on and off states and allow subpicosecond switching. In the on state (out of resonance), destructive interference prevents the light from coupling to the lossy plasmonic section of a device. To validate the approach, we fabricated a plasmonic electro-optic ring modulator. The experiments confirm that low on-chip optical losses, operation at over 100 gigahertz, good energy efficiency, low thermal drift and a compact footprint can be combined in a single device. Our result illustrates that plasmonics has the potential to enable fast, compact on-chip sensing and communications technologies.

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Fig. 1: False-coloured scanning electron microscopy (SEM) image of a plasmonic ring resonator and the corresponding transmittance.
Fig. 2: Theoretical loss advantage of critical coupled resonant device over non-resonant push–pull MZ device.
Fig. 3: Sensitivity and stability of the plasmonic resonator.
Fig. 4: High-speed data experiments with a plasmonic ring resonator used as an electro-optic modulator.

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Acknowledgements

We thank U. Drechsler and H.-R. Benedickter for their technical assistance. We acknowledge partial funding of this project by the EU Project PLASMOFAB (688166), by ERC grant PLASILOR (640478), by the National Science Foundation (NSF) (DMR-1303080) and by the Air Force Office of Scientific Research grants (FA9550-15-1-0319 and FA9550-17-1-0243). N.K. acknowledges support from the Virginia Microelectronics Consortium and the Virginia Commonwealth University Presidential Research Quest Fund. This work was carried out at the BRNC Zurich and ETH Zurich.

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Nature thanks J. Khurgin and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors

Contributions

C.H., N.K., V.M.S., A.B. and J.L. conceived the concept and supervised the project. C.H., D.C., S.S. and T.C. designed the modulator and developed the analytical framework for fast optimization. T.W. designed the photonic grating coupler. C.H., D.C. and Y.F. fabricated the modulator and developed the required process technology. B.C. developed a focused ion beam process to image the cross-section with minimal destructive influence on the suspended bridge. D.L.E., W.H., C.H. and L.R.D. developed, synthesized and implemented the poling procedure of the OEO material for plasmonic ring resonators. C.H. and J.L. designed the experiments. C.H., D.C. and T.C. performed the passive characterization. C.H. performed the temperature sensitivity, d.c. switching and electro-optic bandwidth experiments. B.B., A.J. and C.H. performed the high-speed data experiment. B.B. and A.J. designed, calibrated and automated the high-speed data experiment. B.B. and A.J. developed the digital-signal processing for data generation and analysis of the high-speed data experiment. All authors discussed and analysed the data. C.H., N.K., D.C. and J.L. wrote the manuscript.

Corresponding authors

Correspondence to Christian Haffner or Juerg Leuthold.

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Extended data figures and tables

Extended Data Fig. 1 Resonator performance for various plasmonic materials.

a, Gold, which is interesting for research because of its chemical stability. b, Copper is of interest as it is a CMOS-compatible material. c, Silver features the best plasmonic properties and could be of interest for high-performance applications. d, e, Switching capability of (d) Au and (e) Ag ring resonator for a 2 V bias. The latter uses the newest OEO material, which has a three times larger electro-optic coefficient, r33. The performance improvement enables a considerable reduction in terms of the driving voltage. The number at the bottom right indicates the shift in the resonance wavelength.

Extended Data Fig. 2 Q-factors of various materials filling the slot.

ah, The materials differ in their refractive index, and one can observe that low-n materials are limited by bending loss (diagonal lines) whereas high-n materials are limited by propagation loss (parallel lines). These simulations were performed with 150 nm height of the outer and inner electrode to account for limitations in fabrication processes different from ours.

Extended Data Fig. 3 Tilted SEM image of a processed ring resonator.

The different height of the outer and inner electrodes reduces the bending losses.

Extended Data Fig. 4 Reproducibility of plasmonic ring resonators.

a, Insertion loss and b, extinction ratio histograms. Data are obtained from passive measurements of 23 devices with a designed slot width of 80 nm and radii ranging from 900 nm to 1,100 nm. c, Dependence of the resonance wavelength on ring radius.

Extended Data Fig. 5 Transmission spectrum and the measured bandwidth at the off-resonance, 3 dB and on-resonance operating point.

a, Transmission spectrum; b, measured bandwidth. No bandwidth limitation can be observed up to 110 GHz. The drop at 115 GHz frequencies is due to a limited measurement set-up. Recent studies show that the modulation efficiency at lower radiofrequency is not limited44.

Extended Data Fig. 6 Technology overview in terms of insertion loss and bandwidth of electro-optic modulators.

Ideal candidates should feature low insertion loss with high electro-optic bandwidths.

Extended Data Table 1 Measured d.c. sensitivity
Extended Data Table 2 Overview of the results obtained from data experiments with various devices
Extended Data Table 3 Comparison with state-of-the-art plasmonic electro-optic modulators
Extended Data Table 4 Comparison with state-of-the-art photonic electro-optic modulators

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data, Supplementary Figures 1-10, Supplementary Tables 1-2 and Supplementary References – see contents page for details

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Haffner, C., Chelladurai, D., Fedoryshyn, Y. et al. Low-loss plasmon-assisted electro-optic modulator. Nature 556, 483–486 (2018). https://doi.org/10.1038/s41586-018-0031-4

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