Optical modulators encode electrical signals to the optical domain and thus constitute a key element in high-capacity communication links 1,2 . Ideally, they should feature operation at the highest speed with the least power consumption on the smallest footprint, and at low cost 3 . Unfortunately, current technologies fall short of these criteria 4 . Recently, plasmonics has emerged as a solution offering compact and fast devices 5-7 . Yet, practical implementations have turned out to be rather elusive.Here, we introduce a 70 GHz all-plasmonic Mach-Zehnder modulator that fits into a silicon waveguide of 10 μm length. This dramatic reduction in size by more than two orders of magnitude compared with photonic Mach-Zehnder modulators results in a low energy consumption of 25 fJ per bit up to the highest speeds. The technology suggests a cheap co-integration with electronics.Mach-Zehnder modulators (MZMs) are the most versatile electro-optical converters in high-end communication systems. MZMs are unique, as they can be used to encode multiple bits within one symbol with the highest quality. They are thus instrumental in increasing the capacity of modern communication links 1 . Until now, MZMs have mostly been based on the lithium niobate material system, which requires footprints on the order of cm 2 . Recently, more compact silicon-based modulators have emerged. These devices have already shown operation at bandwidths up to 55 GHz (ref. 8), they are cost-effective, and they feature lengths on the order of hundreds of micrometres to millimetres 2,3,8-12 . Yet, complementary metal-oxide semiconductor electronics (CMOS) house hundreds of transistors on a single μm 2 , making a co-integration of today's silicon MZMs with CMOS electronics impractical 4 . In pursuit of more compact silicon modulators, various approaches have been demonstrated, such as resonant silicon ring modulators 13,14 or germanium-based electro-absorption modulators 15,16 . However, encoding advanced modulation formats is challenging 17 , and high-capacity transmission has, so far, only been achieved with MZMs 2,12 . Instead, plasmonics has drawn significant interest as an alternative solution 6,7 . In plasmonics, optical signals are converted to surface plasmon polaritons (SPPs) propagating at metal-dielectric interfaces, where they can be confined below the diffraction limit of optics 18 . This means that plasmonic devices require only a few µm 2 of footprint 19,20 . With such reduced dimensions, the technology is much closer to bridging the size gap with respect to CMOS electronics. Furthermore, there are various theoretical studies indicating that plasmonic MZMs should offer hundreds of gigahertz of bandwidth 5,21 . To date, however, there is very little experimental evidence to support this claim. Recently, a plasmonic phase modulator demonstrated operation at 40 Gbit s −1 (ref. 22). One could now envision integrating such plasmonic phase modulators into a silicon waveguide MZM configuration. However, by combining plasmonics and silicon photoni...

Graphene has shown great potentials for high-speed photodetection. Yet, the responsivities of graphene-based high-speed photodetectors are commonly limited by the weak effective absorption of atomically thin graphene. Here, we propose and experimentally demonstrate a plasmonically enhanced waveguide-integrated graphene photodetector. The device which combines a 6 m long layer of graphene with field-enhancing nano-sized metallic structures, demonstrates a high external responsivity of 0.5 A/W and a fast photoresponse way beyond 110 GHz. The high efficiency and fast response of the device enables for the first time 100 Gbit/s PAM-2 and 100 Gbit/s PAM-4 data reception with a graphene based device. The results show the potential of graphene as a new technology for highest-speed communication applications.

Intensive efforts have been devoted to exploit novel optoelectronic devices based on two-dimensional (2D) transition-metal dichalcogenides (TMDCs) owing to their strong lightmatter interaction and distinctive material properties [1,2]. In particular, photodetectors featuring both high-speed and high-responsivity performance are of great interest for a vast number of applications such as high-data-rate interconnects operated at standardized telecom wavelengths [3,4]. Yet, the intrinsically small carrier mobilities of TMDCs become a bottleneck for high-speed application use [5].Here, we present high-performance vertical van der Waals heterostructure-based photodetectors integrated on a silicon photonics platform. Our vertical MoTe 2 /graphene heterostructure design minimizes the carrier transit path length in TMDCs and enables a record-high measured bandwidth of at least 24 GHz under a moderate bias voltage of −3 volts. Applying a higher bias or employing thinner MoTe 2 flakes boosts the bandwidth even to 50 GHz. Simultaneously, our device reaches a high external responsivity of 0.2 A/W for incident light at 1300 nm, benefiting from the integrated waveguide design.Our studies shed light on performance trade-offs and present design guidelines for fast and efficient devices. The combination of 2D heterostructures and integrated guided-wave nano photonics defines an attractive platform to realize highperformance optoelectronic devices [6][7][8], such as photodetectors [9], light-emitting devices [10] and electro-optic modulators [11].During the last decade, two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides (TMDCs) have shown great promise for a wide range of photonic and optoelectronic applications [12][13][14]. 2D devices have the potential to outperform established and more mature technologies, particularly in terms of form * These authors contributed equally. † leuthold@ethz.ch ‡ lnovotny@ethz.ch factor, operating conditions and cost-effectiveness. The possibility to integrate 2D materials without constraints of crystal lattice matching is disruptive, as it tremendously simplifies manufacturing and increases possible material combinations. Graphene, which has been widely used for successful 2D device implementations [15][16][17][18][19][20], has an intrinsically weak photosensitivity, though its interaction with light can be enhanced using silicon-based integrated photonics, such as optical resonators [17] or waveguides [21]. Nevertheless, graphene-based devices suffer from other issues stemming from its gapless nature, e.g., large dark currents for photodetectors. Alternatively, TMDCs, a semiconducting class of 2D materials, hold great promise for high-performance optoelectronic devices due to their intrinsically strong light-matter interactions [22]. Yet, the integration with a silicon-based platform is challenging, because direct band-to-band transition energies of TMDCs fall within the absorption band of silicon. Despite of this, few attempts have been made towards the integra...

Plasmonics provides a possible route to overcome both the speed limitations of electronics and the 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, polarization rotators, and active section with phase shifters-are included in a single metal layer. The device can 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, and low-cost technology that might find many applications in a wide range of fields of sensing and communications because it is compatible with and can be placed on a wide variety of materials.

The performance of highly nonlinear organic electro-optic (EO) materials incorporated into nanoscale slots is examined. It is shown that EO coefficients as large as 190 pm/V can be obtained in 150 nm wide plasmonic slot waveguides but that the coefficients decrease for narrower slots. Possible mechanism that lead to such a decrease are discussed. Monte-Carlo computer simulations are performed, confirming that chromophore-surface interactions are one important factor influencing the EO coefficient in narrow plasmonic slots. These highly nonlinear materials are of particular interest for applications in optical modulators. However, in modulators the key parameters are the voltage-length product U_πL and the insertion loss rather than the linear EO coefficients. We show record-low voltage-length products of 70 Vµm and 50 Vµm for slot widths in the order of 50 nm for the materials JRD1 and DLD164, respectively. This is because the nonlinear interaction is enhanced in narrow slot and thereby compensates for the reduced EO coefficient. Likewise, it is found that lowest insertion losses are observed for slot widths in the range 60 to 100 nm.

Photodetectors compatible with CMOS technology have shown great potential in implementing active silicon photonics circuits, yet current technologies are facing fundamental bandwidth limitations. Here, we propose and experimentally demonstrate for the first time a plasmonic photodetector achieving simultaneously record-high bandwidth beyond 100 GHz, an internal quantum efficiency of 36% and low footprint. High-speed data reception at 72 Gbit/s is demonstrated. Such superior performance is attributed to the subwavelength confinement of the optical energy in a photoconductive based plasmonic-germanium waveguide detector that enables shortest drift paths for photogenerated carriers and a very small resistance-capacitance product. In addition, the combination of plasmonic structures with absorbing semiconductors enables efficient and highest-speed photodetection. The proposed scheme may pave the way for a cost-efficient CMOS compatible and low temperature fabricated photodetector solution for photodetection beyond 100 Gbit/s, with versatile applications in fields such as communications, microwave photonics, and THz technologies.

Coherent optical communications provides the largest data transmission capacity with the highest spectral efficiency and therefore has a remarkable potential to satisfy today’s ever-growing bandwidth demands. It relies on so-called in-phase/quadrature (IQ) electro-optic modulators that encode information on both the amplitude and the phase of light. Ideally, such IQ modulators should offer energy-efficient operation and a most compact footprint, which would allow high-density integration and high spatial parallelism. Here, we present compact IQ modulators with an active section occupying a footprint of 4 × 25 µm × 3 µm, fabricated on the silicon platform and operated with sub-1-V driving electronics. The devices exhibit low electrical energy consumptions of only 0.07 fJ bit −1 at 50 Gbit s −1 , 0.3 fJ bit −1 at 200 Gbit s −1 , and 2 fJ bit −1 at 400 Gbit s −1 . Such IQ modulators may pave the way for application of IQ modulators in long-haul and short-haul communications alike.

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