100 GHz Plasmonic Photodetector

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100 GHz Plasmonic Photodetector Yannick Salamin, Ping Ma, Benedikt Baeuerle, Alexandros Emboras, Yuriy Myronovych Fedoryshyn, Wolfgang Heni, Bojun Cheng, Arne Josten, and Juerg Leuthold ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00525 • Publication Date (Web): 24 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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100 GHz Plasmonic Photodetector Yannick Salamin*, #, Ping Ma*, #, Benedikt Baeuerle, Alexandros Emboras, Yuriy Fedoryshyn, Wolfgang Heni, Bojun Cheng, Arne Josten, and Juerg Leuthold* ETH Zurich, Institute of Electromagnetic Fields (IEF), 8092 Zurich, Switzerland KEYWORDS. Photodetector, Photodiodes, Plasmonics, Optoelectronics, Germanium, Silicon photonics, Integrated optics, Waveguide.

ABSTRACT. 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 sub-wavelength confinement of the optical energy in a photoconductive based plasmonic-germanium waveguide detector that enables shortest drift paths for photo-generated 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.

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High-speed photodetectors offering bandwidths beyond 100 GHz are getting increasingly important for the information society. These high speed detectors would fulfil the needs in a wide range of applications such as high-speed optical communications1, microwave photonics2-3, terahertz (THz) communication4-5, imaging and sensing6. For instance, in order to meet the rapidly growing bandwidth demands in an optical network, it is foreseen that optical fiber networks feature 100 Gbit/s data rate per channel in the near future7. To this end, it is crucial that electro-optic components such as optical modulators and photodetectors, which are used to encode and decode electrical signals onto the optical carrier, can meet the requirements of nextgeneration high-data-rate systems8-10. Until now, photodetectors reaching bandwidths of 100 GHz and beyond are mainly based on III-V compound semiconductors by leveraging their strong absorption and high mobility11. Yet, over the last decade, driven by the rapid progress of Silicon (Si) photonics, high-speed photodetectors compatible with the CMOS fabrication standards are desired and tremendous efforts have been made12. Germanium (Ge), a group IV semiconductor material compatible with CMOS processes, has been proven to be a good active material for photodetection in the telecom wavelength range13-15. Recently, Ge p-i-n photodetectors with a bandwidth of 100 GHz14 and data rates up to 100 Gbit/s15 were reported in two separate demonstrations. These designs require complex doping, structuring, and thermal treatment in order to control and optimize the drift and diffusion processes of photo-generated carriers. Still, a 3 dB bandwidth limit around 70 GHz, either resistance-capacitance (RC) product or carrier transit time limited, is usually seen15. Alternatively, photoconductive metal-semiconductor-metal (MSM) detector configurations can be more advantageous for high-speed applications16-17, owing to the small RC product and short transit time of carriers. In addition, MSM structures can significantly reduce fabrication


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complexity and efforts. A narrow active region is crucial to achieve a short transit time for carriers and with this to achieve a fast photoresponse. Most narrow active regions can be obtained by resorting to plasmonics18-19. In plasmonic slot waveguides, one can squeeze optical signals to subwavelength confined scales20-22. Plasmonic Si photodetectors based on hot carrier injection have been proposed and realized23-25. Very recently, a 40 Gbit/s hot-carrier plasmonic Si waveguide photodetector has been shown with a responsivity of 0.12 A/W26. In such plasmonic detectors, the optical energy is transferred to free electrons in the metal, generating hot-carriers at the metal surface. If the plasmon energy is sufficiently high, these hot-carriers can cross the potential barrier of the metal-semiconductor junction27-28. However, this internal carrier injection process is relatively inefficient, as only a small portion of the hot-carriers are contributing to the photo-current, which limits the efficiency of the device. It is therefore of great interest to explore novel concepts that meet the interests of the community and offer highest speed and high efficiency on the smallest possible footprint with a CMOS compatible technology to keep costs low. A possible solution to obtain both a fast response and good confinement can be the photoconductive plasmonic detection scheme29-31. In this work, we demonstrate the first high-speed plasmonic waveguide photodetector relying on a photoconductive effect in an integrated amorphous Germanium (α-Ge) semiconductor material in the plasmonic slot. Highest speed is made possible by the sub-wavelength confinement of the optical energy in the photoconductive based plasmonic-germanium waveguide detector that enables shortest drift paths for photo-generated carriers and a very small resistance-capacitance product. In addition, the combination of plasmonic waveguides with active materials enables efficient photodetection at highest speed. As a result, our device features a high-speed photoresponse (>100 GHz) with an internal quantum efficiency (IQE) of 36 %,


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thanks to the plasmonic design concept. The performance of our device is demonstrated in a 72 Gbit/s data experiment. The proposed design concept suggests a cost-efficient and CMOS compatible alternative technology to crystalline Ge/Si and III-V/Si hybrid photodetectors, and could match future requirements imposed on high-speed photodetectors for a broad range of applications.

Figure 1: Photodetector concept. Three-dimensional rendering of the proposed plasmonic waveguide photodetector. Light is fed to the photoconductive plasmonic detector via a Si access waveguide (indicated in pink) by evanescent coupling. The top insets (I) show the transverse view of the simulated optical and direct current (DC) fields of the plasmonic slot waveguide with a Ge core and Au lateral claddings. The inset at the bottom (II) shows the schematic of the band diagram of the Au-Ge-Au structure under bias. Results Device concept. The proposed photodetector design is illustrated in Fig. 1. The optical signal is guided on chip by passive Si waveguides that are buried in SiO2, and evanescently coupled to the


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active section of the device. The active section of the device consists of a nano-scale MSM slot waveguide with α-Ge as the semiconductor absorbing core and gold (Au) as plasmonic lateral claddings. The evanescently coupled photons are converted into surface plasmon polaritons (SPPs) and propagate along the MSM slot as SPPs. The SPPs are absorbed while propagating in the α-Ge. As the optical energy is almost perfectly confined in the nano-scale active core (Inset I, left image), an efficient photodetection process can be predicted. By applying a bias voltage between the two metallic claddings, a uniform electric field is generated in the Ge (Inset I, right image). Consequently, generated electron-hole pairs are efficiently separated and strongly accelerated by the applied field (Inset II). These separated carriers drift towards metallic claddings, generating a photo-induced current proportional to the intensity of the optical signal. The evaporated α-Ge material behaves like a p-doped material due to intrinsic defect states32. The band diagram for the Au-αGe-Au material system with a slot width of 160 nm is schematically shown in the inset II of Fig. 1. Mechanisms of the ultrafast photoresponse. The ultrafast response of the proposed photodetector is attributed to two main reasons. Firstly, as the detector dimensions are very small, the capacitance of the detector is in the range of a few fF (supplementary information, S1). In addition, the metallic lateral claddings forming the plasmonic waveguide are simultaneously used as low resistive contacts. Thus, the RC limit of the device can reach THz if the device is connected to a 50 Ohm load. Secondly, the subwavelength plasmonic waveguide enables a nanoscale drift path for the photo-generated carriers, yielding a very short transit time for photocarriers in the active material. In addition, the applied Direct Current (DC) field V across the slot width d efficiently induces a strong uniform field, E~V/d, in the active material, enforcing a full drift velocity for all photo-carriers. The proposed scheme is particularly interesting in


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combination with amorphous materials where the carrier mobilities are normally lower than those of the crystalline counterparts32-33. Yet, carriers in amorphous materials can be strongly accelerated in high electrical fields such as offered by the plasmonic structure and to drift at velocities approaching those of their crystalline counterparts34. In addition, the carrier recombination process which could be a major drawback of using amorphous materials for photodetection applications can also be alleviated owing to the short carrier transit path. In other words, the optoelectronic properties of the amorphous materials which lack their long-range order can be significantly improved if used in a short-range structure close to their domain sizes. This can be provided for example by the plasmonic device structure35. As a result, all contributing mechanisms lead to short transit times. In addition, deep-level traps in the amorphous material increases the carrier recombination rate compared to that of crystalline germanium (c-Ge), reducing the carrier lifetime to picoseconds36-37. Short carrier lifetime material systems are widely used in photoconductive based THz generation approaches37. Yet, this comes at a price, as the recombined carriers do not contribute to the photocurrent, the quantum efficiency drops. Using a nano-scale active region helps to minimize this loss, as most of the carriers can reach the contacts in picoseconds. Design of the plasmonic detector. It is well known that plasmonic devices are inherently lossy. Here we discuss the impact of ohmic losses on the maximum achievable quantum efficiency of the plasmonic photodetector by means of three-dimensional (3D) full-wave FEM simulations (supplementary information, S2). It can be predicted that if the absorption coefficient αs of the active material is close or equal to the absorption coefficient of the metal αm, plasmonic losses can reach 70 % of the total absorbed optical energy, which limits the device efficiency. On the other hand, if an active material is used with αs >> αm, the optical energy absorbed in the


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semiconductor per unit length is much stronger. This shortens the interaction of optical energy with metals, and reduces the excess plasmonic losses to 25 %. In this scenario, a maximum quantum efficiency of 75 % can potentially be achieved (supplementary information, S2). Therefore, the proposed design is theoretically capable of achieving much higher bandwidths with only a small decrease of the quantum efficiency (supplementary information, S2). This is a distinct merit of the proposed photoconductive plasmonic detector design, as high-speed photodetectors have commonly suffered from the performance trade-offs between responsivity and bandwidth13. Following this design guideline, we experimentally demonstrate the photodetector around 1310 nm, where the absorption of Ge is stronger. Nevertheless, the proposed high-speed plasmonic photodetector concept can work with different semiconductors and metals for the desired wavelengths.


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Figure 2: Fabricated device and Ge characterization. (a) False-colored scanning electron microscope (SEM) image of the fabricated device. The buried Si waveguide is highlighted by the dashed lines. Ge and Au materials are indicated in a green and yellow color, respectively. (b) A zoom-in cross-sectional SEM image of the Au-Ge-Au plasmonic slot waveguide. (c) Raman spectroscopy of as-deposited α-Ge thin-film and a reference c-Ge bulk material. Devices were fabricated to demonstrate the concept. Figure 2 shows scanning electron microscope (SEM) images of a fabricated device. As visible in Fig. 2b, α-Ge material was smoothly and uniformly filled into the plasmonic slot without cracks and voids. Figure 2c shows the normalized Raman shift spectrum of the evaporated Ge (red curve) and of a reference c-Ge bulk material (blue curve). The Raman spectrum of the evaporated Ge exhibits a broader fullwave half-max (FWHM) and a red shift compared to c-Ge. These observations are similar to the typical Raman spectra of α-Ge as previously reported in the literature38 which confirms the amorphous nature of the evaporated Ge. In addition, resistivity of α-Ge was measured with a


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four-point-probe setup and found to be around 500 Ω cm. This value is very close to previously reported values39.

Figure 3: Static characterization of the fabricated device. (a) Measured typical current-voltage curve under dark (black curve) and illumination (colored curves) conditions, respectively. (b) Photocurrent (left axis) and IQE (right axis) as a function of the applied voltages. (c) Photocurrent as a function of the input light power for various applied voltages. The derived internal quantum efficiency (IQE) in percent is indicated on their respective fitted lines. (d) Wavelength dependence of the photoresponse. A flat spectral response over the O-band is observed. Static optoelectronic response. Figure 3 shows the static performance of the fabricated devices characterized by using a probe-station setup equipped with fiber coupling capabilities. Devices with slot widths of 120 nm and 160 nm were investigated. Devices with slot widths around


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160 nm offer a good balance between detection efficiency and large bandwidth. A continuous wave incident laser light at 1310 nm was coupled via a grating coupler (GC) to the Si access waveguide. A pico-ampere precision source was used to measure the current. Figure 3a plots the current-voltage (IV) measurements under dark (black curve) and illumination (red to yellow curves) conditions for a device with a slot width of 160 nm at low offset biases between 0 and ±1 V. The detector shows the expected electrical characteristics. The dark current (plotted as a black line) is much smaller than what one would expect from a typical MSM photoconductor. This is because the active area of our device is small - and it is small because the plasmonically enhanced waveguide has a small cross section, i.e. it is short and thin. The plot depicts IV curves for incident light at different power levels, corresponding to the light power intensities fed into the plasmonic waveguide. These power levels were estimated with the help of reference structures (with access GCs and passive Si waveguides, and without detector sections) on the same chip and with the help of a 3D electromagnetic simulation of the device (supplementary information, S3). Figure 3b shows the photocurrent as a function of the applied bias voltages for a device with a slot width of 160 nm and a length of 3 µm. The photocurrent strongly increases with the bias voltage, which is expected for amorphous material systems, as the carrier extraction is field dependent38. Figure 3b also plots the extracted IQE of the device and shows an efficiency of up to 36 % (right y-axis, grey curve). Simulations indicate that out of the 64 % of photons that do not contribute to the photocurrent a fraction of 14 % can be attributed to plasmonic losses and the other fraction of 50 % can be attributed to carrier recombination effects in the amorphous Ge. The carrier recombination effects can be alleviated by further developing the amorphous materials, for instance by low-temperature annealing or evaporation32-33.


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Next, it might be of interest to see if the photocurrent and the incident power are linearly related. A linear response would indicate that the device does not suffer from nonlinear effects and saturation – such as one would expect for particularly narrow slot waveguides. Figure 3c shows the measured photocurrent as a function of the coupled light intensity under various bias voltages. A linear dependency can be seen. This is a positive finding, particularly as the experiment was performed with a device featuring a narrow slot of 120 nm and a length of 20 µm. In addition, the IQE has been added to each curve. It was extracted by linear fitting. Figure 3d shows the IQE as a function of the wavelength for the device with the 160 nm wide slot. The flat photoresponse for wavelengths between 1270 and 1330 nm demonstrates the broadband operation of the device, which is only limited by the operating frequency range of the GC. The dark current is plotted in Fig. S3d and further discussed in the following. The dark current is particularly high for low input powers below 20 µW. However, it does not really lead to a penalty for high-speed operations when sufficient optical signal power is available – as will be seen in the next section. The dark current might be an issue for detecting weak signals. We measured a noise equivalent power in the range of 1-5 pW/Hz1/2. Fortunately, the dark current of this diode can be improved by adopting remedies already suggested and demonstrated in literature, including asymmetric metallic contact40, or adding a thin barrier such as a large bandgap material layer41-42. In addition, the performance of the current device generation can be further improved by applying lower coupling loss schemes such as used in the industry. The coupling losses of the in-house fabricated GCs which were used to couple light from the fiber to a Si waveguide were tested to be 7.1 dB. Furthermore, the Si waveguide to plasmonic slot waveguide coupling scheme was not ideal either and led to another 13.8 dB loss so that the fiber-


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to-plasmonic detector losses were about 21 dB. It should be stressed, that state-of-the-art Si photonic fab can couple light with as little as 1.6 dB to a Si waveguide43 and that Si to plasmonic slot waveguide coupling can be performed with 90 % efficiency (see supplementary information, S3). However, due to fabrication imperfections, the shape and dimensions of the fabricated Si access waveguide differed from the design. Mainly a residual Si slab below the plasmonic waveguide remained after oxidation of Si. The Si slab below the plasmonic section was able to retain some of the light from the Si waveguide rather than mapping it to the plasmonic slot. As a result, only a portion of the input light coupled to the Ge plasmonic waveguide. Yet, it is worth mentioning that the inefficient photonic-plasmonic coupling in the present fabricated device is not fundamental, and can be significantly improved with an optimized fabrication process.


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Figure 4: Dynamic characteristics of the fabricated device. (a) Normalized radio frequency (RF) response as a function of the frequency. The two inset show the externally modulated laser setup used for the lower frequency range, and the two-laser beating setup used for the higher frequency range. (b) Detected eye diagram of a 72 Gbit/s On-Off keying pseudo random bit sequence. Frequency response and data experiment. The dynamic behavior of the device was investigated by using two distinct setups in order to cover the frequency range from 100 MHz to 100 GHz. Experiments were performed with a device from the same fabrication batch and same structure as discussed above with a 160 nm slot width, but with a length of 10 µm. Transitioning to a longer device typically would result in a slightly higher efficiency at the expense of a higher capacity and thus a lower bandwidth. The normalized frequency response up to 20 GHz, shown


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in Fig. 4a (blue curve), was measured with a modulated 1310 nm carrier by an RF sinusoidal signal. At a higher frequency range, the normalized frequency response was measured by a two laser beating approach, as shown in Fig. 4a (red curve). The two combined lasers beat in the detector resulting in a RF frequency signal defined by the frequency offset of the two lasers. This way, the RF frequency can be arbitrarily tuned by controlling the wavelength of one laser. We intentionally measured an overlap in frequency in order to normalize both measurements. It can be seen that the frequency response of the device is flat up to 100 GHz without any sign of drop down, demonstrating high-speed capabilities. Measurement beyond 100 GHz was not possible, as the losses in the cable, connectors, and probe yielded an insufficient signal-to-noise ratio (SNR). To verify the usability of the studied plasmonic photodetector in optical communication applications, we carried out a data experiment at 1316 nm. Figure 4b shows the detected electrical eye diagram with a line rate of 72 Gbit/s. The bit-error ratio (BER) has been found to be 1.6·10-2. The line rate is limited by the available modulator at 1300 nm which has a 3 dB bandwidth of 30 GHz. However, the device showed no speed limitation up to 72 Gbit/s. Discussion The subwavelength confinement of the optical energy in the nano-scale active region enables not only shortest drift paths for carriers and smallest RC constants, but also provides an efficient carrier extraction for amorphous materials. The high-speed operation at 72 Gbit/s demonstrates well the performance of the detector. Yet, the considerable optical losses of GC and PPC led to an optical power budget issue. Consequently, the low optical intensity delivered to the device limited the amplitude of the converted electrical signal. Therefore, a 50 Ω RF power amplifier was used to amplify the electrical signal from the device. The impedance mismatch between the photodetector and electrical power amplifier, in combination with the noise figure of the


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amplifier additionally reduced the SNR. Ideally, one would require a trans-impedance amplifier to efficiently amplify the RF signal, which would lead to better BER. In conclusion, we have demonstrated the first plasmonic photoconductive germanium detector achieving highest speed and efficiency simultaneously. The device can achieve a measured bandwidth beyond 100 GHz with static measurements revealing an internal quantum efficiency of up to 36 %, and a broadband operation from 1270 nm to 1330 nm. In addition, high-speed operation of the device at 72 Gbit/s is demonstrated. Our theoretical and experimental results show that the combination of an absorbing semiconductor with plasmonic structures leads to an enhanced conversion efficiency for plasmonic detectors. This platform is a viable path towards ultra-high-speed and high-efficiency photodetectors enabling 200 GBd optical communication. Methods. Device fabrication. First, passive Si waveguides were fabricated on a standard silicon-oninsulator (SOI) wafer by using the LOCal Oxidation of Si (LOCOS) technology44. The buried Si waveguides have a height of 220 nm and an effective width of 450 nm. A 5 nm thick silicon nitride dielectric layer was deposited by atomic layer deposition (ALD) for electrical insulation. Subsequently, the metallic claddings forming the plasmonic waveguide, which are made by 100 nm thick Au, were fabricated by means of e-beam evaporation and a lift-off process. Finally, αGe was deposited and patterned again by room-temperature e-beam evaporation and a lift-off process. In order to obtain a good electrical contact between the Au contact and Ge, a thin Titanium (Ti) adhesion layer was subsequently evaporated on top of the Au layer. Several devices have been fabricated with the same layer structure but of different lengths and different slot widths. The static plots have been measured with a first device with length of 3 um and slot width of 160 nm, and a second device with length of 20 um and slot width of 120 nm, to


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compare their efficiencies. The frequency response has been performed with a device of 160 nm slot width and length of 10 um for a slightly higher external efficiency. For the eye diagram experiment, a device with optimal coupling around 1310 nm was used in order to maximize the signal to noise ratio. The device was 20 um long and had a slot width of 200 nm. IQE Calculation. IQE was calculated according to IQE = 100 ×

·

× , where Iph is

the measured photocurrent, Pabs the absorbed power by the plasmonic detector, λ the wavelength, e is the elementary charge, and h,c the Planck constant and speed of light respectively. Bandwidth measurement. Two distinct setups as shown in the inset of Fig. 4a were employed in order to cover the frequency range from 100 MHz to 100 GHz. For the low frequency range (100 MHz to 20 GHz), an intensity modulator was used to modulate a continuous wave laser. In order to characterize the device performance in a higher frequency range, a two-laser beating approach was employed. By tuning the frequency of one laser the frequency offset between the two lasers can be controlled. Both lasers with equal amplitude were combined with a 3 dB coupler and subsequently amplified by means of a semiconductor optical amplifier (SOA) to 0 dBm. In both scenarios, light was fed into the device via a GC. The resulting RF signal was extracted from the device with high-speed ground-signal (GS) microwave probes and analyzed with an electrical spectrum analyzer (ESA) covering a frequency range up to 110 GHz (Anritsu MS2760A-0110). A bias-tee was used to apply a bias voltage of 7.5 V to the device. Data measurement. For the data reception, a commercially available intensity modulator (u2t MZMO2021) was used to encode a random bit sequence of length 218 bits at a line rate of 72 Gbit/s. The NRZ electrical signal was generated by a high-speed DAC (Micram DAC3 V1) with 72GSa/s. The optical signal was amplified by means of a SOA to 9.5 dBm and fed into the


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device via a GC. The generated electrical signal was read out with a GS microwave probe and a bias-tee was used to apply the DC voltage of 8.5 V. The generated RF signal was amplified by a 50 Ω power amplifier (Centellax UA0L65VM) and recorded by a high sampling rate oscilloscope (Agilent DSO-X 96204Q). Standard signal processing was performed offline to evaluate the BER, including timing and carrier recovery, and linear and non-linear equalization. References 1. Schuh, K.; Buchali, F.; Idler, W.; Eriksson, T. A.; Schmalen, L.; Templ, W.; Schmid, R.; Altenhain, L.; Moeller, M.; Engenhardt, K. In Single Carrier 1.2 Tbit/s Transmission over 300 km with PM-64 QAM at 100 GBaud, Optical Fiber Communication Conference Postdeadline Papers, Los Angeles, California, 2017/03/19; Optical Society of America: Los Angeles, California, 2017; p Th5B.5. 2. Capmany, J.; Novak, D., Microwave photonics combines two worlds. Nature Photon. 2007, 1 (6), 319-330. 3. Yao, J., Microwave photonics. J. Lightw. Technol. 2009, 27 (3), 314-335. 4. Koenig, S.; Lopez-Diaz, D.; Antes, J.; Boes, F.; Henneberger, R.; Leuther, A.; Tessmann, A.; Schmogrow, R.; Hillerkuss, D.; Palmer, R.; Zwick, T.; Koos, C.; Freude, W.; Ambacher, O.; Leuthold, J.; Kallfass, I., Wireless sub-THz communication system with high data rate. Nat. Photonics 2013, 7 (12), 977-981. 5. Nagatsuma, T.; Ducournau, G.; Renaud, C. C., Advances in terahertz communications accelerated by photonics. Nature Photon. 2016, 10 (6), 371-379. 6. Tonouchi, M., Cutting-edge terahertz technology. Nat. Photonics 2007, 1 (2), 97-105. 7. Alliance, E., The 2016 Ethernet Roadmap. White Paper 2016. 8. Heni, W.; Haffner, C.; Baeuerle, B.; Fedoryshyn, Y.; Josten, A.; Hillerkuss, D.; Niegemann, J.; Melikyan, A.; Kohl, M.; Elder, D. L.; Dalton, L. R.; Hafner, C.; Leuthold, J., 108 Gbit/s Plasmonic Mach-Zehnder Modulator with >70 GHz Electrical Bandwidth. J. Lightw. Technol. 2016, 34 (1). 9. Hoessbacher, C.; Josten, A.; Baeuerle, B.; Fedoryshyn, Y.; Hettrich, H.; Salamin, Y.; Heni, W.; Haffner, C.; Kaiser, C.; Schmid, R.; Elder, D. L.; Hillerkuss, D.; Möller, M.; Dalton, L. R.; Leuthold, J., Plasmonic modulator with >170 GHz bandwidth demonstrated at 100 GBd NRZ. Opt. Express 2017, 25 (3), 1762-1768. 10. Chen, X.; Chandrasekhar, S.; Randel, S.; Raybon, G.; Adamiecki, A.; Pupalaikis, P.; Winzer, P. J., All-Electronic 100-GHz Bandwidth Digital-to-Analog Converter Generating PAM Signals up to 190 GBaud. J. Lightw. Technol. 2017, 35 (3), 411-417. 11. Ito, H.; Kodama, S.; Muramoto, Y.; Furuta, T.; Nagatsuma, T.; Ishibashi, T., High-speed and high-output InP-InGaAs unitraveling-carrier photodiodes. IEEE J. Sel. Topics Quantum Electron. 2004, 10 (4), 709-727. 12. Assefa, S.; Xia, F.; Vlasov, Y. A., Reinventing germanium avalanche photodetector for nanophotonic on-chip optical interconnects. Nature 2010, 464 (7285), 80-84. 13. Michel, J.; Liu, J.; Kimerling, L. C., High-performance Ge-on-Si photodetectors. Nat. Photonics 2010, 4, 527.


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Corresponding Author * E-mail: [email protected], [email protected], [email protected]. Author Contributions Y.S. and P.M. conceived the concept, designed and fabricated the devices, designed and performed the experiments. B.B. and A.J. contributed to the data experiments and analysis. A.E. and Y.F. contributed to the device fabrication. W.H. and B.C. contributed to the experiments. Y.S., P.M., and J.L. analyzed the data, and co-wrote the manuscript, with support from all authors. #Y.S. and P.M. contributed equally. Additional information Supplementary Information: S1. Capacitance measurement of the fabricated devices. S2. Optical simulation of plasmonic-germanium waveguide detector. S3. Detailed calculation of the internal quantum efficiency. S4. Discussion on the data experiment. Competing financial interest: The authors declare no competing financial interest. Funding source: The ERC PLASILOR (670478) is acknowledged for partial funding of the work.


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For Table of Contents Use Only

100 GHz Plasmonic Photodetector Yannick Salamin*, #, Ping Ma*, #, Benedikt Baeuerle, Alexandros Emboras, Yuriy Fedoryshyn, Wolfgang Heni, Bojun Cheng, Arne Josten, and Juerg Leuthold* ETH Zurich, Institute of Electromagnetic Fields (IEF), 8092 Zurich, Switzerland We propose and experimentally demonstrate 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. 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.


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100 GHz Plasmonic Photodetector

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