Plasmonically Enhanced Graphene Photodetector Featuring 100 Gbit

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Plasmonically enhanced graphene photodetector featuring 100 Gbit/s data reception, high-responsivity and compact size Ping Ma, Yannick Salamin, Benedikt Baeuerle, Arne Josten, Wolfgang Heni, Alexandros Emboras, and Juerg Leuthold ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01234 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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Plasmonically enhanced graphene photodetector featuring 100 Gbit/s data reception, high-responsivity and compact size Ping Ma,*,1,† Yannick Salamin,*, 1,† Benedikt Baeuerle1, Arne Josten1, Wolfgang Heni1, Alexandros Emboras1 and Juerg Leuthold*,1 1

ETH Zurich, Institute of Electromagnetic Fields (IEF), 8092 Zurich, Switzerland

†These authors contributed equally to this work. KEYWORDS: Graphene, Photodetector, Plasmonics, Optoelectronics, Silicon, Integrated optics, Waveguide, Optical communications.

ABSTRACT. 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 up to at least 110 GHz. The high efficiency and fast response of the device enables for the first time 100 Gbit/s on-off keying and 100 Gbit/s PAM-4 intensity modulated data reception with a graphene-based device. The results show the potential of graphene as a new technology for highest-speed communication applications.

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Graphene is a highly functional two-dimensional (2D) material which has enabled a plethora of new applications and devices ranging from electronics, bioengineering, and photonics. In particular, optoelectronic devices, such as high-speed photodetectors1 and modulators2-4, take full advantage of graphene’s superior electrical and optical characteristics5, including the ultrafast carrier dynamics6, 7, high carrier mobility8, broad spectral photoresponse9-13, and photocarrier multiplication14,

15

. High-speed

photodetectors are crucial components for many applications including high-speed optical communications9, 16, microwave photonics17, and terahertz (THz) technologies18, 19. Of special interest for next generation 100 Gbit/s optical communications20 would be a graphene photodetector featuring simultaneously a high responsivity, high-speed capability and a compact footprint that can be fabricated on an integrated universal platform such as silicon (Si) photonics9, 16. High-speed operation up to a few hundred Gigahertz has been predicted for graphene photodetectors6, 21

. To date, graphene photodetectors have already demonstrated successful data reception up to 50 Gbit/s

two-level on-off keying (OOK) signals22 and 100 Gbit/s four-level PAM-4 intensity encoded signals23. Also, very recently a 110 GHz bandwidth24 and a bandwidth beyond 67 GHz with indications for a 128 GHz bandwidth based on power meter measurements25 have been reported, respectively. Graphene is thus foreseeably in competition with established III/V and Ge high-speed photodetector technologies2628

. Yet, most graphene-based high-speed photodetectors suffer from weak external responsivities. These

have their origin in the weak light-matter interaction with the atomically thin graphene and consequently a small optical absorption, which results in a relatively low external responsivity. To enhance the light interaction with graphene, a variety of solutions have been proposed. For instance, hybrid sensitized graphene photodetectors have been explored by introducing strong light absorbers such as quantum dots29. However, these devices have limited rise times on the order of 10 milliseconds, as they require slow carriers for an efficient multiplication process. Alternatively, graphene can be combined with optical structures such as resonant cavities30,

31

and planar optical waveguides9,

22, 32-36

. While the latter is

potentially the most promising approach for the integration with Si photonics, the mode overlap with


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graphene in a waveguide-integrated photodetector is still limited. Typical devices need to be a few tens of micrometers long to effectively absorb the evanescently coupled incident light, yielding external responsivities of a few 10 mA/W and reaching 0.36 A/W with clever device designs24, 32, 36. In order to further improve the efficiency and scale down the length of the device to a few micrometers, detectors can be enhanced by plasmonics37, 38. Plasmonics offers strong sub-wavelength mode confinement and local field enhancement. Quite a few plasmonic graphene concepts have already been introduced12, 18, 19, 34, 39-46

, demonstrating a responsivity of 0.6 A/W at visible wavelengths for top illuminated structure12.

Surface plasmon polaritons (SPPs) at metal-graphene dielectric interfaces normally exhibit electric fields perpendicular to the interface47, 48. For this reason, realization of a plasmonically enhanced waveguideintegrated graphene photodetector in the telecom wavelengths with highest responsivity at highest speed (e.g. for a 100 GBd application) in a low footprint design is a challenge to this day. In this letter, we present a compact, highly efficient and high-speed plasmonically enhanced waveguide-integrated photodetector comprising a graphene layer. The photodetector is co-integrated with a Si photonic waveguide and operated as a metal-graphene-metal photoconductive detector. Arrayed bowtie-shaped nano-sized metallic structures are integrated to excite SPPs and enhance the external responsivity of the photodetector. The 6 m long device exhibits a high external responsivity of 0.5 A/W under a -0.4 V bias with a frequency response exceeding 110 GHz. Moreover, operation over a broad spectral range is validated (across the S, C, and L bands of the optical communication windows). As a result of the high responsivity and large bandwidth of our device, 100 Gbit/s data reception of two-level OOK and four-level PAM-4 intensity encoded signals are demonstrated. To the best of our knowledge, this is the highest responsivity reported so far for waveguide-integrated graphene photodetectors while providing highest speed capabilities for 100 Gbit/s and beyond optical data communication applications. In addition, the fabless Si photonic chip fabrication and the use of chemical vapor deposition (CVD)grown graphene show the perspective for a large wafer-scale device fabrication and integration.


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Fig. 1: Plasmonically enhanced waveguide-integrated graphene photodetector. (a) Three-dimensional (3D) schematic of the photodetector. Light is accessed by a Si waveguide, evanescently coupled to the graphene photodetector which is enhanced by the integrated bowtie-shaped metallic structures. (b) A cross sectional view of the graphene photodetector showing the layer stack. (c) In-plane view of the magnitude of

and

electric fields, obtained by 3D full-wave finite-element method (FEM)

simulations. Strong plasmonic field enhancements along the edges of the bowtie structures can be seen. Design, Operation Principle and Fabrication Figure 1a and b illustrate the conceptual schematic and the layer stack of the proposed photodetector, respectively. A patterned layer of graphene covers a planarized Si photonics wafer with buried Si waveguides. Bowtie-shaped metallic structures with gaps in the nanometer scale are placed on top of graphene and symmetrically aligned with the Si waveguide underneath.


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The operation principle is as follows: Linearly transverse-electric (quasi-TE)-polarized light is launched through the Si photonic waveguide into the detector. The in-plane electric fields couple evanescently to the graphene photodetector by exciting SPPs in the bowtie-shaped nano-sized metallic structures. The field-enhancing effect of the bowtie-shaped element can be attributed to a combination of different effects. There is on one hand the enhancement of the electric field in the dielectric layer next to a metal (a typical SPP feature). Second, there is the down-tapering effect of the light due to the bowtieshaped metallic structure49. The structure is designed to focus the traveling SPPs into the nano-sized gap resulting in highly-confined SPPs with dominant in-plane electric fields. Third, one can identify the field enhancement at the sharp tips of the bowtie-shaped structures. In literature they have been described as wedge-plasmon modes50. Finally, the dispersion of plasmons result in a slow-down effect in the group velocity, which also contributes to a field enhancement51. The strongly enhanced in-plane electric fields interact with graphene, which are efficiently absorbed24,

47

. The absorbed light then changes the

conductivity in the graphene layer by the bolometric effect. This change in the conductivity leads to a change in the photoconductive current when a bias voltage is applied across the metallic pads. The field enhancement effect in the gap through the plasmonic structure can be best seen with the help of Figure 1c. Figure 1c shows an in-plane view of the magnitude of the

and

electric fields of the

waveguide-integrated graphene photodetector, simulated by three dimensional (3D) full-wave finiteelement method (FEM) (see Supporting Information, S1). The fields are shown for the vertical z position where the graphene layer is located. The simulation shows how the SPPs in the nano-sized metallic gap structure and around the structure induce large field-components parallel to the graphene plane that can be efficiently absorbed in the graphene layer. Numerical simulations predict an enhancement of the graphene absorption by a factor of 8.5 for a single field-enhancing element compared to a monolayer graphene of the same length. To further boost the light-matter interaction and increase the graphene absorption one can add more field-enhancing elements, to e.g. an array of 5 elements. Each additional


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element will contribute less to the overall efficiency with 5 elements being a good number according to simulation studies (see Supporting Information, S1). The photodetection effect in graphene deserves more discussion. It is known that graphene photodetectors commonly rely on the photovoltaic (PV)52, the photo-thermoelectric (PTE)36, 52 or the photo-bolometric (PB) effect53-55. The PV photoresponse is based on photo-excited electrons and holes, which can be separated by an applied electric field. The PTE Seebeck photocurrent is generated by an optically induced temperature gradient and is particularly efficient in combination with spatially varying doping of graphene35. The PB response is induced by photo-generated hot carriers, which modify the channel resistance by either a change to the number of carriers or a change of the temperature dependent carrier mobility56. Under zero bias condition, the PB effect does not contribute to the photoresponse, because it can only be observed in biased devices53. Under biased conditions, all effects could contribute. If the PV effect were dominant, one would find an increased conductivity with the incident light. While, a decrease in the conductivity with incident light is indicative for either the PTE or the PB effect53. In our device the graphene layer is chemically n-doped by the metallic elements and the structure is symmetric with respect to the center of the bowtie element (see Supporting Information, S2)16, 57, 58. Therefore, a net PTE Seebeck current would not be efficiently generated, as PTE currents induced by any potential gradients are eventually cancelled out. From literature it then can be inferred that the PB effect seems to dominate in such biased device35, 53. Thus, if a bias voltage is applied between two contacts, the change of graphene resistance can be detected by the change of the current flowing through the metal-graphenemetal photoconductive device.


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Fig. 2: Device fabrication. (a) A top-view false-color scanning electron microscope (SEM) image of a fabricated photodetector, showing the access Si photonic waveguide (in violet color), five arrayed bowtie-shaped metallic structures (in yellow color) and contact pads (in yellow color). The dotted line indicates the monolayer graphene under the metallic structures. (b) An enlarged-view false-color SEM image of one field-enhancing metallic structure with a gap in nanometer scale. (c) Raman spectroscopy of the monolayer graphene transferred on the chip. For an experimental verification devices have been fabricated on the Si photonic platform. The basic Si photonic structures such as buried waveguides, grating couplers (GCs), and the planarized surface were built by a commercial Si photonic foundry. Before graphene transfer, a 10 nm thick silicon nitride (SiN) layer was first deposited by atomic layer deposition (ALD) to electrically isolate graphene and Si waveguides. The CVD-grown graphene was then transferred to the whole Si photonic chip (≈36 mm2 in size) via a wet transfer technique. Two identical chips with respectively one and double layer graphene were fabricated. Metallic structures including plasmonic field-enhancing structures and contact pads made with 2 nm thick palladium (Pd) and 23 nm thick gold (Au) on top were fabricated by e-beam evaporation and a lift-off process. Pd was used to reduce the contact resistance interfacing with graphene45, 59. The redundant graphene was finally removed by an oxygen plasma dry etching process.


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Figure 2a shows a top-view scanning electron microscope (SEM) image of a fabricated device. Five fieldenhancing metallic elements in an array were placed and aligned to the Si waveguide as indicated. Figure 2b is an enlarged-view SEM image of one element, showing the nanometer-sized gap. The Raman spectrum as shown in Fig. 2c was taken with 561 nm pump light. It exhibits a stronger intensity of the 2D peak compared to that of the G peak, which is a typical feature of a 0.34 nm thick monolayer graphene60. The analysis of the Raman spectrum reveals a full width at half maximum (FWHM) of the 2D peak of 39 cm-1 and a peak intensity ratio for the D and G peaks of ID/IG = 0.066. This corresponds to an estimated point defect density61, 62 in graphene of 1.2×1010 cm-2. A maximum field effect mobility of 1900 cm2/Vs was measured using a reference structure on the same wafer. Static Response of the Photodetectors The static characteristics of the photodetector were examined first. Linearly polarized laser light at wavelengths around 1550 nm was coupled on chip via GCs and fed to the photodetector by Si access waveguides. A high precision voltage source was used to apply the bias voltage (either a positive or negative voltage was applied to one contact pad while the other pad was grounded) and measure the current. The device may operate either way. Yet, small imperfection in fabrication might result in small asymmetries. Photodetectors with five field-enhancing metallic elements were measured under dark and with light illumination at a wavelength of 1550 nm. To investigate the dominant effect governing the photodetection we performed photocurrent measurements. For this, current-voltage (IV) curves were measured under dark and illumination. Figure 3a (top-right) shows a typical IV curve for a double-layer graphene device. The blue curve shows the dark current of the device. The red curve shows the total current for light with 2 mW input power. The difference between the two curves gives the photocurrent. At zero bias one would see a contribution from the PV effect. However, from the IV curves it can be seen that the overall contribution from the PV effect under illumination is small. The IV curves also reveal the resistance of the graphene detectors. For the single- and double-layer device a resistance of 200 and 100  was measured, respectively. Second,


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the sign of the current under illumination and with applied voltage bias should give an indication on whether the PV or PB effect dominates. Figure 3a (bottom-left) shows the change of current as a function of the applied bias. The change of current is the resulting change in total current induced by the incident light, and can be obtained from the measured IV curves under dark and illumination condition. It can be seen that the current (I) decreases with an applied voltage bias

under constant light illumination.

The current decrease ΔI gives testimony of a reduction of the conductivity – which is a characteristic of the PB effect. This reduction is the result of the enhanced electron-acoustic phonon scattering and electron-remote phonon scattering due to surface polar phonons in the underlying dielectric layer at the light-induced elevated temperature63. A more in-depth discussion is given in the Supporting Information, S2. It is important to note that the bolometric detector works in an inverse operation mode as compared to standard photoconductive detectors28. The current signal is high in the off-state under dark and it is lower in the on-state with light incidence. For a good operation, the off-state signal thus ideally should be as large as possible. Further, a strong suppression of the current is desired to achieve a large on-off ratio when an optical signal is applied. This is maximized when the PB effect is most efficient. The bias dependent responsivities for single- and double-layer graphene devices are plotted in Fig. 3b. For the single-layer graphene detector, the derived external responsivity, which is the ratio of the photocurrent and incident optical power delivered to the Si waveguide accessing the graphene photodetector, reaches 0.5 ± 0.05 A/W at a -0.4 V bias and exhibits a quasi-linear dependency. Under low biases the bolometric photocurrent scales with the applied bias according to /

⁄

photocurrent

∆ ,

, where ⁄

∆

is the bias voltage, ∆ the current change which relates to the

corresponds to bolometric coefficient and ∆T is the light-induced temperature

change33, 53. To get a better insight into the photo-absorption and carrier extraction processes it is of interest to calculate the absorption efficiency a and internal quantum efficiency (IQE) i. From simulations we derive a plasmonically enhanced graphene absorption of 46% and an IQE of 87%. In good agreement with our measurements this leads to an external responsivity of 0.5 A/W (see Supporting


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Information, S3). The double-layer graphene detector supports higher bias voltages. At a -0.6 V bias we find a responsivity of 0.4 ± 0.05 A/W. The slope of the plot indicates that larger responsivities could be obtained at even higher bias voltages. We stopped the characterization at this point in order not to destroy the double-layer device.

Fig. 3: Performance characteristics. (a) Measured change of current as a function of the bias voltage of a double-layer device (bottom left). A reduction of the current under bias indicates a bolometric effect. Measured current as a function of the bias voltage under dark and light incidence (top right). The blue curve shows the dark current and red curve shows the total current when light input power is 2 mW. (b) Responsivity as a function of applied bias voltages for a single-layer (blue scatters) and a double-layer graphene device (red scatters). A photoresponsivity of 0.5 A/W is obtained under a bias of -0.4 V for a single-layer graphene device as measured with an input power of 80 W. The double-layer graphene device can handle a higher input power and achieve 0.4 A/W at a bias of -0.6V for an input signal of 1.3 mW. (c) Measured responsivity as a function of the light input power. (d) Measured external responsivity as a function of the wavelength of the incident light at zero bias.


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The absorption of the graphene devices saturates with increasing input light power, resulting in a reduced external responsivity as seen in Fig. 3c. Both Joule heating53 and Pauli blocking64 contribute to this saturation effect. Yet, the double-layer graphene device can handle higher input powers, as they absorb more light and provide a roughly doubled external responsivity. The wavelength-dependent responsivity of the device is checked and shown in Fig. 3d, measured with a tunable laser source. A flat response can be observed, covering from S-band (1480-1565 nm), entire Cband (1530-1565 nm) to L-band (1565-1620 nm) of the optical communication spectral ranges. The measurement range was only limited by the available source and GC spectral response. Simulations show a consistent broadband operation (see Supporting information, S1). High-Frequency Response and Data Experiments To characterize the high-speed performance of the device we measured the frequency response, see Fig. 4. A mostly flat frequency response can be seen spanning the range from 100 kHz up to 110 GHz. Fluctuations in the response are visible but no cut-off trend can be identified. Two distinct setups were employed to cover the broad frequency range. The lower normalized frequency response up to 30 GHz was measured with a continuous wave laser modulated by a commercially available high-speed MachZehnder intensity modulator (MZM, 3dB bandwidth: 30 GHz). At high frequencies, the heterodyne approach was used to generate the radio-frequency (RF) signal from 500 MHz by superposition of two lasers with a varying frequency offset (see Methods for details). A second device from the same chip and with the same structure was used for the heterodyne frequency measurement up to 67 GHz, as the first device was damaged during the preparation of the chip for the 67 GHz bandwidth testing. A frequency overlap was intentionally measured to normalize between two measurements. As displayed in Fig. 4, for a device under -0.4 V bias condition a flat frequency response is clearly visible at least to 67 GHz which is the instrumentation-limit of the electrical spectrum analyzer (ESA) used in the measurement. To test the photodetector towards an even higher frequency, we used an ESA offering bandwidth up to 110 GHz. At the higher frequencies we used a double-layer graphene with five metallic elements. The double-layer


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detector was used as it yielded a better signal-to-noise ratio (SNR) and allowed measurements with less noise up to highest frequencies. As shown in Fig. 4, the device exhibits a fast photoresponse extended further to 110 GHz. Speed limitations of the device may be due to the thermalization time of electrons and holes. The photo-excited electrons and holes thermalize with other carriers and may cool down via interaction with optical phonons. The relaxation time of these hot carriers in graphene is reported to be a few picoseconds7. Eventually, the bandwidth of a PB effect-based photodetector is limited by the product of the electronic heat capacitance and thermal resistance54. As for our device, no apparent sign of a cutoff is observed in the bandwidth measurement, indicating the potentials and capabilities of the device operating at even higher frequencies.

Fig. 4: Flat frequency response of the graphene photodetector in the range of 100 kHz up to 110 GHz. Blue scatters in the lower frequency range show the frequency response measured with an externally modulated laser; blue crosses and red scatters in the higher frequency range show the response measured with a two-laser beating approach for the single-layer and double-layer graphene devices, respectively. The single and double-layer graphene detectors were biased at -0.4 V and -1.0 V, respectively. Inset: Frequency response of the device under zero bias condition (photovoltaic response), measured with an externally modulated laser. The frequency response for device operated at zero bias is shown as inset of Fig. 4. The flat response reveals the corresponding photovoltaic photoresponse is also a fast process, as the drift paths of the photocarriers are very short in the present device24.


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Fig. 5: Data reception. (a) Schematic of the measurement setup as used for the data experiments. The setup consists of a continuous wave laser around 1550 nm, an arbitrary waveform generator (AWG), a Mach-Zehnder modulator (MZM), an erbium-doped fiber amplifier (EDFA), a bias-tee, a radiofrequency (RF) power amplifier, and a digital sampling oscilloscope (DSO). Detected eye diagrams of (b) 50 Gbit/s on-off keying (OOK) (left), 100 Gbit/s PAM-4 intensity modulated (right) optical signals with a single-layer graphene device, and (c) 100 Gbit/s OOK optical signals with a double-layer graphene device. Data reception experiments were performed to demonstrate the capabilities of the developed devices for 100 Gbit/s high-speed optical data communication applications. The measurement setup is shown in Fig. 5a. The commercially available modulator was used to encode a random bit sequence onto a laser source around 1550 nm. An electrical signal with square-root-raised cosine pulse shape spectrum was generated by an arbitrary waveform generator (AWG, sampling rate: 64 GSa/s, electrical 3 dB bandwidth: 25 GHz). The optical signal was amplified with an erbium-doped fiber amplifier (EDFA) and coupled into the single-layer graphene device via GCs and on-chip Si waveguides (~12 dBm). The generated electrical signal was read out with a high-speed ground signal (GS) microwave probe. A bias-tee was used to apply a direct current (DC) bias voltage to the graphene detector. The generated RF signal was amplified by a 50 Ω power amplifier and recorded by a real-time digital storage oscilloscope (DSO,


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sampling rate: 80 GSa/s, electrical 3 dB bandwidth: 33 GHz). Digital signal processing was performed offline to evaluate the bit-error-ratio (BER), including timing recovery, adaptive least mean square (LMS) equalization, non-linear pattern dependent equalization and symbol decision. Figure 5b shows the detected electrical eye diagram for 50 Gbit/s OOK signals below a BER of 2 × 10−4 and the detected eye diagram for 50 GBd PAM-4 intensity modulated signals with a line rate of 100 Gbit/s, which is demonstrated for the first time for a graphene photodetector. A BER of 8.9 × 10−3 was measured at this data rate. The testable line rate is instrumentally limited, for instance, by the modulator used in the experiment which has a 3 dB bandwidth of 30 GHz. A good operation point of the device requires a high signal quality Q – which is related to the SNR65. The signal quality is high, if the on-off ratio of the signal is high and if the total accumulated noise is low. The on-off ratio level can be derived between the on-off rails of the eye diagram as shown in Fig.5. The diagram also shows the total noise of the system. This includes all the noises from the transmitter (the noise of the source and its amplifiers), the system amplifiers (optical) and the receiver. The actual shot noise and thermal noise from the detector makes up thus only a fraction of the total noise. In the example of the 50 Gbit/s eye diagram we measured the onoff ratio of the currents to be in the order of 600 A with signal rails at 2.3 and 1.7 mA. The respective shot noise currents would then account for roughly 4.7 and 4.0 A for the off- and on-states, respectively. The thermal noise (calculated for a Joule heated detector of temperature 390 K at a 0.4 V bias in a 30 GHz electrical bandwidth detector) would be in the order of 2 A. The total shot and thermal noises then account for about 5.1 and 4.47 A, respectively. This actually shows, that the data experiment is neither limited by shot nor thermal noise but rather by the performance of the overall system - such as e.g. the quality of the generated signal, the noise of the amplifiers and etc. Reception of a 100 Gbit/s OOK data signal was tested with the double-layer graphene detector. The higher output power provided the necessary signal power to perform experiments at such high speed. For detection the device was biased with 0.8 V. The power of the signal in the waveguide before the detector was about 11 dBm. For testing a random bit sequence of 218 bits was generated by a digital-to-analog


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converter (DAC) with a sampling rate of 100 GSa/s. Figure 5c shows the eye diagram of the detected 100 Gbit/s OOK signals. A BER of 5.3 × 10−4 was found, which is well below the hard-decision forward error correction limit. CONCLUSION In conclusion, we propose and experimentally demonstrated a compact plasmonically enhanced waveguide-integrated graphene photodetectors by introducing arrayed bowtie-shaped field-enhancing metallic structures. Devices of 6 m length feature a high external responsibility of 0.5 A/W and a broad bandwidth exceeding 110 GHz and are capable of detecting optical data at speeds beyond 100 GBd. The detectors have been tested for reception of 100 Gbit/s OOK and 100 Gbit/s PAM-4 intensity modulated optical data, validating the performance and revealing the great potentials of the proposed graphene photodetectors for next generation optical and microwave photonic communications applications. METHODS Bandwidth measurement: Two different setups were employed in order to cover a wide spectral range from 100 kHz to 110 GHz. For a low frequency range (100 kHz to 30 GHz), an intensity modulator with a 3 dB bandwidth of 30 GHz (u2t MZMO2120) was used to modulate a continuous wave laser light. In order to overcome the bandwidth limitation of the modulator and characterize the photoresponse of the device at high frequencies beyond 30 GHz, the heterodyne approach was adopted to generate the RF signal from 500 MHz to 110 GHz. By tuning the frequency of one laser the frequency offset between two lasers can be controlled. Both lasers with equal amplitudes were combined with a 3 dB coupler. For both cases, light was coupled via GCs and routed to graphene photodetectors by on-chip Si photonic access waveguides. A bias-tee was used to apply bias to a device. The generated RF signal was extracted from the device with high-speed microwave probes and analyzed with three ESAs covering a frequency range up to 50 GHz (Agilent PXA N9030A), 67 GHz (Rohde & Schwarz FSU67) and 110 GHz (Anritsu MS2760A).


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ASSOCIATED CONTENT Supporting Information S1. Design and optical simulations of the plasmonically enhanced graphene photodetectors S2. Photodetection contribution S3. Internal quantum efficiency AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected] Author Contributions †These authors contributed equally to this work. Funding The European ERC PLASILOR project (670478) and ETH project grant ETH-45 14-2 are acknowledged for partial funding of the work. Acknowledgements This work was carried out partially at the Binning and Rohrer Nanotechnology Center (BRNC) and in the FIRST lab cleanroom facility at ETH Zurich. We are grateful to H. R. Benedickter and A. Olziersky for the help with the measurement and fabrication, respectively. References 1. Koppens, F. H. L.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M., Photodetectors based on graphene, other two-dimensional materials and hybrid systems. Nature Nanotechnology 2014, 9, 780. 2. Liu, M.; Yin, X.; Ulin-Avila, E.; Geng, B.; Zentgraf, T.; Ju, L.; Wang, F.; Zhang, X., A graphenebased broadband optical modulator. Nature 2011, 474 (7349), 64-67.


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ToC Figure


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Plasmonically Enhanced Graphene Photodetector Featuring 100 Gbit

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