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 dots. Alternatively, graphene can be combined with optical structures such as resonant cavities 30,31 and planar optical waveguides.
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However, the weak optical absorption of graphene2,3 limits its photoresponsivity. To address this, graphene has been integrated into nanocavities9, microcavities10 and plasmon resonators11,12, but these approaches restrict photodetection to narrow bands.
Hybrid graphene—quantum dot architectures can greatly improve responsivity13, but at the cost of response speed. Here, we demonstrate a waveguide-integrated graphene photodetec- tor that simultaneously exhibits high responsivity, high speed and broad spectral bandwidth.
Using a metal-doped graphene junction coupled evanescently to the waveguide, the detector achieves a photoresponsivity exceeding 0. Under zero-bias operation, we demonstrate response rates exceeding 20 GHz and an instrumentation-limited 12 Gbit s21 optical data link.
Graphene demonstrates ultrafast carrier dynamics for both electrons and holes, and it has been shown that a weak internal electric? This carrier multiplication process is equivalent to inherent gain in graphene photodetection, which exists even without external bias, unlike traditional avalanche detection Despite these attractive features, the low optical absorption in graphene results in low photoresponsivity in vertical-incidence photodetector designs.
Recently it has been revealed that coupling graphene to a bus waveguide can enhance light absorption over a broadband spectrum16, Here, we show that, by integrating a graphene photodetector onto a silicon-on-insulator SOI bus waveguide, it is possible to greatly enhance graphene absorption and the corresponding photodetection ef?
In our device, presented in Fig. A thin SiO2 layer 10 nm deposited on the planarized chip electrically isolates the graphene layer from the underlying silicon structures. The optical waveguide mode couples to the graphene layer through the evanescent?
Two metal electrodes located on opposite sides of the waveguide collect the photocurrent. One of these electrodes is positioned nm from the edge of the waveguide to create a lateral metal-doped junction that overlaps with the waveguide mode. The junction is close enough to the waveguide to ef? Figure 1b presents an optical microscope image of one of the fabricated photodetector devices, and Fig.
This device contains a mm-long graphene bilayer, which provides approximately twice the absorption of monolayer graphene. The graphene sheet was mechanically exfoliated and transferred onto the waveguide.
The two electrodes were created by liftoff patterning with separations of nm and 3. The fabrication of this chip-integrated graphene photodetector see Methods requires only two lithography steps and no implantation, making it potentially simpler than the heterogeneous integration of other semiconductors18, Spatially resolved photocurrent measurements were used to con?
The sample was mounted under a confocal microscope on an x—y translation stage and illuminated from above with a 1, nm continuous-wave c. A scanning re? By correlating the metal edges in the re? Figure 2c maps the photocurrent obtained under zero bias voltage.
Because of the approximately micrometre-scale spot size of the excitation laser, we also observed photocurrent under the metal electrodes. The region of greatest photocurrent coincided with the waveguide and reached 13 nA, corresponding here to an excitation power of 50 mW measured after the objective lens.
This responsivity of 2. By deconvolving the photocurrent pro? These authors contributed equally to this work. All rights reserved. A graphene layer is transferred onto the planarized waveguide with a spacing layer of nm-thick SiO2. Two metal electrodes contact the graphene and conduct the generated photocurrent. One of the electrodes is closer to the waveguide to create a potential difference in the graphene to couple with the evanescent optical?
The waveguide is located by correlating the re? A photocurrent pro? Scale bar applies to all panels. Dashed black and solid white lines show the edges of the metal electrodes and the waveguide, respectively.
The result shows that the graphene has potential gradients around the boundaries of the gold electrodes, yielding the corresponding internal electric? The graphene beneath the two metal contacts has the same p-type doping level, which is lower than the intrinsic doping of the graphene channel. Therefore, band bending with opposing gradients occurs at the two electrode junctions. The bottom panel of Fig.
We also plot the? Here, the top and bottom images are aligned horizontally according to the position of the waveguide. It is thus evident that a strong potential gradient overlaps with the waveguide mode.
Therefore, our optimized asymmetric metal electrode design provides a high internal quantum ef? In testing the performance of the waveguide-integrated graphene detector, light was coupled from lensed? The polarization of the input was controlled to match the TE mode of the waveguide.
From waveguide transmission measurements before and after the graphene transfer, we estimate a transmission loss of 4. The transmission loss indicates an absorption coef? Estimating the absorption from the complex effective index of the simulated guided mode, we obtain a slightly lower absorption coef? We attribute the greater absorption coef? From the simulation, we also calculated the contribution of the nm-thick metal contact to the total waveguide absorption, which indicates an absorption coef?
To measure the photodetection ef? Figure 3b plots the detected photocurrent Iphoto as a function of incident power Pinput obtained at zero bias voltage VB? Here, Pinput is the power reaching the waveguide-graphene detector, estimated by considering the input facet coupling loss and the silicon waveguide transmission loss.
We attribute this dramatic responsivity improvement to the longer light—graphene interaction and the ef? Moreover, the curve plotted in Fig. It was possible to further enhance the external responsivity by applying a bias voltage VB across the photocarrier generation region.
The results are shown in Fig. A 10? The red dashed line denotes the Fermi level. Bottom: simulated electric? The top and bottom images are aligned horizontally by referring to the relative position of the waveguide; the position of the right electrode is symbolic. Inset: photocurrent as a function of excited power from a pulsed OPO laser at a wavelength of 2, nm.
When VB. If VB , 0, the photocurrent decreases due to compensation between the external and internal? The photocurrent changes sign when the bias is decreased further. Note that the responsivity is linear with respect to the bias voltage, without saturation even under a high bias, which indicates that the wide evanescent? Thus, an even higher photocurrent is expected under increased bias. To suppress the enhanced dark current for high bias voltages, future studies may induce a bandgap in the bilayer graphene by the application of a strong perpendicular electric?
Owing to the spectrally? Experimentally, we indeed observed a nearly? The long absorption length of the graphene sheet may be expected to enable operation even at high power, because any saturation towards the front of the graphene would be compensated by additional absorption further along the waveguide. Indeed, experimentally we observed no saturation of photocurrent under c.
Photoresponse measurements were also performed using a pulsed optical parametric oscillator OPO source at a wavelength of 2, nm. The pulse duration was fs. The inset to Fig.
We estimate that, under these conditions, the graphene layer experiences a peak intensity of 6. Unlike the case in conventional semiconductors, both electrons and holes in graphene have very high mobility, and a moderate internal electric? We examined the high-speed response of the device using a commercial lightwave component analyser LCA with an internal laser source and network analyser over a frequency range from 0.
A modulated optical signal at a wavelength of 1, nm with an average power of 1 mW emitted from the LCA was coupled into the device, and the electrical output was measured through a radiofrequency microwave probe. The frequency response of the device was analysed as the S21 parameter of the network analyser. Figure 4 displays the a. Because the extremely high carrier mobility of graphene is estimated to result in an intrinsic photoresponse faster than GHz ref.
To gauge the viability of the waveguide-integrated graphene photodetector in realistic optical applications, we performed an optical data transmission at 12 Gbit s A pulsed pattern generator with a maximum 12 Gbit s21 internal electrical bit stream modulated a 1, nm c. About 10 dBm average optical power was launched into the waveguide graphene detector. The output electrical data stream from the graphene detector was ampli?
As shown in the inset of Fig. Because of the frequency limitation of the pulse pattern 0? The relative a. Inset: 12 Gbit s21 optical data link test of the device, showing a clear eye opening. In summary, we have demonstrated a high-performance waveguide-integrated graphene photodetector. The extended interaction length between the graphene and the silicon waveguide optical mode results in a notable photodetection responsivity of 0.
We expect that this responsivity could be improved through the following re? First, higher graphene absorption for the photodetector may be achieved by extending the graphene length and by coupling the graphene with a transverse magnetic waveguide mode with a stronger evanescent?
Chip-integrated ultrafast graphene photodetector with high responsivity
Optical properties and devices Abstract Graphene is a very attractive material for broadband photodetection in hyperspectral imaging and sensing systems. However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors. Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication within graphene nano-stripes with the ultrafast transport of photocarriers to gold patches before recombination. Through this approach, high-responsivity operation is realized without the use of bandwidth-limiting and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to infrared regime 0.
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However, the weak optical absorption of graphene2,3 limits its photoresponsivity. To address this, graphene has been integrated into nanocavities9, microcavities10 and plasmon resonators11,12, but these approaches restrict photodetection to narrow bands. Hybrid graphene—quantum dot architectures can greatly improve responsivity13, but at the cost of response speed. Here, we demonstrate a waveguide-integrated graphene photodetec- tor that simultaneously exhibits high responsivity, high speed and broad spectral bandwidth. Using a metal-doped graphene junction coupled evanescently to the waveguide, the detector achieves a photoresponsivity exceeding 0. Under zero-bias operation, we demonstrate response rates exceeding 20 GHz and an instrumentation-limited 12 Gbit s21 optical data link. Graphene demonstrates ultrafast carrier dynamics for both electrons and holes, and it has been shown that a weak internal electric?