Supporting Information for: 30 GHz optoelectronic mixing in CVD. graphene

Transcription

1 Supporting Information for: 30 GHz optoelectronic mixing in CVD graphene Alberto Montanaro,, Sana Mzali, Jean-Paul Mazellier, Odile Bezencenet, Christian Larat, Stephanie Molin, Loïc Morvan, Pierre Legagneux, Daniel Dolfi, Bruno Dlubak, Pierre Seneor, Marie-Blandine Martin, Stephan Hofmann, John Robertson, Alba Centeno, and Amaia Zurutuza Thales Research and Technology, 1, Avenue Augustin Fresnel, Palaiseau, France, Unité Mixte de Physique CNRS/Thales, 1, Avenue Augustin Fresnel, Palaiseau, France, Department of Engineering, University of Cambridge, Cambridge CB21PZ, United Kingdom, and Graphenea S.A., Tolosa Hiribidea, 76 E Donostia, Spain To whom correspondence should be addressed Thales Research and Technology, 1, Avenue Augustin Fresnel, Palaiseau, France Unité Mixte de Physique CNRS/Thales, 1, Avenue Augustin Fresnel, Palaiseau, France Department of Engineering, University of Cambridge, Cambridge CB21PZ, United Kingdom Graphenea S.A., Tolosa Hiribidea, 76 E Donostia, Spain 1

2 Optical power and photocurrent calculation As presented in Fabrication and experimental setup, the optical beam was modulated, amplified, and then focused on the device. Only the laser power impinging on the active area (graphene channel) was taken into account to calculate the incident power, while the part of the spot out of the graphene channel was excluded. We experimentally verified that no high-frequency photocurrent was generated if there was no overlap between the laser spot and the graphene channel. Plots 4a and 4c, show direct measurements on the spectrum analyser. All the other measurements took into account the device and cables losses. This correction has been done to extract the intrinsic response of graphene to light excitation. The procedure to calculate the photocurrent and photoresponsivity is described in the following. We first measured the device and cable electrical S parameters, from which we extracted the losses. For plots in figure 3a, 3b and 3c, the laser beam was modulated in intensity at a frequency f opt =5 GHz. We registered the electrical power value of the photodetected signal on the spectrum analyser and we subtracted the losses of the device at 5 GHz, as well as the cable losses between the output of the waveguide and the spectrum analyser. Finally, from this corrected power value, the photocurrent value was extracted taking into account the 50 Ω impedance of the spectrum analyzer. All the plotted photocurrent values are expressed in RMS. The same photocurrent extraction procedure has been adopted for plot 3d, at each recorded frequency. This plot shows the photoresponsivity ( I photo P m ). The P m value was measured with an optical spectrum analyser. This value decreased with frequency due to the MZM bandwidth limitation (20 GHz). The same method was used to determine the power values plotted in figure 4d. 2

3 Downconversion efficiency To calculate the efficiency of our device employed as an OEM, one can refer to figure 4a. The photodetection configuration (red dashed curve) is compared to the optoelectronic mixing configuration (blue curve). In both cases, an electrical power P IN is provided to the device. In the photodetection configuration, it can be calculated from P IN = V DC 2, where R is the R resistance of the device (graphene channel + contacts). R is about 3.3KΩ at the CNP. The consumed electrical power on the device is around 4 dbm, and the photodetected power is P OUT =-94 dbm. In the OEM configuration, The injected AC power is P IN =14 dbm. But a large part of this power is reflected, while only a small part is transmitted and absorbed. Only the non reflected part is useful for the optoelectronic mixing effect targeted. It can be obtained from the S parameters at 400 KHz and is 1 (S 11 ) So, the useful electrical power is around 4 dbm. As a conclusion, in figure 4a, the same optical power and about the same electrical power are involved for the photodetection and OEM process, thus allowing a direct comparison. Raman spectrum measurements Figure S1 shows the Raman spectra of the graphene channel, before and after the overall lithographic process. The D/G peak ratio in the two spectra is very similar. This shows that the device fabrication process does not degrade the graphene structural quality, according to Raman analysis. 3

4 Figure S1: Raman spectrum before and after the overall fabrication process Absorbed optical power calculation Figure S2 shows the path of light passing through our device. To calculate the absorbed power in graphene (A in figure S2) we used the transfer matrix method. 1 The needed material parameters (thickness and refractive index) are listed in figure S2. A graphene thickness of 0.34 nm is used, that corresponds to the extension of graphene π orbitals out of plane. 2 Only the refractive index of graphene is complex. We used the extinction coefficient value found in recent literature 3 for non-doped graphene (Fermi level coincides with Dirac point). The other materials (Al 2 O 3, SiO 2 and high-resistivity silicon) present no absorption at 1.55 µm wavelength, thus exhibiting a real refractive index. Figure S3 shows the simulation of optical absorption as a function of the SiO 2 layer thickness. With a 2µm SiO 2 thick layer (our case), the absorption is around 1.8 % of the total incident 4

5 power (red point in figure S3). A maximum peak of 3.3 % is reached at different thickness values. Figure S2: Path of light in the gcpw, constituted by a stack of different materials. The orange arrow indicates the total incident optical power(i), while the green and blue arrows indicate respectively the reflected (R) and transmitted (T) optical powers. The air and the silicon substrate thickness were considered as semi-infinite. The materials are indicated in the right part of the figure, together with the corresponding refractive indexes and thicknesses. These data were used to simulate the absorbed optical power (A) shown in figure S3. 5

6 Figure S3: Simulation of the absorption as a function of the SiO 2 layer thickness. In our device, the thickness is equal to 2µm (red dot), and the corresponding absorption is around 1.8% of the total incident power. References (1) Saleh B.; Teich M. Fundamentals of Photonics, 2007, Wiley Series in Pure and Applied Optics. Wiley. (2) Pauling L. The Nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Modern Structural Chemistry, 1960, George Fisher Baker Non-Resident Lecture Series. Cornell University Press. (3) Xu F.; Das S.; Gong Y.; Liu Q.; Chien H.-C.; Chiu H.-Y.; Wu J.; Hui R. Applied Physics Letters, 2015, 106,