• electro-absorption
• electro-optical modulator
• graphene
• optoelectronics
• photonics

Over the last few decades, the massive demand of electronic resources has generated a steady improvement over time of the electronic devices' performance. However, the current micro-chip's manufacturing technology is facing its limitations, mainly due to the presence of copper-based interconnections. A possible solution could be replacing them with optical-based links. The electronic components, which constitute the device, would be linked together via optical fibres. This way, the signal will travel along the network thanks to electro-optical modulators coupled with photo-detectors. In order to achieve optical-based interconnections, a novel material such as graphene could step up the pace in enhancing the modulators' performance. This application offers the best opportunity for graphene to outclass the semiconductor-based competitors due to its unique optical and electronic properties. The thesis purpose is to pave the way for graphene-based modulators.

This Master thesis describes my work on studying, developing and characterizing a graphene-based electro-absorption modulator. Through the study of graphene's optical and transport properties we manage to lay robust foundations for our target. The graphene's optical conductivity is evaluated in the linear response theory framework without any approximations. The charge carriers density has been numerically evaluated to avoid inaccuracies whenever a calculation near the charge neutrality point is required, i.e. when the chemical potential lies at the Dirac point. An analytical relationship between the charge carriers scattering time and the electrostatic doping in pristine graphene has been developed. With these improvements of the background theory, we can effectively tune the main device's figures of merit. Another improvement over the electronic properties is the introduction of a high K dielectric material namely Hafnium dioxide. With this approach it is possible to tune the dielectric properties of the modulator, which are critical in the achievement of the Pauli-blocking regime. Numerical optical simulations of the structure allow us to compare experimental data with theoretical predictions and to optimize the device in order to prove the design potential. Finally, we proceeded with the fabrication of many devices. It is possible to subdivide the fabrication process in three parts: mechanical exfoliation of graphene and hexagonal-boron-nitride, Van der Waals assembly process and nano-fabrication stage. My contribution was limited to the mechanical exfoliation stage due to the high level of experience and training required in other tasks. Nevertheless, I was able to witness all the processes and get the heart of the matter.

In this work, we have fabricated a graphene-based EA modulator able to reach both AC modulation efficiencies of 1dB/V and speeds up to 40Gbps. The small device footprint (53 um^2) and the low drive voltage (Vpp = 3.5V) reduce the energy consumption down to 285fJ/bit. The 2D-3D integration of hexagonal-boron-nitride and Hafnium-dioxide allow a symmetric and hysteresis-free operation of the modulator device, while withstanding high voltage loads. Moreover, this dielectric combination allows us to probe experimentally the optical conductivity of graphene even beyond the Pauli-blocking regime.

This project has not only allowed the fabrication of a device which outperforms state-of-the-Art commercial EA modulators in terms of speed and modulation efficiency, but also has highlighted many issues and open questions which have to be answered in order to unlock the full potential of graphene-based modulators.