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GRAPHICS Report Summary

Project ID: 648546
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - GRAPHICS (GRAphene nonlinear PHotonic Integrated CircuitS)

Reporting period: 2015-09-01 to 2017-02-28

Summary of the context and overall objectives of the project

"Over the past 30 years, optical communications have transformed the way we transfer information around the world. Its underlying ultra-high speed fibre-optic network now plays a critical role in our global society and economy. The Internet traffic is growing exponentially, as fueled by new broadband access technologies and services. Yet, the energy requirements for providing this bandwidth can no longer be ignored and should continue to increase along with the amounts of exchanged data. The cost, size, and energy consumption of the present electronic equipment that manages these massive data flows across the network calls for the development of disruptive routing technologies. While the use of light has proved to be the most effective way to carry data over long distances, tasks such as data treatment, storage or routing are performed in electronic routers, creating bottlenecks. More energy-efficient, compact and cooler methods are needed. One promising solution lies in moving to all-optical technologies in which the optical information would be directly processed and redirected across the network via another optical signal, i.e. without being converted back and forth into electrical signals. Such technology crucially requires the development of novel platforms, in which the light–matter interaction is dramatically increased with respect to that in fibers. This implies an appropriate choice of materials and geometries to efficiently manipulate high-speed optical signals in very compact and integrated chips. GRAPHICS will develop new solutions based on graphene/ semiconductor (III-V and SOI) hybrid integration and nanophotonic concepts that promote light matter interaction, in order to create efficient devices for high-speed manipulation of optical signals. The resulting devices will lay the foundations of a novel routing technology that can directly manage, at the heart of the network, high-speed optical data in far more compact platforms that consume less power.
Ideally, the developed technology should be compatible with CMOS processing and architectures, for enabling the convergence of optics and microelectronics. Computer chips meet with severe difficulties to keep up with increasing processing speed performance while maintaining low power consumption. Microelectronics would thus equally benefit from the development of inter- and intra-chip optical interconnects, where the advantages of optics high transfer speed, high data-handling capacity are brought down to the chip level.
While silicon photonics has led to strong advances in this context, crystalline silicon has severe limitations precluding its sole use for the applications mentioned above. GRAPHICS aims to create novel graphene/ semiconductor hybrid platforms for the generation, processing, and manipulation of fast optical signals on-chip at ~1.55m Telecom wavelengths. Our research program will focus on two main classes of nonlinear optical devices: (1) integrated pulsed III-V/ Si microlasers capable of generating short optical pulses on a chip, and (2) all-optical signal processing devices using light to control light in a fast and efficient manner. These devices will rely on two distinct nonlinear features of graphene, i.e. its saturable absorption and its nonlinear Kerr response, respectively. In addition, the capability of electrically tuning graphene optical properties will be explored to create fundamentally flexible and reconfigurable intelligent optical devices. The resulting architectures will be the cornerstone of a novel chip-based optical routing and processing technology that underpins a variety of applications related to telecommunications, datacom, and inter-and intra-chip optical communications.
The two classes of nonlinear devices targeted in the project represent significant achievements in their own right. However, they share some scientific and technological challenges. For instance, relevant strategies must be found for enhancing the typically low interaction of light with the monolayer of carbon atoms, as needed for the device miniaturization. Here, we will combine graphene with advanced microphotonic concepts and structures capable of strongly confining light microcavities, slow light and other optimized waveguide geometries to enhance the light-graphene interaction and realize compact chip-scale devices. More fundamentally, these two classes of nonlinear devices will jointly contribute to shape the long-term vision of a fully integrated photonic platform, in which the pulsed microlaser delivers directly on-chip the optical peak power necessary to trigger all other "intelligent" devices onto the same circuit. GRAPHICS will therefore help to "draw" a novel generation of photonic integrated circuits and architectures, with graphene playing a key role, to be used for managing high-speed optical data."

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project aims to realize three types of optical devices –pulsed lasers on a chip, nonlinear optical devices for routing and processing light signals and electrically tunable devices by exploiting a graphene/ semiconductor hybrid platform. As foreseen in the timeline, efforts during this first period have mainly focused on the realization of integrated pulsed lasers using III-V photonic crystals and graphene. To this aim, much work has been carried out with respect to the design and fabrication of the building blocks for these laser devices, with the main following outcomes:
-Design and optimization of III-V photonic crystal cavities bonded onto low index (silica) substrates. Compact structures have been designed so as to achieve short cavities that sustain a comb of modes with equidistant frequencies by exploiting dispersion engineering of slow light modes. For instance, a resulting 30µm long cavity can provide 9 modes equally spaced across a 12nm bandwidth, i.e. equivalent to ten times longer standard cavities. The associated mode-locked laser is expected to generate a train of sub-picosecond pulses at a 178GHz repetition rate.
-Theoretical evidence of bistable signature under certain conditions through combining graphene and an active photonic crystal laser.
-Nanofabrication of III-V photonic crystal structures that are bonded onto silica substrates for improving heat dissipation and mechanical stability

In parallel, much work has been devoted to the development and nanofabrication of silicon photonic devices, such as Bragg mirrors, for providing first proof of concept devices in which graphene is integrated and strongly interacts with the resonant modes of the structures. These hybrid graphene/ Si devices should enable us to study the optical nonlinear response of graphene, a critical step for the device applications (lasers and nonlinear devices) intended in GRAPHICS.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

"The project has led to the design of new cavity based structures that should provide the basis for the realization of integrated and compact pulsed lasers on a chip. Mode-locked lasers currently consist of stand-alone and generally bulky equipment, which has precluded their widespread use for applications. The development of an integrated photonic platform to be used for managing and/or transferring high-speed optical data, intra-chip and inter-chips, requires "intelligent" all-optical functions that directly control, process, manage, convert or amplify fast rates of optical data without opto-electronic conversions. However, all of these functions rely on external and relatively bulky near-infrared pulsed lasers coupled to the chip with more or less efficient and sophisticated strategies. Similarly, a landmark paper published in Nature late 2015 reported on the co-integration of photonic and microelectronic architectures on a chip for improved functionality and data treatment; yet, the required light source was external to the chip. So far, the heterogeneous integration of III-V semiconductors onto silicon has led to the development of chip-based semiconductor lasers, but these devices only generate continuous-wave optical signals or which can be at best modulated at a few tens of GHz. The realization and integration of near-infrared pulsed lasers on-chip therefore represents a critical milestone in the development of microchips with advanced functionalities, such as fully integrated all-optical signal processing architectures. Here, we will realize on-chip microlasers that generate a train of optical pulses in the Telecom band, with high stability, low pulse duration (sev. 100fs sev. ps), and high repetition rates (sev. 100GHz), much higher than those achievable with electronics. Such devices are key in future high capacity communication systems, photonic switching devices and logic gates, optical interconnects and optical clock distribution and recovery. The foundations for the fabrication of these devices have been laid during this first project period."

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