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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - GRAPHICS (GRAphene nonlinear PHotonic Integrated CircuitS)

Teaser

Summary

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 fuelled 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 is the development of 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 could efficiently complement electronic routers, and crucially requires the development of novel platforms, in which the lightâ€“matter interaction is dramatically increased with respect to that in fibres. 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 ~1.55um 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

Work performed

The project aims to realize three types of optical devices â€“pulsed lasers on a chip, nonlinear optical devices for routing and processing light signals and electrically tunable devicesâ€“ 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 30Âµm 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
-Optical characterization of the fabricated multimode cavities, which exhibit 6 equally spaced modes from 30um long structures. These cavities have been probed by photoluminescence under increasing pump power and exhibit a multimode laser signature. This first step confirms the feasibility of our approach for creating compact chip-based pulsed lasers.

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.

Regarding nonlinear devices, a number of material platforms have been investigated for creating all-optical devices on a chip, namely :
-SiGe/ Si nonlinear waveguides have been realized and provide broadband supercontinuum (between 4um and 8um) generation under femtosecond pulsed excitation. The broad spectral band emitted by these light sources is particularly relevant for biosensing as many molecular agents have strong fingerprint in this window.
-SiN nonlinear waveguides have been realized with a new recipe at CEA-Leti and exhibit fundamental characteristics (dispersion, nonlinear response) that are suitable for nonlinear chip-based devices.

Final results

\"The project has led to the design and realization 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 microlasers, 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 (less than 100um in length) 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 realization of these devices have been laid during the first half period of the project. Preliminary measurements on highly compact optimized cavities have demonstrated the capability to sustain multimode laser operation from equidistant resonance frequencies, thereby representing the first step towards the demonstration of miniaturized pulsed lasers on a chip.

In parallel, the development of chip-based nonlinear devices for all-optical technologies avoiding the use of electronics is attractive for ultra-fast applications, such as signal processing, routing, as well as wavelength conversion and amplification. Harnessing the Kerr nonlinearity of dielectric materials is also promising for the realization of less conventional light sources, such as supercontinuum. GRAPHICS aims at achieving such functions in compact chip based structures that consume less power. Graphene has been identified as a highly nonlinear material, but it requires strategies for its interaction with light to be enhanced adequately. Other materials, such as Silicon nitride, or SiGe/ Si, have been explored in the project, and demonstrated their potential for the generation of continuum and supercontinuum. GRAPHICS will study the opportunity windows offered by the integration of graphene onto dielectric waveguides and resonant structures, for the development of all-optical energy efficient building blocks.

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