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GRAphene nonlinear PHotonic Integrated CircuitS

Periodic Reporting for period 4 - GRAPHICS (GRAphene nonlinear PHotonic Integrated CircuitS)

Reporting period: 2020-03-01 to 2021-05-31

The Internet traffic that sustains our global society and economy is growing exponentially. Yet, the energy requirements for providing this bandwidth can no longer be ignored and will continue to increase 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 in optical fibres, tasks such as data treatment or routing are performed in electronic routers, creating bottlenecks. More energy-efficient and compact methods are needed. One promising solution is the development of all-optical technologies in which the optical information would be directly processed across the network via another optical signal, i.e. without being converted into electrical signals. Such technology could efficiently complement electronic routers, and requires the development of novel platforms, in which the light–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 compact and integrated chips. GRAPHICS aims at developing such devices based on graphene/ semiconductor hybrid integration and nanophotonic concepts that promote light-matter interaction.

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.

GRAPHICS aims to create novel graphene/ semiconductor hybrid platforms, beyond silicon photonics, for the generation, processing, and manipulation of fast optical signals on-chip at Telecom wavelengths. Our research program focuses 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 rely on two distinct nonlinear features of graphene, i.e. its saturable absorption and its nonlinear Kerr response. In addition, the capability of electrically tuning graphene optical properties can enable the creation of reconfigurable intelligent optical devices. The resulting architectures will underpin a variety of applications related to telecommunications, datacom, and inter-and intra-chip optical communications.
The project aims to realize three types of optical devices –pulsed lasers on a chip, nonlinear optical devices and electrically tunable devices.

1/ Towards integrated pulsed lasers:
Optimized III-V photonic crystal cavities bonded onto low index substrates have been designed and fabricated. A new approach has been developed to achieve short cavities that sustain a comb of modes with equidistant frequencies by exploiting dispersion engineering of slow light modes. A resulting 30µm long cavity can provide 9 modes equally spaced across a 12nm bandwidth, i.e. equivalent to ten times longer standard cavities, enabling, in principle the generation of sub-picosecond pulses at 180GHz. The feasibility of these designs has been experimentally validated and a multimode laser signature observed, a first step towards the creation of compact chip-based pulsed lasers.

2/ Reconfigurable/ tunable optoelectronic devices:
A new method to dope graphene has been shown via depositing a high work function WO3 oxide in the surrounding of graphene. It has been exploited in electrically tunable broadband graphene-based Bragg mirrors that demonstrated 20% modulation for low voltage swings of +/-1V [Wood Opt. Express 2020].
Coupled resonators have been studied for increasing the device tuning capabilities offered by graphene [Lhuillier, CLEO 2020]. Metallic plasmonic structures and graphene based plasmons could be made to efficiently interact, providing the basis for low power consumption modulators [Wood JOSAB 2020].

3/ Graphene based nonlinear integrated devices
Several material platforms have been investigated for creating all-optical devices on a chip both at telecom wavelengths and in the mid-infrared.
Ge based nonlinear waveguides have been realized to generate broadband supercontinuum generation in the mid-IR [Sinobad OL 2020]. Besides Telecom applications, the broad spectral band emitted by these light sources is particularly relevant for biosensing. Graphene has been integrated on these chips to explore its potential in this new spectral band.
Hybrid graphene/ SiN nonlinear waveguides have been realized and tested. Fast and fully integrated saturable absorbers have been demonstrated experimentally, supported by appropriate modeling relying on carrier dynamics in graphene [Demongodin, APLP 2019]. Yet, the all-optical response of graphene could not be observed unambiguously. Instead, some nonlinear enhancement has been measured on graphene oxide/ SiN waveguides in collaboration with Swinburne [Qu Adv Mat 2020].
The project has resulted in new cavities that should lay the foundations for integrated and compact pulsed lasers on a chip. Mode-locked lasers currently consist of stand-alone and generally bulky equipment. 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 therefore represents a critical milestone in the development of all-optical signal processing chip-based architectures. Our approach should enable the realization of 100um long on-chip microlasers that generate a train of short optical pulses in the Telecom band. Such devices are key in future high capacity communication systems, photonic switching devices and logic gates, optical interconnects and optical clock distribution and recovery. Our results on highly compact optimized multimode microlasers represent the first step towards the demonstration of miniaturized pulsed lasers on a chip.

In parallel, harnessing the Kerr nonlinearity of dielectric materials is also promising for the realization of less conventional light sources, such as supercontinuum. GRAPHICS contributed to develop such functions out of efficient chip based structures. Si-based 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 at telecom and mid-IR wavelengths. GRAPHICS showed how 2D materials (like graphene, graphene oxide) could be integrated onto dielectric waveguides for the development of all-optical energy efficient devices. Nonlinear graphene chip based saturable absorbers have been demonstrated, and graphene oxide has proved to be a suitable candidate for enhanced nonlinear optics.
Schematic of the GRAPHICS overall objectives
Artist impression of the targeted pulsed laser device integrated onto a silicon photonic circuit