1. Two-dimensional materials and their chemical manipulation for multidisciplinary electronics and optoelectronics
With silicon electronics reaching its fundamental dimensional limits, alternative materials are required for the next generation of electronics. Furthermore, a wide range of fields stand to benefit from systems that are smaller in size whose performance can be enhanced simultaneously. Two-dimensional materials are a fascinating class of atomically thin materials that possess quantum confinement induced properties which can be harnessed for high-speed, low-dimensional energy-efficient devices and systems. This research project will explore a variety of two-dimensional material systems including elemental analogues of graphene and explore their deterministic positiong on structured substrates.
Recently, the exponential demand for computing systems addressing the I/O bottleneck i.e. providing higher communication bandwidths and lower power consumption has fostered a significant amount of work into “More than Moore” approaches. Particularly, analog computing approaches based on stochastic and neuromorphic computing are expected to provide considerable benefits over more traditional transistor-based von Neumann approaches in e.g. machine learning (ML)-based applications thanks to their highly parallel architectures, suitability to implement cost-effectively matrix multiplication operations and to the possibility of exploiting a whole range of emerging technologies e.g. non-volatile memory (NVM) components, potentially leading to better energy consumption and bandwidth performance. More specifically, integrated photonic approaches in Von Neumann architectures are already emerging as a viable solution to solve such bottleneck targeting, at first, high performance and data-centric computing applications –. In this context, the advantages of integrated photonics are currently being explored also for stochastic and neuromorphic computing. However, key photonic devices exploiting e.g. memory effects for such applications have several limitations (e.g. short cavity lifetime, large footprint) in standard Silicon-On-Insulator (SOI) platforms for telecom applications. Hence, the scientific interest of exploring other platforms such as the Lithium Niobate on Insulator (LNOI) platform which provides all the standard photonic components (waveguides, modulators, filters…) as well as photonic memory-dependent devices without the limitations of SOI platforms thus leading to potentially higher performance computing systems.
Point-like defects in wide-bandgap materials are attracting intensive research attention owing to prospective applications in quantum technologies (information processing, sensing) and in near-infrared spectrum bio-imaging. The reason is three-fold: (i) these defects can be considered as artificial atoms with highly efficient optical transitions (single-photon sources realization); (ii) they may encompass charge, orbital and spin degrees of freedom, with possibility for instance of optical control of the spin (Qubit application); (iii) the spin and electronic states can be well isolated from environmental fluctuations leading to record spin coherence. In this context, the nitrogen-vacancy (NV) center in diamond has become a highly mature system, used for a large range of applications. Nevertheless, since 2010, point defects in SiC have been intensively studied. Indeed, SiC presents advantages for these applications: (i) growth at an industrial scale ; (ii) control of the technological steps for devices realization thanks to the upstream of power electronic applications ; (iii) unparalleled properties making SiC an ideal platform for photonic quantum information processing.
After more than 40 years of continuous evolution, our computing systems are reaching their limits. Indeed, the architecture of Von-Neumann, on which our computers are based, physically dissociates the hearts of calculations from the memory. The sequential processing of information is thus confronted with a bottleneck, more commonly known as "Memory Bottleneck". One solution is to draw inspiration from the natural mathematical paradigms of the human brain, in which the data are massively parallel processed with high energy efficiency, realizing the hardware implementation of neuromorphic networks. Currently, we live in a digital and connected world that relies on the rapid transmission and processing of digital data. This is mainly the result of the development of the microelectronics industry, which is the basis of most modern technological devices. This field is currently facing scientific bottlenecks linked to metallic interconnections in microprocessors, preventing further progress in terms of speed and energy efficiency. Electronic/photonic/thermal convergence is currently considered to be one of the most promising solutions for quickly solving this problem. However, silicon, the basic material of the microelectronics industry, is inherently limited in terms of electro-optical properties. In this context, great interest is paid to the heterogeneous integration of new functional materials on silicon substrates. The relevance of this strategy is twofold: on the one hand, it can allow the use of naturally functional materials with electro-optical properties far superior to those of silicon but also with original properties such as changes in the crystallographic phase as well as thermo-optics. And secondly, maintaining silicon - in particular, Silicon On Oxide (SOI) - as a platform allows the integration of electronical, optical and thermal functionalities.
Obtaining light emission from Si or Ge is a topic of high interest because of their abundance and their compatibility with the microelectronic industry. However, because of their indirect band gap these materials suffer from poor light emission efficiency. In the last years several papers have predicted the emergence of new properties such as direct band gap in hexagonal Si or Ge that would make them suitable for integrated light source. However the growth of hexagonal Si or Ge is still a challenge, and the most promising method consist in growing Si or Ge on the facets of wurztite (WZ) III-V epitaxial nanowires. Using self-catalytic nanowire growth by molecular beam epitaxy, we recently shown the possibility to obtain long WZ segment in zinc-blende (ZB) nanowires without introduction of gold in the system. Our method is based on an accurate control of the molecular beam ratio between the group III and group V species, an in situ and real time monitoring of the crystalline structure by Reflective High Energy Diffraction and the utilization of a home-made modelling code of the Vapor-Liquid-Solid growth mechanism.
The Mid-infrared (Mid-IR) wavelength range - from 2.5 to 13 µm - is currently experiencing a huge surge in interest for an enormous range of applications that affect almost every aspect of our society, from compact and highly sensitive biological and chemical sensors, imaging, defense and astronomy. A notable feature of the Mid-IR is that most chemical and biological compounds that relate to our health, safety and environment have a strong spectral signature in this spectral range. The Mid-IR therefore offers unique opportunities for the development of technologies with a high societal (sensor applications, defence, industrial and environmental security, etc.) and fundamental impact (chemistry, biology, astrophysics, etc.). Many actors in the nanophotonics scene have invested in this theme in the USA (Air Force research lab, Harvard, UCLA, Princeton MIRTHE, IBM, Cornell etc...), Australasia and Europe (INL, C2N, University of Surrey, Southampton, University of St Andrews, Ghent/IMEC).
It is well established that photons are suitable for communication and electrons for computing. Yet the border between computing and communication has become somehow blurred when considering emerging research domain such as optical interconnects and photonic neuromorphic computing. In many contexts it has been realized that photonics can help computation by providing an efficient interface for short-distance communication or for performing simple operations but in a massively parallel fashion. Here we consider the problem of dealing with high-speed signals in the optical domain needing to be digitized with very high accuracy. This PhD project will address the most critical operation here, namely extremely accurate sampling with minimal timing error.
The Mid-infrared (Mid-IR) wavelength range - from 3 to 15 µm - is currently experiencing a huge surge in interest for an enormous range of applications that affect almost every aspect of our society, from compact and highly sensitive biological and chemical sensors, imaging, defense and astronomy. A notable feature of the MIR is that most chemical and biological compounds that relate to our health, safety and environment have a strong spectral signature in the medium infrared. The MIR therefore offers unique opportunities for the development of technologies with a high societal (sensor applications, defence, industrial and environmental security, etc.) and fundamental impact (chemistry, biology, astrophysics, etc.).
An optical microcavity (a ring resonator, a photonic crystal cavity…) when properly designed and manufactured can store the light over a very long time and in a small volume potentially leading to high intensities and thus dramatically enhancing the light matter interaction. This is of particular relevance for exploring fundamental aspects, such as non-linear effects, light generation and sensing applications. High-Q microresonators in the mid-IR have been introduced only recently and the topic remains very challenging.
The rise of the field of Silicon Photonics (SiPh) in the last decades has been heavily supported by the need to move ever-growing amount of data more efficiently and faster. In particular, the leveraging of CMOS-compatible platforms has allowed to provide SiPh solutions, which were technologically already achievable thanks to the infrastructure built by the microelectronics industry. However, this opportunity meant also that the vast majority of the solutions investigated were technologically limited to what was readily available in the foundries.
Recently, proposed SiPh solutions are starting to exploit novel technological approaches, often hybrid, to still leverage the benefits from the microelectronics industry, but with the addition of key functionalities. In particular, the field of optical signal processing is constantly seeking solutions that are more and more integrated for scalability. However, integrated solutions in SiPh platforms for applications such as photonic neuromorphic computing, which heavily rely on network reconfigurability properties, present some drawbacks. In fact, key photonic devices exploiting e.g. memory effects for such applications have several limitations (e.g. short cavity lifetime, large footprint) in standard intrinsic Silicon-On-Insulator (SOI) platforms for telecom applications. Hence, the scientific interest of exploring hybrid integration of functional materials, the emerging 2D materials in particular, onto silicon photonics platform to compensate the drawbacks of pure Si will lead to the fully functional neuromorphic photonics for high-end computing on mature silicon photonics platform.
In the past two decades, silicon photonics has emerged as a mature technological platform allowing for multiple optical functions to be integrated onto the same chip . Electro-optic modulators, SiGe photodetectors and low-loss silicon waveguides are now available. However, when it comes to light emission or nonlinear functions, silicon turns out to be intrinsically limited. The heterogeneous integration of III-V materials onto silicon has already provided a way to realize efficient LED or laser devices . Similarly, several material candidates are investigated for their nonlinear properties, with the aim to integrate them onto the mature silicon photonic platform. The nonlinear optical response of materials can enable attractive optical devices such as all-optical switches and even amplifiers that can directly control light signals with other light signals. These all optical nonlinear devices are much faster than their optoelectronic counterparts and perhaps more importantly, they can enable completely new functions such as wavelength conversion, the generation of frequency combs or supercontinuum light. More generally, a wide range of nonlinear devices can be realized for information processing using light control signals . These could advantageously complement the power-hungry and bulky electronic routers that are used in telecommunications. These routers perform data processing and signal routing in the electrical domain, by converting optical signals into the electical domain and after processing back in the optical domain to convey information across the Internet network. However, as the data rate increases so does the energy consumption of the electronic components in routers, calling for the need to develop alternative and disrupting technologies. All-optical devices could play a central role there.