Photonic Spiking Neural Networks based on Q-switched laser integrated on Silicon
Neuromorphic networks for signal and information processing have acquired recently a renewed interest considering the more and more complex tasks that have to be solved automatically in current applications: speech recognition, dynamic image correlation, rapid decision processing integrating a plurality of information sources, behavior optimization, etc… Several types of neuromorphic networks do exist and, among them, the spiking type (SNN), that is, the one closest in behavior to the natural cortical neurons. SNN are the ones who seem to be able to offer a best energy efficiency and thus offer scalability. Several demonstrations have been made in this domain with electronic circuits and more recently with photonic circuits. For these, the dense integration potential of silicon photonics is a real advantage to create complex and highly connected circuits susceptible to lead to complete demonstrations. The PhD goal is to exploit a photonics spiking neuromorphic network architecture based on pulsed (Q-switched) lasers interconnected by a dense and reconfigurable optical network on chip mimicking the synaptic connections. A complete laser, neuron then circuit model is expected with, in the end, the practical demonstration of an application in mathematical data processing (to be defined).
Design, fabrication, and characterization of GeSn alloy-based laser sources for mid-infrared silicon photonics
You will design and fabricate laser and LED sources based on GeSn alloy in a cleanroom environment. These novel group-IV direct-bandgap materials, epitaxially grown on 200 mm Si wafers, are considered CMOS-compatible and hold great promise for the development of low-cost mid-infrared light sources. You will characterize these light sources using a mid-infrared optical test bench, with the goal of their future integration into a Germanium/Silicon photonic platform. Additionally, you will assess the feasibility of gas detection within a concentration range from a few dozen to several thousand ppm.
The objectives of the PhD are to:
• Design efficient GeSn (Si) stack structures that confine both electrons and holes while providing strong optical gain.
• Evaluate the optical gain under optical pumping and electrical injection at different strain levels and doping concentrations.
• Design and fabricate laser cavities with strong optical confinement.
• Characterize the fabricated devices under optical and electrical injection as a function of their strain state at both room and low temperatures.
• Achieve electrically pumped continuous-wave group-IV lasers.
• Understand the physical phenomena that may impact the material and device performance for light emission.
• Characterize the best-fabricated devices for low-cost environmental gas detection applications.
This work will involve collaborations with international laboratories working on the same dynamic research topic.
Study of new photodiode architecture for IR imagers
In the field of high-performance infrared detection, CEA-LETI plays a leading role in the development of the HgCdTe material, which today offers such performance that it is integrated into the James Webb Space Telescope (JWST) and allows the observation and study of deep space with unparalleled precision to date. However, we believe that it is still possible to make a significant step forward in terms of detection performance. Indeed, it seems that a fully depleted structure, called a PiN photodiode, could further reduce the dark current (and thus reduce noise and gain sensitivity at low photonic flux) compared to the non-fully depleted structures currently used. This architecture would represent the ultimate photodiode and would allow either a further increase in performance at a given operating temperature or a significant increase in the operating temperature of the detector, with the potential to open new fields of application by greatly simplifying cryogenics.
Your role in this thesis work will be to contribute to the development of the ultimate photodiode for very high-performance infrared detection, characterize and simulate the PiN photodiodes in HgCdTe technology manufactured on our photonic platform.
Candidate Profile:
You hold a Master's degree in optoelectronics and/or semiconductor material physics and are passionate about applied research.
The main technical skills required are: semiconductor component physics, optoelectronics, data processing, numerical simulations, interest in experimental work to carry out characterizations in a cryogenic environment but also theoretical work to carry out numerical simulations.
The PhD student will be integrated into a multidisciplinary team ranging from the growth of II-VI materials to electro-optical characterization, including microelectronics manufacturing processes in clean rooms and the packaging issues of such objects operating at low temperature.
3D interferometric imaging system with reception module in integrated optics
3D sensing by capturing depth images, is a key function in numerous emerging applications such as augmented reality, robotics and telemedicine. The laboratory has developed an innovative 3D sensing micro-optical prototype, using a frequency modulated Lidar technology with simultaneous illumination of the whole scene. The next step is the miniaturization of the setup with integrated optics. A first PhD is ongoing in the laboratory, focusing on the integration of the illumination module.
The proposed PhD will target the definition of an integrated optics architecture for the reception module. The main objective is to realize the beam recombination with integrated optics, using waveguides and grating couplers, to enable the heterodyne mixing of light back-scattered by the scene with the local oscillator. The candidate will design these integrated optical components in connection with the refractive optical system, simulate the propagation of the beams and interference using Lumericaland Zemaxsoftwares, contribute to device realization in clean room, perform the optical characterization of the components, and experimentally validate the proof of concept of depth imaging with the miniaturized prototype.
Depending on the progress of the developments, the PhD will include the development of a module combining the illumination and reception functions in a single component. Several patents, publications and presentations in international conferences are expected in the framework of this PhD.
development of capacitive IIIV-Silicon modulators for emerging applications in silicon photonics
The proposed thesis work consists in developing phase modulators based on the integration of IIIV-Silicon hybrid capacitors in silicon waveguides, at a wavelength of 1.55µm to meet the emerging demands of photonics (optical computing on chip, LIDAR). Unlike telecom/datacom applications, which have enabled the emergence of integrated silicon photonics, these new application fields involve circuits that require a very large number of phase modulators. All-silicon modulators based on PN junctions, which have optical losses of several dB and centimeter sizes, are a bottleneck to the emergence of these applications.
IIIV-Si hybrid capacitors can allow, thanks to the electro-optical properties of IIIV materials, to reduce the size of silicon modulators by an order of magnitude and improve their energy efficiency (reduction of optical losses). First functional modulators have been designed, fabricated and tested. The first step will be to study in details their performance (losses, efficiency, speed, hysteresis) and to understand their limitations, using the available photonic simulation tools and electrical characterization methods (C(V), interface charge density, DLTS, etc.). In particular, this will involve better understanding the impact of the manufacturing process on the electro-optical properties. In a second step, the doctoral student will propose improvements to the designs and manufacturing processes (in collaboration with our microfabrication specialists), and will validate them experimentally using hybrid capacities and modulators integrating these capacities.
improving effiiciency and directivity in color conversion µLEDs with metasurfaces
In the field of augmented reality, the development of full color µLEDs matrices is a critical step towards miniaturizing and simplifying the optical system. Current pixel architectures in microLEDs displays are based on color conversion. Short wavelength emission from a first active material is absorbed by a second active layer to be re-emitted at longer wavelength. In current architectures, re-emission follows a lambertian profile making them unsuitable for AR/VR applications.
Recent work by the Charles Fabry laboratory - Institut d’Optique, as part of E. Bailly's thesis, has demonstrated that combining metasurfaces with color converters can enable shaping the radiation pattern. The primary goal of this thesis is to apply this innovative method by integrating it with blue GaN µLEDs developed at CEA-LETI.
Throughout this thesis, the student will first design the devices using optical simulations, aiming to optimize them for both efficiency and directional angular radiation pattern. Following this, the student will fabricate the devices in the clean room at LETI and perform opto-electrical characterization.
The initial design phase will primarily take place at the Quantum Nanophotonics and Plasmonics team of Charles Fabry laboratory - Institut d’Optique, in Saclay, under the supervision of the thesis director. The student will then move to CEA-LETI in Grenoble for the fabrication, characterization and comparison with simulation results.
The selected student will benefit from the extensive expertise in nano-photonics and simulation at the Charles Fabry laboratory, as well as the technological, simulation, and characterization expertise in µLEDs at CEA-LETI.
The Quantum Nanophotonics and Plasmonics at Institut d’Optique team investigates the physics and engineering of spontaneous light emission (fluorescence, incandescence, electroluminescence), at different scales (quantum regime with single photon and single atoms, collective effects, photon condensates, condensed matter systems…).
The LITE (Emissive Technologies Integration Laboratory) at CEA-LETI focuses on manufacturing microemitting devices (µLED, OLED, LCD) in a silicon microelectronics foundry-type environment. This includes, for example improving µdisplays performances, made above ASICs, while reducing the pixel size, or demonstrating new use cases of these light sources in the field of biomedical optical sensors.
Attention-based Binarized Visual Encoder for LLM-driven Visual Question Answering
In the context of smart image sensors, there is an increasing demand to go beyond simple inferences such as classification or object detection, to add more complex applications enabling a semantic understanding of the scene. Among these applications, Visual Question Answering (VQA) enables AI systems to answer questions by analyzing images. This project aims to develop an efficient VQA system combining a visual encoder based on Binary Neural Networks (BNN) with a compact language model (tiny LLM). Although LLMs are still far from a complete hardware implementation, this project represents a significant step in this direction by using a BNN to analyze the context and relationship between objects of the scene. This encoder processes images with low resource consumption, allowing real-time deployment on edge devices. Attention mechanisms can be taken into consideration to extract the semantic information necessary for scene understanding. The language model used can be stored locally and adjusted jointly with the BNN to generate precise and contextually relevant answers.
This project offers an opportunity for candidates interested in Tiny Deep Learning and LLMs. It proposes a broad field of research for significant contributions and interesting results for concrete applications. The work will consist of developing a robust BNN topology for semantic scene analysis under certain hardware constraints (memory and computation) and integrating and jointly optimizing the BNN encoder with the LLM, while ensuring a coherent and performant VQA system across different types of inquiries.
Development of micro-optic strcture for uncooled infrared imaging sensor
In this thesis, we aim to incorporate a low-resolution angular sorting function capable of discerning the primary direction of incident infrared flux. This information is crucial for enhancing image processing algorithms, thereby facilitating faster automatic focusing, improved image segmentation, and more accurate depth estimation.
To achieve this functionality, a micro-optics network at pixel level must be designed and realised. At present, we are considering two competitive approaches: refractive microlenses and meta-surfaces. As a PhD student, your responsibilities will include:
?- Establishing the preliminary specifications for these microlenses
?- Designing the micro-optics using numerical simulation and predicting their performance
?- Overseeing the manufacturing of these micro-optics in a clean room environment
?- Characterising the micro-optics on a dedicated laser bench and performing a proof of concept by coupling them with an infrared imager
You will be fully integrated into the Laboratory of Thermal and THz Imaging of the CEA Leti which develops, realizes, and characterizes imaging technologies based on micro-bolometers.
Design and integration of microlasers within a silicon photonics platform
For about ten years, the continuous increase in internet traffic has pushed the electrical interconnections of data centers to their limits in terms of bandwidth, density, and consumption. By replacing these electrical links with optical fibers and integrating all the necessary optical functions on a chip to create transmitters-receivers (transceivers), silicon photonics represents a unique opportunity to address these issues. The integration of a light source within a photonic chip is an essential building block for the development of this technology. While many demonstrations rely on the use of external lasers or bonded laser chips, it is the direct heterogeneous fabrication of a laser within the photonic chip that would allow the desired level of performance while limiting costs.
The objective of this thesis is to provide an innovative solution for the management of very short-distance communications (inter-chip, intra-chip) by realizing, on silicon, III-V membrane microlaserswith buried heterostructures. This type of microlasermeets the numerous challenges of very short-distance links thanks to an efficiency/integrabilitycompromise superior to the state of the art of datacomlasers while being compatible with CMOS fabrication lines.
Based on the work carried out during a previous thesis, the PhD student will be responsible for (i) designing the microlasersusing the available digital simulation tools in the laboratory, then (ii) manufacturing these microlasersby relying on the technological platforms of CEA-LETI, and finally (iii) electro-optically characterizing the components. This thesis work will be carried out in collaboration between CEA-LETI and LTM/CNRS and will constitute a strategic brick necessary for future generations of photonic transceivers.
Design and optimization of color routers for image sensors
Color routers represent a promising technology that could revolutionize the field of image sensors. Composed of nanometricstructures called metasurfaces, these devices allow the modification of light propagation to improve the quantum efficiency of pixels. Thanks to recent technical advances, it is now possible to design and manufacture these structures, paving the way for more efficient image sensors.
The thesis topic focuses on the design and optimization of color routers for image sensors. Several research avenues will be explored, such as the implementation of new metasurfacegeometries (`freeform`) or innovative configurations to reduce pixel pitch (0.5µm or 0.6µm). Various optimization methods can be used, such as the adjointmethod, machine learning, or the use of auto-differentiable solvers. The designs must be resilient to the angle of light incidence and expected variations during manufacturing. After this simulation phase, the proposed structures will be manufactured, and the student will have the mission to characterize the chips and analyze the obtained results (quantum efficiency, modulation transfer function...).
This thesis will be co-supervised by STMicroelectronics and CEA LETI in Grenoble. The student will be integrated into the teams of engineer-researchers working on this project. He/she will be led to collaborate with various specialists in various fields such as lithography and optical characterization.
The student's main activities:
- Optical simulation using numerical methods (FDTD, RCWA)
- Development of optimization methodologies for metasurfacedesign (adjointmethod, topological optimization...)
- Electro-optical characterization and analysis of experimental data