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 demonstration 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 weights. 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).

Semi-polar GaN epitaxy for high frequency µLEDs

Semiconductor nitride-based LEDs have reached a high level of maturity due to their use in the field of lighting. While the internal electric field present in InGaN quantum wells does not limit the efficiency of blue LEDs, it does induce the confined quantum Stark effect (QCSE), which limits the bandwidth of the LEDs and thus their potential use in high-frequency optical communication. In this context, the thesis aim at controlling the MOCVD growth of InGaN/GaN epi-structures on SOI (Silicon on Insulator) along a semi-polar crystallographic orientation [10-11], which mitigates the adverse effects of QCSE. The epitaxy of GaN on SOI poses several challenges that need to be addressed to achieve the buffer quality required for µLED fabrication. These challenges include the chemical reactions between Ga and silicon and the difference in thermal expansion coefficients between GaN and silicon.
This thesis will take place between CNRS-CRHEA in Valbonne, where growth conditions on small substrates will be optimized, and CEA-LETI in Grenoble, where the transfer to larger substrate formats (200mm) will occur. Understanding the growth mechanisms will be crucial for the success of this thesis, requiring in-depth structural characterization of the samples, for example, using electron microscopy or local probe techniques that provide atomic-scale characterization, as well as photo- and cathodo-luminescence techniques, etc.
Finally, this thesis will involve participating in the design and performing the electro-optical characterization of µLEDs (micro LEDs) that will be fabricated in a cleanroom from the epitaxial structures developed on both small and large substrates. The goal of this part of the work is to optimize µLED performance and adapt the epitaxial LED structures to the semi-polar orientation.

Trusted imager: integrated security based on physically unclonable functions

Images, and therefore the sensors that generate them, must respond to the challenges posed by their illicit use, either to divert their content through deep fakes, or for unauthorized access. The concept of trusted imagers responds to the need to ensure the security, authentication or encryption of images as soon as they are acquired.
Based on our first developments, the thesis will consist of searching for innovative solutions to integrate security functions into imagers. Faced with the challenges of robustness and compact integrability, the thesis aims to explore the use of physically non-clonable functions within an image sensor.
After improving the required skills, based in particular on a bibliographical study, and depending on the candidate's interests, the work will consist of:
- Develop compact circuit models in Python to identify and test physically non-clonable functions,
- Validate the proposed physically non-clonable function structures and their associated encryptions
- Analyze their robustness
- Design and simulate integrated circuits corresponding to these functions
With the objective of creating an integrated circuit, the work will take place, within CEA-Léti, with integrated circuit design and software development tools.

Study of new photodiode architecture for IR imagers

CEA-LETI play a leading role in the development of the HgCdTe material, which now performs to such an extent that it is on board the James Webb Space Telescope (JWST), enabling the observation and study of deep space with a precision unrivalled to date.
However, we believe that it is still possible to take an important step forward in terms of detection performance.
We propose to work on a new photodiode architecture that could further reduce the dark current (and therefore reduce noise and gain sensitivity at low photon flux).
Your role in this thesis will be to contribute to the development of the ultimate photodiode for ultra-high-performance IR detection, and to characterize and simulate the HgCdTe photodiodes manufactured on our technology platform.
This experimental and theoretical work will enable us to propose a physical model of objects manufactured at CEA-Leti, and to determine their sensitivity to technological parameters.
You have a Master's degree in optoelectronics or semiconductor materials physics, and a passion for applied research.
The main technical skills required are: physics of semiconductor components, optoelectronics, data processing, numerical simulations, an interest in experimental work to carry out characterizations, and theoretical work to carry out numerical simulations.

Development of a fully passive plenoptic thermal imaging system

Low-resolution plenoptics is becoming widespread in visible imagers for applications such as autofocus, image post-processing and sometimes depth estimation. Its principle is based on the association of three main constitutive elements, a pixels-sized microlens array, a focal plane array of detectors and reconstruction algorithms.
Through this thesis, we would like to evaluate the possibility of achieving such a plenoptic function in the infrared range for uncooled detector technologies (micro-bolometer)

Within the Optics and PhoTonics Department (DOPT), the Thermal and THz Imaging Laboratory (LI2T) designs, develops, produces and characterizes thermal infrared imager technologies, which are then transferred to an industrial partner. As a doctoral student, your role will consist of:
- Establish preliminary specifications for micro lenses adapted with our detectors and with the application of autofocusing.
- Design and simulate the behavior of this micro-optics and propose original designs in refractive solution or in meta-surface
- Manufacture these micro-optics after evaluating the feasibility of these designs in partnership with the people in charge of manufacturing
- Implement an existing reconstruction algorithm that will be identified in the literature
- Characterize the micro-optics on a dedicated bench
To carry out these missions, you will be integrated into the LI2T laboratory where you will be able to interact with different people in order to familiarize yourself with micro bolometer technologies and where you will have access to the calculation resources of CEA Leti.

High speed III-V on Silicon modulators for optical communications and sensing

Worldwide demand for digital interconnections is driven both by increasing data exchanges and by an increasing number of human and non-human users. Optical communications are commonly used to answer this demand, exploiting the network of optical fibers deployed to interconnect data centers and users alike. The data streams across this planet-scale infrastructure are routed at high-speed by multiple electronic-photonic transducer nodes, located everywhere from inside data centers to inside users’ homes (FTTH). Each node in turn requires multiple high performance optical transceivers, to transmit and receive the information along the network.
Photonic Integrated Circuits (PIC) have been a promising technology for low-power & high-performance optical functions in low-volume systems.The availability of relatively low-cost optical telecommunication devices, such as telecom lasers and optical modulators, has gained the interest of other fields of science and technology, working with infrared light. Photonic circuits can be tuned to function at wavelengths centered on spectral absorption rays of gases of interest (H2O, CO, CO2), for sensing purposes.

III-V Lab and CEA-LETI recently demonstrated selective area growth (SAG) of III-V materials on Silicon, a key process enabling the fabrication of different frequency-tailored quantum wells simultaneously. However, to address future needs, the performances of actual III-V on Silicon modulators need to be improved.
This thesis aims at the development of advanced modulators, in order to demonstrate state of the art transceivers, which could be implemented in the future using the previously developed SAG technology.
Such devices based on III-V waveguides on silicon will be modelled, designed, fabricated and tested towards high-speed light phase and intensity modulation.
This work will require opto-electronic optimizations of the InP-based III-V stack and coupling to the SOI waveguides in order to reach low losses, low absorption, and a higher phaseshift per volt, for high bandwidth capabilities reaching 100 GHz.
High-speed phase modulators will be demonstrated in coherent communication systems, targeting beyond 16QAM modulation schemes for 400 Gbps/lane.
High-precision frequency modulation will be demonstrated in single-sideband modulation (oSSBM), for 100 GHz frequency shifting in sensing applications.

The PhD will be based at CEA-LETI (Grenoble), within the Silicon Photonics Integration Laboratory, in the framework of the R&D Photonics Program existing between CEA and III-V Lab that will co-supervise the thesis work.

New architectures in inverse geometry for spectral X-ray imaging

Emerging technologies in the field of X-ray sources and detectors open up possibilities for envisioning new breakthrough systems in 3D imaging. In conventional tomography, a large-area detector captures images of an object exposed to X-rays from a point source. The latest generations of medical scanners also incorporate spectrometric semiconductor detectors, providing a real improvement in image quality.
The proposed thesis aims to shift the paradigm by designing a system that combines numerous distributed X-ray sources with a small-sized spectrometric detector. This inverse geometry is innovative in terms of system architectures, allowing relaxation of constraints on sensor dimensions and reduction of certain artifacts.
The thesis work will revolve around the design and simulation of new systems in inverse geometry, along with the development of associated reconstruction algorithms. These algorithms, based on proximal methods and capable of integrating neural networks, must leverage the rich information provided by the spectrometric detector under conditions of sparse acquisition. The student will utilize simulation and reconstruction tools developed within the laboratory and will also have access to experimental resources for validating the developments. Working within a multidisciplinary laboratory with extensive experience in spectrometric detector and X-ray system design, the student will engage in exchanges with external teams, including radiologists, to incorporate a final need into the research.

Semiconductor perovskites for the future of medical radiography: experimental analysis of doping and link to electrooptical performance

X-rays is the most widely used medical imaging modality for the detection of pathologies, the monitoring of their evolution and during certain surgical procedures.
The objective of this thesis is to study a new semiconductor material based on perovskites for direct X-ray detection. Their use in the form of photoconductive devices in matrix imagers should make it possible to improve the spatial resolution of images and increase the signal, and thus to treat patients better. Prototype X-ray imagers manufactured at the CEA provide better spatial resolution than current systems, but the detector material still needs to be improved.
To this end, the doctoral student, a physicist and experimenter, will study the link between the structural properties of CsPbBr3 layers and the transport properties of charge carriers in these layers. He will then analyse the effect of intrinsic and extrinsic doping of the layers on the dark current, photocurrent and electrical stability of the devices. The results of this thesis will provide a detailed understanding of the mechanisms responsible for the performance of CsPbBr3-based X-ray imagers.

Quantum Cascade III-V/Si laser micro-sources

This thesis project focuses on the development of innovative micro-laser sources by combining III-V Quantum Cascade materials with Silicon Photonic Crystals. By integrating these advanced technologies, we aim to create hybrid lasers emitting in the middle infrared. This approach has significant advantages for medium-infrared spectrometry (MIR), a crucial technique for the chemical detection of gaseous, solid and liquid compounds.
The CEA-LETI Optical Sensor Laboratory offers a state-of-the-art research environment, where the candidate will have the opportunity to design, model, manufacture and characterize these devices. This thesis is part of a competitive but promising context, where technological advances could open new perspectives in areas such as "well-being and the environment". For Master 2 students who are passionate about photonics and emerging technologies, this research offers an opportunity to actively contribute to innovation in a growing field.

Development of integrated superconducting nanowire single photon detectors on silicon for photonic quantum computing

The development of quantum technologies represents a major challenge for the future of our society, in particular to build unhackable communications as well as quantum computers offering computing power well beyond that available with current supercomputers. Photonic quantum bits (or qubits), in the form of single photons, are robust against quantum decoherence and are therefore very attractive for these applications. At CEA-LETI, we are developping an integrated quantum photonics technology on silicon wafers, compatible with industrialization, comprising key building blocks for qubit generation, manipulation and detection on-chip.
The PhD project will be focused on the development of integrated superconducting nanowire single photon detectors, sensitive to the presence of a single photon, required for photonic quantum computing. The objective will be the design of superconducting single photon detectors integrated with ultra-low loss waveguides used for the core of the quantum computing processor, the development of a clean room fabrication process compatible with the existing silicon photonics platform and the characterization of the detector figures of merit (detection efficiency, dark count rate, timing performances) using attenuated lasers. The final goal of the PhD will be the integration of small circuits including several detectors on-chip to characterize the purity and indistinguishability of single photons emitted by a quantum dot source developped in parallel at CEA-IRIG (also located in Grenoble).
This PhD work will be carried out in collaboration between CEA-LETI and CEA-IRIG and will be a strategic cornerstone at the heart of future generations of quantum photonic processors featuring several tens of qubits.

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