Development and Characterization of Terahertz Source Matrices Co-integrated in Silicon and III-V Photonics Technology
The terahertz (THz) range (0.1–10 THz) is increasingly exploited for imaging and spectroscopy (e.g. security scanning, medical diagnostics, non-destructive testing) because many materials are transparent to THz radiation and have unique spectral signatures. However, existing sources struggle to offer both high power and wide tunability: electronic sources (diodes, QCLs) deliver milliwatts but over narrow bands, while photonic emitters (photomixers in III–V semiconductors) are tunable across broad bands but emit only microwatts. This thesis aims to overcome these limitations by developing an integrated matrix of THz sources. The approach is based on photomixing two 1.55 µm lasers in III–V photodiodes to generate a phase-coherent THz current coupled to THz antennas.
Initially, the PhD student will experimentally investigate an existing 16-element THz antenna array (STYX project) CEA-CTReg/DNAQ: setting up the test bench, measuring phase coherence, optical coupling, radiation lobes, and constructive interference. These experiments will provide a scientific foundation for the subsequent design of an integrated photonic array on silicon. The student will simulate the photonic architecture (couplers, waveguides, phase modulators, Si/III–V transitions) synchronizing multiple InGaAs photodiodes. Prototyping will include the fabrication of silicon photonic circuits (CEA-LETI) and THz photodiodes/antennas in InP (III-V Lab or, to be confirmed, Heinrich-Hertz-Institut of the Fraunhofer—HHI), followed by their hybrid integration (bonding, alignment).
This thesis will also rely on close collaboration with the IMS laboratory (Bordeaux), which is nationally and internationally recognized for its expertise in silicon photonics and THz systems. IMS will provide complementary expertise in optical modeling, electromagnetic simulation, and experimental characterization, reinforcing the multidisciplinary strength of the project.
Finally, the ultimate goal of this thesis is to develop a proof-of-concept demonstrator with a few phase-locked THz emitters (e.g. 4–16) will be produced and characterized, showing enhanced beam directivity and output power thanks to constructive interference. This demonstration will pave the way for large-scale THz source arrays with significantly improved range and penetration for advanced THz imaging systems.
Fluid–structure interaction in mixtures: theory, numerical simulations and experiments
This PhD project is part of research on fluid–structure interactions (FSI) in complex media, particularly fluid mixtures involving multiple phases (liquid/liquid or liquid/gas) and/or suspended particles. The objective is to develop a thorough, multi-scale understanding of the coupled mechanisms between deformable structures (such as droplets, interfaces, or flexible walls) and the flows of complex mixtures, by combining theoretical modelling, advanced numerical simulations, and comparison with experimental data.
Fluid-structure coupling with Lattice-Boltzmann approach for the analysis of fast transient dynamics in the context of hydrogen risk
With a view to preparing for the future in the field of high-fidelity, high-performance simulation, the CEA is working with its academic and industrial partners to explore the potential of fluid-structure couplings involving Lattice Boltzmann Methods (LBM). The coupling is part of an open-source standard promoted by the CEA, and promising first steps have been taken for compressible flows interacting with structures undergoing large displacements and rupture. Significant obstacles remain to be overcome, particularly for more complex fluid representations that are representative of industrial needs, especially for the safety of carbon-free energy devices such as batteries and nuclear reactors.
This doctoral work therefore focuses on extending the available basic building blocks to the case of flame propagation in hydrogen/air mixtures, in deflagration and detonation regimes with possible transition between the two, and in interaction with flexible structures undergoing finite displacement. This presupposes, in particular, the consideration of compressible flows with high Mach numbers significantly exceeding those used to date, requiring an in-depth reanalysis of coupling schemes and fluid-structure interaction techniques.
The thesis will be part of a collaboration between the IRESNE Institute (CEA Cadarache) and the M2P2 laboratory (AMU). The work will be mostly localized at M2P2 with a close methodological supervision from IRESNE, especially in the field of coupling techniques.
Localised solidifications in Molten Salt Reactors
In a Molten Salt Reactor (MSR), the nuclear fuel is a liquid, high-temperature salt which acts as its own coolant. Some accidental transients (over-cooling of the fuel, leak) may cause localised solidifications of the fuel salt in the core. These solidifications will have in turn an impact on the salt flow in the core, as well as its neutronic behavior, and could lead to a localised over-heating of the core vessel. Such transients are not well studied, although they have a major impact on the safety and design of an MSR.
The objective of the PhD is to study different accidental transients that would lead to localised solidifications, and to study their impact on the neutronics and thermal-hydraulics of the core. These analyses will require the use of multiphysics, MSR-adapted numerical tools, such as the CFD code TrioCFD and its extensions TRUST-NK (neutronics) and Scorpio (reactive transport), as well as the deterministic neutronic code APOLLO3. In order to balance precision and computation time, different models will be tested, depending on the transient studied: 1D/ turbulent 3D (RANS, LES) models for thermal-hydraulics ; diffusion / SPn transport / Sn transport for neutronics.
AI-Driven Network Management with Large Language Models LLMs
The increasing complexity of heterogeneous networks (satellite, 5G, IoT, TSN) requires an evolution in network management. Intent-Based Networking (IBN), while advanced, still faces challenges in unambiguously translating high-level intentions into technical configurations. This work proposes to overcome this limitation by leveraging Large Language Models (LLMs) as a cognitive interface for complete and reliable automation.
This thesis aims to design and develop an IBN-LLM framework to create the cognitive brain of a closed control loop on the top of an SDN architecture. The work will focus on three major challenges: 1) developing a reliable semantic translator from natural language to network configurations; 2) designing a deterministic Verification Engine (via simulations or digital twins) to prevent LLM "hallucinations"; and 3) integrating real-time analysis capabilities (RAG) for Root Cause Analysis (RCA) and the proactive generation of optimization intents.
We anticipate the design of an IBN-LLM architecture integrated with SDN controllers, along with methodologies for the formal verification of configurations. The core contribution will be the creation of an LLM-based model capable of performing RCA and generating optimization intents in real-time. The validation of the approach will be ensured by a functional prototype (PoC), whose experimental evaluation will allow for the precise measurement of performance in terms of accuracy, latency, and resilience.
DEM-LBM Coupling for simulating the ejection of immersed granular media in compressible Fluid under High Pressure Gradients
In Pressurized Water Reactors (PWRs), the fuel consists of uranium oxide (UO2) pellets stacked in metallic cladding. During a Loss of Coolant Accident (LOCA) scenario, the rapid temperature increase can cause deformation and sometimes rupture of these claddings. This phenomenon can potentially lead to the ejection of fuel fragments into the primary circuit. This phenomenon is known as FFRD (Fuel Fragmentation, Relocation, and Dispersal). Since the cladding is the first safety barrier, it is crucial to evaluate the amount of dispersed fuel. Experimental studies have shown that the size, shape of the fragments, shape of the breach, and internal pressure significantly influence the ejection. However, the speed of the initial depressurization phase makes direct measurements difficult. Numerical approaches, particularly through fluid-grain coupling (LBM-DEM), offer a promising alternative. The IRESNE Institute at CEA Cadarache, through the PLEIADES software platform, is developing these tools to model the behavior of fragments. However, the compressibility of the gas needs to be integrated to accurately reproduce the initial depressurization. In this context, the laboratory M2P2 of the CNRS, a specialist in modeling compressible flows with the LBM method and developer of the ProLB software, brings its expertise to integrate this effect. The thesis therefore aims to design and improve a compressible model in the LBM-DEM coupling, to conduct a parametric study, and to develop a 3D HPC demonstrator capable of leveraging modern supercomputers.
This CEA thesis will be conducted in close collaboration between the Fuels Research Department (DEC) of the IRESNE Institute at CEA Cadarache and the laboratory M2P2 (CNRS). You will be primarily located at M2P2 but will make regular visits to CEA within the Fuel Simulation Laboratory, to which you will be affiliated. The approaches developed in this thesis ensure a high scientific level with numerous potential industrial applications both within and outside the nuclear field.
Chemical and mechanical properties of N-A-S-H aluminosilicates of geopolymer
Management of low- and medium-level nuclear waste relies primarily on cements, but their limitations with regard to certain types of waste (reactive metals, oil) require the exploration of new, more effective materials. Geopolymers, particularly those composed of hydrated sodium aluminosilicates (Na2O–Al2O3–SiO2–H2O, or N–A–S–H), appear to be a promising alternative thanks to their chemical compatibility with certain types of waste.
However, despite the growing interest in geopolymers, scientific obstacles remain: 1) The available thermodynamic data on N-A-S-H is still incomplete, making it difficult to predict their long-term stability via modeling, 2) The role of their atomic structure in regard to their reactivity remains unclear, and 3) The links between chemical composition (in terms of Si/Al ratio) and mechanical properties are not established, limiting the representativeness of the models created.
By combining experimentation and modeling in order to link atomic structure and properties, this thesis aims to obtain robust and novel data on the chemical and mechanical properties of N-A-S-H. The thesis is organized around three major objectives: 1) determining the impact of N-A-S-H composition on dissolution and establishing thermodynamic solubility constants, 2) characterizing their atomic structure (aluminols, silanols, and hydrated environments) using advanced NMR spectroscopy, and 3) linking their mechanical properties, measured by nanoindentation, to their structure and composition using molecular dynamics modeling.
Study of oxygen and hydrogen diffusion processes in pre- and post-transitional oxide layers formed on zirconium alloys
The corrosion mechanisms of zirconium alloys in pressurised water reactors are still a subject of debate more than half a century after the first research on this material. The literature reports two distinct mechanisms for the transport of diffusing species in oxide layers: one favours the molecular diffusion of oxygen and hydrogen through interconnected nanopore channels during the pre-transient regime, while the other favours diffusion via short circuits (grain boundaries, etc.) in the oxide layer. In the latter case, the oxide layer is considered to be relatively homogeneous and impermeable to the oxidising medium, in this case the water in the primary circuit. On the other hand, the first interpretation is based on the principle that there is a layer that is permeable to the medium due to an interconnected network of nanopores, even during the pre-transient regime, with the density of percolated nanopores increasing over time.
Technically speaking, how can we decide between these two divergent interpretations in terms of the diffusion mechanism, which consequently leads to different solutions for protection against degradation? What is the reaction mechanism that ultimately leads to the hydration of Zr alloys and their oxidation?
To address this challenge, we will explore diffusion processes by studying the dissociation-recombination rates of molecular species at different temperatures in equi-isotopic gas mixtures such as H2/D2, 18O2/16O2, H218O/D216O, H218O/D2, etc., using an experimental device equipped with a mass spectrometer that tracks the molecular species of interest in real time.
Physical-attack-assisted cryptanalysis for error-correcting code-based schemes
The security assessment of post-quantum cryptography, from the perspective of physical attacks, has been extensively studied in the literature, particularly with regard to the ML-KEM and ML-DSA standards, which are based on Euclidean lattices. Furthermore, in March 2025, the HQC scheme, based on error-correcting codes, was standardized as an alternative key encapsulation mechanism to ML-KEM. Recently, Soft-Analytical Side-Channel Attacks (SASCA) have been used on a wide variety of algorithms to combine information related to intermediate variables in order to trace back to the secret, providing a form of “correction” to the uncertainty associated with profiled attacks. SASCA is based on probabilistic models called “factor graphs,” to which a “belief propagation” algorithm is applied. In the case of attacks on post-quantum cryptosystems, it is theoretically possible to use the underlying mathematical structure to process the output of a SASCA attack in the form of cryptanalysis. This has been demonstrated, for example, on ML-KEM. The objective of this thesis is to develop a methodology and the necessary tools for cryptanalysis and residual complexity calculation for cryptography based on error-correcting codes. These tools will need to take into account information (“hints”) obtained from a physical attack. A second part of the thesis will be to study the impact that this type of tool can have on the design of countermeasures.
Surface technologies for enhanced superconducting Qubits lifetimes
Materials imperfections in superconducting quantum circuits—in particular, two-level-system (TLS) defects—are a major source of decoherence, ultimately limiting the performance of qubits. Thus, identifying the microscopic origin of possible TLS defects in these devices and developing strategies to eliminate them is key to superconducting qubit performance improvement. This project proposes an original approach that combines the passivation of the superconductor’s surface with films deposited by Atomic Layer Deposition (ALD), which inherently have lower densities of TLS defects, and thermal treatments designed to dissolve the initially present native oxides. These passivating layers will be tested on 3D Nb resonators than implemented in 2D resonators and Qubits and tested to measure their coherence time. The project will also perform systematic material studies with complementary characterization techniques in order to correlate improvements in qubit performances with the chemical and crystalline alteration of the surface.