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.
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.
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
Understanding and Modeling Laser Cutting Mechanisms for Dismantling
For over 30 years, the Assembly Technologies Laboratory (LTA) at CEA Saclay has been conducting research to develop innovative tools for the dismantling of nuclear facilities, by designing laser cutting processes to work in hostile environments. This technology is suitable to cut thick materials, either in air or underwater, and has proven particularly effective for dismantling operations due to its precision and ability to limit aerosol generation. Today, this technology is considered safe and reliable, thanks to the efforts achieved through the European project "LD-SAFE".
However, technical challenges remain, particularly the management of residual laser energy, which, by propagating beyond the cut piece, can damage surrounding structures.
Initial studies, including a PhD thesis, have made it possible to develop numerical models to predict and control this energy, yielding significant advancements. Nevertheless, technological challenges remain, such as handling thicker materials (>10 mm), cutting multi-plate configurations, and considering the addition of oxygen to improve cutting efficiency.
The objective of the PhD is to address these challenges and to gain a better understanding of the laser cutting process and the propagation of residual laser energy. The doctoral student will refine the numerical model to predict its impact on background structures, particularly for thick materials and multi-plate configurations. The work will include the development of a multiphysics model, validated by experiments, with a particular focus on the effect of oxygen, the creation of simplified models, and adaptation for use by operators.
The PhD will be conducted in collaboration between the Assembly Technologies Laboratory (LTA) at CEA Saclay and the Dupuy de Lôme Research Institute (IRDL - UMR CNRS 6027) at the University of South Brittany (Lorient).
Experimental study of the two-phase natural convection and vaporization regimes in the cooling pool of a nuclear facility
Nuclear energy, with low CO2 emissions, is one of the major players in France's energy transition. In this context, the management of the cooling of irradiated fuel elements is a matter of utmost importance. This thesis focuses on two-phase natural convection flows and vaporization phenomena that can develop in the cooling pools of various nuclear facilities, particularly those having a significant vertical variation in the saturation temperature of the coolant due to their great depth. These pools are used to dissipate the residual heat from irradiated fuels in many types of nuclear reactors, both existing and planned. In an accident scenario with a significant heat release from the fuels, the water in these pools can vaporize, eventually limiting their cooling capability. Among the possible phase change mechanisms in deep pools is the gravity-driven flashing, a phenomenon found in various natural or industrial systems analogous to vertical channels heated from below. However, this phenomenon has been little studied in the specific configuration of a pool and was only recently observed in this context. Therefore, the objective of this thesis is to better understand the phenomenon, as well as the turbulence induced within the coolant by the bubbles it generates, in order to improve state-of-the-art thermal-hydraulic models for simulating such pools. The proposed research, of an experimental nature, will be conducted in collaboration with the Catholic University of Louvain (UCLouvain, Belgium) and the LEGI laboratory of CNRS Grenoble, with a significant portion of the research carried out at UCLouvain. The candidate will be affiliated to the Core and Circuit Thermal-hydraulics Laboratory (LTHC) of CEA IRESNE, specialized in the study of two-phase flows in nuclear facilities. During the thesis, finely resolved experimental data in both space and time will be acquired and interpreted, contributing to a better understanding of the phenomenon. To achieve this, advanced techniques such as stereo particle image velocimetry (3D PIV) in two-phase media, thermometry and shadowgraphy will be employed. During this thesis project, the PhD student will be able to develop skills in the field of experimental thermal-hydraulics through the definition, execution, and interpretation of tests, as well as the use of advanced two-phase flow measurement techniques.
Sperm 3D - Male infertility diagnostic tool using holography for imaging and 3D tracking
Infertility is a growing problem in all developed countries. The standard methods for the diagnostic of male infertility examine the concentration, motility and morphological anomalies of individual sperm cells. However, one in five male infertility cases remain unexplained with the standard diagnostic tools.
In this thesis, we will explore the possibility to determine the male infertility causes from the detailed analysis of 3D trajectories and morphology of sperms swimming freely in the environment mimicking the conditions in the female reproductive tract. For this challenging task, we will develop a dedicated microscope based on holography for fast imaging and tracking of individual sperm cells. Along with classical numerical methods, we will use up-to date artificial intelligence algorithms for improving the imaging quality as well as for analysis of multi-dimensional data.
Throughout the project we will closely collaborate with medical research institute (CHU/IAB) specialized in Assisted Reproductive Technologies (ART). We will be examining real patient samples in order to develop a new tool for male infertility diagnosis.
Development of a lensless microfluidic instrument for in-situ measurement of facies-dependent dissolution kinetics
This thesis is part of an ambitious program designated as a priority research program. This project identifies the subsoil as a major reservoir of resources necessary for the energy transition.
One of the major issues is the dissolution of ores in the context of mining and extractive metallurgy. In particular, with the objective of process industrialization, the dissolution kinetics of ores must be compatible with the footprint of the installations, biocompatibility and the volume of reagents consumed.
The observation today is the very strong mismatch between the volume of experimental data produced and those necessary to model the chemical processes essential to demonstrate the viability of industrial processes.
This thesis proposes to develop a millifluidic prototype bench for mass kinetic data acquisition using lensless imaging techniques. This will make it possible to measure dissolution reaction kinetics using 3D reconstitution techniques, in-situ, under stable chemical conditions and with statistical representativeness allowing the original properties of the solid to be taken into account.
A large part of the research will be directed towards the development of the lensless optical technique in a millifluidic device and the mass production of chemical kinetic data for catalytic dissolution models.
The desired profile is that of a general physics and chemistry student, with a strong desire to learn in areas they are least familiar with, such as microfluidics or optics. At the end of this thesis, the student will acquire solid professional experience in applied research and will learn to evolve in a multithematic environment.