PhD defense
(in french)
Centre de nanosciences et de nanotechnologies, Amphithéâtre, Palaiseau
PhD defense
Identical quantum dot-based single-photon sources
Centre de nanosciences et de nanotechnologies, Amphithéâtre, Palaiseau
PhD defense
Semiconductor quantum dots embedded in micropillar cavities are bright and deterministic emitters of indistinguishable single photons. Scaling up quantum photonic technologies requires interfering photons emitted from independent sources, which in turn demands perfectly identical emitters. Achieving such reproducibility relies on precise cavity fabrication, robust tuning mechanisms, and environmental noise mitigation.
In this thesis, we address all three of these challenges. We first adjust the micropillar cavity diameters individually and narrow the acceptable spectral range of the QD transitions to fabricate sources with nearly identical optical properties. Secondly, we study the noise affecting our single-photon source and find lower QD density across the grown wafers can reduce the charge noise. Finally, making use of our p-i-n diode structure and a fiber on top of the source, we implement both electrical and strain tuning on the emitters.
We demonstrate state-of-the-art two-photon interference of 88% between remote cavity-embedded QDs. This value is in close agreement with the estimated upper bound of 88.3(1)% , calculated by considering only the individual source Hong–Ou–Mandel interference visibilities and the difference in decay rates between the QDs, demonstrating the near-perfect stability of our emitters. These results mark a critical milestone for the development of multi-source quantum devices and pave the way toward the implementation of scalable quantum networks and photonic quantum computers.
Jury members :
Figure : Scanning electron microscope image of three micropillar single-photon sources.
Control of mid-infrared light dispersion and polarization with SiGe photonics / Contrôle de la dispersion et de la polarisation de la lumière infrarouge moyenne via la photonique SiGe
Centre de nanosciences et de nanotechnologies, Amphithéâtre, Palaiseau
PhD defense
The mid-infrared is a key spectral range for spectroscopy and chemical sensing. Such applications could become accessible to a broad audience through photonic integration. In this context, this thesis explores the use of silicon-germanium (SiGe) photonics to control two properties of quantum cascade lasers (QCLs): dispersion and polarization. First, dispersion control using variable-period Bragg gratings combined with a multimode interferometer enabled the generation of 1.39 ps ultrashort pulses around 8 µm from a QCL frequency comb. Second, a polarization rotator consisting of a mode-evolution waveguide and an asymmetric directional coupler was developed, demonstrating a conversion rate exceeding 88% between 7.5 and 8.9 µm. These achievements constitute essential building blocks for the development of fully integrated spectroscopic systems.
Jury members :
(in french)
Centre de nanosciences et de nanotechnologies, Amphithéâtre, Palaiseau
PhD defense
Electric-field-assisted phase switching in semiconducting nanowires Contrôle par champ électrique de la structure cristalline de nanofils semi-conducteurs
Centre de nanosciences et de nanotechnologies, , Palaiseau
PhD defense
Polytypism, namely the existence of different stacking sequences of the crystal planes of the same material, provides a route to heterostructure formation in semiconductor nanowires. In III–V nanowires, such crystal phase heterostructures can be obtained through the controlled stacking of the cubic (zinc blende) and hexagonal (wurtzite) structures. One may thus form crystal phase quantum dots (CPQDs), which are promising building blocks for quantum devices.
This thesis investigates the precise control of crystal phases in GaAs nanowires for CPQD fabrication using electric-field (E-field) assisted phase switching. Growth is performed by metalorganic chemical vapor deposition in the in situ NanoMAX transmission electron microscope, using specially designed cantilevers that provide a heated substrate and allow one to apply the E-field. The nanowires grow epitaxially in the vapor-liquid-solid mode with a liquid catalyst droplet at their tips. The method consists in changing quasi-instantaneously the shape of the droplet, and therefore its contact angle, by applying the E-field.
A major contribution is the demonstration of reversible crystal phase switching with atomically abrupt interfaces. Crystal phase insertion sizes down to a single monolayer are achieved. Several factors influencing CPQD formation are investigated. In addition to the droplet contact angle, a high enough temperature is found to be another key parameter for wurtzite growth. This is attributed to the temperature-dependent decomposition of metalorganic precursors, which may modify the surface energies of the nanowire facets and consequently affect crystal phase selection. In addition, the polarity of the E-field is found to affect the growth rate. Upon E-field reversal, growth rates become lower or higher than under no E-field. The possible causes of this effect are discussed.
Numerical simulations are used to investigate the effect of the E-field on the geometry of the droplet and to elucidate the mechanism of E-field-induced phase switching. The critical contact angle for phase selection is calculated from a combination of simulations and classical nucleation theory, and is compared with the experiments.
Jury members :
Rapporteure/rapporteur:
Examinateur/examinatrice:
Figure: By applying an electric field (E-field) during the growth of GaAs nanowires, we achieve reversible crystal phase switching between zinc blende (ZB) and wurtzite (WZ). The E-field dynamically deforms the catalyst droplet, reducing its contact angle j below the critical value for WZ formation and thereby triggering the ZB-to-WZ transition with monolayer precision. The TEM image shows a sharp crystal phase heterostructure obtained by this method, with WZ segments formed under E-field.
(in french)
Centre de nanosciences et de nanotechnologies, Amphithéâtre, Palaiseau
PhD defense
Métaplasticité Bayésienne : l’Incertitude comme Principe Algorithmique pour l’Apprentissage Embarqué
Centre de nanosciences et de nanotechnologies, , Palaiseau
PhD defense
Artificial intelligence is increasingly moving from data centers to edge devices, where neural networks must operate and learn continuously under severe constraints on memory, computation, and energy. At the same time, real-world environments are dynamic, requiring models to adapt to new information without overwriting what they have already learned. This thesis investigates a fundamental question for edge AI: can a neural network regulate its own plasticity and determine which information should be learned, retained, or forgotten?
The thesis explores Bayesian metaplasticity as a general framework for addressing this challenge. It first introduces MESU, a Bayesian approach to continual learning in which each network parameter maintains an estimate of its uncertainty. By incorporating uncertainty into the learning dynamics and retaining a bounded memory of past information, MESU allows parameters to adapt their plasticity according to their confidence. This provides a principled balance between stability and adaptation without requiring explicit task boundaries or replay buffers.
The thesis then introduces BiMU, a binary Bayesian extension designed for resource-constrained computing. By combining one-bit parameters with uncertainty-driven plasticity, BiMU reduces the memory and computational requirements of continual learning while preserving uncertainty over long, non-stationary data streams. This uncertainty can also be exploited to identify the most informative samples and enable selective adaptation through active learning, further improving the efficiency of learning at the edge.
Finally, the thesis investigates how the same principle can be implemented at the hardware level through magneto-ionic synapses. The intrinsic electrochemical dynamics of these devices introduce history dependence and nonlinear update dynamics, providing a potential physical substrate for metaplasticity in which the synapse itself regulates the magnitude of its updates according to its internal state and history.
Overall, this thesis connects Bayesian inference, continual learning, low-precision neural networks, and neuromorphic hardware around a common principle: uncertainty can act as a mechanism for regulating plasticity. This perspective points toward edge AI systems whose synapses can autonomously determine when and how strongly they should adapt to their environment, enabling continuous and energy-efficient learning under severe hardware constraints.
Figure 1. Bayesian metaplasticity, from the problem to the device. A chip implanted in the brain must keep learning for years without the cloud and with almost no memory or energy, so each synapse has to decide by itself how much to change. The thesis lets a weight's uncertainty set its plasticity: a large variance (gold) leaves the weight free to learn, a small variance (purple) protects it, data narrows the distribution (Bayesian learning) and time widens it back (Bayesian forgetting). The principle is brought onto the chip in two ways: in the algorithm, weights shrink to one bit while their variance survives as a hidden confidence; in the hardware, a magneto-ionic memory cell shows history-dependent updates of its own.
Jury members:
Crystallographic alignment and electric field control in bilayer graphene and boron nitride heterostructures
Centre de nanosciences et de nanotechnologies, , Palaiseau
PhD defense
Two-dimensional materials can be stacked into van der Waals heterostructures, exhibiting novel properties tunable through layer combination, interlayer spacing and crystallographic alignment. This thesis experimentally investigates the effects of moiré superlattice and perpendicular electric field on the electronic properties of heterostructures based on Bernal bilayer graphene (BBG) and hexagonal boron nitride (BN). Using dynamically rotatable devices, we tune the relative angle between graphene and the top BN layer via AFM tip driven rotation. Dual gated rotatable devices, developed during the thesis, allow to independently tune carrier density and displacement field while retaining angular control.
Charge transport measurements in local and non-local configurations, from low to high temperatures, reveal distinct electronic properties at 0° and 60° alignments of BBG and BN. Numerical simulations attribute these differences to distinct in-plane atomic relaxation at the two alignments, arising from inequivalent stacking configurations. Using dual gated devices, we disentangle the respective impact of moiré superlattice and displacement field in opening an energy gap at the charge neutrality point of BBG, revealing a non-trivial angular dependence of the built-in crystal field, in agreement with simulations. We further observe the anomalous gating effect in BN/BBG/BN heterostructures, marked by gate-induced hysteresis and gate ineffectiveness, and we identify the 30° ± 15° rotation between the two BN layers as the triggering parameter. We show that this effect can be switched ON and OFF by rotating the top BN layer, with angle control shifting the device between three distinct metastable states.
Together, these results show that the twist angle affects the properties of van der Waals heterostructures not only through the alignment of the main active layers, but through the angular configuration of all layers involved.
Jury members :
Figure 1. (a) and (b), Schematics of a dual gated rotatable device and its cross section. The device is composed of: Si/SiO2 substrate, graphite bottom gate, bottom BN flake, BBG Hall bar, top gate made of BN/Ti/Au rotator and flexible graphite electrode.
Active and passive devices on III-V platform for mid-IR integrated photonics
Centre de nanosciences et de nanotechnologies, , Palaiseau
PhD defense
The mid-infrared (2–20 𝜇𝑚) is of great interest for spectroscopy and optical communications, thanks to its characteristic molecular absorptions and its two atmospheric transparency windows (3–5 𝜇𝑚 and 8–12 𝜇𝑚). Advances in materials and epitaxial growth are driving the development of mid-IR photonic integrated circuits (PICs), offering the prospect to realize compact, robust, and energy-efficient systems for portable spectroscopy and real-time detection applications. The research on mid-IR PICs focuses on reducing propagation losses of dielectric waveguides. In addition, efforts are devoted to develop high-𝑄 resonators, versatile passive devices useful for filtering, wavelength division multiplexing and Kerr frequency comb generation (as demonstrated in the near infrared region). Furthermore, efficient and ultrafast integrated mid-IR modulators need developments, especially in the long-wave infrared region. They allow for an improved functionality of photonic integrated circuits, enabling electro-optic comb generation, synchronous detection, and high-speed data transmission for optical communications. The choice of a PIC platform depends heavily on the transparency of its constituent materials: III–V semiconductors are particularly attractive due to their low optical losses and their ability to monolithically integrate active and passive components. In this context, we investigated the performance of III–V passive devices, notably low-loss waveguides and high-Q resonators. In addition, we fabricated ultrafast amplitude modulators operating in the 8–12 μm band, which exploit intersubband transitions in asymmetric coupled quantum wells to achieve modulation.
Jury members:
(in french)
Centre de nanosciences et de nanotechnologies, , Palaiseau
PhD defense
