This thesis is part of the ANR MAXSAW project, which aims to develop new components operating in the RF domain, adapted to the new 5G frequencies. Surface acoustic wave (SAW) filters are widely used to distinguish the different frequencies of RF signals. Unfortunately, the frequency of conventional SAW filters is limited to 3.7 GHz. Epitaxial thin films of LiNbO₃ on sapphire host guided acoustic waves that meet the demand for higher frequencies and efficiency.

We first produced LiNbO₃ thin films on sapphire substrates. Thin films of good crystalline quality were obtained. We also studied their acoustic properties using simulations, and confirmed the frequencies achievable with these structures.

Then, we simulated, designed and characterized SAW resonators based on LiNbO₃ thin films deposited on sapphire, and compared them with the state of the art. Promising acoustic devices were obtained. Finally, an application to the MAXSAW project is presented.

electrical characterisation, physical properties, piezoelectricity, acoustic waves, lithium niobate

This Thesis is embargoed until 23/09/2026.

The development of wide-band RF filters operating at high frequency is urgently needed for the implementation of the 5th generation (5G) communication infrastructure. LiNbO3 is a promising material for integration into bulk acoustic wave (BAW) resonators/filters adapted to high-frequency applications owing to its high piezoelectric properties. However, integrating this material typically involves ion-slicing/polishing techniques of single crystals, which pose challenges for thickness homogeneity and thus industrial-scale production.

In this thesis, we investigated the integration of highly-coupled 32.8°Y-LiNbO3 thin films grown by DLI-CVD in BAW resonators. We initially focused on optimizing the growth of pure phase LiNbO3 with (01-12) textured growth on a LaNiO3 seed layer. We then assessed the ferroelectric, pyroelectric, and piezoelectric properties of the grown LN films, obtaining values of P_s = 52 μC/cm², pi = 60 μC/m². K and e_(31,f) = -2.81 C/m², which are comparable to those of LN single crystal.

Next, we optimized the fabrication process for integration into a basic HBAR structure to evaluate the performance of the grown films. The HBARs demonstrated a k_eff² up to 22.4 % at a resonance frequency of 5.6 GHz. For integrating the grown films into SMRs, we first optimized the growth and studied the thermal stability of the ZnO/Pt Bragg reflector. This reflector was then used to fabricate 32.8°Y-LN based SMRs. Electrical characterization of the fabricated SMRs showed resonances at frequencies in the range of 5.2-5.7 GHz, indicating great potential for high-frequency RF filtering applications.

BAW resonators, thin films, CVD, structural properties, piezoelectricity

Confidential thesis until 03/10/2034.

Lead-free piezoelectric materials are actively investigated for energy harvesting, sensor and high-frequency acoustic wave devices. In this manuscript, different architectures and microfabrication processes based on lead-free LiNbO₃ and KTa1-xNb_xO₃ crystals single crystals are investigated.

In a first part, energy harvester of LiNbO₃ on silicon substrate is fabricated by using wafer bonding and polishing. The transducers attained one of the highest power densities (965 µW/cm²/g²) compared to Pb and Pb-free vibrational harvesting devices. Then, the scalability of the LiNbO₃/Si to MEMS technology devices is investigated with LiNbO₃ and silicon etching. The etching of LiNbO₃ have been performed by implementing a pulsed mode reactive ion etching by using Ar/SF₆ gas. Accelerometric sensor has been demonstrated.

In a second part, our interest moved toward flexible metallic substrates. A big step has been achieved by developing Au-Au bonding of LiNbO₃ to metal substrates. The performances of bimorph beam of LiNbO₃-stainless steel-LiNbO₃ attained 209.7 µW/cm²/g² at 39.3 Hz.

Finally, we investigate alternative lead-free piezoelectric materials of KTa1-xNb_xO₃ crystals, for SAW devices application. First, structural and microstructural characterization of the crystal was carried out followed by fabrication and characterization of one-port SAW resonator. An electrotechnical coupling of 80 % was achieved while 49 % was obtained for KNbO₃ resonators.

alkaline niobates and tantalates, piezoelectric energy harvesting, saw resonators, microfabrication

Confidential thesis until 15/07/2034.

This thesis explores the development of a transportable, ultra-narrow linewidth laser integrating a high-finesse Fabry-Perot cavity made from ultra-low expansion glass with optically contacted Fused Silica mirrors, aiming to minimize thermal and mechanical perturbations and enhance frequency stability. A novel digital frequency stabilization method using an FPGA-based platform is introduced, targeting a fractional frequency stability of 1×10^-15 at 1 s integration. This approach contrasts traditional analog systems by offering increased stability and reduced complexity. The study also examines several limitations of ultra-stable lasers like phase noise, thermal noise, etc. and several approaches to mitigate these types of noise. Additionally, an optical frequency dissemination system using FPGA-based phase-locked loops and optical fiber links is detailed, ensuring stable signal transmission over laboratory distances.

optical frequency standard, time and frequency metrology, ultra-stable oscillator, optical phase noise measurement, laser frequency stabilization

Full document (EN) : TEL-04823081

 Optical clocks have now reached accuracies close to 1×10⁻¹⁸. They are used for various applications, such as chronometric geodesy, tests of General Relativity, the search for physics beyond the Standard Model, and the redefinition of the SI second. 

Among neutral species, mercury has several attractive properties for an optical lattice clock, including a low sensitivity to blackbody radiation and a high vapor pressure at room temperature. Until now, the ¹⁹⁹Hg fermionic isotope was the only isotope used in mercury clocks. However, its limited lifetime in the excited state restricts the full potential of the upcoming generation of ultrastable lasers. Using bosonic isotopes instead offers a way to overcome this limitation, thanks to their potentially unlimited lifetime.

This thesis reports the first observation of the ¹⁹⁸Hg bosonic transition in an optical lattice clock, which was achieved through several key experimental advancements and a challenging search for a narrow transition across a wide uncertainty range. The bosonic clock transition is forbidden but it can become weakly allowed via a high magnetic field, a technique known as the quenching method. This approach enables longer probing times that can be adjusted to the laser properties. Therefore, the first critical step was developing a setup capable of generating a sufficiently large magnetic field to induce the bosonic transition with the highest possible coupling. Another challenge involved implementing a widely tunable and flexible probe laser while preserving its ultra-low noise characteristics, allowing the probing of any mercury isotope without introducing  additional noise. Since the coupling also increases with probe power, a major milestone was significantly boosting the power of our deep UV ultrastable light source.

Despite these experimental improvements, our calculations indicated that the coupling remained relatively weak, leading to a narrow-line transition that needed to be found over a broad frequency range. We conducted various measurements and checks, to optimize our chances of finding the transition. Thanks to these cumulative efforts, the search for the ¹⁹⁸Hg transition was successful, marking the first observation of a bosonic mercury isotope transition.

Building on this achievement, we established an operational optical lattice clock with the bosonic ¹⁹⁸Hg, already achieving a stability of 10⁻¹⁵ at 1 s. We have undertaken several studies of this new transition, including measuring the quadratic Zeeman shift coefficient with sufficient precision to control this shift to 10⁻¹⁷ or better. We have also begun investigating other systematic effects, such as the light shift, cold collisional shift, and lattice light shift, along with measuring the ¹⁹⁸Hg magic wavelength. We made a first series of comparing to ⁸⁷Sr and obtained a stability of 1.2×10⁻¹⁵ at 1 s for this comparison, paving the way for a first measurement of the ¹⁹⁸Hg/⁸⁷Sr optical frequency ratio. The work on the bosonic isotope will shortly lead to the possibility to implement more sophisticated probing methods (Hyper-Ramsey spectroscopy) that will improve the uncertainty to the limit of our current experimental setup.

This thesis also presents analyses and results obtained with the ¹⁹⁹Hg fermionic isotope during a fiber link clock comparison with several European institutes conducted in March/April 2023. 

optical lattice clock, mercury, fermionic and bosonic isotope

Full document (EN) : TEL-05034816

This thesis present the latest work realized on the absolute colds atoms gravimeter of the LNE-SYRTE. To measure g, this device, called CAG, realize an atomic interferometer with Raman pulses on a free falling atomic cloud of Rubidium 87. The accuracy of the CAG’s measurements are limited by the Raman beam, its phase and intensity profile. During the thesis, the apparatus was moved to the Observatoire de Paris to be upgraded, in order to reduce these effects and extending the limits of the CAG further.

After the restart of the device, which able us to reach similar sensibilities than previously at 20×10⁻⁹·g Hz^−1/2, the studies related to the Raman beam have been continued. The manuscript details this work, which guides to study the stability of the intensity of each Raman beam, as well as the impact of inhomogeneity in the intensity profile. This inhomogeneity has been proven to cause a light shift distribution within the atomic cloud, resulting in a loss of contrast. This phenomenon also cause a bias on the measurement of g, of the order of 5×10⁻⁹·g, which is not eliminated by the measurement algorithm, and should be taken into account into the uncertainty budget. We also present the first results of the optimal control of the Raman transitions. This experimental method aims to improve the efficiency of the transitions and, consequently, the contrast of the interferometer.

atomic interferometry, gravimeter, inertial sensor, colds atoms

Full document (FR) : TEL-05035515

Ultra-stable lasers based on spectral hole burning are a promising alternative scheme to overcome the thermal noise limitation of the traditional ultra-stable Fabry-Perot cavity scheme. In this dissertation, a Eu³⁺Y₂SiO⁵ crystal at cryogenic temperatures is used to realize narrow spectral holes as a frequency reference. A flexible and versatile multi-mode heterodyne laser probing scheme based on software defined radio is realized to achieve low detection noises.

The crystal is further cooled down to sub-kelvin temperatures to gain better thermal noise and temperature sensitivity performances. Meanwhile, some relevant spectral hole properties are characterized at this temperature regime, and some useful properties are found, which differ from theoretical predictions based on simple models.

Furthermore, a previously unobserved phenomenon, that I call “glowing”' is reported, together with preliminary analysis based on several characterization measurements, providing new insights to understand the system of Eu3+Y2SiO5 crystal.

Remarkably, the fractional frequency stability of the laser, based on the spectral hole burning scheme, has been encouragingly enhanced to 4(1)×10⁻¹⁶ at 1 s, approximately 2 times better than the previously reported results. This provides compelling evidence of the potential of the spectral hole burning approach for metrological applications.

ultra-stable laser, Eu:YSO, spectral hole, frequency metrology

Full document (EN) : TEL-04822912

Ultralight dark matter (ULDM), as a class of low mass (< 1 eV) dark matter (DM) candidates, is a compelling alternative to historically dominant models such as WIMPs and has recently gained significant attention in the scientific community. In this thesis, we study various experimental schemes for the direct detection of ULDM, both on ground and in space. More precisely, we propose a theoretical modeling of current and futuristic experiments, and we derive an estimation of their respective sensitivity.

We mainly concentrate on three distinct phenomenologies.

The first one is the coupling between a DM (1) field, known as the dark photon (DP), and electromagnetism, which induces a small electric field oscillating at the DP Compton frequency. We first propose an innovative way of detecting this small electric field by measuring the quadratic Stark shift of Rydberg atoms inside a microwave cavity, and we show that such an experiment could reach competitive constraints compared to existing laboratory experiments. Another possibility of detecting this electric field is to use a spherical mirror, which reflects it and focuses the electromagnetic power at its center of curvature, where a horn antenna is located (e.g. SHUKET experiment at CEA Saclay). We analytically investigate the effects of diffraction and mode matching in this type of experiment, and we show that the expected signal intensity can be significantly reduced compared to usual estimates. In this study, we also propose an optimization of the experimental parameters in order to increase the signal.

The second main phenomenology considered in this thesis is the oscillation of rest mass and transition frequencies of atoms and test masses. These oscillations could be produced by the non-universal coupling of standard matter with a scalar ULDM candidate (dilaton or axion-like particle). We extensively study the impact of such oscillations on various atom interferometer schemes and classical tests of the universality of free fall, and we demonstrate how these different experiments could probe unconstrained regions of the parameter space. The oscillation of rest mass could also be observed in space-based gravitational wave (GW) detectors, such as LISA, and we investigate the possibility of such detection using more realistic orbits of spacecraft compared to previous studies. In particular, using Bayesian methods, we show that LISA could disentangle scalar ULDM signals from monochromatic GWs. We also show that the small velocity of the DM wave is not resolvable for most frequencies in the LISA band, which induces a decrease in sensitivity to scalar ULDM couplings, with respect to previous studies.

 Finally, we study the effect of vacuum birefringence and dichroism induced by the coupling between axions and photons, and how it could be detected with optical cavities, fibers, and LISA. In particular, we show that a slight modification of LISA’s optical benches would make LISA the most sensitive experiment to the axion-photon coupling at low axion masses.

ultralight dark matter, axion, dilaton, dark photon, quantum sensors, atomic clock, atom interferometry, optical interferometry, LISA, equivalence principle, fundamental physics

Full document (EN) : TEL-04750272

Depuis le début du 20^e siècle, les observations géodésiques permettent de suivre les variations de la rotation de la Terre et ainsi d’étudier les effets dynamiques externes et internes à l’origine de ces variations. Les années 1980 ont vu les toutes premières tentatives d’estimation des variations sub-diurnes de la rotation terrestre grâce aux sessions intensives VLBI. Elles ont été suivies par des campagnes d’observation continue (CONT) du réseau VLBI puis les traitements des observations du GNSS. Alors que la capacité du réseau VLBI à estimer la variation subdiurne de la rotation de la Terre est encore limitée par la fréquence insuffisante des sessions d’observation régulières, le GNSS est avantagé par un réseau plus étendu et de ses observations beaucoup plus fréquentes.

Dans ce travail, nous utilisons les données de la constellation américaine GPS et de la constellation européenne Galileo couvrant la période 2017 à 2022 pour déterminer à la fois des solutions mono-constellation et des solutions multi-GNSS des ERP avec une résolution horaire avec le logiciel GINS/DYNAMO, développé et maintenu par le centre d’analyse français IGS GRG, géré par le CNES/CLS. Le choix de cette période assure une performance comparable entre les deux constellations. Nous avons testé les contraintes pour bloquer la bande rétrograde diurne (la nutation par convention).

Nous avons trouvé leur application bénéfique pour la détermination des autres bandes sub-diurnes, c’est-à-dire la bande diurne prograde et semi-diurnes prograde et rétrograde. Nous montrons également que pour une estimation horaire de UT1, il est nécessaire et suffisant de fixer les valeurs de UT1 à celles déterminées par les observations VLBI une fois par jour. Nous avons validé les solutions en comparant les réseaux terrestres et célestes résolus en même temps que les solutions ERP horaires aux réseaux des solutions finales GRG dans le cadre de la campagne IGS Repro3. Nous comparons les deux constellations par des analyses spectrales et harmoniques. Les séries temporelles des ERP après avoir retiré des effets de marées océaniques sont confrontées aux excitations géophysiques qui résultent de la circulation atmosphérique et océanique non-“maréale”, ainsi qu’à un développement harmonique dérivé des observations VLBI.

En parallèle, nous avons développé un nouveau modèle des effets sub-diurnes produits par les marées océaniques sur les ERP en nous fondant sur les derniers développements de la théorie du mouvement du pôle et de l’atlas des marées océaniques FES2014b. Une comparaison de ces résultats avec le développement harmonique VLBI met en évidence l’existence des termes de libration résultant de l’effet des marées luni-solaire sur la distribution asymétrique de la masse de la Terre dans le mouvement du pôle et l’UT1.

GNSS, mouvement du pôle, rotation de la Terre, marée océanique

Consulter la thèse (EN) : TEL-04792592

This thesis presents the first force measurements at close range between an atom and a surface, which was the ultimate objective of the Forca-G project.

The force is measured by atom interferometry with atoms trapped in potential wells formed by a vertical optical lattice. Stimulated Raman transitions are used to measure the energy difference between two wells, i.e. the force applied to the atoms. By moving the atoms to different distances from the mirror in a controlled manner using a Bloch lift, it is possible to measure variations in surface forces with a spatial resolution of the order of a micrometre. In the vicinity of the surface, a force measurement sensitivity of 3.4×10^-28 N has been achieved, which is state-of-the-art for surface force measurements. At atom-surface distances of less than a hundred micrometres, electrostatic forces dominate, due to the electrostatic fields generated by atoms adsorbed on the surface. By modelling these fields, another force is highlighted: the Casimir-Polder force. These parasitic electric fields have been measured directly with the trapped atoms, making it possible to correct, albeit imperfectly, the measurements of the impact of the electrostatic forces.

Finally, we have shown that the amplitude and the orientation of the atom-surface forces are modified by the illumination of the surface with UV light, which charges the surface.

quantum, interferometry, cold atoms, Casimir-Polder, trapped atoms

Full document (FR) : TEL-04680406